diff --git a/packages/opencode/src/mcp/oauth-callback.ts b/packages/opencode/src/mcp/oauth-callback.ts
index dd1d886fc1..0be49033e7 100644
--- a/packages/opencode/src/mcp/oauth-callback.ts
+++ b/packages/opencode/src/mcp/oauth-callback.ts
@@ -5,6 +5,10 @@ import { OAUTH_CALLBACK_PORT, OAUTH_CALLBACK_PATH } from "./oauth-provider"
 
 const log = Log.create({ service: "mcp.oauth-callback" })
 
+function escapeHtml(str: string): string {
+  return str.replace(/&/g, "&amp;").replace(/</g, "&lt;").replace(/>/g, "&gt;").replace(/"/g, "&quot;").replace(/'/g, "&#39;")
+}
+
 const HTML_SUCCESS = `<!DOCTYPE html>
 <html>
 <head>
@@ -41,7 +45,7 @@ const HTML_ERROR = (error: string) => `<!DOCTYPE html>
   <div class="container">
     <h1>Authorization Failed</h1>
     <p>An error occurred during authorization.</p>
-    <div class="error">${error}</div>
+    <div class="error">${escapeHtml(error)}</div>
   </div>
 </body>
 </html>`
diff --git a/packages/opencode/test/mcp/oauth-callback-xss.test.ts b/packages/opencode/test/mcp/oauth-callback-xss.test.ts
new file mode 100644
index 0000000000..89cd4c6742
--- /dev/null
+++ b/packages/opencode/test/mcp/oauth-callback-xss.test.ts
@@ -0,0 +1,48 @@
+import { describe, expect, test } from "bun:test"
+
+/**
+ * CWE-79: Cross-Site Scripting (XSS)
+ * File: packages/opencode/src/mcp/oauth-callback.ts
+ *
+ * HTML_ERROR interpolated error string directly into HTML template.
+ * Since error comes from URL query params (error, error_description),
+ * an attacker could craft a malicious OAuth callback URL to inject scripts.
+ */
+
+function escapeHtml(str: string): string {
+  return str.replace(/&/g, "&amp;").replace(/</g, "&lt;").replace(/>/g, "&gt;").replace(/"/g, "&quot;").replace(/'/g, "&#39;")
+}
+
+const HTML_ERROR = (error: string) => `<div class="error">${escapeHtml(error)}</div>`
+
+describe("CWE-79: XSS in oauth-callback.ts HTML_ERROR", () => {
+  test("should escape script tags in error message", () => {
+    const result = HTML_ERROR('<script>alert("xss")</script>')
+    expect(result).not.toContain("<script>")
+    expect(result).toContain("&lt;script&gt;")
+  })
+
+  test("should escape img onerror payload", () => {
+    const result = HTML_ERROR('<img src=x onerror=alert(1)>')
+    expect(result).not.toContain("<img")
+    expect(result).toContain("&lt;img")
+  })
+
+  test("should escape HTML entities in error_description", () => {
+    const result = HTML_ERROR('access_denied&error_description=<b>bold</b>')
+    expect(result).not.toContain("<b>")
+    expect(result).toContain("&lt;b&gt;")
+  })
+
+  test("should escape quotes to prevent attribute injection", () => {
+    const result = HTML_ERROR('" onmouseover="alert(1)')
+    expect(result).toContain("&quot;")
+    expect(result).not.toContain(' onmouseover="alert')
+  })
+
+  test("should render normal error messages correctly", () => {
+    const result = HTML_ERROR("access_denied")
+    expect(result).toContain("access_denied")
+    expect(result).toContain('class="error"')
+  })
+})
