stack/packages/harness/README.md

# harness

What a managed role session runs from and runs as (slice 1 S6, issue #1523):
the launch bundle generated from one resolved role instance, the tool gate
both harnesses share, the typed bus tools, the Pi extension, the Claude Code
hook and MCP server, and the session runner. The host's launcher
(`packages/cli/src/launcher.mjs`, `packages/seat/src/session.mjs`) builds the bundle and
starts the runner; nothing here spawns a session on its own.

Plain ESM, no dependencies, Node 24 or newer. Tests: `npm test` in this
directory (the Pi and Claude Code session tests drive the real CLIs against
a scripted Messages API on 127.0.0.1; the Claude Code ones skip when
`claude` isn't on PATH).

## What S6 relies on

Every harness layer below is a row S0 line
(`docs/plans/2026-10-04_slice-1.md`, "What S6 can rely on"), and each
bundle's `manifest.json` names the lines it relies on (`reliesOn`):

| | Pi | Claude Code |
|---|---|---|
| 1. block a tool | `tool_call` handler returns a block | PreToolUse command hook exits 2; `--bare` is never passed |
| 2. gate crash | a throw in the handler blocks | the hook runs as `<gate> \|\| exit 2` |
| 3. gate missing | a missing `-e` file refuses to start | the wrapper blocks |
| 4. gate hang | the runner's wall clock plus the `agent_end` marker | `timeout -k 2 10 <gate> \|\| exit 2`, hook timeout 20 |
| 5. bash | limited only by the tool limit | limited only by the tool limit |

Claude Code also runs with `--restricted`: no user, project or local
settings, file tools confined to the working directory, and no `CLAUDE.md`
file (user, parent or workspace) or auto-memory in the prompt. None of the
S0 lines depend on it, and the gate doesn't rely on it for paths. It is the
layer that keeps founder and repository memory out of the session's
prompt, though, so `claude-session.test.mjs` fails if the adapter stops
passing it, and shows the memory reaching the model without it.

## The bundle

`buildBundle({dir, resolved, contract, arbiter, tracker, credentials, session, piVersion, claudeVersion})`
writes the bundle once into `dir` (0700; every file 0600, created with `wx`,
so an existing bundle is never overwritten):

- `prompt.md`: the role contract, then a generated "This session" section
  (business, instance, run, who launched it, workspace, typed tools).
- `policy.json`: harness, workspace, built-in tools (`limits.tools`), typed
  tool names, network, the services the role needs, and the credential
  status the host read from the broker (metadata only, never a token).
- `tools.json`: the typed tools (below).
- `manifest.json`: bundle version, the instance and run, the resolved
  digest, harness, provider, model, thinking, the Pi or Claude Code version, built-in and
  typed tools, the actions without a typed tool, the S0 lines, the gate
  timeout, and the sha256 of every bundle file and of the gate, extension,
  MCP server and tools sources.
- Claude Code only: `settings.json` (the wrapped PreToolUse hook, matcher
  `*`) and `mcp.json` (one stdio server, `mosaic`, serving the typed tools).

`sessionModel(resolved)` picks the harness, provider and model: the agent
layer's `harness` and `model` variables win, else the system's execution
adapter and model. Only `pi` and `claude-code` are accepted.

## Typed tools

A session acts on the bus only through typed tools. Each one maps onto one
broker verb, or for `launch` onto the host's launch socket. A session gets
a tool only for an action its resolved instance holds (within-role or
cross-role), plus the reads and `raise_decision`. The broker still decides
every call; a tool being present is not permission.

The harness never holds the capability. Tools call the runner's tool socket
(`tools.sock`, 0600) with `{tool, args}`; the runner checks the name
against `tools.json` and forwards the call with its own capability. A
refusal comes back to the model as `refused: <code>`.

## The gate

`gate.mjs` `decide(policy, tool, input)` answers one tool call: the
policy's built-in tools and typed tools pass, anything else is blocked, and
every path argument of a file tool (and a `find`/`Glob` pattern) must
resolve inside the workspace after symlinks and Pi's own path
normalisation. A path that reaches a dangling symlink, at any depth, is
refused even when the link points inside: a write through it would create
the target wherever it names, and Pi's `write` makes the missing
directories first. Pi calls it from the extension, Claude Code from
`claude-gate.mjs`.

A relative path is resolved the way Pi resolves it: against its working
directory. Both adapters `cd` into the workspace, and a process's working
directory is the real path, so `..` climbs the real path's parents. With
the workspace or the dataRoot behind a symlink, those differ from the
parents of the path as given, and `../../data/ws/x` can be inside as given
but outside for Pi. The gate requires a relative path to be inside from
both the real path and the path as given. The second check is stricter
than Pi: a path that only climbs out and back in through the real path's
parents is refused. That keeps the gate fail-closed whichever directory it
runs in. Claude Code isn't affected the same way: it makes a path absolute
against its own (real) working directory before the `PreToolUse` hook
sees it, so the gate gets the path Claude Code will open.

Pi's `read` doesn't always open the name it is given. When that name
doesn't exist, it tries other spellings of the whole resolved path: a
narrow no-break space (U+202F) before ` AM.` or ` PM.`, the NFD form, a
curly apostrophe (U+2019) for `'`, and NFD with the curly apostrophe. So
for `read`, the gate also checks every one of those spellings that exists
as a name, built from both the workspace as given and its real path (Pi's
working directory). If any of them resolves outside the workspace or goes
through a dangling symlink, the read is refused, whichever one Pi would
pick. A spelling that doesn't exist is ignored. Claude Code's `Read` gets
the same check, though in Darkwing's round 2 probe Claude Code's own
symlink check already stopped the one variant it tries (AM/PM). `write`,
`edit`, `grep`, `find` and `ls` take the name as given in Pi 0.85.1, so
they get no such check.

