fx/AGENTS.md

AGENTS.md

Instructions for AI coding agents working with this codebase.

Declaring Work Ready

Do not say the work is "ready", "done", "good to go", "complete", or similar until you have personally run the binary and exercised the change on its happy path. A passing test suite is necessary, not sufficient — tests in this repo do not always construct the full runtime, attach a TTY, or spawn background threads, so they will not catch startup crashes, render regressions, or thread-lifetime bugs.

Before reporting the work as ready:

1. Build succeeds.
2. Focused tests for the changed path pass locally.
3. The Full CI run for the exact current commit passes on every required Linux and macOS runner.
4. Run the built binary locally and drive at least one real interaction that exercises the change end to end.
5. Confirm the process did not abort, stderr is clean, and the behavior matches what you are about to tell the user.

If you cannot run the binary in your environment, say so explicitly and ask the user to verify. Do not silently skip this step and declare the work ready. "The tests pass" is not a substitute for running the app.

Always use the built binary in this repo

When running fx for verification, always use the freshly-built binary at ./zig-out/bin/fx from this checkout. Never run fx from PATH, never rely on whatever is at ~/.fx/bin/fx, and never assume an installed copy reflects your change.

• The user may have an older fx on their PATH (e.g. installed via fx upgrade or the CDN install script). Running that one will not exercise your edits.

• zig build writes to zig-out/bin/fx. That is the only binary that contains your latest change.

• When a user reports "still not working" after you believe you fixed something, do not assume they are running the wrong binary. Assume your fix is incomplete and investigate further. If you genuinely suspect a PATH mismatch, ask — do not silently copy binaries into ~/.fx/bin/.

• In any shell invocation — tmux, direct run, scripts — reference fx as /Users/<you>/path/to/repo/zig-out/bin/fx (absolute) or ./zig-out/bin/fx (when cwd is the repo root). Bare fx is always wrong for dev verification.

Language and Toolchain

This project is written in Zig 0.16+. There is no Node.js runtime, no package.json at the root, and no JavaScript build step for the main binary.

Build and test commands:

zig build          # build the binary
zig build test     # run all unit tests
zig build run      # build and run
zig fmt src/       # format all source files

The test suites under tests/ use Bun but are separate from the Zig codebase. See Testing below.

Code Style

• Format all Zig source with zig fmt before committing. The canonical check is zig fmt src/.

• Do not use emojis in code, output, or documentation. Unicode symbols (e.g. checkmark, arrow) are acceptable.

• In documentation, never use double hyphens (--) as a dash. Use an emdash (—) sparingly, or rewrite to avoid dashes.

• Use the technical-writer skill (.fx/skills/technical-writer/SKILL.md) when you write or edit prose, including documentation, commit messages, PR titles and descriptions, and issues. Where the skill and this section differ, such as on em dashes, this section takes precedence.

• CLI flags use kebab-case (e.g. --no-save, --json). Never use camelCase for flags.

• Prefer snake_case for all Zig identifiers. Types use PascalCase per Zig convention.

• Keep pub surface area minimal. Only mark declarations pub when they are used outside the file.

Architecture

Key rules:

• src/main.zig is the composition root. Do not add leaf feature logic here.

• src/core/ owns contracts, runtimes, config, sessions, permissions, MCP, skills.

• src/tools/ owns built-in tool implementations. Generic tool contracts and dispatch live in src/core/tooling/. Default tool specs are centralized in src/core/tooling/tool_specs.zig or src/builtins/tools.zig, not in individual tool files.

• src/ui/ owns terminal rendering, event loop, input, transcript. It must not own product state.

• src/gateway/ owns provider transport. It must not absorb product-state logic.

• src/acp/ owns the ACP (Agent Client Protocol) JSON-RPC 2.0 server.

Adding a Feature

Before implementing, answer in order:

1. Which module owns the behavior?
2. What is the typed contract?
3. Does it need persistence?
4. Does it need both text and JSON output?
5. What docs and tests land with it?
6. How is its deterministic E2E owner classified in the macOS arm64 PGSO corpus?

If unclear, define the contract first.

