refactor: remove legacy edit mode

packages/opencode/src/acp/agent.ts

@@ -347,21 +347,13 @@ export namespace ACP {
                 ]
 
                 if (kind === "edit") {
-                  const input = part.state.input
-                  const filePath = typeof input["filePath"] === "string" ? input["filePath"] : ""
-                  const oldText = typeof input["oldString"] === "string" ? input["oldString"] : ""
-                  const newText =
-                    typeof input["newString"] === "string"
-                      ? input["newString"]
-                      : typeof input["content"] === "string"
-                        ? input["content"]
-                        : ""
-                  content.push({
-                    type: "diff",
-                    path: filePath,
-                    oldText,
-                    newText,
-                  })
+                  const diff = editDiff(part.state.input, part.state.metadata)
+                  if (diff) {
+                    content.push({
+                      type: "diff",
+                      ...diff,
+                    })
+                  }
                 }
 
                 if (part.tool === "todowrite") {
@@ -862,21 +854,13 @@ export namespace ACP {
               ]
 
               if (kind === "edit") {
-                const input = part.state.input
-                const filePath = typeof input["filePath"] === "string" ? input["filePath"] : ""
-                const oldText = typeof input["oldString"] === "string" ? input["oldString"] : ""
-                const newText =
-                  typeof input["newString"] === "string"
-                    ? input["newString"]
-                    : typeof input["content"] === "string"
-                      ? input["content"]
-                      : ""
-                content.push({
-                  type: "diff",
-                  path: filePath,
-                  oldText,
-                  newText,
-                })
+                const diff = editDiff(part.state.input, part.state.metadata)
+                if (diff) {
+                  content.push({
+                    type: "diff",
+                    ...diff,
+                  })
+                }
               }
 
               if (part.tool === "todowrite") {
@@ -1630,6 +1614,40 @@ export namespace ACP {
     }
   }
 
+  function editDiff(input: Record<string, any>, metadata: unknown) {
+    const meta = typeof metadata === "object" && metadata ? (metadata as Record<string, any>) : undefined
+    const filediff =
+      meta && typeof meta["filediff"] === "object" && meta["filediff"]
+        ? (meta["filediff"] as Record<string, any>)
+        : undefined
+    const path =
+      typeof filediff?.["file"] === "string"
+        ? filediff["file"]
+        : typeof input["filePath"] === "string"
+          ? input["filePath"]
+          : ""
+    const oldText =
+      typeof filediff?.["before"] === "string"
+        ? filediff["before"]
+        : typeof input["oldString"] === "string"
+          ? input["oldString"]
+          : ""
+    const newText =
+      typeof filediff?.["after"] === "string"
+        ? filediff["after"]
+        : typeof input["newString"] === "string"
+          ? input["newString"]
+          : typeof input["content"] === "string"
+            ? input["content"]
+            : ""
+    if (!path && !oldText && !newText) return
+    return {
+      path,
+      oldText,
+      newText,
+    }
+  }
+
   function getNewContent(fileOriginal: string, unifiedDiff: string): string | undefined {
     const result = applyPatch(fileOriginal, unifiedDiff)
     if (result === false) {

packages/opencode/src/cli/cmd/tui/routes/session/index.tsx

@@ -2065,7 +2065,7 @@ function Edit(props: ToolProps<typeof EditTool>) {
       </Match>
       <Match when={true}>
         <InlineTool icon="←" pending="Preparing edit..." complete={props.input.filePath} part={props.part}>
-          Edit {normalizePath(props.input.filePath!)} {input({ replaceAll: props.input.replaceAll })}
+          Edit {normalizePath(props.input.filePath!)} {input(props.input, ["edits", "filePath"])}
         </InlineTool>
       </Match>
     </Switch>

packages/opencode/src/config/config.ts

@@ -1151,6 +1151,12 @@ export namespace Config {
         .object({
           disable_paste_summary: z.boolean().optional(),
           batch_tool: z.boolean().optional().describe("Enable the batch tool"),
+          hashline_autocorrect: z
+            .boolean()
+            .optional()
+            .describe(
+              "Enable hashline autocorrect cleanup for copied prefixes and formatting artifacts (default true)",
+            ),
           openTelemetry: z
             .boolean()
             .optional()

packages/opencode/src/session/prompt/codex_header.txt

@@ -5,7 +5,7 @@ You are an interactive CLI tool that helps users with software engineering tasks
 ## Editing constraints
 - Default to ASCII when editing or creating files. Only introduce non-ASCII or other Unicode characters when there is a clear justification and the file already uses them.
 - Only add comments if they are necessary to make a non-obvious block easier to understand.
-- Try to use apply_patch for single file edits, but it is fine to explore other options to make the edit if it does not work well. Do not use apply_patch for changes that are auto-generated (i.e. generating package.json or running a lint or format command like gofmt) or when scripting is more efficient (such as search and replacing a string across a codebase).
+- Prefer the edit tool for file edits. Use apply_patch only when it is available and clearly a better fit. Do not use apply_patch for changes that are auto-generated (i.e. generating package.json or running a lint or format command like gofmt) or when scripting is more efficient (such as search and replacing a string across a codebase).
 
 ## Tool usage
 - Prefer specialized tools over shell for file operations:

packages/opencode/src/tool/edit.ts

@@ -1,13 +1,9 @@
-// the approaches in this edit tool are sourced from
-// https://github.com/cline/cline/blob/main/evals/diff-edits/diff-apply/diff-06-23-25.ts
-// https://github.com/google-gemini/gemini-cli/blob/main/packages/core/src/utils/editCorrector.ts
-// https://github.com/cline/cline/blob/main/evals/diff-edits/diff-apply/diff-06-26-25.ts
-
 import z from "zod"
 import * as path from "path"
+import * as fs from "fs/promises"
+import { createTwoFilesPatch, diffLines } from "diff"
 import { Tool } from "./tool"
 import { LSP } from "../lsp"
-import { createTwoFilesPatch, diffLines } from "diff"
 import DESCRIPTION from "./edit.txt"
 import { File } from "../file"
 import { FileWatcher } from "../file/watcher"
@@ -17,567 +13,325 @@ import { Filesystem } from "../util/filesystem"
 import { Instance } from "../project/instance"
 import { Snapshot } from "@/snapshot"
 import { assertExternalDirectory } from "./external-directory"
+import {
+  HashlineEdit,
+  applyHashlineEdits,
+  hashlineOnlyCreates,
+  parseHashlineContent,
+  serializeHashlineContent,
+} from "./hashline"
+import { Config } from "../config/config"
 
 const MAX_DIAGNOSTICS_PER_FILE = 20
+const HASHLINE_EDIT_MODE = "hashline"
+const LEGACY_KEYS = ["oldString", "newString", "replaceAll"] as const
+
+const EditParams = z
+  .object({
+    filePath: z.string().describe("The absolute path to the file to modify"),
+    edits: z.array(HashlineEdit).optional(),
+    delete: z.boolean().optional(),
+    rename: z.string().optional(),
+  })
+  .strict()
+  .superRefine((value, ctx) => {
+    if (value.edits !== undefined) return
+    ctx.addIssue({
+      code: "custom",
+      message: "Hashline payload requires edits (use [] when only delete or rename is intended).",
+    })
+  })
+
+type EditParams = z.infer<typeof EditParams>
+
+function formatValidationError(error: z.ZodError) {
+  const legacy = error.issues.some((issue) => {
+    if (issue.code !== "unrecognized_keys") return false
+    if (!("keys" in issue) || !Array.isArray(issue.keys)) return false
+    return issue.keys.some((key) => LEGACY_KEYS.includes(key as (typeof LEGACY_KEYS)[number]))
+  })
+  if (legacy) {
+    return "Legacy edit payload has been removed. Use hashline fields: { filePath, edits, delete?, rename? }."
+  }
+  return `Invalid parameters for tool 'edit':\n${error.issues.map((issue) => `- ${issue.message}`).join("\n")}`
+}
 
 function normalizeLineEndings(text: string): string {
   return text.replaceAll("\r\n", "\n")
 }
 
-export const EditTool = Tool.define("edit", {
-  description: DESCRIPTION,
-  parameters: z.object({
-    filePath: z.string().describe("The absolute path to the file to modify"),
-    oldString: z.string().describe("The text to replace"),
-    newString: z.string().describe("The text to replace it with (must be different from oldString)"),
-    replaceAll: z.boolean().optional().describe("Replace all occurrences of oldString (default false)"),
-  }),
-  async execute(params, ctx) {
-    if (!params.filePath) {
-      throw new Error("filePath is required")
+async function withLocks(paths: string[], fn: () => Promise<void>) {
+  const unique = Array.from(new Set(paths)).sort((a, b) => a.localeCompare(b))
+  const recurse = async (idx: number): Promise<void> => {
+    if (idx >= unique.length) return fn()
+    await FileTime.withLock(unique[idx], () => recurse(idx + 1))
+  }
+  await recurse(0)
+}
+
+function createFileDiff(file: string, before: string, after: string): Snapshot.FileDiff {
+  const filediff: Snapshot.FileDiff = {
+    file,
+    before,
+    after,
+    additions: 0,
+    deletions: 0,
+  }
+  for (const change of diffLines(before, after)) {
+    if (change.added) filediff.additions += change.count || 0
+    if (change.removed) filediff.deletions += change.count || 0
+  }
+  return filediff
+}
+
+async function diagnosticsOutput(filePath: string, output: string) {
+  await LSP.touchFile(filePath, true)
+  const diagnostics = await LSP.diagnostics()
+  const normalizedFilePath = Filesystem.normalizePath(filePath)
+  const issues = diagnostics[normalizedFilePath] ?? []
+  const errors = issues.filter((item) => item.severity === 1)
+  if (errors.length === 0) {
+    return {
+      output,
+      diagnostics,
+    }
+  }
+
+  const limited = errors.slice(0, MAX_DIAGNOSTICS_PER_FILE)
+  const suffix =
+    errors.length > MAX_DIAGNOSTICS_PER_FILE ? `\n... and ${errors.length - MAX_DIAGNOSTICS_PER_FILE} more` : ""
+  return {
+    output:
+      output +
+      `\n\nLSP errors detected in this file, please fix:\n<diagnostics file="${filePath}">\n${limited.map(LSP.Diagnostic.pretty).join("\n")}${suffix}\n</diagnostics>`,
+    diagnostics,
+  }
+}
+
+async function executeHashline(
+  params: EditParams,
+  ctx: Tool.Context,
+  autocorrect: boolean,
+  aggressiveAutocorrect: boolean,
+) {
+  const sourcePath = path.isAbsolute(params.filePath) ? params.filePath : path.join(Instance.directory, params.filePath)
+  const targetPath = params.rename
+    ? path.isAbsolute(params.rename)
+      ? params.rename
+      : path.join(Instance.directory, params.rename)
+    : sourcePath
+  const edits = params.edits ?? []
+
+  await assertExternalDirectory(ctx, sourcePath)
+  if (params.rename) {
+    await assertExternalDirectory(ctx, targetPath)
+  }
+
+  if (params.delete && edits.length > 0) {
+    throw new Error("delete=true cannot be combined with edits")
+  }
+  if (params.delete && params.rename) {
+    throw new Error("delete=true cannot be combined with rename")
+  }
+
+  let diff = ""
+  let before = ""
+  let after = ""
+  let noop = 0
+  let deleted = false
+  let changed = false
+  let diagnostics: Awaited<ReturnType<typeof LSP.diagnostics>> = {}
+  await withLocks([sourcePath, targetPath], async () => {
+    const sourceStat = Filesystem.stat(sourcePath)
+    if (sourceStat?.isDirectory()) throw new Error(`Path is a directory, not a file: ${sourcePath}`)
+    const exists = Boolean(sourceStat)
+
+    if (params.rename && !exists) {
+      throw new Error("rename requires an existing source file")
     }
 
