feat(tool-fs): result-time applied-hunk diffs for write/edit

fs write/edit now emit a result-time contextual-diff tool_call_update
(the applied hunk with ±3 context lines, one hunk per replace_all site),
matching what claude-agent-acp sends and what makes an editor render the
change in place. The call-time snippet diff stays; the result hunk
supersedes it (ACP content-replace).

Mechanism:
- A persisted tool-private `meta` channel: execute may return
  `{ content, meta }`; `meta` (JsonValue) rides on the tool/result event
  and is handed back to presentResult, so the diff reproduces on replay
  (event-sourced). JsonValue is now exported from dsh-session.
- The backend returns raw before/after text (storage facts) on
  FsWriteOutcome/FsEditOutcome; the tool computes the hunk via the npm
  `diff` package's structuredPatch. A create has no before → no result
  diff; a failed/aborted mutation carries no meta.
- ToolResultView gains a DiffResultView; the bridge's result-side switch
  renders it as {type:'diff'} content blocks.

RFC: docs/rfc/implemented/architecture/2026-07-02-result-time-applied-hunk-diffs.md
(justifies the npm `diff` runtime dep over vendoring and the meta channel);
the render-intent-union RFC's Non-goal is updated to record this shipped.
All fs snapshot goldens re-recorded; edit/overwrite gain the contextual
result diff, create/read/policy-reject unchanged in structure.

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

@@ -382,6 +382,25 @@ export async function readForEdit(
   return { content: normalizeLineEndings(raw), lineEndings: detectLineEndings(raw) }
 }
 
+/**
+ * Best-effort read of a file's current text for a before/after diff basis, used
+ * by an overwrite. Returns the LF-normalized decoded content, or `null` when the
+ * file is binary or not valid UTF-8 — a write must succeed regardless of the
+ * prior bytes, so an undiffable prior file simply yields no contextual diff
+ * (the caller treats `null` the same as an absent file: call-time card only).
+ */
+export async function readTextForDiff(absolutePath: string, signal?: AbortSignal): Promise<string | null> {
+  const buffer = await readFileAbortable(absolutePath, 'read', signal)
+  if (buffer.includes(0)) return null
+  try {
+    return normalizeLineEndings(new TextDecoder('utf-8', { fatal: true }).decode(buffer))
+  } catch (error: unknown) {
+    /* v8 ignore next 2 -- TextDecoder({fatal}) only throws TypeError on invalid bytes; any other throw is an unreachable runtime fault. */
+    if (!(error instanceof TypeError)) throw error
+    return null
+  }
+}
+
 /**
  * Apply a literal replacement to LF-normalized content. Throws
  * `FS_EDIT_NOT_FOUND` on empty `oldString` or zero matches and

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

@@ -28,6 +28,7 @@ import {
   applyLiteralEdit,
   probe,
   readForEdit,
+  readTextForDiff,
   readWholeText,
   resolveLocalTarget,
   restoreLineEndings,
@@ -41,6 +42,7 @@ export {
   applyLiteralEdit,
   probe,
   readForEdit,
+  readTextForDiff,
   readWholeText,
   resolveLocalTarget,
   restoreLineEndings,
@@ -143,11 +145,18 @@ export class LocalFileSystem extends FileSystem {
       // provider) — no version guard, no read-first requirement. Still atomic
       // (the per-target lock is unconditional), so the write is never torn.
 
+      // Capture the prior text (the before/after diff basis) BEFORE the write.
+      // `null` for a create (no existing file) OR an existing-but-undiffable
+      // file (binary/invalid-UTF-8) — a consumer renders no result-time diff for
+      // either, only the call-time whole-file card.
+      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)
       return {
         operation: existing ? 'update' : 'create',
         version: this.versionAfterWrite(after, target),
+        before,
+        after: content,
       }
     })
   }
@@ -183,6 +192,10 @@ export class LocalFileSystem extends FileSystem {
         replacements: edited.replacements,
         replaceAll: edit.replaceAll,
         version: this.versionAfterWrite(after, target),
+        // The LF-normalized before/after text (the applied-hunk diff basis);
+        // line-ending restoration is a storage detail the diff ignores.
+        before: original.content,
+        after: edited.content,
       }
     })
   }

packages/fs/fs-local/tests/filesystem.spec.ts

@@ -188,6 +188,49 @@ describe('writeText', () => {
     await expect(fs.writeText(target, 'x')).rejects.toMatchObject({ code: 'FS_NOT_REGULAR_FILE' })
   })
 