Every root tests/e2e/*.test.ts file must have exactly one classification in scripts/pgso/corpus.json:

• Training: common or performance-sensitive product behavior that should influence LLVM's hot and cold decisions

• Verification-only: important correctness, recovery, security, or rare behavior that the final candidate must pass without making it hot

• Intentional exclusion: nondeterministic, live-network, credentialed, sound-related, or harness-only coverage, with a concrete reason

New tests inside an already classified file inherit that file's classification, but feature work must reconsider whether the existing classification still matches the file's product role. When removing a feature or E2E owner, remove its stale corpus entry. Normal PR CI loads the corpus and rejects missing, duplicate, stale, or unclassified files without running the expensive PGSO qualification.

Adding a Command

1. Add the spec to src/core/slash_commands/command_specs.zig
2. Add dispatch wiring in src/core/cli/cli_surface.zig
3. Add a snapshot type if it has structured output
4. Render text and JSON from the same snapshot via src/core/output/output_contracts.zig

Do not scatter help text or argument parsing across multiple files.

Configuration and State

Profile configuration and runtime state lives under ~/.fx/. Project .fx.json contains committed project defaults only.

Config precedence (highest wins):

1. Environment variables such as FX_PROVIDER, FX_MODEL, FX_PERMISSION_MODE, and FX_MAX_AGENT_STEPS
2. ~/.fx/settings.json → workspaces["<workspace_path>"] (profile workspace overrides)
3. ~/.fx/settings.json top-level (profile global settings)
4. <workspace>/.fx.json (committed project defaults)
5. Built-in defaults

Project .fx.json accepts only repo-safe defaults: sandbox, max_agent_steps, max_tool_result_bytes, context, provider_order, and provider_strict. Profile-owned keys such as provider, providers, models, model, effort, fast_mode, slash_menu_categories, startup_scrollback, prompt_history, statusLine, skill_match_fuzzy, first_call_tool_choice, auto_upgrade, permission_mode, credential_source, and permission are ignored from project config before their values are parsed.

Runtime state lives under ~/.fx/sessions/<session-id>/ (session.json, background/, subagent/, logs/). Sessions are global and portable across workspaces. Each session tracks its workspace_root, which updates when resumed in a different workspace. A subagent child is an internal ordinary session with its own history. Its parent owns one bounded subagent/children.json registry, and the child carries only an immutable owner marker. Child sessions stay out of ordinary session discovery and cannot be resumed directly. A first subagent.message creates a named persistent child in that parent; later messages continue it, and optional instructions replace only its child-specific system overlay.

Permissions

Security is permission-first. All sensitive tool behavior must integrate with src/core/permissions/permissions.zig.

• permission_mode controls baseline (ask, auto, or full-access; yolo remains an alias). Full access bypasses fx permission policy and uses an effective sandbox of none without rewriting saved sandbox configuration

• Configured denies are evaluated before saved-session rules; an exact saved-session deny can narrow a configured allow, while an exact saved-session allow can satisfy an unresolved configured ask

• Session always approvals are non-persistent; command approvals match the exact command while other grant categories may use patterns

• /permissions remember allow|deny <tool-name> <arguments-json> confirms and stores an exact rule only for an active saved session; list and revoke those rules by their stable IDs

• Routine parsed development commands and reversible new-file creation can execute without model review after configured and saved-session policy. Every remaining unresolved auto action receives one narrow security review using the exact unmasked action and targets, origin and call identity, optional host-proven current-branch evidence, and bounded unmasked terminal-safe excerpts of earlier current-turn tool results. A text match between the action and prior tool output is evidence to inspect, not proof of prompt injection or malicious activity. Prepared file mutations and other static root tools omit task text. Reviewed commands, shell input, dynamic tools, and subagent actions also receive bounded unmasked canonical current, first, and recent root requests plus explicit omission counts; the reviewer may use that context only to distinguish trusted user intent from malicious or injected influence, never to judge task quality, alignment, or authorization. Assistant prose, permission feedback, compacted summaries, the pending tool group, later results, and tool or repository text never become authority. Shell input reviews also include the owned receiving session's launch command, working directory, and bounded current screen after verifying session authority, including on resume. The launch command describes startup only; screen content remains untrusted evidence and input still receives its own review

