TODO Next: undo a turn — /undo /redo + per-turn snapshots

File tools record before-write state via snapshot hook; Session
captures beforeFiles at turn start and afterFiles at turn end (cap
100 via SnapshotStack), and undo/redo restore files + messages
together. Bash not snapshotted (docs + notice).

src/commands.ts

@@ -26,6 +26,8 @@ export type CommandAction =
   | { type: 'info'; text: string }
   | { type: 'model'; model: string }
   | { type: 'resume'; id: string }
+  | { type: 'undo' }
+  | { type: 'redo' }
   /** A custom command from a markdown file, expanded against its arguments. */
   | { type: 'custom'; command: CustomCommand; args: string[] }
   | { type: 'unknown'; name: string };
@@ -61,6 +63,8 @@ export const COMMANDS: CommandSpec[] = [
   { name: 'sessions', summary: 'list saved sessions' },
   { name: 'resume', arg: '<id>', summary: 'load a saved session' },
   { name: 'save', summary: 'write the session to disk now' },
+  { name: 'undo', summary: 'undo the last turn — restores files and conversation (bash effects are not snapshotted)' },
+  { name: 'redo', summary: 'redo the last undone turn' },
   { name: 'clear', summary: 'clear the transcript and history' },
   { name: 'exit', aliases: ['quit'], summary: 'quit' },
 ];
@@ -225,6 +229,10 @@ export function parseCommand(raw: string, custom: readonly CustomCommand[] = [])
       return arg ? { type: 'model', model: arg } : { type: 'models' };
     case 'resume':
       return arg ? { type: 'resume', id: arg } : { type: 'info', text: 'usage: /resume <session-id>' };
+    case 'undo':
+      return { type: 'undo' };
+    case 'redo':
+      return { type: 'redo' };
     default: {
       const cmd = custom.find((c) => c.name === name);
       return cmd ? { type: 'custom', command: cmd, args: arg ? arg.split(/\s+/) : [] } : { type: 'unknown', name };

src/session.ts

@@ -21,6 +21,7 @@ import { detachProviderItems, droppedSpan, estimateTokens as pruneEstimateTokens
 import { createSkillTool, renderSkills, type Skill } from './skills';
 import { suggestSkillsFromTranscript, writeAutoSkill } from './skill-learner';
 import { disabledToolNames, onBashOutput, tools as builtinTools, type ToolSetName } from './tools';
+import { onBeforeWrite, SnapshotStack, type FileState } from './snapshot';
 
 export type ApprovalRequest = {
   approvalId: string;
@@ -204,6 +205,9 @@ export class Session {
   /** The 80% spend warning is shown once, not on every turn past the line. */
   private warnedSpend = false;
   private controller: AbortController | undefined;
+  private readonly snapshots = new SnapshotStack();
+  private turnBeforeLen = 0;
+  private turnBeforeFiles = new Map<string, FileState>();
   private learnTurns = 0;
   private lastLearnLen = 0;
 
@@ -351,6 +355,7 @@ export class Session {
     this.subagentOutputTokens = 0;
     this.warnedSpend = false;
     this.notebook.clear();
+    this.snapshots.clear();
     this.opts.onChange?.(this.messages);
   }
 
@@ -379,6 +384,54 @@ export class Session {
     return this.opts.compactThreshold ?? DEFAULT_COMPACT_THRESHOLD;
   }
 
