diff --git a/packages/app/src/context/global-sync.tsx b/packages/app/src/context/global-sync.tsx
index ad3d124b2c..12fe37acf3 100644
--- a/packages/app/src/context/global-sync.tsx
+++ b/packages/app/src/context/global-sync.tsx
@@ -839,6 +839,23 @@ function createGlobalSync() {
         )
         break
       }
+      case "message.part.delta": {
+        const parts = store.part[event.properties.messageID]
+        if (!parts) break
+        const result = Binary.search(parts, event.properties.partID, (p) => p.id)
+        if (!result.found) break
+        setStore(
+          "part",
+          event.properties.messageID,
+          produce((draft) => {
+            const part = draft[result.index]
+            const field = event.properties.field as keyof typeof part
+            const existing = part[field] as string | undefined
+            ;(part[field] as string) = (existing ?? "") + event.properties.delta
+          }),
+        )
+        break
+      }
       case "message.part.removed": {
         const messageID = event.properties.messageID
         const parts = store.part[messageID]
diff --git a/packages/opencode/src/acp/agent.ts b/packages/opencode/src/acp/agent.ts
index cc9a029a04..2af375b996 100644
--- a/packages/opencode/src/acp/agent.ts
+++ b/packages/opencode/src/acp/agent.ts
@@ -365,46 +365,68 @@ export namespace ACP {
                 return
             }
           }
+          return
+        }
 
-          if (part.type === "text") {
-            const delta = props.delta
-            if (delta && part.ignored !== true) {
-              await this.connection
-                .sessionUpdate({
-                  sessionId,
-                  update: {
-                    sessionUpdate: "agent_message_chunk",
-                    content: {
-                      type: "text",
-                      text: delta,
-                    },
+        case "message.part.delta": {
+          const props = event.properties
+          const session = this.sessionManager.tryGet(props.sessionID)
+          if (!session) return
+          const sessionId = session.id
+
+          const message = await this.sdk.session
+            .message(
+              {
+                sessionID: props.sessionID,
+                messageID: props.messageID,
+                directory: session.cwd,
+              },
+              { throwOnError: true },
+            )
+            .then((x) => x.data)
+            .catch((error) => {
+              log.error("unexpected error when fetching message", { error })
+              return undefined
+            })
+
+          if (!message || message.info.role !== "assistant") return
+
+          const part = message.parts.find((p) => p.id === props.partID)
+          if (!part) return
+
+          if (part.type === "text" && props.field === "text" && part.ignored !== true) {
+            await this.connection
+              .sessionUpdate({
+                sessionId,
+                update: {
+                  sessionUpdate: "agent_message_chunk",
+                  content: {
+                    type: "text",
+                    text: props.delta,
                   },
-                })
-                .catch((error) => {
-                  log.error("failed to send text to ACP", { error })
-                })
-            }
+                },
+              })
+              .catch((error) => {
+                log.error("failed to send text delta to ACP", { error })
+              })
             return
           }
 
-          if (part.type === "reasoning") {
-            const delta = props.delta
-            if (delta) {
-              await this.connection
-                .sessionUpdate({
-                  sessionId,
-                  update: {
-                    sessionUpdate: "agent_thought_chunk",
-                    content: {
-                      type: "text",
-                      text: delta,
-                    },
+          if (part.type === "reasoning" && props.field === "text") {
+            await this.connection
+              .sessionUpdate({
+                sessionId,
+                update: {
+                  sessionUpdate: "agent_thought_chunk",
+                  content: {
+                    type: "text",
+                    text: props.delta,
                   },
-                })
-                .catch((error) => {
-                  log.error("failed to send reasoning to ACP", { error })
-                })
-            }
+                },
+              })
+              .catch((error) => {
+                log.error("failed to send reasoning delta to ACP", { error })
+              })
           }
           return
         }
diff --git a/packages/opencode/src/cli/cmd/tui/context/sync.tsx b/packages/opencode/src/cli/cmd/tui/context/sync.tsx
index eb8ed2d9bb..269ed7ae0b 100644
--- a/packages/opencode/src/cli/cmd/tui/context/sync.tsx
+++ b/packages/opencode/src/cli/cmd/tui/context/sync.tsx
@@ -299,6 +299,24 @@ export const { use: useSync, provider: SyncProvider } = createSimpleContext({
           break
         }
 
+        case "message.part.delta": {
+          const parts = store.part[event.properties.messageID]
+          if (!parts) break
+          const result = Binary.search(parts, event.properties.partID, (p) => p.id)
+          if (!result.found) break
+          setStore(
+            "part",
+            event.properties.messageID,
+            produce((draft) => {
+              const part = draft[result.index]
+              const field = event.properties.field as keyof typeof part
+              const existing = part[field] as string | undefined
+              ;(part[field] as string) = (existing ?? "") + event.properties.delta
+            }),
+          )
+          break
+        }
+
         case "message.part.removed": {
           const parts = store.part[event.properties.messageID]
           const result = Binary.search(parts, event.properties.partID, (p) => p.id)
diff --git a/packages/opencode/src/session/index.ts b/packages/opencode/src/session/index.ts
index 2d506e2e96..e87e4e4e44 100644
--- a/packages/opencode/src/session/index.ts
+++ b/packages/opencode/src/session/index.ts
@@ -603,21 +603,9 @@ export namespace Session {
     },
   )
 