+  it('a create reports before:null and after = the written content (no prior file)', async () => {
+    const target = await fs.resolve('new.txt')
+    const outcome = await fs.writeText(target, 'fresh')
+    expect(outcome.before).toBeNull()
+    expect(outcome.after).toBe('fresh')
+  })
+
+  it('an overwrite reports before = the OLD content and after = the new content', async () => {
+    await writeFile(join(dir, 'a.txt'), 'old body')
+    const target = await fs.resolve('a.txt')
+    const outcome = await fs.writeText(target, 'new body')
+    expect(outcome.before).toBe('old body')
+    expect(outcome.after).toBe('new body')
+  })
+
+  it('an overwrite of a CRLF file returns LF-normalized before content', async () => {
+    await writeFile(join(dir, 'a.txt'), 'a\r\nb\r\n')
+    const target = await fs.resolve('a.txt')
+    const outcome = await fs.writeText(target, 'a\nB\n')
+    expect(outcome.before).toBe('a\nb\n')
+  })
+
+  it('an overwrite of a BINARY prior file reports before:null (undiffable), still succeeds', async () => {
+    await writeFile(join(dir, 'a.bin'), Buffer.from([0x00, 0x01, 0x02]))
+    const target = await fs.resolve('a.bin')
+    const outcome = await fs.writeText(target, 'now text')
+    expect(outcome.operation).toBe('update')
+    expect(outcome.before).toBeNull()
+    expect(outcome.after).toBe('now text')
+  })
+
+  it('an overwrite of an INVALID-UTF-8 (non-NUL) prior file reports before:null, still succeeds', async () => {
+    // 0xff is never valid UTF-8 but is not a NUL, so it exercises the decoder's
+    // fatal-throw path (not the NUL-scan short-circuit): an undiffable prior file
+    // still yields a successful write with no before-content basis.
+    await writeFile(join(dir, 'a.bin'), Buffer.from([0x68, 0xff, 0x69]))
+    const target = await fs.resolve('a.bin')
+    const outcome = await fs.writeText(target, 'now valid')
+    expect(outcome.operation).toBe('update')
+    expect(outcome.before).toBeNull()
+    expect(outcome.after).toBe('now valid')
+  })
+
   it('releases per-target mutation locks after success and failure', async () => {
     const target = await fs.resolve('a.txt')
     await fs.writeText(target, 'created', { kind: 'createIfAbsent' })
@@ -242,6 +285,17 @@ describe('editText', () => {
     expect(await readFile(join(dir, 'a.txt'), 'utf8')).toBe('hello there')
   })
 
+  it('reports before/after content (the applied-hunk basis), LF-normalized', async () => {
+    await writeFile(join(dir, 'a.txt'), 'a\r\nOLD\r\nb\r\n')
+    const target = await fs.resolve('a.txt')
+    const outcome = await fs.editText(target, { oldString: 'OLD', newString: 'NEW', replaceAll: false })
+    expect(outcome.before).toBe('a\nOLD\nb\n')
+    expect(outcome.after).toBe('a\nNEW\nb\n')
+    // The written file keeps the original CRLF endings (before/after are the
+    // LF-normalized diff basis, not the on-disk bytes).
+    expect(await readFile(join(dir, 'a.txt'), 'utf8')).toBe('a\r\nNEW\r\nb\r\n')
+  })
+
   it('checks the stale version BEFORE literal matching', async () => {
     await writeFile(join(dir, 'a.txt'), 'hello world')
     const target = await fs.resolve('a.txt')

packages/fs/fs/src/types.ts

@@ -101,6 +101,14 @@ export interface FsWriteOutcome {
   operation: 'create' | 'update'
   /** Opaque version of the file after the write. */
   version: FsVersion
+  /**
+   * The file's content BEFORE the write, or `null` when the file did not exist
+   * (a create). Raw storage text (LF-normalized by the backend), never a diff —
+   * a consumer computes the result-time contextual diff from `before`/`after`.
+   */
+  before: string | null
+  /** The file's content AFTER the write (the text that was written). */
+  after: string
 }
 
 /** A literal-replacement edit request. */
@@ -121,6 +129,14 @@ export interface FsEditOutcome {
   replaceAll: boolean
   /** Opaque version of the file after the edit. */
   version: FsVersion
+  /**
+   * The file's content BEFORE the edit. Raw storage text (LF-normalized by the
+   * backend), never a diff — a consumer computes the result-time contextual diff
+   * (the applied hunk with context) from `before`/`after`.
+   */
+  before: string
+  /** The file's content AFTER the edit. */
+  after: string
 }
 