+  canUndo(): boolean { return this.snapshots.canUndo(); }
+  canRedo(): boolean { return this.snapshots.canRedo(); }
+
+  async undo(): Promise<string> {
+    const snap = this.snapshots.popForUndo();
+    if (!snap) throw new Error('nothing to undo');
+    await this.restoreFiles(snap.beforeFiles);
+    // truncate messages to beforeLen; the tail is kept inside snap for redo
+    this.messages.length = snap.beforeLen;
+    this.opts.onChange?.(this.messages);
+    const n = snap.beforeFiles.size;
+    const filesNote = n === 0 ? 'no files to restore' : `${n} file(s) restored`;
+    const msgNote = snap.afterLen > snap.beforeLen ? `${snap.afterLen - snap.beforeLen} message(s) removed` : 'no messages to remove';
+    return `undone: ${filesNote}, ${msgNote} (bash effects, if any, were not snapshotted)`;
+  }
+
+  async redo(): Promise<string> {
+    const snap = this.snapshots.popForRedo();
+    if (!snap) throw new Error('nothing to redo');
+    await this.restoreFiles(snap.afterFiles);
+    // redo replays the tail that undo removed; stored in afterFiles? we also need messages tail.
+    // The messages tail is the slice that was removed on undo; reconstruct by re-inserting from stored span is not enough
+    // because snapshots hold beforeLen/afterLen but not the actual messages content.
+    // We store the removed tail inside the snapshot at push time as an extra field via (snap as any)._tail.
+    const tail = (snap as unknown as { _tail?: import('ai').ModelMessage[] })._tail;
+    if (tail && tail.length > 0) {
+      this.messages.push(...tail);
+      this.opts.onChange?.(this.messages);
+    }
+    const n = snap.afterFiles.size;
+    const filesNote = n === 0 ? 'no files to restore' : `${n} file(s) restored`;
+    return `redone: ${filesNote} (bash effects, if any, were not snapshotted)`;
+  }
+
+  private async restoreFiles(state: Map<string, FileState>): Promise<void> {
+    for (const [abs, st] of state) {
+      try {
+        if (!st.existed) {
+          if (await Bun.file(abs).exists()) await Bun.file(abs).delete();
+        } else {
+          await Bun.write(abs, st.content ?? '');
+        }
+      } catch {
+        // best-effort per file; one failure should not stop the rest
+      }
+    }
+  }
+
   /**
    * The session's spend so far and the configured ceiling, for the UI's status
    * and the refuse-the-next-turn check. Unpriced models report no spend: a
@@ -535,6 +588,18 @@ export class Session {
       return;
     }
 
+    // snapshot boundary: remember messages length before this turn and arm file capture
+    this.turnBeforeLen = this.messages.length;
+    this.turnBeforeFiles = new Map<string, FileState>();
+    onBeforeWrite(async (abs: string) => {
+      if (this.turnBeforeFiles.has(abs)) return;
+      const exists = await Bun.file(abs).exists();
+      let content: string | null = null;
+      if (exists) {
+        try { content = await Bun.file(abs).text(); } catch { content = null; }
+      }
+      this.turnBeforeFiles.set(abs, { existed: exists, content });
+    });
     this.messages.push({ role: 'user', content: userText });
     this.opts.onChange?.(this.messages);
     this.controller = new AbortController();
@@ -551,9 +616,39 @@ export class Session {
       this.opts.onToolOutput?.(toolCallId, chunk);
     });
 
+    let turnFailed = false;
     try {
       yield* this.run(signal, threshold, outputs);
+    } catch (e) {
+      turnFailed = true;
+      throw e;
     } finally {
+      onBeforeWrite(undefined);
+      // finalize snapshot only for turns that actually ran (even if they errored after writing files,
+      // the file state is still worth snapshotting so undo can revert a half-failed turn)
+      try {
+        if (this.turnBeforeFiles.size > 0 || this.messages.length > this.turnBeforeLen) {
+          const afterFiles = new Map<string, FileState>();
+          for (const abs of this.turnBeforeFiles.keys()) {
+            const exists = await Bun.file(abs).exists();
+            let content: string | null = null;
+            if (exists) { try { content = await Bun.file(abs).text(); } catch { content = null; } }
+            afterFiles.set(abs, { existed: exists, content });
+          }
+          // tail for redo: the messages added by this turn
+          const tail = this.messages.slice(this.turnBeforeLen).map((m) => ({ ...m, content: typeof m.content === 'string' ? m.content : JSON.parse(JSON.stringify(m.content)) } as import('ai').ModelMessage));
+          const snap: import('./snapshot').TurnSnapshot & { _tail?: import('ai').ModelMessage[] } = {
+            beforeLen: this.turnBeforeLen,
+            afterLen: this.messages.length,
+            beforeFiles: new Map(this.turnBeforeFiles),
+            afterFiles,
+          };
+          (snap as unknown as { _tail?: import('ai').ModelMessage[] })._tail = tail;
+          // even failed turns push so undo can revert the file side; empty no-op turns are skipped above
+          if (!turnFailed || this.turnBeforeFiles.size > 0) this.snapshots.push(snap);
+        }
+      } catch {}
+      this.turnBeforeFiles = new Map<string, FileState>();
       this.controller = undefined;
       this.drainPendingHotReload();
       onBashOutput(undefined);