-    if (params.oldString === params.newString) {
-      throw new Error("No changes to apply: oldString and newString are identical.")
-    }
-
-    const filePath = path.isAbsolute(params.filePath) ? params.filePath : path.join(Instance.directory, params.filePath)
-    await assertExternalDirectory(ctx, filePath)
-
-    let diff = ""
-    let contentOld = ""
-    let contentNew = ""
-    await FileTime.withLock(filePath, async () => {
-      if (params.oldString === "") {
-        const existed = await Filesystem.exists(filePath)
-        contentNew = params.newString
-        diff = trimDiff(createTwoFilesPatch(filePath, filePath, contentOld, contentNew))
-        await ctx.ask({
-          permission: "edit",
-          patterns: [path.relative(Instance.worktree, filePath)],
-          always: ["*"],
-          metadata: {
-            filepath: filePath,
-            diff,
-          },
-        })
-        await Filesystem.write(filePath, params.newString)
-        await Bus.publish(File.Event.Edited, {
-          file: filePath,
-        })
-        await Bus.publish(FileWatcher.Event.Updated, {
-          file: filePath,
-          event: existed ? "change" : "add",
-        })
-        FileTime.read(ctx.sessionID, filePath)
+    if (params.delete) {
+      if (!exists) {
+        noop = 1
         return
       }
-
-      const stats = Filesystem.stat(filePath)
-      if (!stats) throw new Error(`File ${filePath} not found`)
-      if (stats.isDirectory()) throw new Error(`Path is a directory, not a file: ${filePath}`)
-      await FileTime.assert(ctx.sessionID, filePath)
-      contentOld = await Filesystem.readText(filePath)
-      contentNew = replace(contentOld, params.oldString, params.newString, params.replaceAll)
-
-      diff = trimDiff(
-        createTwoFilesPatch(filePath, filePath, normalizeLineEndings(contentOld), normalizeLineEndings(contentNew)),
-      )
+      await FileTime.assert(ctx.sessionID, sourcePath)
+      before = await Filesystem.readText(sourcePath)
+      diff = trimDiff(createTwoFilesPatch(sourcePath, sourcePath, normalizeLineEndings(before), ""))
       await ctx.ask({
         permission: "edit",
-        patterns: [path.relative(Instance.worktree, filePath)],
+        patterns: [path.relative(Instance.worktree, sourcePath)],
         always: ["*"],
         metadata: {
-          filepath: filePath,
+          filepath: sourcePath,
           diff,
         },
       })
-
-      await Filesystem.write(filePath, contentNew)
+      await fs.rm(sourcePath, { force: true })
       await Bus.publish(File.Event.Edited, {
-        file: filePath,
+        file: sourcePath,
       })
       await Bus.publish(FileWatcher.Event.Updated, {
-        file: filePath,
-        event: "change",
+        file: sourcePath,
+        event: "unlink",
       })
-      contentNew = await Filesystem.readText(filePath)
-      diff = trimDiff(
-        createTwoFilesPatch(filePath, filePath, normalizeLineEndings(contentOld), normalizeLineEndings(contentNew)),
-      )
-      FileTime.read(ctx.sessionID, filePath)
+      deleted = true
+      changed = true
+      return
+    }
+
+    if (!exists && !hashlineOnlyCreates(edits)) {
+      throw new Error("Missing file can only be created with append/prepend hashline edits")
+    }
+    if (exists) {
+      await FileTime.assert(ctx.sessionID, sourcePath)
+    }
+
+    const parsed = exists
+      ? parseHashlineContent(await Filesystem.readBytes(sourcePath))
+      : {
+          bom: false,
+          eol: "\n",
+          trailing: false,
+          lines: [] as string[],
+          text: "",
+          raw: "",
+        }
+
+    before = parsed.raw
+    const next = applyHashlineEdits({
+      lines: parsed.lines,
+      trailing: parsed.trailing,
+      edits,
+      autocorrect,
+      aggressiveAutocorrect,
     })
+    const output = serializeHashlineContent({
+      lines: next.lines,
+      trailing: next.trailing,
+      eol: parsed.eol,
+      bom: parsed.bom,
+    })
+    after = output.text
 
-    const filediff: Snapshot.FileDiff = {
-      file: filePath,
-      before: contentOld,
-      after: contentNew,
-      additions: 0,
-      deletions: 0,
-    }
-    for (const change of diffLines(contentOld, contentNew)) {
-      if (change.added) filediff.additions += change.count || 0
-      if (change.removed) filediff.deletions += change.count || 0
+    if (before === after && sourcePath === targetPath) {
+      noop = 1
+      diff = trimDiff(
+        createTwoFilesPatch(sourcePath, sourcePath, normalizeLineEndings(before), normalizeLineEndings(after)),
+      )
+      return
     }
 
-    ctx.metadata({
+    diff = trimDiff(
+      createTwoFilesPatch(sourcePath, targetPath, normalizeLineEndings(before), normalizeLineEndings(after)),
+    )
+    const patterns = [path.relative(Instance.worktree, sourcePath)]
+    if (sourcePath !== targetPath) patterns.push(path.relative(Instance.worktree, targetPath))
+    await ctx.ask({
+      permission: "edit",
+      patterns: Array.from(new Set(patterns)),
+      always: ["*"],
       metadata: {
+        filepath: sourcePath,
         diff,
-        filediff,
-        diagnostics: {},
       },
     })
 
-    let output = "Edit applied successfully."
-    await LSP.touchFile(filePath, true)
-    const diagnostics = await LSP.diagnostics()
-    const normalizedFilePath = Filesystem.normalizePath(filePath)
-    const issues = diagnostics[normalizedFilePath] ?? []
-    const errors = issues.filter((item) => item.severity === 1)
-    if (errors.length > 0) {
-      const limited = errors.slice(0, MAX_DIAGNOSTICS_PER_FILE)
-      const suffix =
-        errors.length > MAX_DIAGNOSTICS_PER_FILE ? `\n... and ${errors.length - MAX_DIAGNOSTICS_PER_FILE} more` : ""
-      output += `\n\nLSP errors detected in this file, please fix:\n<diagnostics file="${filePath}">\n${limited.map(LSP.Diagnostic.pretty).join("\n")}${suffix}\n</diagnostics>`
+    if (sourcePath === targetPath) {
+      await Filesystem.write(sourcePath, output.bytes)
+      await Bus.publish(File.Event.Edited, {
+        file: sourcePath,
+      })
+      await Bus.publish(FileWatcher.Event.Updated, {
+        file: sourcePath,
+        event: exists ? "change" : "add",
+      })
+      FileTime.read(ctx.sessionID, sourcePath)
+      changed = true
+      return
     }
 
+    const targetExists = await Filesystem.exists(targetPath)
+    await Filesystem.write(targetPath, output.bytes)
+    await fs.rm(sourcePath, { force: true })
+    await Bus.publish(File.Event.Edited, {
+      file: sourcePath,
+    })
+    await Bus.publish(File.Event.Edited, {
+      file: targetPath,
+    })
+    await Bus.publish(FileWatcher.Event.Updated, {
+      file: sourcePath,
+      event: "unlink",
+    })
+    await Bus.publish(FileWatcher.Event.Updated, {
+      file: targetPath,
+      event: targetExists ? "change" : "add",
+    })
+    FileTime.read(ctx.sessionID, targetPath)
+    changed = true
+  })
+
+  const file = deleted ? sourcePath : targetPath
+  const filediff = createFileDiff(file, before, after)
+  ctx.metadata({
+    metadata: {
+      diff,
+      filediff,
+      diagnostics,
+      edit_mode: HASHLINE_EDIT_MODE,
+      noop,
+    },
+  })
+
+  if (!deleted && (changed || noop === 0)) {
+    const result = await diagnosticsOutput(targetPath, noop > 0 ? "No changes applied." : "Edit applied successfully.")
+    diagnostics = result.diagnostics
     return {
       metadata: {
         diagnostics,
         diff,
         filediff,
+        edit_mode: HASHLINE_EDIT_MODE,
+        noop,
       },
-      title: `${path.relative(Instance.worktree, filePath)}`,
-      output,
+      title: `${path.relative(Instance.worktree, targetPath)}`,
+      output: result.output,
     }
+  }
+
+  return {
+    metadata: {
+      diagnostics,
+      diff,
+      filediff,
+      edit_mode: HASHLINE_EDIT_MODE,
+      noop,
+    },
+    title: `${path.relative(Instance.worktree, file)}`,
+    output: deleted ? "Edit applied successfully." : "No changes applied.",
+  }
+}
+
+export const EditTool = Tool.define("edit", {
+  description: DESCRIPTION,
+  parameters: EditParams,
+  formatValidationError,
+  async execute(params, ctx) {
+    if (!params.filePath) {
+      throw new Error("filePath is required")
+    }
+
+    const config = await Config.get()
+    return executeHashline(
+      params,
+      ctx,
+      config.experimental?.hashline_autocorrect !== false || Bun.env.OPENCODE_HL_AUTOCORRECT === "1",
+      Bun.env.OPENCODE_HL_AUTOCORRECT === "1",
+    )
   },
 })
 