 /**

packages/fs/fs/tests/service.spec.ts

@@ -39,14 +39,15 @@ class FakeFileSystem extends FileSystem {
     return (async function* () { yield content })()
   }
   override async writeText(target: FsTarget, content: string, _expected?: FsWriteIntent): Promise<FsWriteOutcome> {
-    const existed = this.files.has(target.targetKey)
+    const before = this.files.get(target.targetKey) ?? null
     this.files.set(target.targetKey, content)
-    return { operation: existed ? 'update' : 'create', version: FsVersion('v2') }
+    return { operation: before !== null ? 'update' : 'create', version: FsVersion('v2'), before, after: content }
   }
   override async editText(target: FsTarget, edit: FsEditRequest): Promise<FsEditOutcome> {
     const content = this.files.get(target.targetKey) ?? ''
-    this.files.set(target.targetKey, content.split(edit.oldString).join(edit.newString))
-    return { replacements: 1, replaceAll: edit.replaceAll, version: FsVersion('v3') }
+    const after = content.split(edit.oldString).join(edit.newString)
+    this.files.set(target.targetKey, after)
+    return { replacements: 1, replaceAll: edit.replaceAll, version: FsVersion('v3'), before: content, after }
   }
 }
 

packages/fs/tool-fs/package.json

@@ -21,9 +21,13 @@
     "src"
   ],
   "license": "BSD-3-Clause",
+  "dependencies": {
+    "diff": "^9.0.0"
+  },
   "peerDependencies": {
     "@deepseek-ai/dsh-fs": "^0.0.1",
     "@deepseek-ai/dsh-llm": "^0.0.1",
+    "@deepseek-ai/dsh-session": "^0.0.1",
     "@deepseek-ai/dsh-system-prompt": "^0.0.1",
     "@deepseek-ai/dsh-tools": "^0.0.1",
     "cordis": "^4.0.0-rc.6"

packages/fs/tool-fs/src/diff.ts

@@ -0,0 +1,92 @@
+/**
+ * Result-time contextual-diff computation for the `write`/`edit` tools. Turns a
+ * before/after pair of file texts into one {@link FileDiff} per applied hunk —
+ * each hunk's `oldText`/`newText` reconstructed from the unified-diff lines with
+ * ±{@link DIFF_CONTEXT} surrounding context lines, matching how claude-agent-acp
+ * renders an editor inline diff.
+ *
+ * This is display-only presentation vocabulary (a UI concern), so it lives in
+ * the model-facing tool, NOT the `dsh-fs` storage seam — the backend returns
+ * only the raw before/after text (storage facts) and the tool computes the diff.
+ *
+ * @module @deepseek-ai/dsh-tool-fs/src/diff
+ */
+
+import { structuredPatch } from 'diff'
+import type { FileDiff } from '@deepseek-ai/dsh-tools'
+import type { JsonValue } from '@deepseek-ai/dsh-session'
+
+/** Context lines shown on each side of an applied hunk (matches claude-agent-acp). */
+export const DIFF_CONTEXT = 3
+
+/**
+ * The `write`/`edit` tools' private `tool/result` `meta` payload: the applied
+ * contextual-diff hunks. A {@link JsonValue} (persisted with the session log, so
+ * `presentResult` reproduces the diff card on replay). The producing tool owns
+ * this shape; the bridge only sees the opaque `meta` and the tool narrows it back
+ * via {@link diffsFromMeta}.
+ */
+export type FsDiffMeta = { diffs: FileDiff[] }
+
+/**
+ * Compute one {@link FileDiff} per hunk between `before` and `after`, each
+ * carrying the applied change plus {@link DIFF_CONTEXT} context lines. Returns an
+ * empty array when the texts are identical (no hunks). For a scattered
+ * `replace_all` edit the patch yields multiple hunks, so multiple `FileDiff`s
+ * come back — matching the editor rendering one diff block per site.
+ *
+ * Each hunk's `oldText` is its `-` (removed) and context lines joined by `\n`;
+ * `newText` is its `+` (added) and context lines. A hunk with no old lines
+ * (a pure insertion) reports `oldText: null` (nothing to diff against), mirroring
+ * the call-time card's new-file convention. The unified-diff "\ No newline at end
+ * of file" markers are dropped — they annotate the patch, not file content.
+ */
+export function computeHunkDiffs(path: string, before: string, after: string): FileDiff[] {
+  const patch = structuredPatch('', '', before, after, undefined, undefined, { context: DIFF_CONTEXT })
+  const diffs: FileDiff[] = []
+  for (const hunk of patch.hunks) {
+    const oldLines: string[] = []
+    const newLines: string[] = []
+    for (const line of hunk.lines) {
+      // The unified-diff marker for a missing trailing newline annotates the
+      // patch, not the content — skip it so it never leaks into a diff block.
+      if (line.startsWith('\\')) continue
+      const text = line.slice(1)
+      if (line.startsWith('-')) {
+        oldLines.push(text)
+      } else if (line.startsWith('+')) {
+        newLines.push(text)
+      } else {
+        // A context (unchanged) line appears on both sides.
+        oldLines.push(text)
+        newLines.push(text)
+      }
+    }
+    diffs.push({ path, oldText: oldLines.length > 0 ? oldLines.join('\n') : null, newText: newLines.join('\n') })
+  }
+  return diffs
+}
+
+/** Whether `value` is a valid {@link FileDiff} (defensive narrowing from opaque `meta`). */
+function isFileDiff(value: JsonValue): value is FileDiff & JsonValue {
+  if (typeof value !== 'object' || value === null || Array.isArray(value)) return false
+  const { path, oldText, newText } = value
+  return typeof path === 'string'
+    && (oldText === null || typeof oldText === 'string')
+    && typeof newText === 'string'
+}
+
+/**
+ * Narrow an opaque `tool/result` `meta` back to this tool's {@link FileDiff}
+ * hunks, or `undefined` when it is absent/malformed. `presentResult` runs on
+ * arbitrary logged `meta` (possibly from an older shape or a hand-edited log), so
+ * it validates defensively rather than trusting the payload — a bad `meta` yields
+ * no diff card (the generic result rendering) instead of a thrown presenter.
+ */
+export function diffsFromMeta(meta: JsonValue | undefined): FileDiff[] | undefined {
+  if (typeof meta !== 'object' || meta === null || Array.isArray(meta)) return undefined
+  const diffs = meta.diffs
+  if (!Array.isArray(diffs) || diffs.length === 0 || !diffs.every(isFileDiff)) return undefined
+  return diffs
+}
+