-  const UpdatePartInput = z.union([
-    MessageV2.Part,
-    z.object({
-      part: MessageV2.TextPart,
-      delta: z.string(),
-    }),
-    z.object({
-      part: MessageV2.ReasoningPart,
-      delta: z.string(),
-    }),
-  ])
+  const UpdatePartInput = MessageV2.Part
 
-  export const updatePart = fn(UpdatePartInput, async (input) => {
-    const part = "delta" in input ? input.part : input
-    const delta = "delta" in input ? input.delta : undefined
+  export const updatePart = fn(UpdatePartInput, async (part) => {
     const { id, messageID, sessionID, ...data } = part
     const time = Date.now()
     Database.use((db) => {
@@ -634,13 +622,25 @@ export namespace Session {
       Database.effect(() =>
         Bus.publish(MessageV2.Event.PartUpdated, {
           part,
-          delta,
         }),
       )
     })
     return part
   })
 
+  export const updatePartDelta = fn(
+    z.object({
+      sessionID: z.string(),
+      messageID: z.string(),
+      partID: z.string(),
+      field: z.string(),
+      delta: z.string(),
+    }),
+    async (input) => {
+      Bus.publish(MessageV2.Event.PartDelta, input)
+    },
+  )
+
   export const getUsage = fn(
     z.object({
       model: z.custom<Provider.Model>(),
diff --git a/packages/opencode/src/session/message-v2.ts b/packages/opencode/src/session/message-v2.ts
index f5a4d7abbd..1b6bf04a84 100644
--- a/packages/opencode/src/session/message-v2.ts
+++ b/packages/opencode/src/session/message-v2.ts
@@ -417,7 +417,16 @@ export namespace MessageV2 {
       "message.part.updated",
       z.object({
         part: Part,
-        delta: z.string().optional(),
+      }),
+    ),
+    PartDelta: BusEvent.define(
+      "message.part.delta",
+      z.object({
+        sessionID: z.string(),
+        messageID: z.string(),
+        partID: z.string(),
+        field: z.string(),
+        delta: z.string(),
       }),
     ),
     PartRemoved: BusEvent.define(
diff --git a/packages/opencode/src/session/processor.ts b/packages/opencode/src/session/processor.ts
index 2707105618..826d0842cc 100644
--- a/packages/opencode/src/session/processor.ts
+++ b/packages/opencode/src/session/processor.ts
@@ -63,17 +63,19 @@ export namespace SessionProcessor {
                   if (value.id in reasoningMap) {
                     continue
                   }
-                  reasoningMap[value.id] = {
+                  const reasoningPart = {
                     id: Identifier.ascending("part"),
                     messageID: input.assistantMessage.id,
                     sessionID: input.assistantMessage.sessionID,
-                    type: "reasoning",
+                    type: "reasoning" as const,
                     text: "",
                     time: {
                       start: Date.now(),
                     },
                     metadata: value.providerMetadata,
                   }
+                  reasoningMap[value.id] = reasoningPart
+                  await Session.updatePart(reasoningPart)
                   break
 
                 case "reasoning-delta":
@@ -81,7 +83,13 @@ export namespace SessionProcessor {
                     const part = reasoningMap[value.id]
                     part.text += value.text
                     if (value.providerMetadata) part.metadata = value.providerMetadata
-                    if (part.text) await Session.updatePart({ part, delta: value.text })
+                    await Session.updatePartDelta({
+                      sessionID: part.sessionID,
+                      messageID: part.messageID,
+                      partID: part.id,
+                      field: "text",
+                      delta: value.text,
+                    })
                   }
                   break
 
@@ -288,17 +296,20 @@ export namespace SessionProcessor {
                     },
                     metadata: value.providerMetadata,
                   }
+                  await Session.updatePart(currentText)
                   break
 
                 case "text-delta":
                   if (currentText) {
                     currentText.text += value.text
                     if (value.providerMetadata) currentText.metadata = value.providerMetadata
-                    if (currentText.text)
-                      await Session.updatePart({
-                        part: currentText,
-                        delta: value.text,
-                      })
+                    await Session.updatePartDelta({
+                      sessionID: currentText.sessionID,
+                      messageID: currentText.messageID,
+                      partID: currentText.id,
+                      field: "text",
+                      delta: value.text,
+                    })
                   }
                   break
 
diff --git a/packages/sdk/js/src/v2/gen/types.gen.ts b/packages/sdk/js/src/v2/gen/types.gen.ts
index 8555e84384..20fc5920df 100644
--- a/packages/sdk/js/src/v2/gen/types.gen.ts
+++ b/packages/sdk/js/src/v2/gen/types.gen.ts
@@ -480,7 +480,17 @@ export type EventMessagePartUpdated = {
   type: "message.part.updated"
   properties: {
     part: Part
-    delta?: string
+  }
+}
+
+export type EventMessagePartDelta = {
+  type: "message.part.delta"
+  properties: {
+    sessionID: string
+    messageID: string
+    partID: string
+    field: string
+    delta: string
   }
 }
 
@@ -650,10 +660,6 @@ export type Todo = {
    * Priority level of the task: high, medium, low
    */
   priority: string
-  /**
-   * Unique identifier for the todo item
-   */
-  id: string
 }
 
 export type EventTodoUpdated = {
@@ -896,6 +902,7 @@ export type Event =
   | EventMessageUpdated
   | EventMessageRemoved
   | EventMessagePartUpdated
+  | EventMessagePartDelta
   | EventMessagePartRemoved
   | EventPermissionAsked
   | EventPermissionReplied