-export type Replacer = (content: string, find: string) => Generator<string, void, unknown>
-
-// Similarity thresholds for block anchor fallback matching
-const SINGLE_CANDIDATE_SIMILARITY_THRESHOLD = 0.0
-const MULTIPLE_CANDIDATES_SIMILARITY_THRESHOLD = 0.3
-
-/**
- * Levenshtein distance algorithm implementation
- */
-function levenshtein(a: string, b: string): number {
-  // Handle empty strings
-  if (a === "" || b === "") {
-    return Math.max(a.length, b.length)
-  }
-  const matrix = Array.from({ length: a.length + 1 }, (_, i) =>
-    Array.from({ length: b.length + 1 }, (_, j) => (i === 0 ? j : j === 0 ? i : 0)),
-  )
-
-  for (let i = 1; i <= a.length; i++) {
-    for (let j = 1; j <= b.length; j++) {
-      const cost = a[i - 1] === b[j - 1] ? 0 : 1
-      matrix[i][j] = Math.min(matrix[i - 1][j] + 1, matrix[i][j - 1] + 1, matrix[i - 1][j - 1] + cost)
-    }
-  }
-  return matrix[a.length][b.length]
-}
-
-export const SimpleReplacer: Replacer = function* (_content, find) {
-  yield find
-}
-
-export const LineTrimmedReplacer: Replacer = function* (content, find) {
-  const originalLines = content.split("\n")
-  const searchLines = find.split("\n")
-
-  if (searchLines[searchLines.length - 1] === "") {
-    searchLines.pop()
-  }
-
-  for (let i = 0; i <= originalLines.length - searchLines.length; i++) {
-    let matches = true
-
-    for (let j = 0; j < searchLines.length; j++) {
-      const originalTrimmed = originalLines[i + j].trim()
-      const searchTrimmed = searchLines[j].trim()
-
-      if (originalTrimmed !== searchTrimmed) {
-        matches = false
-        break
-      }
-    }
-
-    if (matches) {
-      let matchStartIndex = 0
-      for (let k = 0; k < i; k++) {
-        matchStartIndex += originalLines[k].length + 1
-      }
-
-      let matchEndIndex = matchStartIndex
-      for (let k = 0; k < searchLines.length; k++) {
-        matchEndIndex += originalLines[i + k].length
-        if (k < searchLines.length - 1) {
-          matchEndIndex += 1 // Add newline character except for the last line
-        }
-      }
-
-      yield content.substring(matchStartIndex, matchEndIndex)
-    }
-  }
-}
-
-export const BlockAnchorReplacer: Replacer = function* (content, find) {
-  const originalLines = content.split("\n")
-  const searchLines = find.split("\n")
-
-  if (searchLines.length < 3) {
-    return
-  }
-
-  if (searchLines[searchLines.length - 1] === "") {
-    searchLines.pop()
-  }
-
-  const firstLineSearch = searchLines[0].trim()
-  const lastLineSearch = searchLines[searchLines.length - 1].trim()
-  const searchBlockSize = searchLines.length
-
-  // Collect all candidate positions where both anchors match
-  const candidates: Array<{ startLine: number; endLine: number }> = []
-  for (let i = 0; i < originalLines.length; i++) {
-    if (originalLines[i].trim() !== firstLineSearch) {
-      continue
-    }
-
-    // Look for the matching last line after this first line
-    for (let j = i + 2; j < originalLines.length; j++) {
-      if (originalLines[j].trim() === lastLineSearch) {
-        candidates.push({ startLine: i, endLine: j })
-        break // Only match the first occurrence of the last line
-      }
-    }
-  }
-
-  // Return immediately if no candidates
-  if (candidates.length === 0) {
-    return
-  }
-
-  // Handle single candidate scenario (using relaxed threshold)
-  if (candidates.length === 1) {
-    const { startLine, endLine } = candidates[0]
-    const actualBlockSize = endLine - startLine + 1
-
-    let similarity = 0
-    let linesToCheck = Math.min(searchBlockSize - 2, actualBlockSize - 2) // Middle lines only
-
-    if (linesToCheck > 0) {
-      for (let j = 1; j < searchBlockSize - 1 && j < actualBlockSize - 1; j++) {
-        const originalLine = originalLines[startLine + j].trim()
-        const searchLine = searchLines[j].trim()
-        const maxLen = Math.max(originalLine.length, searchLine.length)
-        if (maxLen === 0) {
-          continue
-        }
-        const distance = levenshtein(originalLine, searchLine)
-        similarity += (1 - distance / maxLen) / linesToCheck
-
-        // Exit early when threshold is reached
-        if (similarity >= SINGLE_CANDIDATE_SIMILARITY_THRESHOLD) {
-          break
-        }
-      }
-    } else {
-      // No middle lines to compare, just accept based on anchors
-      similarity = 1.0
-    }
-
-    if (similarity >= SINGLE_CANDIDATE_SIMILARITY_THRESHOLD) {
-      let matchStartIndex = 0
-      for (let k = 0; k < startLine; k++) {
-        matchStartIndex += originalLines[k].length + 1
-      }
-      let matchEndIndex = matchStartIndex
-      for (let k = startLine; k <= endLine; k++) {
-        matchEndIndex += originalLines[k].length
-        if (k < endLine) {
-          matchEndIndex += 1 // Add newline character except for the last line
-        }
-      }
-      yield content.substring(matchStartIndex, matchEndIndex)
-    }
-    return
-  }
-
-  // Calculate similarity for multiple candidates
-  let bestMatch: { startLine: number; endLine: number } | null = null
-  let maxSimilarity = -1
-
-  for (const candidate of candidates) {
-    const { startLine, endLine } = candidate
-    const actualBlockSize = endLine - startLine + 1
-
-    let similarity = 0
-    let linesToCheck = Math.min(searchBlockSize - 2, actualBlockSize - 2) // Middle lines only
-
-    if (linesToCheck > 0) {
-      for (let j = 1; j < searchBlockSize - 1 && j < actualBlockSize - 1; j++) {
-        const originalLine = originalLines[startLine + j].trim()
-        const searchLine = searchLines[j].trim()
-        const maxLen = Math.max(originalLine.length, searchLine.length)
-        if (maxLen === 0) {
-          continue
-        }
-        const distance = levenshtein(originalLine, searchLine)
-        similarity += 1 - distance / maxLen
-      }
-      similarity /= linesToCheck // Average similarity
-    } else {
-      // No middle lines to compare, just accept based on anchors
-      similarity = 1.0
-    }
-
-    if (similarity > maxSimilarity) {
-      maxSimilarity = similarity
-      bestMatch = candidate
-    }
-  }
-
-  // Threshold judgment
-  if (maxSimilarity >= MULTIPLE_CANDIDATES_SIMILARITY_THRESHOLD && bestMatch) {
-    const { startLine, endLine } = bestMatch
-    let matchStartIndex = 0
-    for (let k = 0; k < startLine; k++) {
-      matchStartIndex += originalLines[k].length + 1
-    }
-    let matchEndIndex = matchStartIndex
-    for (let k = startLine; k <= endLine; k++) {
-      matchEndIndex += originalLines[k].length
-      if (k < endLine) {
-        matchEndIndex += 1
-      }
-    }
-    yield content.substring(matchStartIndex, matchEndIndex)
-  }
-}
-
-export const WhitespaceNormalizedReplacer: Replacer = function* (content, find) {
-  const normalizeWhitespace = (text: string) => text.replace(/\s+/g, " ").trim()
-  const normalizedFind = normalizeWhitespace(find)
-
-  // Handle single line matches
-  const lines = content.split("\n")
-  for (let i = 0; i < lines.length; i++) {
-    const line = lines[i]
-    if (normalizeWhitespace(line) === normalizedFind) {
-      yield line
-    } else {
-      // Only check for substring matches if the full line doesn't match
-      const normalizedLine = normalizeWhitespace(line)
-      if (normalizedLine.includes(normalizedFind)) {
-        // Find the actual substring in the original line that matches
-        const words = find.trim().split(/\s+/)
-        if (words.length > 0) {
-          const pattern = words.map((word) => word.replace(/[.*+?^${}()|[\]\\]/g, "\\$&")).join("\\s+")
-          try {
-            const regex = new RegExp(pattern)
-            const match = line.match(regex)
-            if (match) {
-              yield match[0]
-            }
-          } catch (e) {
-            // Invalid regex pattern, skip
-          }
-        }
-      }
-    }
-  }
-
-  // Handle multi-line matches
-  const findLines = find.split("\n")
-  if (findLines.length > 1) {
-    for (let i = 0; i <= lines.length - findLines.length; i++) {
-      const block = lines.slice(i, i + findLines.length)
-      if (normalizeWhitespace(block.join("\n")) === normalizedFind) {
-        yield block.join("\n")
-      }
-    }
-  }
-}
-
-export const IndentationFlexibleReplacer: Replacer = function* (content, find) {
-  const removeIndentation = (text: string) => {
-    const lines = text.split("\n")
-    const nonEmptyLines = lines.filter((line) => line.trim().length > 0)
-    if (nonEmptyLines.length === 0) return text
-
-    const minIndent = Math.min(
-      ...nonEmptyLines.map((line) => {
-        const match = line.match(/^(\s*)/)
-        return match ? match[1].length : 0
-      }),
-    )
-
-    return lines.map((line) => (line.trim().length === 0 ? line : line.slice(minIndent))).join("\n")
-  }
-
-  const normalizedFind = removeIndentation(find)
-  const contentLines = content.split("\n")
-  const findLines = find.split("\n")
-
-  for (let i = 0; i <= contentLines.length - findLines.length; i++) {
-    const block = contentLines.slice(i, i + findLines.length).join("\n")
-    if (removeIndentation(block) === normalizedFind) {
-      yield block
-    }
-  }
-}
-
-export const EscapeNormalizedReplacer: Replacer = function* (content, find) {
-  const unescapeString = (str: string): string => {
-    return str.replace(/\\(n|t|r|'|"|`|\\|\n|\$)/g, (match, capturedChar) => {
-      switch (capturedChar) {
-        case "n":
-          return "\n"
-        case "t":
-          return "\t"
-        case "r":
-          return "\r"
-        case "'":
-          return "'"
-        case '"':
-          return '"'
-        case "`":
-          return "`"
-        case "\\":
-          return "\\"
-        case "\n":
-          return "\n"
-        case "$":
-          return "$"
-        default:
-          return match
-      }
-    })
-  }
-
-  const unescapedFind = unescapeString(find)
-
-  // Try direct match with unescaped find string
-  if (content.includes(unescapedFind)) {
-    yield unescapedFind
-  }
-
-  // Also try finding escaped versions in content that match unescaped find
-  const lines = content.split("\n")
-  const findLines = unescapedFind.split("\n")
-
-  for (let i = 0; i <= lines.length - findLines.length; i++) {
-    const block = lines.slice(i, i + findLines.length).join("\n")
-    const unescapedBlock = unescapeString(block)
-
-    if (unescapedBlock === unescapedFind) {
-      yield block
-    }
-  }
-}
-
-export const MultiOccurrenceReplacer: Replacer = function* (content, find) {
-  // This replacer yields all exact matches, allowing the replace function
-  // to handle multiple occurrences based on replaceAll parameter
-  let startIndex = 0
-
-  while (true) {
-    const index = content.indexOf(find, startIndex)
-    if (index === -1) break
-
-    yield find
-    startIndex = index + find.length
-  }
-}
-
-export const TrimmedBoundaryReplacer: Replacer = function* (content, find) {
-  const trimmedFind = find.trim()
-
-  if (trimmedFind === find) {
-    // Already trimmed, no point in trying
-    return
-  }
-
-  // Try to find the trimmed version
-  if (content.includes(trimmedFind)) {
-    yield trimmedFind
-  }
-
-  // Also try finding blocks where trimmed content matches
-  const lines = content.split("\n")
-  const findLines = find.split("\n")
-
-  for (let i = 0; i <= lines.length - findLines.length; i++) {
-    const block = lines.slice(i, i + findLines.length).join("\n")
-
-    if (block.trim() === trimmedFind) {
-      yield block
-    }
-  }
-}
-
-export const ContextAwareReplacer: Replacer = function* (content, find) {
-  const findLines = find.split("\n")
-  if (findLines.length < 3) {
-    // Need at least 3 lines to have meaningful context
-    return
-  }
-
-  // Remove trailing empty line if present
-  if (findLines[findLines.length - 1] === "") {
-    findLines.pop()
-  }
-
-  const contentLines = content.split("\n")
-
-  // Extract first and last lines as context anchors
-  const firstLine = findLines[0].trim()
-  const lastLine = findLines[findLines.length - 1].trim()
-
-  // Find blocks that start and end with the context anchors
-  for (let i = 0; i < contentLines.length; i++) {
-    if (contentLines[i].trim() !== firstLine) continue
-
-    // Look for the matching last line
-    for (let j = i + 2; j < contentLines.length; j++) {
-      if (contentLines[j].trim() === lastLine) {
-        // Found a potential context block
-        const blockLines = contentLines.slice(i, j + 1)
-        const block = blockLines.join("\n")
-
-        // Check if the middle content has reasonable similarity
-        // (simple heuristic: at least 50% of non-empty lines should match when trimmed)
-        if (blockLines.length === findLines.length) {
-          let matchingLines = 0
-          let totalNonEmptyLines = 0
-
-          for (let k = 1; k < blockLines.length - 1; k++) {
-            const blockLine = blockLines[k].trim()
-            const findLine = findLines[k].trim()
-
-            if (blockLine.length > 0 || findLine.length > 0) {
-              totalNonEmptyLines++
-              if (blockLine === findLine) {
-                matchingLines++
-              }
-            }
-          }
-
-          if (totalNonEmptyLines === 0 || matchingLines / totalNonEmptyLines >= 0.5) {
-            yield block
-            break // Only match the first occurrence
-          }
-        }
-        break
-      }
-    }
-  }
-}
-
 export function trimDiff(diff: string): string {
   const lines = diff.split("\n")
   const contentLines = lines.filter(
@@ -613,42 +367,3 @@ export function trimDiff(diff: string): string {
 
   return trimmedLines.join("\n")
 }
-
-export function replace(content: string, oldString: string, newString: string, replaceAll = false): string {
-  if (oldString === newString) {
-    throw new Error("No changes to apply: oldString and newString are identical.")
-  }
-
-  let notFound = true
-
-  for (const replacer of [
-    SimpleReplacer,
-    LineTrimmedReplacer,
-    BlockAnchorReplacer,
-    WhitespaceNormalizedReplacer,
-    IndentationFlexibleReplacer,
-    EscapeNormalizedReplacer,
-    TrimmedBoundaryReplacer,
-    ContextAwareReplacer,
-    MultiOccurrenceReplacer,
-  ]) {
-    for (const search of replacer(content, oldString)) {
-      const index = content.indexOf(search)
-      if (index === -1) continue
-      notFound = false
-      if (replaceAll) {
-        return content.replaceAll(search, newString)
-      }
-      const lastIndex = content.lastIndexOf(search)
-      if (index !== lastIndex) continue
-      return content.substring(0, index) + newString + content.substring(index + search.length)
-    }
-  }
-
-  if (notFound) {
-    throw new Error(
-      "Could not find oldString in the file. It must match exactly, including whitespace, indentation, and line endings.",
-    )
-  }
-  throw new Error("Found multiple matches for oldString. Provide more surrounding context to make the match unique.")
-}

packages/opencode/src/tool/edit.txt

@@ -1,10 +1,23 @@
-Performs exact string replacements in files. 
+Performs file edits with hashline anchors.
 
 Usage:
-- You must use your `Read` tool at least once in the conversation before editing. This tool will error if you attempt an edit without reading the file. 
-- When editing text from Read tool output, ensure you preserve the exact indentation (tabs/spaces) as it appears AFTER the line number prefix. The line number prefix format is: line number + colon + space (e.g., `1: `). Everything after that space is the actual file content to match. Never include any part of the line number prefix in the oldString or newString.
+- You must use your `Read` tool at least once before editing an existing file. This tool rejects stale edits when file contents changed since read.
 - ALWAYS prefer editing existing files in the codebase. NEVER write new files unless explicitly required.
 - Only use emojis if the user explicitly requests it. Avoid adding emojis to files unless asked.
-- The edit will FAIL if `oldString` is not found in the file with an error "oldString not found in content".
-- The edit will FAIL if `oldString` is found multiple times in the file with an error "Found multiple matches for oldString. Provide more surrounding lines in oldString to identify the correct match." Either provide a larger string with more surrounding context to make it unique or use `replaceAll` to change every instance of `oldString`. 
-- Use `replaceAll` for replacing and renaming strings across the file. This parameter is useful if you want to rename a variable for instance.
+- `{ filePath, edits, delete?, rename? }`
+- `edits` is required; use `[]` when only delete or rename is intended.
+- Use strict anchor references from `Read` output: `LINE#ID`.
+- Autocorrect cleanup is on by default and can be turned off with `experimental.hashline_autocorrect: false`.
+- Default autocorrect only strips copied `LINE#ID:`/`>>>` prefixes; set `OPENCODE_HL_AUTOCORRECT=1` to opt into heavier cleanup heuristics.
+- When `Read` returns `LINE#ID:<content>`, prefer hashline operations.
+- Operations:
+  - `set_line { line, text }`
+  - `replace_lines { start_line, end_line, text }`
+  - `insert_after { line, text }`
+  - `insert_before { line, text }`
+  - `insert_between { after_line, before_line, text }`
+  - `append { text }`
+  - `prepend { text }`
+  - `replace { old_text, new_text, all? }`
+- Provide the exact `LINE#ID` anchors from the latest `Read` result. Mismatched anchors are rejected and should be retried with the returned `retry with` anchors.
+- GPT-family models can use `apply_patch` as fallback when needed.