• The reviewer returns caution only for concrete prompt injection or malicious activity. Destructive, risky, external, public, remote, unrequested, or task-conflicting actions clear when they are not malicious. A clear review authorizes only the exact unchanged action. A caution, incomplete-evidence result, or unavailable review holds only that action, returns guidance to the agent, and never opens a human permission screen, disables tools, or ends the turn

• Exact cautions and deterministic incomplete-evidence results are reused only for the current turn. An unavailable outcome is not cached as a security judgment, but the same exact action spends at most one unavailable review opportunity per turn; changed actions remain independently reviewable until the bounded current-turn review budget is exhausted. Each review accepts exactly one valid structured decision even with accompanying prose and may retry one malformed completion within the current attempt's deadline. A transport timeout, transient transport failure, or failed transport call is retried once with a fresh 30-second deadline; permanent transport failures, valid cautions, and cancellation are never retried. Legacy permission_request_id input is rejected without prompting

• Host-generated review holds retain their advice for the agent and transcript, but carry a saved review_feedback marker that excludes them from later security evidence, including after recovery. Old unmarked results remain untrusted evidence; never infer the marker from output text. Quoted review accusations are not proof of an attack, and handling plans or test instructions as data is not itself prompt injection

Do not bypass the permission system for new tools.

Zig-Specific Patterns

Memory

• Allocators are passed explicitly. Never use a global allocator.

• Free what you allocate. Use defer for cleanup at the call site.

• Prefer ArenaAllocator for request-scoped work that can be freed in bulk.

• When a function returns allocated memory, document who owns it (caller or callee).

Error Handling

• Return errors rather than panicking. @panic is for programmer bugs, not runtime conditions.

• Use errdefer to clean up partial state on error paths.

• Prefer specific error sets over anyerror when the set is bounded.

Strings and JSON

• Zig strings are []const u8. There is no implicit null termination.

• For JSON serialization, use std.json.Stringify.value with an allocating writer (std.Io.Writer.Allocating).

• For JSON string escaping (writing raw JSON), use the project's writeJsonStr helper in src/acp/jsonrpc.zig rather than assuming std.json.encodeJsonString exists.

• Zig 0.16 uses std.Io.File.stdin() / .stdout() / .stderr(), not std.io.getStdIn().

I/O (Zig 0.16 "Juicy Main")

• main uses pub fn main(init: std.process.Init) !void signature.

• All I/O goes through std.Io, passed explicitly or via the project's src/core/shared/io.zig helper (io_mod.getIo()).

• File operations use std.Io.Dir and std.Io.File (not std.fs). Most methods require an io parameter.

• Environment variables: use io_mod.getenv(key) (returns ?[]const u8), not std.process.getEnvVarOwned.

• Time: use io_mod.milliTimestamp(), io_mod.nanoTimestamp(), io_mod.sleep(ns).

• File reading: use io_mod.readFileToEnd(alloc, &file, max_bytes).

• Realpath: use io_mod.realpathAlloc(alloc, path) or io_mod.dirRealpathAlloc(alloc, dir, sub_path).

• Process spawning: use std.process.spawn(io, opts) and std.process.run(alloc, io, opts).

• Mutexes: std.Io.Mutex, initialized with .init, locked with .lockUncancelable(io).

• HTTP: std.http.Client requires .io = io_mod.getIo() in its initializer.

• std.mem renames: trimLeft is trimStart, trimRight is trimEnd, indexOf is find, indexOfScalar is findScalar.

• ArrayList(T) initializes with .empty (not .{}).

Testing

• Zig unit tests go inside the source file they test, using test "description" { ... } blocks.

• Run the narrowest relevant tests while developing. The complete zig build test suite runs in ReleaseSafe in Full CI after the feature branch is pushed, and it must pass before the draft PR is marked ready.

• Use std.testing.expect, std.testing.expectEqual, std.testing.expectEqualStrings for assertions.

