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, built-in and typed tools). The built-in tools are named as the harness names them, as the manifest lists them: limits.tools for Pi, claudeBuiltins for Claude Code (Read, Write, Edit, Grep, Glob).
• 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>.

A reply goes to the model as compact JSON, from the MCP server and the Pi extension alike (replyText in tools.mjs). Over REPLY_CAP, 40000 characters, the tool returns an error that names the reply's size and the cap and asks the session to narrow the query, never a cut-off reply. The pinned Claude CLI (2.1.296) saves an MCP result over 50000 characters to a file outside the workspace, which the gate refuses to read (row 89, #1583; agents/dewey/work/queue-89/build.md has how that size was found).

list_tasks (tracker holders) takes the broker's tasks filters, done, assignee, label, requirement, limit and after, and sends done: false when the session leaves done out, so its default is open tasks, 50 at a time, without descriptions. get_task takes a task_ref and returns that task in full. packages/bus/README.md has the reply.

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 the sandbox. It gates the harness's own tool calls. bash, where the role's tool limit grants it (S0 line 5), reaches what the session's sandbox holds (packages/seat/README.md, "The sandbox"): the system directories, the runner's own repository files, its workspace and its run directory. The data root, other workspaces and runs, ~/.config, ~/.mosaic, ~/.ssh, ~/.claude and the rest of the checkout aren't there. HOME and /tmp are the run's own. The session's credential variable is in its environment, so bash (or Claude Code's Bash) can read it (packages/cli/README.md, "Sessions").
• Same UID. The sandbox's user namespace maps the session to the host's user. It can't reach the host's other files, but anything outside the sandbox that it can talk to (below) acts with that user's full rights.
• Network and abstract sockets aren't confined. The session shares the host's network namespace: it reaches the network, every service on 127.0.0.1, and every abstract Unix socket (a name starting with a NUL byte, which no mount hides). A service listening there, an X server or a session bus that offers one, acts for the session outside the sandbox. The shared namespace also shows up in /proc/net/unix, which lists the host's Unix sockets by name, path ones included; a name gives no access to a path socket the mounts hide. The policy's network field is recorded, not enforced.
• The broker socket is bound in. A session that can run bash can connect to it; it still needs a capability to do anything there. The tool socket is in the run directory and acts with the session's own capability only. The launch socket is bound in only for the role whose launch tool calls it.
• PID namespace. Each session's runner is pid 1 of its own namespace (bwrap --unshare-pid --as-pid-1), with its own IPC namespace. A session sees no other process, so it can't signal or ptrace one by pid. The runner doesn't reap zombies; turns are reaped by the runner's own waits, and the namespace dies with it.
• mosaic stop is same-UID process control from outside the sandbox: 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.
• The reply cap is per reply. Claude Code also persists the largest results of one message when their total passes 200000 characters, so six or more replies near the cap in one turn can still be saved to a file. A single task whose full row is over the cap (a description near 40000 characters) can't be read through get_task. The cap matches Claude CLI 2.1.296; the binary the bus unit names updates itself, so a later version can move the threshold.
• --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 outside the sandbox; 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.