packages/opencode/src/tool/grep.ts

@@ -9,6 +9,7 @@ import DESCRIPTION from "./grep.txt"
 import { Instance } from "../project/instance"
 import path from "path"
 import { assertExternalDirectory } from "./external-directory"
+import { hashlineRef } from "./hashline"
 
 const MAX_LINE_LENGTH = 2000
 
@@ -129,7 +130,7 @@ export const GrepTool = Tool.define("grep", {
       }
       const truncatedLineText =
         match.lineText.length > MAX_LINE_LENGTH ? match.lineText.substring(0, MAX_LINE_LENGTH) + "..." : match.lineText
-      outputLines.push(`  Line ${match.lineNum}: ${truncatedLineText}`)
+      outputLines.push(`  ${hashlineRef(match.lineNum, match.lineText)}:${truncatedLineText}`)
     }
 
     if (truncated) {

packages/opencode/src/tool/grep.txt

@@ -2,7 +2,8 @@
 - Searches file contents using regular expressions
 - Supports full regex syntax (eg. "log.*Error", "function\s+\w+", etc.)
 - Filter files by pattern with the include parameter (eg. "*.js", "*.{ts,tsx}")
-- Returns file paths and line numbers with at least one match sorted by modification time
+- Returns file paths with matching lines sorted by modification time
+- Output format uses hashline anchors: `N#ID:<content>`
 - Use this tool when you need to find files containing specific patterns
 - If you need to identify/count the number of matches within files, use the Bash tool with `rg` (ripgrep) directly. Do NOT use `grep`.
 - When you are doing an open-ended search that may require multiple rounds of globbing and grepping, use the Task tool instead

packages/opencode/src/tool/hashline.ts

@@ -0,0 +1,646 @@
+// hashline autocorrect heuristics in this file are inspired by
+// https://github.com/can1357/oh-my-pi (mit license), adapted for opencode.
+
+import z from "zod"
+
+export const HASHLINE_ALPHABET = "ZPMQVRWSNKTXJBYH"
+
+const HASHLINE_ID_LENGTH = 2
+const HASHLINE_ID_REGEX = new RegExp(`^[${HASHLINE_ALPHABET}]{${HASHLINE_ID_LENGTH}}$`)
+const HASHLINE_REF_REGEX = new RegExp(`(\\d+)#([${HASHLINE_ALPHABET}]{${HASHLINE_ID_LENGTH}})(?=$|\\s|:)`)
+const LOW_SIGNAL_CONTENT_RE = /^[^a-zA-Z0-9]+$/
+
+type TextValue = string | string[]
+
+export const HashlineText = z.union([z.string(), z.array(z.string())])
+
+export const HashlineEdit = z.discriminatedUnion("type", [
+  z
+    .object({
+      type: z.literal("set_line"),
+      line: z.string(),
+      text: HashlineText,
+    })
+    .strict(),
+  z
+    .object({
+      type: z.literal("replace_lines"),
+      start_line: z.string(),
+      end_line: z.string(),
+      text: HashlineText,
+    })
+    .strict(),
+  z
+    .object({
+      type: z.literal("insert_after"),
+      line: z.string(),
+      text: HashlineText,
+    })
+    .strict(),
+  z
+    .object({
+      type: z.literal("insert_before"),
+      line: z.string(),
+      text: HashlineText,
+    })
+    .strict(),
+  z
+    .object({
+      type: z.literal("insert_between"),
+      after_line: z.string(),
+      before_line: z.string(),
+      text: HashlineText,
+    })
+    .strict(),
+  z
+    .object({
+      type: z.literal("append"),
+      text: HashlineText,
+    })
+    .strict(),
+  z
+    .object({
+      type: z.literal("prepend"),
+      text: HashlineText,
+    })
+    .strict(),
+  z
+    .object({
+      type: z.literal("replace"),
+      old_text: z.string(),
+      new_text: HashlineText,
+      all: z.boolean().optional(),
+    })
+    .strict(),
+])
+
+export type HashlineEdit = z.infer<typeof HashlineEdit>
+
+function isLowSignalContent(normalized: string) {
+  if (normalized.length === 0) return true
+  if (normalized.length <= 2) return true
+  return LOW_SIGNAL_CONTENT_RE.test(normalized)
+}
+
+export function hashlineID(lineNumber: number, line: string): string {
+  let normalized = line
+  if (normalized.endsWith("\r")) normalized = normalized.slice(0, -1)
+  normalized = normalized.replace(/\s+/g, "")
+  const seed = isLowSignalContent(normalized) ? `${normalized}:${lineNumber}` : normalized
+  const hash = Bun.hash.xxHash32(seed) & 0xff
+  const high = (hash >>> 4) & 0x0f
+  const low = hash & 0x0f
+  return `${HASHLINE_ALPHABET[high]}${HASHLINE_ALPHABET[low]}`
+}
+
+export function hashlineRef(lineNumber: number, line: string): string {
+  return `${lineNumber}#${hashlineID(lineNumber, line)}`
+}
+
+export function hashlineLine(lineNumber: number, line: string): string {
+  return `${hashlineRef(lineNumber, line)}:${line}`
+}
+
+export function parseHashlineRef(input: string, label: string) {
+  const match = input.match(HASHLINE_REF_REGEX)
+  if (!match) {
+    throw new Error(`${label} must contain a LINE#ID reference`)
+  }
+
+  const line = Number.parseInt(match[1], 10)
+  if (!Number.isInteger(line) || line < 1) {
+    throw new Error(`${label} has invalid line number: ${match[1]}`)
+  }
+
+  const id = match[2]
+  if (!HASHLINE_ID_REGEX.test(id)) {
+    throw new Error(`${label} has invalid hash id: ${id}`)
+  }
+
+  return {
+    raw: `${line}#${id}`,
+    line,
+    id,
+  }
+}
+
+function toLines(text: TextValue) {
+  if (Array.isArray(text)) return text
+  return text.split(/\r?\n/)
+}
+
+const HASHLINE_PREFIX_RE = /^\s*(?:>>>|>>)?\s*\d+#[ZPMQVRWSNKTXJBYH]{2}:/
+const WRAPPER_PREFIX_RE = /^\s*(?:>>>|>>)\s?/
+
+function stripByMajority(lines: string[], test: (line: string) => boolean, rewrite: (line: string) => string) {
+  const nonEmpty = lines.filter((line) => line.length > 0)
+  if (nonEmpty.length === 0) return lines
+
+  const matches = nonEmpty.filter(test).length
+  if (matches === 0 || matches < nonEmpty.length * 0.5) return lines
+
+  return lines.map(rewrite)
+}
+
+function stripNewLinePrefixes(lines: string[]) {
+  const stripped = stripByMajority(
+    lines,
+    (line) => HASHLINE_PREFIX_RE.test(line),
+    (line) => line.replace(HASHLINE_PREFIX_RE, ""),
+  )
+  return stripByMajority(
+    stripped,
+    (line) => WRAPPER_PREFIX_RE.test(line),
+    (line) => line.replace(WRAPPER_PREFIX_RE, ""),
+  )
+}
+
+function equalsIgnoringWhitespace(a: string, b: string) {
+  if (a === b) return true
+  return a.replace(/\s+/g, "") === b.replace(/\s+/g, "")
+}
+
+function leadingWhitespace(line: string) {
+  const match = line.match(/^\s*/)
+  if (!match) return ""
+  return match[0]
+}
+
+function restoreLeadingIndent(template: string, line: string) {
+  if (line.length === 0) return line
+  const templateIndent = leadingWhitespace(template)
+  if (templateIndent.length === 0) return line
+  const indent = leadingWhitespace(line)
+  if (indent.length > 0) return line
+  return templateIndent + line
+}
+
+function restoreIndentForPairedReplacement(oldLines: string[], newLines: string[]) {
+  if (oldLines.length !== newLines.length) return newLines
+  let changed = false
+  const out = new Array<string>(newLines.length)
+  for (let idx = 0; idx < newLines.length; idx++) {
+    const restored = restoreLeadingIndent(oldLines[idx], newLines[idx])
+    out[idx] = restored
+    if (restored !== newLines[idx]) changed = true
+  }
+  if (changed) return out
+  return newLines
+}
+
+function stripAllWhitespace(s: string) {
+  return s.replace(/\s+/g, "")
+}
+
+function restoreOldWrappedLines(oldLines: string[], newLines: string[]) {
+  if (oldLines.length === 0 || newLines.length < 2) return newLines
+
+  const canonToOld = new Map<string, { line: string; count: number }>()
+  for (const line of oldLines) {
+    const canon = stripAllWhitespace(line)
+    const bucket = canonToOld.get(canon)
+    if (bucket) bucket.count++
+    if (!bucket) canonToOld.set(canon, { line, count: 1 })
+  }
+
+  const candidates: Array<{ start: number; len: number; replacement: string; canon: string }> = []
+  for (let start = 0; start < newLines.length; start++) {
+    for (let len = 2; len <= 10 && start + len <= newLines.length; len++) {
+      const canonSpan = stripAllWhitespace(newLines.slice(start, start + len).join(""))
+      const old = canonToOld.get(canonSpan)
+      if (old && old.count === 1 && canonSpan.length >= 6) {
+        candidates.push({
+          start,
+          len,
+          replacement: old.line,
+          canon: canonSpan,
+        })
+      }
+    }
+  }
+  if (candidates.length === 0) return newLines
+
+  const canonCounts = new Map<string, number>()
+  for (const candidate of candidates) {
+    canonCounts.set(candidate.canon, (canonCounts.get(candidate.canon) ?? 0) + 1)
+  }
+
+  const unique = candidates.filter((candidate) => (canonCounts.get(candidate.canon) ?? 0) === 1)
+  if (unique.length === 0) return newLines
+
+  unique.sort((a, b) => b.start - a.start)
+  const out = [...newLines]
+  for (const candidate of unique) {
+    out.splice(candidate.start, candidate.len, candidate.replacement)
+  }
+
+  return out
+}
+
+function stripInsertAnchorEchoAfter(anchorLine: string, lines: string[]) {
+  if (lines.length <= 1) return lines
+  if (equalsIgnoringWhitespace(lines[0], anchorLine)) return lines.slice(1)
+  return lines
+}
+
+function stripInsertAnchorEchoBefore(anchorLine: string, lines: string[]) {
+  if (lines.length <= 1) return lines
+  if (equalsIgnoringWhitespace(lines[lines.length - 1], anchorLine)) return lines.slice(0, -1)
+  return lines
+}
+
+function stripInsertBoundaryEcho(afterLine: string, beforeLine: string, lines: string[]) {
+  let out = lines
+  if (out.length > 1 && equalsIgnoringWhitespace(out[0], afterLine)) out = out.slice(1)
+  if (out.length > 1 && equalsIgnoringWhitespace(out[out.length - 1], beforeLine)) out = out.slice(0, -1)
+  return out
+}
+
+function stripRangeBoundaryEcho(fileLines: string[], startLine: number, endLine: number, lines: string[]) {
+  const count = endLine - startLine + 1
+  if (lines.length <= 1 || lines.length <= count) return lines
+
+  let out = lines
+  const beforeIdx = startLine - 2
+  if (beforeIdx >= 0 && equalsIgnoringWhitespace(out[0], fileLines[beforeIdx])) {
+    out = out.slice(1)
+  }
+
+  const afterIdx = endLine
+  if (
+    afterIdx < fileLines.length &&
+    out.length > 0 &&
+    equalsIgnoringWhitespace(out[out.length - 1], fileLines[afterIdx])
+  ) {
+    out = out.slice(0, -1)
+  }
+
+  return out
+}
+
+function ensureText(text: TextValue, label: string) {
+  const value = Array.isArray(text) ? text.join("") : text
+  if (value.length > 0) return
+  throw new Error(`${label} must be non-empty`)
+}
+
+function applyReplace(content: string, oldText: string, newText: TextValue, all = false) {
+  if (oldText.length === 0) throw new Error("replace.old_text must be non-empty")
+
+  const next = toLines(newText).join("\n")
+  const first = content.indexOf(oldText)
+  if (first < 0) throw new Error(`replace.old_text not found: ${JSON.stringify(oldText)}`)
+
+  if (all) return content.replaceAll(oldText, next)
+
+  const last = content.lastIndexOf(oldText)
+  if (first !== last) {
+    throw new Error("replace.old_text matched multiple times. Set all=true or provide a more specific old_text.")
+  }
+
+  return content.slice(0, first) + next + content.slice(first + oldText.length)
+}
+
+function mismatchContext(lines: string[], line: number) {
+  if (lines.length === 0) return ">>> (file is empty)"
+  const start = Math.max(1, line - 1)
+  const end = Math.min(lines.length, line + 1)
+  return Array.from({ length: end - start + 1 }, (_, idx) => start + idx)
+    .map((num) => {
+      const marker = num === line ? ">>>" : "   "
+      return `${marker} ${hashlineLine(num, lines[num - 1])}`
+    })
+    .join("\n")
+}
+
+function mismatchSummary(lines: string[], mismatch: { expected: string; line: number }) {
+  if (mismatch.line < 1 || mismatch.line > lines.length) {
+    return `- expected ${mismatch.expected} -> line ${mismatch.line} is out of range (1-${Math.max(lines.length, 1)})`
+  }
+  return `- expected ${mismatch.expected} -> retry with ${hashlineRef(mismatch.line, lines[mismatch.line - 1])}`
+}
+
+function throwMismatch(lines: string[], mismatches: Array<{ expected: string; line: number }>) {
+  const seen = new Set<string>()
+  const unique = mismatches.filter((mismatch) => {
+    const key = `${mismatch.expected}:${mismatch.line}`
+    if (seen.has(key)) return false
+    seen.add(key)
+    return true
+  })
+
+  const preview = unique.slice(0, 2).map((mismatch) => mismatchSummary(lines, mismatch))
+  const hidden = unique.length - preview.length
+  const count = unique.length
+  const linesOut = [
+    `Hashline edit rejected: ${count} anchor mismatch${count === 1 ? "" : "es"}. Re-read the file and retry with the updated anchors below.`,
+    ...preview,
+    ...(hidden > 0 ? [`- ... and ${hidden} more mismatches`] : []),
+  ]
+
+  if (Bun.env.OPENCODE_HL_MISMATCH_DEBUG === "1") {
+    const body = unique
+      .map((mismatch) => {
+        if (mismatch.line < 1 || mismatch.line > lines.length) {
+          return [
+            `>>> expected ${mismatch.expected}`,
+            `>>> current line ${mismatch.line} is out of range (1-${Math.max(lines.length, 1)})`,
+          ].join("\n")
+        }
+        return [
+          `>>> expected ${mismatch.expected}`,
+          mismatchContext(lines, mismatch.line),
+          `>>> retry with ${hashlineRef(mismatch.line, lines[mismatch.line - 1])}`,
+        ].join("\n")
+      })
+      .join("\n\n")
+    linesOut.push("", body)
+  }
+
+  throw new Error(linesOut.join("\n"))
+}
+
+function validateAnchors(lines: string[], refs: Array<{ raw: string; line: number; id: string }>) {
+  const mismatches = refs
+    .filter((ref) => {
+      if (ref.line < 1 || ref.line > lines.length) return true
+      return hashlineID(ref.line, lines[ref.line - 1]) !== ref.id
+    })
+    .map((ref) => ({ expected: ref.raw, line: ref.line }))
+
+  if (mismatches.length > 0) throwMismatch(lines, mismatches)
+}
+
+function splitLines(text: string) {
+  if (text === "") {
+    return {
+      lines: [] as string[],
+      trailing: false,
+    }
+  }
+
+  const trailing = text.endsWith("\n")
+  const lines = text.split(/\r?\n/)
+  if (trailing) lines.pop()
+
+  return { lines, trailing }
+}
+
+export function parseHashlineContent(bytes: Buffer) {
+  const raw = bytes.toString("utf8")
+  let text = raw
+  const bom = raw.startsWith("\uFEFF")
+  if (bom) text = raw.slice(1)
+
+  const eol = text.includes("\r\n") ? "\r\n" : "\n"
+  const { lines, trailing } = splitLines(text)
+
+  return {
+    bom,
+    eol,
+    trailing,
+    lines,
+    text,
+    raw,
+  }
+}
+
+export function serializeHashlineContent(input: { lines: string[]; bom: boolean; eol: string; trailing: boolean }) {
+  let text = input.lines.join(input.eol)
+  if (input.trailing && input.lines.length > 0) text += input.eol
+  if (input.bom) text = `\uFEFF${text}`
+  return {
+    text,
+    bytes: Buffer.from(text, "utf8"),
+  }
+}
+
+type Splice = {
+  start: number
+  del: number
+  text: string[]
+  order: number
+  kind: "set_line" | "replace_lines" | "insert_after" | "insert_before" | "insert_between" | "append" | "prepend"
+  sortLine: number
+  precedence: number
+  startLine?: number
+  endLine?: number
+  anchorLine?: number
+  beforeLine?: number
+  afterLine?: number
+}
+
+export function applyHashlineEdits(input: {
+  lines: string[]
+  trailing: boolean
+  edits: HashlineEdit[]
+  autocorrect?: boolean
+  aggressiveAutocorrect?: boolean
+}) {
+  const lines = [...input.lines]
+  const originalLines = [...input.lines]
+  let trailing = input.trailing
+  const refs: Array<{ raw: string; line: number; id: string }> = []
+  const replaceOps: Array<Extract<HashlineEdit, { type: "replace" }>> = []
+  const ops: Splice[] = []
+  const autocorrect = input.autocorrect ?? Bun.env.OPENCODE_HL_AUTOCORRECT === "1"
+  const aggressiveAutocorrect = input.aggressiveAutocorrect ?? Bun.env.OPENCODE_HL_AUTOCORRECT === "1"
+  const parseText = (text: TextValue) => {
+    const next = toLines(text)
+    if (!autocorrect) return next
+    return stripNewLinePrefixes(next)
+  }
+
+  input.edits.forEach((edit, order) => {
+    if (edit.type === "replace") {
+      replaceOps.push(edit)
+      return
+    }
+
+    if (edit.type === "append") {
+      ensureText(edit.text, "append.text")
+      ops.push({
+        start: lines.length,
+        del: 0,
+        text: parseText(edit.text),
+        order,
+        kind: "append",
+        sortLine: lines.length + 1,
+        precedence: 1,
+      })
+      return
+    }
+
+    if (edit.type === "prepend") {
+      ensureText(edit.text, "prepend.text")
+      ops.push({
+        start: 0,
+        del: 0,
+        text: parseText(edit.text),
+        order,
+        kind: "prepend",
+        sortLine: 0,
+        precedence: 2,
+      })
+      return
+    }
+
+    if (edit.type === "set_line") {
+      const line = parseHashlineRef(edit.line, "set_line.line")
+      refs.push(line)
+      ops.push({
+        start: line.line - 1,
+        del: 1,
+        text: parseText(edit.text),
+        order,
+        kind: "set_line",
+        sortLine: line.line,
+        precedence: 0,
+        startLine: line.line,
+        endLine: line.line,
+      })
+      return
+    }
+
+    if (edit.type === "replace_lines") {
+      const start = parseHashlineRef(edit.start_line, "replace_lines.start_line")
+      const end = parseHashlineRef(edit.end_line, "replace_lines.end_line")
+      refs.push(start)
+      refs.push(end)
+
+      if (start.line > end.line) {
+        throw new Error("replace_lines.start_line must be less than or equal to replace_lines.end_line")
+      }
+
+      ops.push({
+        start: start.line - 1,
+        del: end.line - start.line + 1,
+        text: parseText(edit.text),
+        order,
+        kind: "replace_lines",
+        sortLine: end.line,
+        precedence: 0,
+        startLine: start.line,
+        endLine: end.line,
+      })
+      return
+    }
+
+    if (edit.type === "insert_after") {
+      const line = parseHashlineRef(edit.line, "insert_after.line")
+      ensureText(edit.text, "insert_after.text")
+      refs.push(line)
+      ops.push({
+        start: line.line,
+        del: 0,
+        text: parseText(edit.text),
+        order,
+        kind: "insert_after",
+        sortLine: line.line,
+        precedence: 1,
+        anchorLine: line.line,
+      })
+      return
+    }
+
+    if (edit.type === "insert_before") {
+      const line = parseHashlineRef(edit.line, "insert_before.line")
+      ensureText(edit.text, "insert_before.text")
+      refs.push(line)
+      ops.push({
+        start: line.line - 1,
+        del: 0,
+        text: parseText(edit.text),
+        order,
+        kind: "insert_before",
+        sortLine: line.line,
+        precedence: 2,
+        anchorLine: line.line,
+      })
+      return
+    }
+
+    const after = parseHashlineRef(edit.after_line, "insert_between.after_line")
+    const before = parseHashlineRef(edit.before_line, "insert_between.before_line")
+    ensureText(edit.text, "insert_between.text")
+    refs.push(after)
+    refs.push(before)
+
+    if (after.line >= before.line) {
+      throw new Error("insert_between.after_line must be less than insert_between.before_line")
+    }
+
+    ops.push({
+      start: after.line,
+      del: 0,
+      text: parseText(edit.text),
+      order,
+      kind: "insert_between",
+      sortLine: before.line,
+      precedence: 3,
+      afterLine: after.line,
+      beforeLine: before.line,
+    })
+  })
+
+  validateAnchors(lines, refs)
+
+  const sorted = [...ops].sort((a, b) => {
+    if (a.sortLine !== b.sortLine) return b.sortLine - a.sortLine
+    if (a.precedence !== b.precedence) return a.precedence - b.precedence
+    return a.order - b.order
+  })
+
+  sorted.forEach((op) => {
+    if (op.start < 0 || op.start > lines.length) {
+      throw new Error(`line index ${op.start + 1} is out of range`)
+    }
+
+    let text = op.text
+    if (autocorrect && aggressiveAutocorrect) {
+      if (op.kind === "set_line" || op.kind === "replace_lines") {
+        const start = op.startLine ?? op.start + 1
+        const end = op.endLine ?? start + op.del - 1
+        const old = originalLines.slice(start - 1, end)
+        text = stripRangeBoundaryEcho(originalLines, start, end, text)
+        text = restoreOldWrappedLines(old, text)
+        text = restoreIndentForPairedReplacement(old, text)
+      }
+
+      if ((op.kind === "insert_after" || op.kind === "append") && op.anchorLine) {
+        text = stripInsertAnchorEchoAfter(originalLines[op.anchorLine - 1], text)
+      }
+
+      if ((op.kind === "insert_before" || op.kind === "prepend") && op.anchorLine) {
+        text = stripInsertAnchorEchoBefore(originalLines[op.anchorLine - 1], text)
+      }
+
+      if (op.kind === "insert_between" && op.afterLine && op.beforeLine) {
+        text = stripInsertBoundaryEcho(originalLines[op.afterLine - 1], originalLines[op.beforeLine - 1], text)
+      }
+    }
+
+    lines.splice(op.start, op.del, ...text)
+  })
+
+  if (replaceOps.length > 0) {
+    const content = `${lines.join("\n")}${trailing && lines.length > 0 ? "\n" : ""}`
+    const replaced = replaceOps.reduce(
+      (acc, op) =>
+        applyReplace(acc, op.old_text, autocorrect ? stripNewLinePrefixes(toLines(op.new_text)) : op.new_text, op.all),
+      content,
+    )
+    const split = splitLines(replaced)
+    lines.splice(0, lines.length, ...split.lines)
+    trailing = split.trailing
+  }
+
+  return {
+    lines,
+    trailing,
+  }
+}
+
+export function hashlineOnlyCreates(edits: HashlineEdit[]) {
+  return edits.every((edit) => edit.type === "append" || edit.type === "prepend")
+}