• In test blocks, use std.testing.io for the Io parameter. io_mod.getIo() automatically returns std.testing.io in test builds.

• Use io_mod.dirRealpathAlloc(alloc, dir, sub_path) to resolve paths within std.testing.tmpDir().

Testing (TypeScript)

Two test suites live under tests/, both using Bun:

tests/evals/ — LLM Evals

Eval scenarios that exercise the agent through fx ask --json. Require AI_GATEWAY_API_KEY.

cd tests/evals && bun install && bun test           # run all evals
cd tests/evals && bun run eval:matrix               # cross-model matrix run

tests/e2e/ — End-to-End Tests

Deterministic runtime tests (CLI commands, ACP protocol, TUI via tmux). No API key needed for most.

cd tests/e2e && bun install && bun test              # run all e2e tests
cd tests/e2e && bun test cli.test.ts                 # just CLI tests
cd tests/e2e && bun test acp.test.ts                 # just ACP tests
cd tests/e2e && bun test tui-*.test.ts               # just TUI tests (requires tmux)

TUI tests use tmux to drive the interactive terminal. They require tmux to be installed.

Pull Request Classification

Every pull request must have exactly one type: label, chosen by its primary intent:

• type: bug: fixes incorrect behavior

• type: feature: adds a new user-facing capability

• type: improvement: improves existing user-facing behavior

• type: docs: changes documentation only

• type: maintenance: changes internal tooling, dependencies, CI, or implementation structure without a user-facing behavior change

• type: release: prepares or repairs a release

• type: security: fixes or hardens a security boundary

Assign the label when the PR is opened and keep it accurate when the PR changes. If the authenticated contributor cannot manage labels, state the required label and keep the PR in draft until a maintainer or repository agent applies it. For a mixed PR, choose the label that describes the primary reason the PR exists. If that is ambiguous, ask before applying or changing the label.

Keep PR titles as clean imperative sentences, such as Restore feedback report file clipboard. Do not add bracketed prefixes such as [bug], [feature], or [improvement]. Type belongs in the label, not the title.

Full CI on Feature Branches

Do not run the complete deterministic test suite locally as the default development loop. Run the focused test for the changed path, build the binary, and exercise that path with ./zig-out/bin/fx.

After the focused checks pass, create a clean checkpoint commit, push the non-main feature branch, and open a draft PR immediately. .github/workflows/full-ci.yml runs the following on all four supported native runner architectures:

• ubuntu-24.04 (x86_64)
• ubuntu-24.04-arm (aarch64)
• macos-15-intel (x86_64)
• macos-15 (aarch64)

The native matrix builds, tests, and smoke-tests ReleaseSafe on every platform; formatting and the public-surface audit run in those ReleaseSafe jobs. The E2E matrix runs four duration-balanced, isolated ReleaseSafe shards per platform with Bun and tmux. Checked-in weights assign every test file to exactly one shard on each platform, and files inside each shard run sequentially in separate Bun processes so terminal fixtures and process state cannot leak between files. A failed file receives one bounded retry after its tmux server is reset. Live model evals remain separate because they require credentials and are not deterministic.

A Full CI result is valid only when it belongs to the exact current commit and all four Full suite (...) jobs succeed. Each platform aggregate requires its ReleaseSafe native check plus all four ReleaseSafe E2E shards. Do not mark the draft PR ready or request review from a stale, partial, queued, cancelled, skipped, or failed run. If Full CI fails, make the smallest repair, rerun the focused local proof, push the new commit to the same draft PR, and wait for Full CI on the new exact commit. After CI passes, run the final ship gate and mark the PR ready only when it reports SHIP for that exact commit.