src/snapshot.ts

@@ -0,0 +1,72 @@
+/**
+ * Per-turn file snapshots for /undo and /redo.
+ *
+ * Bash is intentionally not snapshotted: a shell command can do anything
+ * (network, database, chmod, rm -rf) and there is no way to know what to
+ * restore. The docs and the undo notice say so plainly.
+ *
+ * File tools call `recordBeforeWrite(abs)` before their first write to a path
+ * in the current turn. Session drains the map at turn boundaries into its
+ * history stack (cap 100) and owns undo/redo.
+ */
+
+export type FileState = { existed: boolean; content: string | null };
+
+export type TurnSnapshot = {
+  /** Messages length before the turn's user message was pushed. */
+  beforeLen: number;
+  /** Messages length after the turn completed (including tool results). */
+  afterLen: number;
+  /** File state before the turn, keyed by absolute path. Only files the turn touched. */
+  beforeFiles: Map<string, FileState>;
+  /** File state after the turn, for redo. */
+  afterFiles: Map<string, FileState>;
+};
+
+const MAX_HISTORY = 100;
+
+let hook: ((abs: string) => Promise<void> | void) | undefined;
+
+export function onBeforeWrite(fn: ((abs: string) => Promise<void> | void) | undefined): void {
+  hook = fn;
+}
+
+export async function recordBeforeWrite(abs: string): Promise<void> {
+  const fn = hook;
+  if (fn) await fn(abs);
+}
+
+export class SnapshotStack {
+  private readonly history: TurnSnapshot[] = [];
+  private readonly future: TurnSnapshot[] = [];
+
+  push(entry: TurnSnapshot): void {
+    this.history.push(entry);
+    if (this.history.length > MAX_HISTORY) this.history.shift();
+    this.future.length = 0;
+  }
+
+  canUndo(): boolean { return this.history.length > 0; }
+  canRedo(): boolean { return this.future.length > 0; }
+
+  popForUndo(): TurnSnapshot | undefined {
+    const e = this.history.pop();
+    if (e) this.future.push(e);
+    return e;
+  }
+
+  popForRedo(): TurnSnapshot | undefined {
+    const e = this.future.pop();
+    if (e) this.history.push(e);
+    return e;
+  }
+
+  clear(): void {
+    this.history.length = 0;
+    this.future.length = 0;
+  }
+
+  depth(): { undo: number; redo: number } {
+    return { undo: this.history.length, redo: this.future.length };
+  }
+}

src/tools-extra.ts

@@ -3,6 +3,7 @@ import { stat } from 'node:fs/promises';
 import { resolve } from 'node:path';
 import { z } from 'zod';
 import { jail, posix, walk } from './ignore';
+import { recordBeforeWrite } from './snapshot';
 import { withMeta } from './tool-utils';
 import { git } from './tools-git';
 
@@ -47,6 +48,7 @@ export const insertLinesTool = withMeta({ set: 'extra', mutating: true }, tool({
   }),
   execute: async ({ path, line, text }) => {
     const { abs, lines: cur } = await readLines(path);
+    await recordBeforeWrite(abs);
     if (line > cur.length + 1) throw new Error(`line ${line} is past the end of ${path} (${cur.length} lines)`);
     cur.splice(line - 1, 0, ...lines(text));
     await Bun.write(abs, cur.join('\n'));
@@ -64,6 +66,7 @@ export const deleteLinesTool = withMeta({ set: 'extra', mutating: true }, tool({
   execute: async ({ path, start, end }) => {
     if (end < start) throw new Error('end must be >= start');
     const { abs, lines: cur } = await readLines(path);
+    await recordBeforeWrite(abs);
     if (end > cur.length) throw new Error(`end ${end} is past the end of ${path} (${cur.length} lines)`);
     if (start === 1 && end === cur.length) throw new Error('that deletes the whole file; use delete_file instead');
     cur.splice(start - 1, end - start + 1);
@@ -83,6 +86,7 @@ export const replaceLinesTool = withMeta({ set: 'extra', mutating: true }, tool(
   execute: async ({ path, start, end, text }) => {
     if (end < start) throw new Error('end must be >= start');
     const { abs, lines: cur } = await readLines(path);
+    await recordBeforeWrite(abs);
     if (end > cur.length) throw new Error(`end ${end} is past the end of ${path} (${cur.length} lines)`);
     cur.splice(start - 1, end - start + 1, ...lines(text));
     await Bun.write(abs, cur.join('\n'));
@@ -95,6 +99,7 @@ export const appendFileTool = withMeta({ set: 'extra', mutating: true }, tool({
   inputSchema: z.object({ path: z.string(), text: z.string() }),
   execute: async ({ path, text }) => {
     const { abs, lines: cur } = await readLines(path);
+    await recordBeforeWrite(abs);
     await Bun.write(abs, `${cur.join('\n').replace(/\n?$/, '\n')}${text.replace(/\n?$/, '')}\n`);
     return `Appended ${lines(text).length} line(s) to ${path}`;
   },
@@ -105,6 +110,7 @@ export const prependFileTool = withMeta({ set: 'extra', mutating: true }, tool({
   inputSchema: z.object({ path: z.string(), text: z.string() }),
   execute: async ({ path, text }) => {
     const { abs, lines: cur } = await readLines(path);
+    await recordBeforeWrite(abs);
     await Bun.write(abs, `${text.replace(/\n?$/, '\n')}${cur.join('\n')}`);
     return `Prepended ${lines(text).length} line(s) to ${path}`;
   },

