fix(acp): relativize the completed diff card title

The result-time diff card sent view.title raw, so a completed edit/write
of an absolute in-workspace path flipped the card header back from the
relativized `Edit src/b.ts` to the absolute path — the pending card
relativizes, the result did not, and tool_call_update.title replaces the
header. Apply displayTitle to the result diff arm using the diff path,
mirroring the call-side card. Regression test proven red on the unfixed
arm.

Also record the overwrite diff-basis pre-read as a bounded follow-up
(TODO(overwrite-diff-bound) + RFC non-goal): overwriting a large file
reads the whole prior text into memory for a UI-only diff.

packages/fs/fs-local/src/index.ts

@@ -151,6 +151,9 @@ export class LocalFileSystem extends FileSystem {
       // file (binary/invalid-UTF-8) — a null `before` gives no contextual-hunk
       // basis, so a consumer falls back to a whole-file diff (the tool still
       // renders a result-time diff card, not the raw result text).
+      // TODO(overwrite-diff-bound): this reads the whole prior file into memory
+      // for a UI-only diff; bound the pre-read and fall back to no contextual
+      // basis above a size threshold (see the applied-hunk-diffs RFC non-goals).
       const before = existing ? await readTextForDiff(target.targetKey, signal) : null
       await writeFileAtomic(target.targetKey, content, existing?.mode, signal, this.internals)
       const after = await probe(target.targetKey)

packages/ui/acp/src/index.ts

@@ -1178,12 +1178,17 @@ function toolResultUpdate(callId: CallId, view: ToolResultView, isError: boolean
       // the diff the pending card installed (and keeps the model-facing result
       // text from clobbering it).
       const content: AcpToolCallContent[] = view.diffs.map(d => ({ type: 'diff', path: d.path, oldText: d.oldText, newText: d.newText }))
+      // Relativize the replacement title against the session cwd from the diff
+      // path, exactly as the call-side card does — `tool_call_update.title`
+      // replaces the card header, so a raw absolute path here would undo the
+      // pending card's relativized title.
+      const title = view.title !== undefined ? displayTitle(view.title, view.diffs[0]?.path, terminal.cwd) : undefined
       return {
         sessionUpdate: 'tool_call_update',
         toolCallId: callId,
         status,
         ...content.length > 0 ? { content } : {},
-        ...view.title !== undefined ? { title: view.title } : {},
+        ...title !== undefined ? { title } : {},
       }
     }
     default:

packages/ui/acp/tests/stream-update.spec.ts

@@ -675,6 +675,32 @@ describe('result-time diff card (REAL fs edit tool → tool_call_update diff blo
     await ctx.fiber.dispose()
   })
 
+  it('the completed diff TITLE relativizes against the session cwd (the result title replaces the card header)', async () => {
+    // A `tool_call_update.title` replaces the card header, so the result-side
+    // diff must relativize its title exactly as the pending card did — otherwise
+    // a completed absolute-path edit flips `Edit src/b.ts` back to the raw
+    // absolute path. The diff/location paths stay absolute (the editor opens the
+    // real path). Drive the REAL fs edit tool with an absolute in-workspace path.
+    const ctx = await fsCtx()
+    const presenter = new ToolPresenter(ctx.tools)
+    const args = JSON.stringify({ file_path: '/work/proj/src/b.ts', old_string: 'OLD', new_string: 'NEW' })
+    const meta = { diffs: [{ path: '/work/proj/src/b.ts', oldText: 'a\nOLD\nb', newText: 'a\nNEW\nb' }] }
+    const out: SessionNotification['update'][] = []
+    const rendering = { enabled: false, cwd: '/work/proj' }
+    for (const event of [
+      evt('tool/call', { turn: 1, step: 1, callId: CallId('e1'), name: 'edit', arguments: args }),
+      evt('tool/result', { turn: 1, step: 1, callId: CallId('e1'), content: [{ type: 'text', text: 'ok' }], isError: false, meta }),
+    ]) streamSessionEventUpdate(SessionId('s1'), event, n => out.push(n.update), presenter, rendering)
+    expect(out[1]).toEqual({
+      sessionUpdate: 'tool_call_update',
+      toolCallId: 'e1',
+      status: 'completed',
+      title: 'Edit src/b.ts',
+      content: [{ type: 'diff', path: '/work/proj/src/b.ts', oldText: 'a\nOLD\nb', newText: 'a\nNEW\nb' }],
+    })
+    await ctx.fiber.dispose()
+  })
+
   it('a diff result with an EMPTY diffs array and no title omits both keys (nothing to send)', () => {
     // A synthetic tool whose presentResult yields a `diff` card with no hunks and
     // no title — the shipping fs tools never emit this (edit always has a hunk;