packages/fs/tool-fs/src/edit.ts

@@ -14,11 +14,12 @@
 
 import type { Context } from 'cordis'
 import { defineTool } from '@deepseek-ai/dsh-tools'
-import type { DiffCallView } from '@deepseek-ai/dsh-tools'
+import type { DiffCallView, DiffResultView, ToolResult } from '@deepseek-ai/dsh-tools'
 import type { ContentBlock } from '@deepseek-ai/dsh-llm'
 import type { FsEditOutcome } from '@deepseek-ai/dsh-fs'
 import type {} from '@deepseek-ai/dsh-fs'
 import type {} from '@deepseek-ai/dsh-system-prompt'
+import { computeHunkDiffs, diffsFromMeta, type FsDiffMeta } from './diff.ts'
 import { sessionCwd } from './session-cwd.ts'
 
 /** Validated `edit` arguments after defaulting. */
@@ -66,7 +67,7 @@ export function applyEditTool(ctx: Context): void {
       new_string: { type: 'string', required: true, description: 'Literal replacement text. Use an empty string to delete the match.' },
       replace_all: { type: 'boolean', description: 'Replace all matches. Defaults to false; when false, old_string must appear exactly once.' },
     },
-    async execute(args, exec): Promise<ContentBlock[]> {
+    async execute(args, exec): Promise<{ content: ContentBlock[]; meta?: FsDiffMeta }> {
       const input = parseEditArgs(args)
       const cwd = sessionCwd(exec)
       const target = await ctx.fs.resolve(input.filePath, cwd !== undefined ? { cwd } : undefined)
@@ -82,7 +83,17 @@ export function applyEditTool(ctx: Context): void {
       )
       // Record the observed version (a no-op when no policy plugin listens).
       ctx.emit('fs/observed', target, outcome.version, exec)
-      return [{ type: 'text', text: formatEditOutput(target.displayPath, outcome) }]
+      // The result-time applied-hunk diff (before→after with context lines). An
+      // edit always changes content (parseEditArgs requires old_string to differ
+      // and editText matches at least once), so there is always at least one hunk.
+      // The bridge renders these as an inline diff that supersedes the call-time
+      // snippet; the display path is the model-facing `file_path` (the bridge
+      // relativizes it).
+      const diffs = computeHunkDiffs(input.filePath, outcome.before, outcome.after)
+      return {
+        content: [{ type: 'text', text: formatEditOutput(target.displayPath, outcome) }],
+        meta: { diffs },
+      }
     },
     // Pure display: a diff card of the literal replacement (old_string →
     // new_string), derived from the call args. `oldText: old_string || null`
@@ -96,5 +107,15 @@ export function applyEditTool(ctx: Context): void {
         locations: [{ path: args.file_path }],
       }
     },
+    // Result-time display: the applied contextual-diff hunks carried on `meta`.
+    // On success with diffs, a `diff` result card supersedes the call-time
+    // snippet; on error (nothing applied) or malformed meta, fall through to the
+    // generic "updated successfully" rendering.
+    presentResult(args, result: ToolResult): DiffResultView | undefined {
+      if (result.isError) return undefined
+      const diffs = diffsFromMeta(result.meta)
+      if (diffs === undefined) return undefined
+      return { card: 'diff', title: `Edit ${args.file_path}`, diffs }
+    },
   }))
 }

