Add /fork and /rewind slash commands

.gitignore

@@ -55,3 +55,7 @@ benchmarks/baseline.json
 .vibe/
 .windsurf/
 .zencoder/
+
+# Python bytecode caches from scripts/
+__pycache__/
+*.pyc

README.md

@@ -87,6 +87,30 @@ fx session resume last
 fx session resume --id <id>
 ```
 
+Inside the interactive shell, use `/resume` to open the session picker, `/resume last` for the latest workspace session, or `/resume <id>` for an exact session.
+
+`fx session <id>` prints the saved conversation with a `[turn N]` label above every turn. Those labels are the boundaries the branch and undo commands take:
+
+```bash
+fx session fork <id> --at 7
+fx session rewind <id> --by 2
+```
+
+`fork` copies the first 7 turns into a brand-new session and prints the new ID to resume. The source session is left exactly as it was. `rewind` drops the last 2 turns from the session in place, keeping its ID. Both accept `--id <id>` and `--json`.
+
+Rewound turns are not erased. The rewind is recorded as a new revision in the session's event log, and the files those turns referenced stay on disk.
+
+The interactive shell has the same two operations for the session it is already in:
+
+```
+/fork 7
+/rewind 2
+```
+
+`/fork` branches the whole session at its current point; `/fork 7` branches at turn 7. Both forms name the source ID and the new one and leave you in the branch, while the source session keeps every turn it had. Bare `/rewind` opens a turn picker that restores to before the selected prompt and puts that prompt back in the composer. `/rewind 2` asks first: the message says how many turns it will drop and how many remain, and a second identical `/rewind 2` carries it out. Any other command in between cancels it.
+
+Neither command reverts file edits, commands, commits, or API calls. They change the conversation only.
+
 Each interactive session names its terminal tab. The title prefers the session name, falls back to the workspace name, and keeps the active model as secondary context. Renaming or resuming a session updates the tab, and exiting clears the fx-owned title. Noninteractive commands do not emit terminal-title controls.
 
 Run `/feedback` to open the feedback form at `fx.sh/feedback`. It does not create a diagnostic or change the clipboard.

scripts/pgso/corpus.json

@@ -113,6 +113,8 @@
   ],
   "verification_scenarios": [
     {"name": "verify-auto-mode-reliability", "argv": ["bun", "test", "--max-concurrency", "1", "./auto-mode-reliability.test.ts"], "test_file": "auto-mode-reliability.test.ts"},
+    {"name": "verify-session-fork", "argv": ["bun", "test", "--max-concurrency", "1", "./session-fork.test.ts"], "test_file": "session-fork.test.ts", "requires_tmux": false},
+    {"name": "verify-tui-session-fork", "argv": ["bun", "test", "--max-concurrency", "1", "./tui-session-fork.test.ts"], "test_file": "tui-session-fork.test.ts", "requires_tmux": true},
     {"name": "verify-oauth-keychain-migration", "argv": ["bun", "test", "--max-concurrency", "1", "./oauth-keychain-migration.test.ts"], "test_file": "oauth-keychain-migration.test.ts", "allow_keychain": true},
     {"name": "verify-tui-auth-source-selection", "argv": ["bun", "test", "--max-concurrency", "1", "./tui-auth-source-selection.test.ts"], "test_file": "tui-auth-source-selection.test.ts"},
     {"name": "verify-tui-composer-edit-contracts", "argv": ["bun", "test", "--max-concurrency", "1", "./tui-composer-edit-contracts.test.ts"], "test_file": "tui-composer-edit-contracts.test.ts"},

src/builtins/commands.zig

@@ -193,14 +193,16 @@ pub const top_level_specs = [_]TopLevelSpec{
     .{
         .kind = .session,
         .token = "session",
-        .usage = "session <last|id>|--id <id> [--json] | session resume [last|<id>] | session resume --id <id> | session migrate <id>|--id <id> [--allow-large] [--json] | session recover <id>|--id <id> [--json]",
-        .summary = "Inspect, resume, migrate, or recover saved sessions",
+        .usage = "session <last|id>|--id <id> [--json] | session resume [last|<id>] | session resume --id <id> | session migrate <id>|--id <id> [--allow-large] [--json] | session recover <id>|--id <id> [--json] | session fork <id>|--id <id> --at <turn> [--json] | session rewind <id>|--id <id> --by <count> [--json]",
+        .summary = "Inspect, resume, migrate, recover, fork, or rewind saved sessions",
         .options = &.{
