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/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 }
+    },
   }))
 }