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

@@ -32,6 +32,8 @@ export { applyWriteTool, formatWriteOutput, parseWriteArgs } from './write.ts'
 export { applyEditTool, formatEditOutput, parseEditArgs } from './edit.ts'
 export { READ_MAX_BYTES, READ_MAX_LINE_LENGTH, buildWindow, formatReadOutput } from './read-render.ts'
 export type { FileReadOutcome, FileTextLine, ReadWindow, WindowResult } from './read-render.ts'
+export { DIFF_CONTEXT, computeHunkDiffs, diffsFromMeta } from './diff.ts'
+export type { FsDiffMeta } from './diff.ts'
 
 /** Cordis plugin name used by loader diagnostics. */
 export const name = 'tool-fs'

packages/fs/tool-fs/src/write.ts

@@ -13,11 +13,12 @@
 
 import type { Context } from 'cordis'
 import { defineTool } from '@deepseek-ai/dsh-tools'
-import type { DiffCallView } from '@deepseek-ai/dsh-tools'
+import type { DiffCallView, DiffResultView, ToolResult } from '@deepseek-ai/dsh-tools'
 import type { ContentBlock } from '@deepseek-ai/dsh-llm'
 import type { FsWriteOutcome } from '@deepseek-ai/dsh-fs'
 import type {} from '@deepseek-ai/dsh-fs'
 import type {} from '@deepseek-ai/dsh-system-prompt'
+import { computeHunkDiffs, diffsFromMeta, type FsDiffMeta } from './diff.ts'
 import { sessionCwd } from './session-cwd.ts'
 
 /** Validate value constraints the schema DSL can't express. */
@@ -51,7 +52,7 @@ export function applyWriteTool(ctx: Context): void {
       file_path: { type: 'string', required: true, description: 'Path to write, resolved by the filesystem backend.' },
       content: { type: 'string', required: true, description: 'Full UTF-8 text content to write.' },
     },
-    async execute(args, exec): Promise<ContentBlock[]> {
+    async execute(args, exec): Promise<{ content: ContentBlock[]; meta?: FsDiffMeta }> {
       const input = parseWriteArgs(args)
       const cwd = sessionCwd(exec)
       const target = await ctx.fs.resolve(input.filePath, cwd !== undefined ? { cwd } : undefined)
@@ -61,7 +62,14 @@ export function applyWriteTool(ctx: Context): void {
       const outcome = await ctx.fs.writeText(target, input.content, intent, exec.signal)
       // Record the observed version (a no-op when no policy plugin listens).
       ctx.emit('fs/observed', target, outcome.version, exec)
-      return [{ type: 'text', text: formatWriteOutput(target.displayPath, outcome) }]
+      // Result-time contextual diff ONLY for an overwrite (a before-version
+      // exists). A create has no "before" — `outcome.before` is null — so it
+      // carries no result diff, leaving just the call-time whole-file card.
+      const diffs = outcome.before !== null ? computeHunkDiffs(input.filePath, outcome.before, outcome.after) : []
+      return {
+        content: [{ type: 'text', text: formatWriteOutput(target.displayPath, outcome) }],
+        ...diffs.length > 0 ? { meta: { diffs } } : {},
+      }
     },
     // Pure display: a diff card (an editor renders write as a new-file / full-
     // replace diff). `oldText: null` — a call-time presenter has no access to the
@@ -75,5 +83,15 @@ export function applyWriteTool(ctx: Context): void {
         locations: [{ path: args.file_path }],
       }
     },
+    // Result-time display: for an OVERWRITE, the applied contextual-diff hunks on
+    // `meta` supersede the call-time whole-file snippet. A create carries no meta
+    // (no "before"), so this returns undefined and the call-time new-file card
+    // stands; an error or malformed meta also falls through to generic rendering.
+    presentResult(args, result: ToolResult): DiffResultView | undefined {
+      if (result.isError) return undefined
+      const diffs = diffsFromMeta(result.meta)
+      if (diffs === undefined) return undefined
+      return { card: 'diff', title: `Write ${args.file_path}`, diffs }
+    },
   }))
 }