packages/opencode/src/tool/multiedit.ts

@@ -1,6 +1,7 @@
 import z from "zod"
 import { Tool } from "./tool"
 import { EditTool } from "./edit"
+import { WriteTool } from "./write"
 import DESCRIPTION from "./multiedit.txt"
 import path from "path"
 import { Instance } from "../project/instance"
@@ -22,17 +23,32 @@ export const MultiEditTool = Tool.define("multiedit", {
   }),
   async execute(params, ctx) {
     const tool = await EditTool.init()
+    const write = await WriteTool.init()
     const results = []
-    for (const [, edit] of params.edits.entries()) {
-      const result = await tool.execute(
-        {
-          filePath: params.filePath,
-          oldString: edit.oldString,
-          newString: edit.newString,
-          replaceAll: edit.replaceAll,
-        },
-        ctx,
-      )
+    for (const edit of params.edits) {
+      const result =
+        edit.oldString === ""
+          ? await write.execute(
+              {
+                filePath: params.filePath,
+                content: edit.newString,
+              },
+              ctx,
+            )
+          : await tool.execute(
+              {
+                filePath: params.filePath,
+                edits: [
+                  {
+                    type: "replace",
+                    old_text: edit.oldString,
+                    new_text: edit.newString,
+                    all: edit.replaceAll,
+                  },
+                ],
+              },
+              ctx,
+            )
       results.push(result)
     }
     return {

packages/opencode/src/tool/read.ts

@@ -11,6 +11,7 @@ import { Instance } from "../project/instance"
 import { assertExternalDirectory } from "./external-directory"
 import { InstructionPrompt } from "../session/instruction"
 import { Filesystem } from "../util/filesystem"
+import { hashlineRef } from "./hashline"
 
 const DEFAULT_READ_LIMIT = 2000
 const MAX_LINE_LENGTH = 2000
@@ -156,6 +157,7 @@ export const ReadTool = Tool.define("read", {
     const offset = params.offset ?? 1
     const start = offset - 1
     const raw: string[] = []
+    const full: string[] = []
     let bytes = 0
     let lines = 0
     let truncatedByBytes = false
@@ -179,6 +181,7 @@ export const ReadTool = Tool.define("read", {
         }
 
         raw.push(line)
+        full.push(text)
         bytes += size
       }
     } finally {
@@ -191,7 +194,8 @@ export const ReadTool = Tool.define("read", {
     }
 
     const content = raw.map((line, index) => {
-      return `${index + offset}: ${line}`
+      const lineNumber = index + offset
+      return `${hashlineRef(lineNumber, full[index])}:${line}`
     })
     const preview = raw.slice(0, 20).join("\n")
 

packages/opencode/src/tool/read.txt

@@ -7,7 +7,9 @@ Usage:
 - To read later sections, call this tool again with a larger offset.
 - Use the grep tool to find specific content in large files or files with long lines.
 - If you are unsure of the correct file path, use the glob tool to look up filenames by glob pattern.
-- Contents are returned with each line prefixed by its line number as `<line>: <content>`. For example, if a file has contents "foo\n", you will receive "1: foo\n". For directories, entries are returned one per line (without line numbers) with a trailing `/` for subdirectories.
+- Contents are returned with a line prefix.
+- Default format: `LINE#ID:<content>` (example: `1#AB:foo`). Use these anchors for hashline edits.
+- For directories, entries are returned one per line (without line numbers) with a trailing `/` for subdirectories.
 - Any line longer than 2000 characters is truncated.
 - Call this tool in parallel when you know there are multiple files you want to read.
 - Avoid tiny repeated slices (30 line chunks). If you need more context, read a larger window.