The gate copies Pi 0.85.1 code, so recheck these files on a Pi upgrade:
`dist/utils/paths.js` (`normalizePath`), `dist/core/tools/path-utils.js`
(`resolveToCwd`, `resolveReadPathAsync`) and which tools in
`dist/core/tools/` call `resolveReadPath*`. The `pi` binary runs the
bundled copy under `dist/bundle/`; in 0.85.1 it holds the same code, and
only `read` and the CLI's own file arguments call `resolveReadPath*`.

## The runner

`node runner.mjs <run dir>`, with `{"cap": "..."}` and a newline on stdin.
The host writes the capability there after the broker bound the launch, so
it never touches the disk and isn't in the environment or argv.

1. Founder-credential stop (REQ-CRED-2): a known founder credential
   variable in the environment (`GITEA_TOKEN`, `GH_TOKEN`, `SSH_AUTH_SOCK`
   and the rest of `FOUNDER_ENV`), or a service the role needs with no
   usable role token, exits 20 before the role is claimed.
2. `role.claim`.
3. The tool socket serves the typed tools.
4. One harness turn per received message, through the adapter
   (`/bin/sh <adapter.sh>`: the repository keeps adapters 0644 and only the
   image sets the mode), in its own process group, under the wall clock
   (`turnTimeout`, default 900 s). A turn counts only if it exits 0 and,
   on Pi, the extension wrote the turn marker at `agent_end` (S0 line 4).
   Its stdout goes back to the sender as a RESULT; a failed turn replies
   `The turn failed: <reason>`; an incoming RESULT never gets an automatic
   reply. Then `message.read`.
5. SIGTERM or SIGINT kills the turn's process group, releases the role and
   exits 0. The handlers go in before anything else, because pid 1 of a
   PID namespace never sees a signal it has no handler for; a stop before
   the claim exits 0 without claiming. Node's own startup still leaves a
   short window, so the host and `mosaic stop` send SIGTERM again every
   second until the runner is gone.

| Exit | Meaning | Launch end reason |
|---|---|---|
| 0 | stopped | `stopped` |
| 2 | bad run directory or stdin | `failed` |
| 20 | founder credentials | `founder-credentials` |
| 21 | `role.claim` refused | `claim-refused` |
| 22 | the capability or claim stopped working (run ended, claim revoked) | `session-invalid` |
| 23 | `role.claim` got no answer, or the broker stayed unreachable for `brokerRetries` polls (default 30) | `broker-unreachable` |

Each turn's stderr goes to `turns/NNNN.stderr` (0600) in the run directory.

## Limits

These are limits, not bugs, and nothing in this package claims otherwise.

- **The gate is not a sandbox.** It gates the harness's own tool calls.
  `bash`, where the role's tool limit grants it, can reach anything the OS
  user can (S0 line 5).
- **Same UID.** Every session runs as the host's user. Founder files in
  `HOME` (`~/.git-credentials`, the tea config, `~/.ssh`) are readable
  through `bash`; only the environment is filtered. `/tmp` is shared. The
  PID namespace (below) hides other processes, so a session can't signal
  or `ptrace` another one by pid.
- **The broker socket is shared.** A session that can run `bash` can
  connect to it; it still needs a capability to do anything there. The
  tool socket is reachable through `bash` as well, and acts with the
  session's own capability only.
- **Network.** Abstract Unix sockets and the network are not confined; the
  policy's `network` field is recorded, not enforced.
- **PID namespace.** Each session's runner is pid 1 of its own namespace
  (`unshare --user --map-current-user --pid --fork --kill-child
  --mount-proc`). It doesn't reap zombies; turns are reaped by the runner's
  own waits, and the namespace dies with it. A same-UID process can still
  ask something outside its tree (a systemd user manager, an existing tmux
  server) to run a command.
- **`mosaic stop`** is same-UID process control: it checks the registry's
  start time and that the process is a runner before sending SIGTERM.
- **If the host dies,** sessions keep running until their broker polls fail
  `brokerRetries` times, then exit 23. The next host kills what still runs
  and ends those launches as `host-lost` (`packages/cli/README.md`,
  "Sessions").
- **Resolution** runs without the project layer, there is no skills source
  (bundles list none), and the resolved `thinking` level is recorded in the
  manifest but not applied to either harness.
- **`--restricted`** (Claude Code) is not one of the S0 lines. It is what
  keeps `CLAUDE.md` files and auto-memory out of the prompt (above).
- **Other spellings.** A read is refused when a spelling Pi might open
  points outside the workspace, even if the name as given exists and is
  inside, and Pi would open that one. The same holds when a directory
  next to the workspace has another spelling of its path, for example
  `jo’s/ws` beside a workspace in `jo's/ws`.
- **The gate checks a path when the call is made.** A link created or
  changed between the check and the tool's own open (by `bash`, or by
  another process of the same user) isn't seen. That is the same reach as
  `bash` itself.
- **The system prompt is in argv** for both adapters, so the same UID can
  read it in `/proc/<pid>/cmdline`; the PID namespace hides it from other
  sessions. It holds no secret.
- **Pi adapter paths** with spaces break its unquoted `-e` and skill
  splitting; the launcher's paths don't contain spaces.