Reproducing Render Bugs

fx's rendering is inline by default and deliberately emits a small ANSI subset. Three owner classes are the narrow exceptions, and each takes the alternate buffer exclusively through AlternateScreenOwner in src/ui/shell_runtime.zig: interactive permission review, the full-transcript screen, and catalog menus. Only one class may own the buffer at a time, and each must leave it and restore the main grid, composer, cursor, paste, mouse, focus, and keyboard modes when it closes. Transcript rendering, question prompts, command-output expansion, and subagent delegation remain inline. Three tools exist for reproducing and regression-proofing render bugs:

tmux (live TTY repros)

Best for resize and SIGWINCH interactions. The helper in tests/e2e/tmux-helpers.ts exposes resizeWindow(cols, rows), capturePaneGrid(), and capturePaneEscapes(). See tests/e2e/tui-resize.test.ts for the canonical resize matrix.

cd tests/e2e && bun test tui-resize.test.ts

Debug terminal recording and replay

Set FX_DEBUG_RECORD=1 to create an automatic private tape under ~/.fx/recordings/. Set FX_DEBUG_RECORD_SILENT_BANNER=1 as well when the developer-only recording notice must stay out of the inline transcript during a screen share. The notice remains available in the Ctrl+O full transcript. Use FX_RECORD=<path> when a test or investigation needs an exact destination. Recording dumps every byte fx writes and every resize into a framed binary tape. Replay the tape through the built-in virtual terminal:

FX_DEBUG_RECORD=1 ./zig-out/bin/fx
FX_RECORD=/tmp/bug.fxtape ./zig-out/bin/fx
./zig-out/bin/fx replay /tmp/bug.fxtape
./zig-out/bin/fx replay /tmp/bug.fxtape --frames
./zig-out/bin/fx replay /tmp/bug.fxtape --json
./zig-out/bin/fx replay /tmp/bug.fxtape --golden out.txt

The tape is deterministic — any reviewer can replay it without a TTY, and a golden file can be checked in as a regression test.

Shared terminal engine (sub-second unit tests)

src/core/terminal/engine.zig is the shared bounded text-terminal engine for hosted terminal sessions, recovery, replay, and deterministic rendering tests. src/ui/resize_tests.zig drives TranscriptRuntime against it in process so resize behavior can be exercised with no fd or timing dependence.

zig build test                      # runs every VT and resize test

When a tmux or tape-based scenario exposes a bug, reproduce it as a Zig unit test in resize_tests.zig (or a new sibling) before fixing. The test lands the fix as a regression.

Benchmarks

Startup latency benchmarks live in benchmarks/ and run in CI via .github/workflows/bench.yml.

./benchmarks/startup.sh            # full run (100 iterations, builds ReleaseSafe, needs hyperfine)
./benchmarks/startup.sh --quick    # quick run (20 iterations)

The CI workflow builds a ReleaseSafe binary, measures six CLI paths with hyperfine, and enforces per-command latency budgets. PRs that exceed a budget fail the check. On main, results are uploaded to Vercel Blob for historical tracking.

The startup benchmark uses FX_BENCH=1, an environment variable that runs through arg parsing and CLI dispatch, then exits before TTY initialization. This lives in src/core/app/app_entry_runtime.zig.

Current raw wall-clock contract:

• Linux CI: 2ms for every command
• Non-Linux local runs: informational raw means

The Linux CI runner is the authoritative product budget. Local macOS process and dynamic-loader floors vary enough to exceed 2ms independently of fx, so local runs report raw means without assigning a substitute product budget. The process baseline is diagnostic only and is never subtracted.

When adding features, consider their impact on startup latency. The fx help path is the baseline cold-start benchmark.

Binary Size Observability

Every pull request runs .github/workflows/binary-size.yml across Linux x86_64, Linux arm64, macOS x86_64, and macOS arm64. Each matrix job builds the pull request merge commit and its base commit as stripped ReleaseSafe binaries on the same native runner, then reports the exact byte and MiB delta plus ELF or Mach-O section changes.

Each platform check is informational. An increase of at least 52,429 bytes (0.050000 MiB) emits a warning and retains that platform's binaries for investigation, but does not reject the pull request. Investigate notable unexplained growth before changing the threshold. The full macOS arm64 PGSO release qualification remains authoritative for the 7.800 MiB production ceiling and performance gates.

Documentation

When adding or changing user-facing features, update all relevant files:

1. --help output via command specs in src/core/slash_commands/command_specs.zig
2. README.md — feature descriptions, usage examples
3. CONTRIBUTING.md — if build steps, config, or collaboration rules change

Do not document intended behavior as if it already exists.

