# ADR 0018: The Agent Execution Surface Is Bounded By The Project Workspace

## Status

Accepted, and implemented. The mechanism is settled: the Claude Code SDK's
built-in sandbox (bubblewrap on Linux, seatbelt on macOS) confines the agent
process and its Bash subprocesses at the OS level. `bypassPermissions`
remains (headless server — no interactive prompts), but the sandbox is the
hard boundary, not the permission-prompt layer. Constitution rule 4
divergence (ADR-0017's unbounded tools) is resolved.

Builds on **ADR-0001** (project group as collaboration space), **ADR-0002**
(project lock scoped to `run_id`), **ADR-0004** (permission grants govern
`triggerAgent`), **ADR-0007** (the engineering file is a directory tree on
disk), and **ADR-0017** (Claude Code SDK as the agent runtime).

## Context

ADR-0001/0002/0004 together cover *collaboration governance* up to the
moment a teacher `@Claude`s: who may trigger an `AgentRun`, that the run
holds a project-scoped lock, that the group is a long-lived space. They stop
at `triggerAgent`. What the agent may do *after* the trigger — which files it
may read or write, what commands it may run — is not named in any ADR, not
modeled in `spec/`, and not enforced by any contract-level invariant.

The gap is not theoretical. ADR-0017 adopted the Claude Code SDK with
`allowedTools: ["Read", "Write", "Bash", "Glob", "Grep"]` and
`permissionMode: "bypassPermissions"`. Under that configuration the SDK's
built-in tools operate over the whole host filesystem with no confinement, and
`Bash` executes arbitrary shell with no approval gate. The Hub's
`hub/src/agent/workspace.ts` was *intended* to be "the agent's only file
surface" (its module doc says so) and ships a `confine()` path validator that
rejects `..` and absolute escapes. But ADR-0017's cutover left those custom
tools unwired — the SDK's built-in tools replaced them — so `confine()` is
dead code and the boundary it was meant to enforce does not exist at runtime.
Compounding this, the systemd unit's default `SERVICE_USER` is `root`
(`hub/deploy/install_service.sh`), so the unbounded agent runs as root.

This is a real divergence point by the constitution's test: "if unwritten,
will the developer and the agent make different assumptions?" They already
have. ADR-0017 assumed the boundary was an implementation concern; the
`workspace.ts` author assumed it was a contract. The next person to touch the
runner has nothing telling them the boundary must be enforced, and ADR-0017
silently overrode the one that existed.

## Decision

### The agent execution surface is bounded by the run's project workspace

During an `AgentRun`, every file operation the agent performs — read, write,
glob, grep target — must fall **within the run's project workspace directory**
(the on-disk engineering-file tree of ADR-0007). Operations whose target path
escapes that tree are refused. This is a platform core invariant, pinned in
`Spec.System.AgentSurface`.

The boundary is **per-run, anchored to the run's project**, the same scope as
the Lock (ADR-0002). This keeps the two invariants composable:

- `Lock` (ADR-0002) bounds **concurrency** — who may be modifying the project.
- `AgentSurface` (this ADR) bounds **blast radius** — what the agent may touch.

Both scope to `run × project`. Acquiring the lock and entering the surface
happen together at run start; releasing the lock and leaving the surface
happen together at run termination.

### Shell execution is subject to the same boundary, by effect

A shell command the agent runs executes with the workspace as its working
directory (already true for the `cph` subprocess, ADR-0016) and its **file
effects** — files read, written, created, or deleted — are subject to the same
workspace boundary. The agent may invoke **named external tools** whose
binaries live outside the workspace (notably `cph` via `CPH_BIN`); the
boundary governs *file effects*, not *binary location*. Arbitrary
`cat /etc/shadow` or `curl … > /etc/...` is out of bounds regardless of how it
is invoked.

### The Hub process runs as a non-privileged service user

The systemd unit must not run as `root`. The default in
`hub/deploy/install_service.sh` changes from `root` to a dedicated unprivileged
user. This is a deployment invariant, recorded here because the unbounded-agent
risk is compounded by root; it is enforced in the unit, not in `spec/` (it is
not a semantic divergence between developer and agent — it is operational
discipline).

### The mechanism: SDK sandbox (OS-level, per agent process)

The boundary is enforced by the Claude Code SDK's built-in sandbox
(bubblewrap plus `socat` on Linux, seatbelt on macOS), configured via `query()` options in
`hub/src/agent/runner.ts`:

- `sandbox.enabled: true` + `failIfUnavailable: true` — the agent process and
  its Bash subprocesses run sandboxed; if the sandbox can't start, `query()`
  emits an error and exits rather than running unsandboxed.
- `sandbox.allowUnsandboxedCommands: false` — a tool cannot opt out with the
  SDK's `dangerouslyDisableSandbox` input.
- `sandbox.filesystem.denyRead: ["/"]` with `allowRead` for the canonical
  current workspace and a small named system-runtime set — normal reads stay
  in the run's workspace while `/bin`, shared libraries, CA certificates,
  fonts and the configured `cph` executable remain available as the external
  tool exception described above.
- `sandbox.filesystem.allowWrite: [workspaceDir]` confines every write to the
  canonical ADR-0007 workspace. Config/cache/home stay beneath
  `.cph/agent-runtime/`. `TMPDIR`, `TMP`, and `TEMP` use the absolute
  workspace-local `.cph/t/` path so tools remain anchored after `cd`;
  `CLAUDE_CODE_TMPDIR` uses the short relative `.cph/t` prefix, resolved from
  the canonical workspace cwd, because the SDK's socat bridge otherwise falls
  back to a host temp directory when its Unix-socket prefix is too long.
  Root is denied for writes and only the canonical workspace is re-opened, so
  `/tmp`, `/var/tmp`, sibling projects and every other host path are rejected.
  There is no writable scratch exception outside the project directory.
- The SDK subprocess environment replaces rather than spreads `process.env`.
  Only provider protocol variables and non-secret runtime variables cross the
  boundary; database, Feishu and Hub session credentials never enter it.
  `sandbox.credentials` denies provider token variables inside Bash and denies
  configured credential files. `CPH_SANDBOX_EXTRA_DENY_READ` names additional
  deployment-specific secret paths on the Hub side only.
- `settingSources: []` and strict MCP configuration prevent an untrusted
  workspace or service-user config from widening tools, hooks, MCP servers, or
  sandbox paths.
- Network: open (see Open Questions).

`bypassPermissions` is kept (headless server — no interactive prompts); the
sandbox is the hard boundary, not the permission-prompt layer. This is
subprocess-level, not process-level: the Hub host process is not sandboxed
(it manages DB, Feishu ws, logs), only the Claude Code agent process and its
children. This keeps the sandbox config surface small and lets future
external CLIs (Gitea, Lark) run inside the sandbox by giving their binaries
read access, without widening the Hub's own filesystem surface.

The contract pins the **invariant** (operations stay in the workspace), not
the **mechanism**. The SDK sandbox is the current mechanism; switching to a
different one (tool wrappers, container) would not be a spec change as long
as it upholds `AgentFileOp.Authorized`.

## Consequences

- `Spec.System.AgentSurface` gains an `AgentFileOp` structure and an
  `Authorized` predicate mirroring `Memory.McpReadRequest.Authorized`: the op
  carries a `run` and a `path`; `Authorized` holds iff the path falls within
  the run's workspace, via platform-provided `runWorkspace` and `pathWithin`
  (representations `OPEN`).
- `hub/src/agent/workspace.ts`'s `confine()` intent is now contract-backed.
  Re-wiring it (or replacing it with an OS sandbox) is **mandatory**, not a
  preference. Its current dead-code status under ADR-0017 is a divergence to
  be addressed, not silently retained.
- ADR-0017's `bypassPermissions` + unrestricted built-in tools is a
  **divergence from this ADR** and is surfaced as such (constitution rule 4).
  The reconciliation — restrict the SDK tool set, wrap Bash, or sandbox at the
  OS level — is the open mechanism question above. Whichever is chosen must
  uphold the `AgentSurface` invariant.
- Inbound file/image delivery (`hub/src/feishu/trigger.ts`) lands files under
  `${workspaceDir}/.cph/inbox/`, which is inside the boundary — consistent, no
  change to the delivery path.
- The `cph` subprocess (ADR-0016) already runs with `cwd = workspaceDir`
  (`hub/src/agent/cph.ts`), so it is inside the boundary by construction.

## Open Questions / Deferred

- **Network egress.** Decided: network is **open** in the initial
  production deployment. The sandbox confines filesystem writes to the
  workspace and denies reads of sensitive host paths, but does not restrict
  outbound network. Rationale: future Gitea/Lark CLI integration needs
  unconstrained network egress; `cph build` may fetch typst packages. The
  exfiltration risk (agent `curl`s workspace content out) is accepted as a
  tradeoff for CLI integration simplicity. This is revisitable — a network
  allowlist (`sandbox.network.allowedDomains`) can be added without a spec
  change if the threat model tightens.
- **Named system-runtime read set.** The implementation allows only the
  platform runtime directories/files needed for shell, dynamic libraries,
  fonts, TLS/DNS and `cph` execution in addition to the project workspace.
  Expanding that set requires an explicit reviewed code change. Deployments
  name credential files with `CPH_SANDBOX_EXTRA_DENY_READ`; these paths are
  protected through the SDK credential layer and are not passed to the Agent
  environment. The exact runtime set remains plumbing and is not spec-pinned.
- **Shell command vocabulary.** Whether the agent's Bash is restricted to a
  whitelist (e.g. `cph`, `ls`, `grep`) or allowed arbitrary commands whose
  *effects* are then confined, is a policy choice left to implementation. The
  invariant governs effects either way; current choice is arbitrary commands
  (sandbox confines effects).

## Related Decisions

- **Rate limiting and token usage.** ADR-0022 requires multi-dimensional hard
  request limits while keeping token thresholds as attributed soft alerts in
  the initial release.
- **Inbound file limits.** ADR-0022 requires hard file, attachment, archive,
  Project-storage, and Organization-storage limits with explicit failure and
  partial-file cleanup.