packages/fs/tool-fs/tests/diff.spec.ts

@@ -0,0 +1,113 @@
+/**
+ * Unit tests for the result-time contextual-diff computation (`src/diff.ts`):
+ * the pure before/after → {@link FileDiff}[] hunk builder and the defensive
+ * `meta` narrowing. These pin the exact hunk reconstruction (context lines,
+ * multi-hunk replaceAll, pure insertion/deletion, no-op) the ACP bridge renders.
+ */
+
+import { describe, expect, it } from 'vitest'
+import { computeHunkDiffs, diffsFromMeta, DIFF_CONTEXT } from '@deepseek-ai/dsh-tool-fs'
+import type { JsonValue } from '@deepseek-ai/dsh-session'
+
+const lines = (n: number): string => Array.from({ length: n }, (_, i) => `line${i + 1}`).join('\n') + '\n'
+
+describe('computeHunkDiffs', () => {
+  it('a single-line change yields one hunk with ±context lines on both sides', () => {
+    const before = lines(8)
+    const after = before.replace('line4', 'CHANGED')
+    const diffs = computeHunkDiffs('f.txt', before, after)
+    expect(diffs).toEqual([{
+      path: 'f.txt',
+      oldText: 'line1\nline2\nline3\nline4\nline5\nline6\nline7',
+      newText: 'line1\nline2\nline3\nCHANGED\nline5\nline6\nline7',
+    }])
+  })
+
+  it('a scattered replace_all yields one FileDiff PER hunk (matching per-site editor blocks)', () => {
+    const before = lines(20)
+    const after = before.replace('line3', 'A').replace('line16', 'B')
+    const diffs = computeHunkDiffs('f.txt', before, after)
+    expect(diffs).toHaveLength(2)
+    expect(diffs[0]?.path).toBe('f.txt')
+    expect(diffs[0]?.oldText).toContain('line3')
+    expect(diffs[0]?.newText).toContain('A')
+    expect(diffs[1]?.oldText).toContain('line16')
+    expect(diffs[1]?.newText).toContain('B')
+    // The two hunks are distinct sites, not one merged block.
+    expect(diffs[0]?.newText).not.toContain('B')
+    expect(diffs[1]?.newText).not.toContain('A')
+  })
+
+  it('identical before/after (a no-op) yields no hunks', () => {
+    expect(computeHunkDiffs('f.txt', 'same\n', 'same\n')).toEqual([])
+  })
+
+  it('a pure insertion into empty content reports oldText null (nothing to diff against)', () => {
+    const diffs = computeHunkDiffs('f.txt', '', 'brand new\n')
+    expect(diffs).toEqual([{ path: 'f.txt', oldText: null, newText: 'brand new' }])
+  })
+
+  it('a pure deletion of the whole file reports newText empty', () => {
+    const diffs = computeHunkDiffs('f.txt', 'gone\n', '')
+    expect(diffs).toEqual([{ path: 'f.txt', oldText: 'gone', newText: '' }])
+  })
+
+  it('drops the "\\ No newline at end of file" marker from a no-trailing-newline change', () => {
+    const diffs = computeHunkDiffs('f.txt', 'x', 'y')
+    // The marker line (starting with "\\") must never leak into a diff block.
+    expect(diffs).toEqual([{ path: 'f.txt', oldText: 'x', newText: 'y' }])
+    expect(diffs[0]?.oldText).not.toContain('\\')
+    expect(diffs[0]?.newText).not.toContain('\\')
+  })
+
+  it('uses DIFF_CONTEXT (3) surrounding lines', () => {
+    expect(DIFF_CONTEXT).toBe(3)
+    const before = lines(20)
+    const after = before.replace('line10', 'CHANGED')
+    const [diff] = computeHunkDiffs('f.txt', before, after)
+    // 3 context above (7,8,9) + the change + 3 below (11,12,13) = 7 lines a side.
+    expect(diff?.oldText?.split('\n')).toHaveLength(7)
+    expect(diff?.newText.split('\n')).toHaveLength(7)
+    expect(diff?.oldText?.split('\n')[0]).toBe('line7')
+  })
+})
+
+describe('diffsFromMeta (defensive narrowing)', () => {
+  // The narrowing accepts an opaque JsonValue; a malformed payload is not a
+  // statically-valid JsonValue, so route every case through one cast helper that
+  // mirrors how a hand-edited/older session log delivers arbitrary shapes.
+  const m = (value: unknown): JsonValue | undefined => value as JsonValue | undefined
+  const good = { diffs: [{ path: 'f.txt', oldText: 'a', newText: 'b' }] }
+
+  it('narrows a well-formed { diffs } payload', () => {
+    expect(diffsFromMeta(m(good))).toEqual(good.diffs)
+  })
+
+  it('accepts a diff whose oldText is null (a create-style hunk)', () => {
+    const meta = { diffs: [{ path: 'f.txt', oldText: null, newText: 'x' }] }
+    expect(diffsFromMeta(m(meta))).toEqual(meta.diffs)
+  })
+
+  it('rejects undefined / non-object / array meta', () => {
+    expect(diffsFromMeta(undefined)).toBeUndefined()
+    expect(diffsFromMeta(null)).toBeUndefined()
+    expect(diffsFromMeta(m('nope'))).toBeUndefined()
+    expect(diffsFromMeta(m([]))).toBeUndefined()
+  })
+
+  it('rejects a missing / empty / non-array diffs field', () => {
+    expect(diffsFromMeta(m({}))).toBeUndefined()
+    expect(diffsFromMeta(m({ diffs: [] }))).toBeUndefined()
+    expect(diffsFromMeta(m({ diffs: 'x' }))).toBeUndefined()
+  })
+
+  it('rejects a diffs array containing a malformed entry', () => {
+    expect(diffsFromMeta(m({ diffs: [{ path: 'f.txt', oldText: 'a' }] }))).toBeUndefined()
+    expect(diffsFromMeta(m({ diffs: [{ path: 1, oldText: 'a', newText: 'b' }] }))).toBeUndefined()
+    expect(diffsFromMeta(m({ diffs: [{ path: 'f', oldText: 5, newText: 'b' }] }))).toBeUndefined()
+    expect(diffsFromMeta(m({ diffs: [{ path: 'f', oldText: 'a', newText: 7 }] }))).toBeUndefined()
+    expect(diffsFromMeta(m({ diffs: [null] }))).toBeUndefined()
+    expect(diffsFromMeta(m({ diffs: ['x'] }))).toBeUndefined()
+    expect(diffsFromMeta(m({ diffs: [[]] }))).toBeUndefined()
+  })
+})