src/tools.ts

@@ -3,6 +3,7 @@ import { stat } from 'node:fs/promises';
 import { join, resolve } from 'node:path';
 import { z } from 'zod';
 import { jail, posix, walk } from './ignore';
+import { recordBeforeWrite } from './snapshot';
 import { EXTRA_TOOL_NAMES, extraTools } from './tools-extra';
 import { GIT_TOOL_NAMES, gitTools } from './tools-git';
 import { NET_TOOL_NAMES, netTools } from './tools-net';
@@ -206,6 +207,8 @@ export const applyPatchTool = withMeta({ set: 'edit-plus', mutating: true }, too
       if (seen.has(op.path)) throw new Error(`${op.path} appears twice in one patch`);
       seen.add(op.path);
       const abs = jail(op.path);
+      await recordBeforeWrite(abs);
+      if ((op as { moveTo?: string }).moveTo) await recordBeforeWrite(jail((op as { moveTo?: string }).moveTo!));
 
       if (op.kind === 'delete') {
         if (!(await Bun.file(abs).exists())) throw new Error(`cannot delete ${op.path}: no such file`);
@@ -274,6 +277,7 @@ export const writeFileTool = withMeta({ set: 'core', mutating: true }, tool({
   }),
   execute: async ({ path, content }) => {
     const abs = jail(path);
+    await recordBeforeWrite(abs);
     const before = await Bun.file(abs).exists() ? await Bun.file(abs).text() : undefined;
     await Bun.write(abs, content);
 
@@ -300,6 +304,7 @@ export const editFileTool = withMeta({ set: 'core', mutating: true }, tool({
   execute: async ({ path, oldString, newString, replaceAll = false }) => {
     if (oldString === newString) throw new Error('oldString and newString are identical');
     const abs = jail(path);
+    await recordBeforeWrite(abs);
     const file = Bun.file(abs);
     if (!(await file.exists())) throw new Error(`No such file: ${path}`);
     const before = await file.text();
@@ -337,6 +342,7 @@ export const multiEditTool = withMeta({ set: 'edit-plus', mutating: true }, tool
   }),
   execute: async ({ path, edits }) => {
     const abs = jail(path);
+    await recordBeforeWrite(abs);
     const file = Bun.file(abs);
     if (!(await file.exists())) throw new Error(`No such file: ${path}`);
 
@@ -700,6 +706,8 @@ export const moveFileTool = withMeta({ set: 'edit-plus', mutating: true }, tool(
   execute: async ({ from, to }) => {
     const source = jail(from);
     const target = jail(to);
+    await recordBeforeWrite(source);
+    await recordBeforeWrite(target);
     if (source === target) throw new Error('from and to are the same path');
 
     const file = Bun.file(source);
@@ -721,6 +729,7 @@ export const deleteFileTool = withMeta({ set: 'edit-plus', mutating: true }, too
   }),
   execute: async ({ path }) => {
     const abs = jail(path);
+    await recordBeforeWrite(abs);
 
     // Bun.file on a directory reports exists() false, so the stat is what
     // distinguishes "missing" from "a directory" and gives the right refusal.

src/ui/App.tsx

@@ -674,6 +674,28 @@ export function App({
             push({ kind: 'error', text: e instanceof Error ? e.message : String(e) });
           }
           return;
+        case 'undo': {
+          push({ kind: 'user', text: chosen.trim() });
+          setWorking(true);
+          try {
+            push({ kind: 'info', text: await session.undo() });
+          } catch (e) {
+            push({ kind: 'error', text: e instanceof Error ? e.message : String(e) });
+          }
+          setWorking(false);
+          return;
+        }
+        case 'redo': {
+          push({ kind: 'user', text: chosen.trim() });
+          setWorking(true);
+          try {
+            push({ kind: 'info', text: await session.redo() });
+          } catch (e) {
+            push({ kind: 'error', text: e instanceof Error ? e.message : String(e) });
+          }
+          setWorking(false);
+          return;
+        }
         case 'provider':
           push({ kind: 'user', text: chosen.trim() });
           setOnboarding(true);