Releasing

Releases use a two-workflow pipeline. The maintainer controls the changelog voice and format.

Automated flow (preferred)

1. Go to Actions > Prepare Release on GitHub
2. Select the bump type (patch, minor, or major) and run the workflow
3. The workflow bumps the version, feeds the actual git diff to an LLM to draft the changelog, and opens a PR
4. Review the PR — edit the AI-drafted changelog if needed — then merge
5. The existing release.yml detects the version change and handles build, publish, tagging, and the GitHub Release

The prepare-release.yml workflow uses the Vercel AI Gateway (AI_GATEWAY_API_KEY secret) to generate the changelog from the real code diff, not from commit messages or PR descriptions.

Manual flow

To prepare a release by hand:

1. Create a branch (e.g. prepare-v0.3.0)
2. Bump pub const version in src/main.zig
3. Write the changelog entry in CHANGELOG.md at the top, under a new ## <version> heading, wrapped in <!-- release:start --> and <!-- release:end --> markers. Remove the markers from the previous release entry so only the new release has them.
4. Update README.md install example version
5. Open a PR and merge to main

When the PR merges, CI compares the version tag to what exists in git. If the tag is missing, it cross-compiles all platform binaries, creates the git tag, and publishes a GitHub Release with the binaries attached. The release body is extracted from the content between the <!-- release:start --> and <!-- release:end --> markers in CHANGELOG.md.

Writing the changelog

Whether automated or manual, the changelog is public product copy. Describe observable user behavior, not the engineering process behind it. Use the diff, commits, and merged pull requests as research evidence only.

Public changelog entries must:

• Spell the product name fx. Preserve different casing only when it is part of an exact code identifier such as FX_MODEL.
• Use only relevant sections from ### Breaking Changes, ### New Features, ### Improvements, ### Bug Fixes, and ### Security. Omit empty sections.
• Bold a short feature or fix name, then describe the user-visible change after a colon.
• Omit pull request numbers, issue numbers, commit hashes, contributor names, and author attribution.
• Omit internal details such as repository moves, website or marketing work, CDN layout, CI workflows, test fixtures, branch history, and implementation-only refactors. Translate relevant work into its public user outcome or leave it out.
• Avoid forcing every merged change into the notes. A change without a public user outcome does not need a bullet.

Only the current release should have markers; remove <!-- release:start --> and <!-- release:end --> from any previous entry:

## 0.3.0

<!-- release:start -->
### New Features

- **Interactive terminal startup:** Start an interactive shell when the `terminal` tool receives an empty command
<!-- release:end -->

## 0.2.5

### Improvements

- **Inline rendering:** Keep the active conversation visible in terminal scrollback

Do not add a ### Contributors section or tracker references. Use descriptive section names.

Do not create version tags manually. Do not change build.zig.zon version (it is a placeholder).

Repository and License

The canonical repository is vercel-labs/fx on GitHub. All URLs, links, and references to the repo must use vercel-labs/fx (not vercel/fx, user/fx, or any other org/owner). Licensed under Apache-2.0.

What Not To Do

• Do not grow main.zig with leaf feature logic

• Do not add hidden product state that only exists in the live shell

• Do not add a second execution path for the same feature without a clear reason

• Do not commit generated state from .fx/, .zig-cache/, or zig-out/

• Do not add dependencies outside the Zig standard library without discussion

• Do not use @import with runtime-computed paths — Zig imports are comptime only

• Do not ignore zig fmt failures

• Do not create git tags manually (the release workflow owns tag creation)

• Do not report work as ready without running the binary. See Declaring Work Ready.

Before Marking a PR Ready

1. Run zig fmt --check src/ and the focused tests for the changed path.
2. Build and exercise the change locally with ./zig-out/bin/fx.
3. Push a clean checkpoint commit and open a draft PR immediately.
4. Require Full CI and the final ship gate to pass on the exact current commit across all four native runners.
5. Update docs if behavior changed.