packages/fs/tool-fs/tests/tools.spec.ts

@@ -57,16 +57,17 @@ class FakeFs extends FileSystem {
   override async writeText(target: FsTarget, content: string, expected?: FsWriteIntent): Promise<FsWriteOutcome> {
     this.throwIfArmed()
     this.writeIntents.push(expected)
-    const existed = this.files.has(target.targetKey)
+    const before = this.files.get(target.targetKey) ?? null
     this.files.set(target.targetKey, content)
-    return { operation: existed ? 'update' : 'create', version: FsVersion('v2') }
+    return { operation: before !== null ? 'update' : 'create', version: FsVersion('v2'), before, after: content }
   }
   override async editText(target: FsTarget, edit: FsEditRequest, expected?: { version: FsVersion }): Promise<FsEditOutcome> {
     this.throwIfArmed()
     this.editIntents.push(expected)
     const content = this.files.get(target.targetKey) ?? ''
-    this.files.set(target.targetKey, content.split(edit.oldString).join(edit.newString))
-    return { replacements: 1, replaceAll: edit.replaceAll, version: FsVersion('v3') }
+    const after = content.split(edit.oldString).join(edit.newString)
+    this.files.set(target.targetKey, after)
+    return { replacements: 1, replaceAll: edit.replaceAll, version: FsVersion('v3'), before: content, after }
   }
 }
 