             .{ .flag = "last", .description = "Inspect the current workspace session" },
             .{ .flag = "--id <id>", .description = "Inspect a saved session by exact id" },
             .{ .flag = "resume [last|<id>]", .description = "Resume the latest workspace session or a session by id" },
             .{ .flag = "migrate <id>", .description = "Migrate a saved session to the current format" },
             .{ .flag = "recover <id>", .description = "Copy a recoverable corrupt session into a new session" },
+            .{ .flag = "fork <id> --at <turn>", .description = "Branch a session into a new session holding its first <turn> turns" },
+            .{ .flag = "rewind <id> --by <count>", .description = "Drop the last <count> turns from a session in place" },
             .{ .flag = "--allow-large", .description = "Permit migrating an oversized session" },
             json_option,
         },
@@ -312,6 +314,8 @@ pub const top_level_help_groups = [_]TopLevelHelpGroup{
         .{ .usage = "session resume [last|id]", .summary = "Resume the latest workspace session or a session by id" },
         .{ .usage = "session migrate <id>", .summary = "Migrate a saved session to the current format" },
         .{ .usage = "session recover <id>", .summary = "Copy a recoverable corrupt session" },
+        .{ .usage = "session fork <id> --at <turn>", .summary = "Branch a session at a turn into a new session" },
+        .{ .usage = "session rewind <id> --by <count>", .summary = "Drop the last turns from a session in place" },
     } },
     .{ .entries = &.{
         .{ .kind = .login, .usage = "login [vercel|codex|grok]" },
@@ -435,9 +439,11 @@ pub const slash_specs = [_]SlashSpec{
     .{ .kind = .clear_screen, .command = "/clear", .help_entry = "/clear", .completion_description = "start a fresh session and keep background processes", .presentation_category = .general, .show_in_welcome = true },
     .{ .kind = .new_session, .command = "/new", .help_entry = "/new", .completion_description = "start a fresh session", .presentation_category = .session, .show_in_welcome = true },
     .{ .kind = .reset_session, .command = "/reset", .help_entry = "/reset", .completion_description = "reset the current session context", .presentation_category = .session },
-    .{ .kind = .resume_session, .command = "/resume", .help_entry = "/resume", .completion_description = "resume a saved session", .presentation_category = .session },
+    .{ .kind = .resume_session, .command = "/resume", .help_entry = "/resume [last|<id>]", .completion_description = "resume a saved session", .presentation_category = .session, .has_args = true, .accepts_payload = true },
     .{ .kind = .continue_recovery, .command = "/continue", .help_entry = "/continue", .completion_description = "continue a paused model response", .presentation_category = .session, .requires_prompt_credential = true },
     .{ .kind = .rename_session, .command = "/rename", .help_entry = "/rename <title>", .completion_description = "rename the current session", .presentation_category = .session, .has_args = true, .accepts_payload = true },
+    .{ .kind = .fork_session, .command = "/fork", .help_entry = "/fork [turn]", .completion_description = "branch this session at a turn into a new one", .presentation_category = .session, .has_args = true, .accepts_payload = true },
+    .{ .kind = .rewind_session, .command = "/rewind", .help_entry = "/rewind [count]", .completion_description = "drop the last turns from this session", .presentation_category = .session, .has_args = true, .accepts_payload = true },
     .{ .kind = .login, .command = "/login", .help_entry = "/login", .completion_description = "choose Vercel or Codex sign-in", .presentation_category = .account },
     .{ .kind = .logout, .command = "/logout", .help_entry = "/logout [vercel|codex|grok]", .completion_description = "sign out of a provider session", .presentation_category = .account, .has_args = true, .accepts_payload = true },
     .{ .kind = .setup, .command = "/setup", .help_entry = "/setup", .completion_description = "manage accounts and AI Gateway access", .presentation_category = .account },
@@ -538,6 +544,8 @@ test "built-in slash commands register exact active order" {
         "/resume",
         "/continue",
         "/rename",
+        "/fork",
+        "/rewind",
         "/login",
         "/logout",
         "/setup",