packages/opencode/src/tool/registry.ts

@@ -136,6 +136,8 @@ export namespace ToolRegistry {
     agent?: Agent.Info,
   ) {
     const tools = await all()
+    const usePatch =
+      model.modelID.includes("gpt-") && !model.modelID.includes("oss") && !model.modelID.includes("gpt-4")
     const result = await Promise.all(
       tools
         .filter((t) => {
@@ -144,11 +146,7 @@ export namespace ToolRegistry {
             return model.providerID === "opencode" || Flag.OPENCODE_ENABLE_EXA
           }
 
-          // use apply tool in same format as codex
-          const usePatch =
-            model.modelID.includes("gpt-") && !model.modelID.includes("oss") && !model.modelID.includes("gpt-4")
           if (t.id === "apply_patch") return usePatch
-          if (t.id === "edit" || t.id === "write") return !usePatch
 
           return true
         })

packages/opencode/test/config/config.test.ts

@@ -102,6 +102,48 @@ test("loads JSONC config file", async () => {
   })
 })
 
+test("parses experimental.hashline_autocorrect", async () => {
+  await using tmp = await tmpdir({
+    init: async (dir) => {
+      await writeConfig(dir, {
+        $schema: "https://opencode.ai/config.json",
+        experimental: {
+          hashline_autocorrect: true,
+        },
+      })
+    },
+  })
+  await Instance.provide({
+    directory: tmp.path,
+    fn: async () => {
+      const config = await Config.get()
+      expect(config.experimental?.hashline_autocorrect).toBe(true)
+    },
+  })
+})
+
+test("ignores removed experimental.hashline_edit", async () => {
+  await using tmp = await tmpdir({
+    init: async (dir) => {
+      await writeConfig(dir, {
+        $schema: "https://opencode.ai/config.json",
+        experimental: {
+          hashline_edit: true,
+          hashline_autocorrect: true,
+        },
+      })
+    },
+  })
+  await Instance.provide({
+    directory: tmp.path,
+    fn: async () => {
+      const config = await Config.get()
+      expect(config.experimental?.hashline_autocorrect).toBe(true)
+      expect((config.experimental as Record<string, unknown>)?.hashline_edit).toBeUndefined()
+    },
+  })
+})
+
 test("merges multiple config files with correct precedence", async () => {
   await using tmp = await tmpdir({
     init: async (dir) => {

packages/opencode/test/tool/edit.test.ts

@@ -5,6 +5,7 @@ import { EditTool } from "../../src/tool/edit"
 import { Instance } from "../../src/project/instance"
 import { tmpdir } from "../fixture/fixture"
 import { FileTime } from "../../src/file/time"
+import { hashlineLine, hashlineRef } from "../../src/tool/hashline"
 
 const ctx = {
   sessionID: "test-edit-session",
@@ -18,479 +19,506 @@ const ctx = {
 }
 
 describe("tool.edit", () => {
-  describe("creating new files", () => {
-    test("creates new file when oldString is empty", async () => {
-      await using tmp = await tmpdir()
-      const filepath = path.join(tmp.path, "newfile.txt")
-
-      await Instance.provide({
-        directory: tmp.path,
-        fn: async () => {
-          const edit = await EditTool.init()
-          const result = await edit.execute(
-            {
-              filePath: filepath,
-              oldString: "",
-              newString: "new content",
-            },
-            ctx,
-          )
-
-          expect(result.metadata.diff).toContain("new content")
-
-          const content = await fs.readFile(filepath, "utf-8")
-          expect(content).toBe("new content")
-        },
-      })
+  test("rejects legacy payloads", async () => {
+    await using tmp = await tmpdir({
+      init: async (dir) => {
+        await fs.writeFile(path.join(dir, "file.txt"), "a\nb\nc", "utf-8")
+      },
     })
+    const filepath = path.join(tmp.path, "file.txt")
 
-    test("creates new file with nested directories", async () => {
-      await using tmp = await tmpdir()
-      const filepath = path.join(tmp.path, "nested", "dir", "file.txt")
-
-      await Instance.provide({
-        directory: tmp.path,
-        fn: async () => {
-          const edit = await EditTool.init()
-          await edit.execute(
+    await Instance.provide({
+      directory: tmp.path,
+      fn: async () => {
+        const edit = await EditTool.init()
+        await expect(
+          edit.execute(
             {
               filePath: filepath,
-              oldString: "",
-              newString: "nested file",
-            },
+              oldString: "b",
+              newString: "B",
+            } as any,
             ctx,
-          )
-
-          const content = await fs.readFile(filepath, "utf-8")
-          expect(content).toBe("nested file")
-        },
-      })
-    })
-
-    test("emits add event for new files", async () => {
-      await using tmp = await tmpdir()
-      const filepath = path.join(tmp.path, "new.txt")
-
-      await Instance.provide({
-        directory: tmp.path,
-        fn: async () => {
-          const { Bus } = await import("../../src/bus")
-          const { File } = await import("../../src/file")
-          const { FileWatcher } = await import("../../src/file/watcher")
-
-          const events: string[] = []
-          const unsubEdited = Bus.subscribe(File.Event.Edited, () => events.push("edited"))
-          const unsubUpdated = Bus.subscribe(FileWatcher.Event.Updated, () => events.push("updated"))
-
-          const edit = await EditTool.init()
-          await edit.execute(
-            {
-              filePath: filepath,
-              oldString: "",
-              newString: "content",
-            },
-            ctx,
-          )
-
-          expect(events).toContain("edited")
-          expect(events).toContain("updated")
-          unsubEdited()
-          unsubUpdated()
-        },
-      })
+          ),
+        ).rejects.toThrow("Legacy edit payload has been removed")
+      },
     })
   })
 
-  describe("editing existing files", () => {
-    test("replaces text in existing file", async () => {
-      await using tmp = await tmpdir()
-      const filepath = path.join(tmp.path, "existing.txt")
-      await fs.writeFile(filepath, "old content here", "utf-8")
-
-      await Instance.provide({
-        directory: tmp.path,
-        fn: async () => {
-          FileTime.read(ctx.sessionID, filepath)
-
-          const edit = await EditTool.init()
-          const result = await edit.execute(
-            {
-              filePath: filepath,
-              oldString: "old content",
-              newString: "new content",
-            },
-            ctx,
-          )
-
-          expect(result.output).toContain("Edit applied successfully")
-
-          const content = await fs.readFile(filepath, "utf-8")
-          expect(content).toBe("new content here")
-        },
-      })
+  test("replaces a single line", async () => {
+    await using tmp = await tmpdir({
+      init: async (dir) => {
+        await fs.writeFile(path.join(dir, "file.txt"), "a\nb\nc", "utf-8")
+      },
     })
+    const filepath = path.join(tmp.path, "file.txt")
 
-    test("throws error when file does not exist", async () => {
-      await using tmp = await tmpdir()
-      const filepath = path.join(tmp.path, "nonexistent.txt")
-
-      await Instance.provide({
-        directory: tmp.path,
-        fn: async () => {
-          FileTime.read(ctx.sessionID, filepath)
-
-          const edit = await EditTool.init()
-          await expect(
-            edit.execute(
+    await Instance.provide({
+      directory: tmp.path,
+      fn: async () => {
+        FileTime.read(ctx.sessionID, filepath)
+        const edit = await EditTool.init()
+        const result = await edit.execute(
+          {
+            filePath: filepath,
+            edits: [
               {
-                filePath: filepath,
-                oldString: "old",
-                newString: "new",
+                type: "set_line",
+                line: hashlineRef(2, "b"),
+                text: "B",
               },
-              ctx,
-            ),
-          ).rejects.toThrow("not found")
-        },
-      })
-    })
+            ],
+          },
+          ctx,
+        )
 
-    test("throws error when oldString equals newString", async () => {
-      await using tmp = await tmpdir()
-      const filepath = path.join(tmp.path, "file.txt")
-      await fs.writeFile(filepath, "content", "utf-8")
-
-      await Instance.provide({
-        directory: tmp.path,
-        fn: async () => {
-          const edit = await EditTool.init()
-          await expect(
-            edit.execute(
-              {
-                filePath: filepath,
-                oldString: "same",
-                newString: "same",
-              },
-              ctx,
-            ),
-          ).rejects.toThrow("identical")
-        },
-      })
-    })
-
-    test("throws error when oldString not found in file", async () => {
-      await using tmp = await tmpdir()
-      const filepath = path.join(tmp.path, "file.txt")
-      await fs.writeFile(filepath, "actual content", "utf-8")
-
-      await Instance.provide({
-        directory: tmp.path,
-        fn: async () => {
-          FileTime.read(ctx.sessionID, filepath)
-
-          const edit = await EditTool.init()
-          await expect(
-            edit.execute(
-              {
-                filePath: filepath,
-                oldString: "not in file",
-                newString: "replacement",
-              },
-              ctx,
-            ),
-          ).rejects.toThrow()
-        },
-      })
-    })
-
-    test("throws error when file was not read first (FileTime)", async () => {
-      await using tmp = await tmpdir()
-      const filepath = path.join(tmp.path, "file.txt")
-      await fs.writeFile(filepath, "content", "utf-8")
-
-      await Instance.provide({
-        directory: tmp.path,
-        fn: async () => {
-          const edit = await EditTool.init()
-          await expect(
-            edit.execute(
-              {
-                filePath: filepath,
-                oldString: "content",
-                newString: "modified",
-              },
-              ctx,
-            ),
-          ).rejects.toThrow("You must read file")
-        },
-      })
-    })
-
-    test("throws error when file has been modified since read", async () => {
-      await using tmp = await tmpdir()
-      const filepath = path.join(tmp.path, "file.txt")
-      await fs.writeFile(filepath, "original content", "utf-8")
-
-      await Instance.provide({
-        directory: tmp.path,
-        fn: async () => {
-          // Read first
-          FileTime.read(ctx.sessionID, filepath)
-
-          // Wait a bit to ensure different timestamps
-          await new Promise((resolve) => setTimeout(resolve, 100))
-
-          // Simulate external modification
-          await fs.writeFile(filepath, "modified externally", "utf-8")
-
-          // Try to edit with the new content
-          const edit = await EditTool.init()
-          await expect(
-            edit.execute(
-              {
-                filePath: filepath,
-                oldString: "modified externally",
-                newString: "edited",
-              },
-              ctx,
-            ),
-          ).rejects.toThrow("modified since it was last read")
-        },
-      })
-    })
-
-    test("replaces all occurrences with replaceAll option", async () => {
-      await using tmp = await tmpdir()
-      const filepath = path.join(tmp.path, "file.txt")
-      await fs.writeFile(filepath, "foo bar foo baz foo", "utf-8")
-
-      await Instance.provide({
-        directory: tmp.path,
-        fn: async () => {
-          FileTime.read(ctx.sessionID, filepath)
-
-          const edit = await EditTool.init()
-          await edit.execute(
-            {
-              filePath: filepath,
-              oldString: "foo",
-              newString: "qux",
-              replaceAll: true,
-            },
-            ctx,
-          )
-
-          const content = await fs.readFile(filepath, "utf-8")
-          expect(content).toBe("qux bar qux baz qux")
-        },
-      })
-    })
-
-    test("emits change event for existing files", async () => {
-      await using tmp = await tmpdir()
-      const filepath = path.join(tmp.path, "file.txt")
-      await fs.writeFile(filepath, "original", "utf-8")
-
-      await Instance.provide({
-        directory: tmp.path,
-        fn: async () => {
-          FileTime.read(ctx.sessionID, filepath)
-
-          const { Bus } = await import("../../src/bus")
-          const { File } = await import("../../src/file")
-          const { FileWatcher } = await import("../../src/file/watcher")
-
-          const events: string[] = []
-          const unsubEdited = Bus.subscribe(File.Event.Edited, () => events.push("edited"))
-          const unsubUpdated = Bus.subscribe(FileWatcher.Event.Updated, () => events.push("updated"))
-
-          const edit = await EditTool.init()
-          await edit.execute(
-            {
-              filePath: filepath,
-              oldString: "original",
-              newString: "modified",
-            },
-            ctx,
-          )
-
-          expect(events).toContain("edited")
-          expect(events).toContain("updated")
-          unsubEdited()
-          unsubUpdated()
-        },
-      })
+        expect(await fs.readFile(filepath, "utf-8")).toBe("a\nB\nc")
+        expect(result.metadata.edit_mode).toBe("hashline")
+      },
     })
   })
 
-  describe("edge cases", () => {
-    test("handles multiline replacements", async () => {
-      await using tmp = await tmpdir()
-      const filepath = path.join(tmp.path, "file.txt")
-      await fs.writeFile(filepath, "line1\nline2\nline3", "utf-8")
-
-      await Instance.provide({
-        directory: tmp.path,
-        fn: async () => {
-          FileTime.read(ctx.sessionID, filepath)
-
-          const edit = await EditTool.init()
-          await edit.execute(
-            {
-              filePath: filepath,
-              oldString: "line2",
-              newString: "new line 2\nextra line",
-            },
-            ctx,
-          )
-
-          const content = await fs.readFile(filepath, "utf-8")
-          expect(content).toBe("line1\nnew line 2\nextra line\nline3")
-        },
-      })
+  test("supports replace operations with all=true", async () => {
+    await using tmp = await tmpdir({
+      init: async (dir) => {
+        await fs.writeFile(path.join(dir, "file.txt"), "foo bar foo baz foo", "utf-8")
+      },
     })
+    const filepath = path.join(tmp.path, "file.txt")
 