@@ -395,3 +396,80 @@ describe('tool-owned presentation (pure presentCall)', () => {
     })
   })
 })
+
+describe('result-time contextual diff (meta + presentResult)', () => {
+  // An edit records the applied contextual hunk on `tool/result` meta, and the
+  // tool's presentResult narrows it back into a `diff` result card the bridge
+  // renders. Drive execute end-to-end so the meta is the REAL computed hunk.
+  const withContext = 'a\nb\nc\nOLD\nd\ne\nf\n'
+
+  it('edit: execute attaches the applied hunk as meta { diffs }', async () => {
+    const { ctx, fs } = await setup()
+    const session = { header: {} }
+    fs.files.set('key:a.txt', withContext)
+    await call(ctx, 'read', { file_path: 'a.txt' }, { session })
+    const result = await call(ctx, 'edit', { file_path: 'a.txt', old_string: 'OLD', new_string: 'NEW' }, { session })
+    expect(result.isError).toBe(false)
+    expect(result.meta).toEqual({
+      diffs: [{ path: 'a.txt', oldText: 'a\nb\nc\nOLD\nd\ne\nf', newText: 'a\nb\nc\nNEW\nd\ne\nf' }],
+    })
+  })
+
+  it('edit: presentResult turns the meta into a diff result card', async () => {
+    const { ctx, fs } = await setup()
+    const session = { header: {} }
+    fs.files.set('key:a.txt', withContext)
+    await call(ctx, 'read', { file_path: 'a.txt' }, { session })
+    const result = await call(ctx, 'edit', { file_path: 'a.txt', old_string: 'OLD', new_string: 'NEW' }, { session })
+    const view = ctx.tools.get('edit')?.presentResult?.({ file_path: 'a.txt', old_string: 'OLD', new_string: 'NEW' }, result)
+    expect(view).toEqual({
+      card: 'diff', title: 'Edit a.txt',
+      diffs: [{ path: 'a.txt', oldText: 'a\nb\nc\nOLD\nd\ne\nf', newText: 'a\nb\nc\nNEW\nd\ne\nf' }],
+    })
+  })
+
+  it('write OVERWRITE: execute attaches a contextual hunk; presentResult renders a diff card', async () => {
+    const { ctx, fs } = await setup()
+    const session = { header: {} }
+    fs.files.set('key:a.txt', withContext)
+    await call(ctx, 'read', { file_path: 'a.txt' }, { session })
+    const result = await call(ctx, 'write', { file_path: 'a.txt', content: 'a\nb\nc\nNEW\nd\ne\nf\n' }, { session })
+    expect(result.isError).toBe(false)
+    expect(result.meta).toEqual({ diffs: [{ path: 'a.txt', oldText: 'a\nb\nc\nOLD\nd\ne\nf', newText: 'a\nb\nc\nNEW\nd\ne\nf' }] })
+    const view = ctx.tools.get('write')?.presentResult?.({ file_path: 'a.txt', content: 'x' }, result)
+    expect(view).toEqual({ card: 'diff', title: 'Write a.txt', diffs: [{ path: 'a.txt', oldText: 'a\nb\nc\nOLD\nd\ne\nf', newText: 'a\nb\nc\nNEW\nd\ne\nf' }] })
+  })
+
+  it('write CREATE: no before-version → no meta, presentResult returns undefined (call-time card stands)', async () => {
+    const { ctx } = await setup()
+    const session = { header: {} }
+    const result = await call(ctx, 'write', { file_path: 'new.txt', content: 'fresh\n' }, { session })
+    expect(result.isError).toBe(false)
+    expect(result.meta).toBeUndefined()
+    expect(ctx.tools.get('write')?.presentResult?.({ file_path: 'new.txt', content: 'fresh\n' }, result)).toBeUndefined()
+  })
+
+  it('write OVERWRITE with identical content: a before exists but yields no hunk → no meta', async () => {
+    const { ctx, fs } = await setup()
+    const session = { header: {} }
+    fs.files.set('key:a.txt', 'same\n')
+    await call(ctx, 'read', { file_path: 'a.txt' }, { session })
+    const result = await call(ctx, 'write', { file_path: 'a.txt', content: 'same\n' }, { session })
+    expect(result.isError).toBe(false)
+    expect(result.meta).toBeUndefined()
+  })
+
+  it('presentResult returns undefined on an error result (nothing applied)', async () => {
+    const { ctx } = await setup()
+    const errorResult = { content: [{ type: 'text' as const, text: 'Error: boom' }], isError: true }
+    expect(ctx.tools.get('edit')?.presentResult?.({ file_path: 'a.txt', old_string: 'x', new_string: 'y' }, errorResult)).toBeUndefined()
+    expect(ctx.tools.get('write')?.presentResult?.({ file_path: 'a.txt', content: 'y' }, errorResult)).toBeUndefined()
+  })
+
+  it('presentResult returns undefined on malformed meta (defensive narrowing)', async () => {
+    const { ctx } = await setup()
+    const badMeta = { content: [{ type: 'text' as const, text: 'ok' }], isError: false, meta: { diffs: 'nope' } }
+    expect(ctx.tools.get('edit')?.presentResult?.({ file_path: 'a.txt', old_string: 'x', new_string: 'y' }, badMeta)).toBeUndefined()
+    expect(ctx.tools.get('write')?.presentResult?.({ file_path: 'a.txt', content: 'y' }, badMeta)).toBeUndefined()
+  })
+})