-    test("handles CRLF line endings", async () => {
-      await using tmp = await tmpdir()
-      const filepath = path.join(tmp.path, "file.txt")
-      await fs.writeFile(filepath, "line1\r\nold\r\nline3", "utf-8")
-
-      await Instance.provide({
-        directory: tmp.path,
-        fn: async () => {
-          FileTime.read(ctx.sessionID, filepath)
-
-          const edit = await EditTool.init()
-          await edit.execute(
-            {
-              filePath: filepath,
-              oldString: "old",
-              newString: "new",
-            },
-            ctx,
-          )
-
-          const content = await fs.readFile(filepath, "utf-8")
-          expect(content).toBe("line1\r\nnew\r\nline3")
-        },
-      })
-    })
-
-    test("throws error when oldString equals newString", async () => {
-      await using tmp = await tmpdir()
-      const filepath = path.join(tmp.path, "file.txt")
-      await fs.writeFile(filepath, "content", "utf-8")
-
-      await Instance.provide({
-        directory: tmp.path,
-        fn: async () => {
-          const edit = await EditTool.init()
-          await expect(
-            edit.execute(
+    await Instance.provide({
+      directory: tmp.path,
+      fn: async () => {
+        FileTime.read(ctx.sessionID, filepath)
+        const edit = await EditTool.init()
+        await edit.execute(
+          {
+            filePath: filepath,
+            edits: [
               {
-                filePath: filepath,
-                oldString: "",
-                newString: "",
+                type: "replace",
+                old_text: "foo",
+                new_text: "qux",
+                all: true,
               },
-              ctx,
-            ),
-          ).rejects.toThrow("identical")
-        },
-      })
-    })
+            ],
+          },
+          ctx,
+        )
 
-    test("throws error when path is directory", async () => {
-      await using tmp = await tmpdir()
-      const dirpath = path.join(tmp.path, "adir")
-      await fs.mkdir(dirpath)
-
-      await Instance.provide({
-        directory: tmp.path,
-        fn: async () => {
-          FileTime.read(ctx.sessionID, dirpath)
-
-          const edit = await EditTool.init()
-          await expect(
-            edit.execute(
-              {
-                filePath: dirpath,
-                oldString: "old",
-                newString: "new",
-              },
-              ctx,
-            ),
-          ).rejects.toThrow("directory")
-        },
-      })
-    })
-
-    test("tracks file diff statistics", async () => {
-      await using tmp = await tmpdir()
-      const filepath = path.join(tmp.path, "file.txt")
-      await fs.writeFile(filepath, "line1\nline2\nline3", "utf-8")
-
-      await Instance.provide({
-        directory: tmp.path,
-        fn: async () => {
-          FileTime.read(ctx.sessionID, filepath)
-
-          const edit = await EditTool.init()
-          const result = await edit.execute(
-            {
-              filePath: filepath,
-              oldString: "line2",
-              newString: "new line a\nnew line b",
-            },
-            ctx,
-          )
-
-          expect(result.metadata.filediff).toBeDefined()
-          expect(result.metadata.filediff.file).toBe(filepath)
-          expect(result.metadata.filediff.additions).toBeGreaterThan(0)
-        },
-      })
+        expect(await fs.readFile(filepath, "utf-8")).toBe("qux bar qux baz qux")
+      },
     })
   })
 
-  describe("concurrent editing", () => {
-    test("serializes concurrent edits to same file", async () => {
-      await using tmp = await tmpdir()
-      const filepath = path.join(tmp.path, "file.txt")
-      await fs.writeFile(filepath, "0", "utf-8")
+  test("supports range replacement and insert modes", async () => {
+    await using tmp = await tmpdir({
+      init: async (dir) => {
+        await fs.writeFile(path.join(dir, "file.txt"), "a\nb\nc\nd", "utf-8")
+      },
+    })
+    const filepath = path.join(tmp.path, "file.txt")
 
-      await Instance.provide({
-        directory: tmp.path,
-        fn: async () => {
-          FileTime.read(ctx.sessionID, filepath)
+    await Instance.provide({
+      directory: tmp.path,
+      fn: async () => {
+        FileTime.read(ctx.sessionID, filepath)
+        const edit = await EditTool.init()
+        await edit.execute(
+          {
+            filePath: filepath,
+            edits: [
+              {
+                type: "replace_lines",
+                start_line: hashlineRef(2, "b"),
+                end_line: hashlineRef(3, "c"),
+                text: ["B", "C"],
+              },
+              {
+                type: "insert_before",
+                line: hashlineRef(2, "b"),
+                text: "x",
+              },
+              {
+                type: "insert_after",
+                line: hashlineRef(3, "c"),
+                text: "y",
+              },
+            ],
+          },
+          ctx,
+        )
 
-          const edit = await EditTool.init()
+        expect(await fs.readFile(filepath, "utf-8")).toBe("a\nx\nB\nC\ny\nd")
+      },
+    })
+  })
 
-          // Two concurrent edits
-          const promise1 = edit.execute(
+  test("creates missing files from append and prepend operations", async () => {
+    await using tmp = await tmpdir()
+    const filepath = path.join(tmp.path, "created.txt")
+
+    await Instance.provide({
+      directory: tmp.path,
+      fn: async () => {
+        const edit = await EditTool.init()
+        await edit.execute(
+          {
+            filePath: filepath,
+            edits: [
+              {
+                type: "prepend",
+                text: "start",
+              },
+              {
+                type: "append",
+                text: "end",
+              },
+            ],
+          },
+          ctx,
+        )
+
+        expect(await fs.readFile(filepath, "utf-8")).toBe("start\nend")
+      },
+    })
+  })
+
+  test("requires a prior read for existing files", async () => {
+    await using tmp = await tmpdir({
+      init: async (dir) => {
+        await fs.writeFile(path.join(dir, "file.txt"), "content", "utf-8")
+      },
+    })
+    const filepath = path.join(tmp.path, "file.txt")
+
+    await Instance.provide({
+      directory: tmp.path,
+      fn: async () => {
+        const edit = await EditTool.init()
+        await expect(
+          edit.execute(
             {
               filePath: filepath,
-              oldString: "0",
-              newString: "1",
+              edits: [
+                {
+                  type: "set_line",
+                  line: hashlineRef(1, "content"),
+                  text: "changed",
+                },
+              ],
             },
             ctx,
-          )
+          ),
+        ).rejects.toThrow("You must read file")
+      },
+    })
+  })
 
-          // Need to read again since FileTime tracks per-session
-          FileTime.read(ctx.sessionID, filepath)
+  test("rejects files modified since read", async () => {
+    await using tmp = await tmpdir({
+      init: async (dir) => {
+        await fs.writeFile(path.join(dir, "file.txt"), "original", "utf-8")
+      },
+    })
+    const filepath = path.join(tmp.path, "file.txt")
 
-          const promise2 = edit.execute(
+    await Instance.provide({
+      directory: tmp.path,
+      fn: async () => {
+        FileTime.read(ctx.sessionID, filepath)
+        await new Promise((resolve) => setTimeout(resolve, 100))
+        await fs.writeFile(filepath, "external", "utf-8")
+
+        const edit = await EditTool.init()
+        await expect(
+          edit.execute(
             {
               filePath: filepath,
-              oldString: "0",
-              newString: "2",
+              edits: [
+                {
+                  type: "set_line",
+                  line: hashlineRef(1, "original"),
+                  text: "changed",
+                },
+              ],
             },
             ctx,
-          )
+          ),
+        ).rejects.toThrow("modified since it was last read")
+      },
+    })
+  })
 
-          // Both should complete without error (though one might fail due to content mismatch)
-          const results = await Promise.allSettled([promise1, promise2])
-          expect(results.some((r) => r.status === "fulfilled")).toBe(true)
+  test("rejects missing files for non-append and non-prepend edits", async () => {
+    await using tmp = await tmpdir()
+    const filepath = path.join(tmp.path, "missing.txt")
+
+    await Instance.provide({
+      directory: tmp.path,
+      fn: async () => {
+        const edit = await EditTool.init()
+        await expect(
+          edit.execute(
+            {
+              filePath: filepath,
+              edits: [
+                {
+                  type: "replace",
+                  old_text: "a",
+                  new_text: "b",
+                },
+              ],
+            },
+            ctx,
+          ),
+        ).rejects.toThrow("Missing file can only be created")
+      },
+    })
+  })
+
+  test("rejects directories", async () => {
+    await using tmp = await tmpdir({
+      init: async (dir) => {
+        await fs.mkdir(path.join(dir, "adir"))
+      },
+    })
+    const filepath = path.join(tmp.path, "adir")
+
+    await Instance.provide({
+      directory: tmp.path,
+      fn: async () => {
+        const edit = await EditTool.init()
+        await expect(
+          edit.execute(
+            {
+              filePath: filepath,
+              edits: [
+                {
+                  type: "append",
+                  text: "x",
+                },
+              ],
+            },
+            ctx,
+          ),
+        ).rejects.toThrow("directory")
+      },
+    })
+  })
+
+  test("tracks file diff statistics", async () => {
+    await using tmp = await tmpdir({
+      init: async (dir) => {
+        await fs.writeFile(path.join(dir, "file.txt"), "line1\nline2\nline3", "utf-8")
+      },
+    })
+    const filepath = path.join(tmp.path, "file.txt")
+
+    await Instance.provide({
+      directory: tmp.path,
+      fn: async () => {
+        FileTime.read(ctx.sessionID, filepath)
+        const edit = await EditTool.init()
+        const result = await edit.execute(
+          {
+            filePath: filepath,
+            edits: [
+              {
+                type: "replace_lines",
+                start_line: hashlineRef(2, "line2"),
+                end_line: hashlineRef(2, "line2"),
+                text: ["new line a", "new line b"],
+              },
+            ],
+          },
+          ctx,
+        )
+
+        expect(result.metadata.filediff).toBeDefined()
+        expect(result.metadata.filediff.file).toBe(filepath)
+        expect(result.metadata.filediff.additions).toBeGreaterThan(0)
+      },
+    })
+  })
+
+  test("emits change events for existing files", async () => {
+    await using tmp = await tmpdir({
+      init: async (dir) => {
+        await fs.writeFile(path.join(dir, "file.txt"), "a\nb", "utf-8")
+      },
+    })
+    const filepath = path.join(tmp.path, "file.txt")
+
+    await Instance.provide({
+      directory: tmp.path,
+      fn: async () => {
+        FileTime.read(ctx.sessionID, filepath)
+
+        const { Bus } = await import("../../src/bus")
+        const { File } = await import("../../src/file")
+        const { FileWatcher } = await import("../../src/file/watcher")
+
+        const events: string[] = []
+        const unsubEdited = Bus.subscribe(File.Event.Edited, () => events.push("edited"))
+        const unsubUpdated = Bus.subscribe(FileWatcher.Event.Updated, () => events.push("updated"))
+
+        const edit = await EditTool.init()
+        await edit.execute(
+          {
+            filePath: filepath,
+            edits: [
+              {
+                type: "set_line",
+                line: hashlineRef(2, "b"),
+                text: "B",
+              },
+            ],
+          },
+          ctx,
+        )
+
+        expect(events).toContain("edited")
+        expect(events).toContain("updated")
+        unsubEdited()
+        unsubUpdated()
+      },
+    })
+  })
+
+  test("applies hashline autocorrect prefixes through config", async () => {
+    await using tmp = await tmpdir({
+      config: {
+        experimental: {
+          hashline_autocorrect: true,
         },
-      })
+      },
+      init: async (dir) => {
+        await fs.writeFile(path.join(dir, "file.txt"), "a\nb\nc", "utf-8")
+      },
+    })
+    const filepath = path.join(tmp.path, "file.txt")
+
+    await Instance.provide({
+      directory: tmp.path,
+      fn: async () => {
+        FileTime.read(ctx.sessionID, filepath)
+        const edit = await EditTool.init()
+        await edit.execute(
+          {
+            filePath: filepath,
+            edits: [
+              {
+                type: "set_line",
+                line: hashlineRef(2, "b"),
+                text: hashlineLine(2, "B"),
+              },
+            ],
+          },
+          ctx,
+        )
+
+        expect(await fs.readFile(filepath, "utf-8")).toBe("a\nB\nc")
+      },
+    })
+  })
+
+  test("supports delete and rename flows", async () => {
+    await using tmp = await tmpdir({
+      init: async (dir) => {
+        await fs.writeFile(path.join(dir, "src.txt"), "a\nb", "utf-8")
+        await fs.writeFile(path.join(dir, "delete.txt"), "delete me", "utf-8")
+      },
+    })
+    const source = path.join(tmp.path, "src.txt")
+    const target = path.join(tmp.path, "renamed.txt")
+    const doomed = path.join(tmp.path, "delete.txt")
+
+    await Instance.provide({
+      directory: tmp.path,
+      fn: async () => {
+        const edit = await EditTool.init()
+
+        FileTime.read(ctx.sessionID, source)
+        await edit.execute(
+          {
+            filePath: source,
+            rename: target,
+            edits: [
+              {
+                type: "set_line",
+                line: hashlineRef(2, "b"),
+                text: "B",
+              },
+            ],
+          },
+          ctx,
+        )
+
+        expect(await fs.readFile(target, "utf-8")).toBe("a\nB")
+        await expect(fs.stat(source)).rejects.toThrow()
+
+        FileTime.read(ctx.sessionID, doomed)
+        await edit.execute(
+          {
+            filePath: doomed,
+            delete: true,
+            edits: [],
+          },
+          ctx,
+        )
+
+        await expect(fs.stat(doomed)).rejects.toThrow()
+      },
+    })
+  })
+
+  test("serializes concurrent edits to the same file", async () => {
+    await using tmp = await tmpdir({
+      init: async (dir) => {
+        await fs.writeFile(path.join(dir, "file.txt"), "0", "utf-8")
+      },
+    })
+    const filepath = path.join(tmp.path, "file.txt")
+
+    await Instance.provide({
+      directory: tmp.path,
+      fn: async () => {
+        FileTime.read(ctx.sessionID, filepath)
+        const edit = await EditTool.init()
+
+        const first = edit.execute(
+          {
+            filePath: filepath,
+            edits: [
+              {
+                type: "set_line",
+                line: hashlineRef(1, "0"),
+                text: "1",
+              },
+            ],
+          },
+          ctx,
+        )
+
+        FileTime.read(ctx.sessionID, filepath)
+        const second = edit.execute(
+          {
+            filePath: filepath,
+            edits: [
+              {
+                type: "set_line",
+                line: hashlineRef(1, "0"),
+                text: "2",
+              },
+            ],
+          },
+          ctx,
+        )
+
+        const results = await Promise.allSettled([first, second])
+        expect(results.some((result) => result.status === "fulfilled")).toBe(true)
+      },
     })
   })
 })

packages/opencode/test/tool/grep.test.ts

@@ -37,6 +37,29 @@ describe("tool.grep", () => {
     })
   })
 
+  test("emits hashline anchors by default", async () => {
+    await using tmp = await tmpdir({
+      init: async (dir) => {
+        await Bun.write(path.join(dir, "test.txt"), "alpha\nbeta")
+      },
+    })
+
+    await Instance.provide({
+      directory: tmp.path,
+      fn: async () => {
+        const grep = await GrepTool.init()
+        const result = await grep.execute(
+          {
+            pattern: "alpha",
+            path: tmp.path,
+          },
+          ctx,
+        )
+        expect(result.output).toMatch(/\b1#[ZPMQVRWSNKTXJBYH]{2}:alpha\b/)
+      },
+    })
+  })
+
   test("no matches returns correct output", async () => {
     await using tmp = await tmpdir({
       init: async (dir) => {
@@ -61,10 +84,8 @@ describe("tool.grep", () => {
   })
 
   test("handles CRLF line endings in output", async () => {
-    // This test verifies the regex split handles both \n and \r\n
     await using tmp = await tmpdir({
       init: async (dir) => {
-        // Create a test file with content
         await Bun.write(path.join(dir, "test.txt"), "line1\nline2\nline3")
       },
     })

packages/opencode/test/tool/hashline.test.ts

@@ -0,0 +1,210 @@
+import { describe, expect, test } from "bun:test"
+import { applyHashlineEdits, hashlineID, hashlineLine, hashlineRef, parseHashlineRef } from "../../src/tool/hashline"
+
+function swapID(ref: string) {
+  const [line, id] = ref.split("#")
+  const next = id[0] === "Z" ? `P${id[1]}` : `Z${id[1]}`
+  return `${line}#${next}`
+}
+
+function errorMessage(run: () => void) {
+  try {
+    run()
+    return ""
+  } catch (error) {
+    return error instanceof Error ? error.message : String(error)
+  }
+}
+
+describe("tool.hashline", () => {
+  test("hash computation is stable and 2-char alphabet encoded", () => {
+    const a = hashlineID(1, "  const x = 1")
+    const b = hashlineID(1, "constx=1")
+    const c = hashlineID(99, "constx=1")
+    expect(a).toBe(b)
+    expect(a).toBe(c)
+    expect(a).toMatch(/^[ZPMQVRWSNKTXJBYH]{2}$/)
+  })
+
+  test("low-signal lines mix line index into hash id", () => {
+    const a = hashlineID(1, "")
+    const b = hashlineID(2, "")
+    const c = hashlineID(1, "{}")
+    const d = hashlineID(2, "{}")
+    expect(a).not.toBe(b)
+    expect(c).not.toBe(d)
+  })
+
+  test("autocorrect strips copied hashline prefixes when enabled", () => {
+    const old = Bun.env.OPENCODE_HL_AUTOCORRECT
+    Bun.env.OPENCODE_HL_AUTOCORRECT = "1"
+    try {
+      const result = applyHashlineEdits({
+        lines: ["a"],
+        trailing: false,
+        edits: [
+          {
+            type: "set_line",
+            line: hashlineRef(1, "a"),
+            text: hashlineLine(1, "a"),
+          },
+        ],
+      })
+      expect(result.lines).toEqual(["a"])
+    } finally {
+      if (old === undefined) delete Bun.env.OPENCODE_HL_AUTOCORRECT
+      else Bun.env.OPENCODE_HL_AUTOCORRECT = old
+    }
+  })
+
+  test("default autocorrect does not rewrite non-prefix content", () => {
+    const result = applyHashlineEdits({
+      lines: ["a"],
+      trailing: false,
+      edits: [
+        {
+          type: "set_line",
+          line: hashlineRef(1, "a"),
+          text: "+a",
+        },
+      ],
+      autocorrect: true,
+      aggressiveAutocorrect: false,
+    })
+    expect(result.lines).toEqual(["+a"])
+  })
+
+  test("parses strict LINE#ID references with tolerant extraction", () => {
+    const ref = parseHashlineRef(">>> 12#ZP:const value = 1", "line")
+    expect(ref.line).toBe(12)
+    expect(ref.id).toBe("ZP")
+    expect(ref.raw).toBe("12#ZP")
+
+    expect(() => parseHashlineRef("12#ab", "line")).toThrow("LINE#ID")
+  })
+
+  test("reports compact mismatch errors with retry anchors", () => {
+    const lines = ["alpha", "beta", "gamma"]
+    const wrong = swapID(hashlineRef(2, lines[1]))
+
+    const message = errorMessage(() =>
+      applyHashlineEdits({
+        lines,
+        trailing: false,
+        edits: [
+          {
+            type: "set_line",
+            line: wrong,
+            text: "BETA",
+          },
+        ],
+      }),
+    )
+
+    expect(message).toContain("anchor mismatch")
+    expect(message).toContain("retry with")
+    expect(message).not.toContain(">>>")
+    expect(message.length).toBeLessThan(260)
+  })
+
+  test("applies batched line edits bottom-up for stable results", () => {
+    const lines = ["a", "b", "c", "d"]
+    const one = hashlineRef(1, lines[0])
+    const two = hashlineRef(2, lines[1])
+    const three = hashlineRef(3, lines[2])
+    const four = hashlineRef(4, lines[3])
+
+    const result = applyHashlineEdits({
+      lines,
+      trailing: false,
+      edits: [
+        {
+          type: "replace_lines",
+          start_line: two,
+          end_line: three,
+          text: ["B", "C"],
+        },
+        {
+          type: "insert_after",
+          line: one,
+          text: "A1",
+        },
+        {
+          type: "set_line",
+          line: four,
+          text: "D",
+        },
+      ],
+    })
+
+    expect(result.lines).toEqual(["a", "A1", "B", "C", "D"])
+  })
+
+  test("orders append and prepend deterministically on empty files", () => {
+    const result = applyHashlineEdits({
+      lines: [],
+      trailing: false,
+      edits: [
+        {
+          type: "append",
+          text: "end",
+        },
+        {
+          type: "prepend",
+          text: "start",
+        },
+      ],
+    })
+
+    expect(result.lines).toEqual(["start", "end"])
+  })
+
+  test("validates ranges, between constraints, and non-empty insert text", () => {
+    const lines = ["a", "b", "c"]
+    const one = hashlineRef(1, lines[0])
+    const two = hashlineRef(2, lines[1])
+
+    expect(() =>
+      applyHashlineEdits({
+        lines,
+        trailing: false,
+        edits: [
+          {
+            type: "replace_lines",
+            start_line: two,
+            end_line: one,
+            text: "x",
+          },
+        ],
+      }),
+    ).toThrow("start_line")
+
+    expect(() =>
+      applyHashlineEdits({
+        lines,
+        trailing: false,
+        edits: [
+          {
+            type: "insert_between",
+            after_line: two,
+            before_line: one,
+            text: "x",
+          },
+        ],
+      }),
+    ).toThrow("insert_between.after_line")
+
+    expect(() =>
+      applyHashlineEdits({
+        lines,
+        trailing: false,
+        edits: [
+          {
+            type: "append",
+            text: "",
+          },
+        ],
+      }),
+    ).toThrow("append.text")
+  })
+})

packages/opencode/test/tool/read.test.ts

@@ -6,6 +6,7 @@ import { Filesystem } from "../../src/util/filesystem"
 import { tmpdir } from "../fixture/fixture"
 import { PermissionNext } from "../../src/permission/next"
 import { Agent } from "../../src/agent/agent"
+import { hashlineLine } from "../../src/tool/hashline"
 
 const FIXTURES_DIR = path.join(import.meta.dir, "fixtures")
 
@@ -269,10 +270,10 @@ describe("tool.read truncation", () => {
       fn: async () => {
         const read = await ReadTool.init()
         const result = await read.execute({ filePath: path.join(tmp.path, "offset.txt"), offset: 10, limit: 5 }, ctx)
-        expect(result.output).toContain("10: line10")
-        expect(result.output).toContain("14: line14")
-        expect(result.output).not.toContain("9: line10")
-        expect(result.output).not.toContain("15: line15")
+        expect(result.output).toContain(hashlineLine(10, "line10"))
+        expect(result.output).toContain(hashlineLine(14, "line14"))
+        expect(result.output).not.toContain(hashlineLine(9, "line9"))
+        expect(result.output).not.toContain(hashlineLine(15, "line15"))
         expect(result.output).toContain("line10")
         expect(result.output).toContain("line14")
         expect(result.output).not.toContain("line0")
@@ -443,6 +444,27 @@ root_type Monster;`
   })
 })
 
+describe("tool.read hashline output", () => {
+  test("returns LINE#ID prefixes by default", async () => {
+    await using tmp = await tmpdir({
+      init: async (dir) => {
+        await Bun.write(path.join(dir, "hashline.txt"), "foo\nbar")
+      },
+    })
+
+    await Instance.provide({
+      directory: tmp.path,
+      fn: async () => {
+        const read = await ReadTool.init()
+        const result = await read.execute({ filePath: path.join(tmp.path, "hashline.txt") }, ctx)
+        expect(result.output).toContain(hashlineLine(1, "foo"))
+        expect(result.output).toContain(hashlineLine(2, "bar"))
+        expect(result.output).not.toContain("1: foo")
+      },
+    })
+  })
+})
+
 describe("tool.read loaded instructions", () => {
   test("loads AGENTS.md from parent directory and includes in metadata", async () => {
     await using tmp = await tmpdir({

packages/opencode/test/tool/registry-hashline.test.ts

@@ -0,0 +1,42 @@
+import { describe, expect, test } from "bun:test"
+import { tmpdir } from "../fixture/fixture"
+import { Instance } from "../../src/project/instance"
+import { ToolRegistry } from "../../src/tool/registry"
+
+describe("tool.registry hashline routing", () => {
+  test("keeps edit and apply_patch for GPT models", async () => {
+    await using tmp = await tmpdir()
+
+    await Instance.provide({
+      directory: tmp.path,
+      fn: async () => {
+        const tools = await ToolRegistry.tools({
+          providerID: "openai",
+          modelID: "gpt-5",
+        })
+        const ids = tools.map((tool) => tool.id)
+        expect(ids).toContain("edit")
+        expect(ids).toContain("write")
+        expect(ids).toContain("apply_patch")
+      },
+    })
+  })
+
+  test("keeps edit and removes apply_patch for non-GPT models", async () => {
+    await using tmp = await tmpdir()
+
+    await Instance.provide({
+      directory: tmp.path,
+      fn: async () => {
+        const tools = await ToolRegistry.tools({
+          providerID: "anthropic",
+          modelID: "claude-3-7-sonnet",
+        })
+        const ids = tools.map((tool) => tool.id)
+        expect(ids).toContain("edit")
+        expect(ids).toContain("write")
+        expect(ids).not.toContain("apply_patch")
+      },
+    })
+  })
+})

packages/ui/src/components/message-part.tsx

@@ -1751,11 +1751,11 @@ ToolRegistry.register({
                   mode="diff"
                   before={{
                     name: props.metadata?.filediff?.file || props.input.filePath,
-                    contents: props.metadata?.filediff?.before || props.input.oldString,
+                    contents: props.metadata?.filediff?.before ?? props.input.oldString,
                   }}
                   after={{
                     name: props.metadata?.filediff?.file || props.input.filePath,
-                    contents: props.metadata?.filediff?.after || props.input.newString,
+                    contents: props.metadata?.filediff?.after ?? props.input.newString,
                   }}
                 />
               </div>