# Turn loop + MCP

How the harness wakes up, what it asks claude to do, and what tools
claude has access to in return.

## The loop

Each agent harness (`hive-agent` — one serve-loop binary for all
agents) runs:

0. Check the pause marker (`<harness>/paused`). While it exists the
   loop does nothing but re-stat it every 5 s — no broker poll, no
   claude process. Because step 1 is never reached, messages stay
   queued and unacked, so a resume drains the backlog instead of
   losing it; reminders and todo wakes buffer in their channels. Set
   it with `hivectl agent <name> pause` or the dashboard toggle; see
   [persistence](../persistence.md#-harnesspaused-per-agent).
1. Long-poll `Recv` on its socket. The host-side broker
   (`broker.rs::recv_blocking_batch`) returns immediately if there's
   a pending message, otherwise waits up to 30 s for a broker `Sent`
   event for this recipient.
2. Pop one message. Peek the remaining inbox depth with `Status`.
3. Emit `LiveEvent::TurnStart { from, body, unread }` onto the SSE
   bus.
4. Spawn claude (one process per turn) and pipe the wake prompt
   over stdin.
5. Stream stdout (JSON lines) into the bus as
   `LiveEvent::Stream(value)`. Pump stderr as `Note`.
6. Wait for claude to exit and classify the turn's outcome from the
   stream + exit — success, compaction, rate-limit, auth-failure, or
   hard failure. The outcome drives the post-turn action (see
   [Turn outcomes](#turn-outcomes)); compaction is handled inside the
   session (see
   [Compaction](claude-invocation.md#compaction)). Rate-limit
   and auth-failure detection is described [below](#failure-detection-and-login).
7. Emit `LiveEvent::TurnEnd { ok, note }`. Sleep `poll_ms` to avoid
   tight loops on transient failures.

### Failure detection and login

- **Rate limit** — a `429` / `rate_limit` marker on stderr, or a parsed
  `{"type":"error"}` rate-limit event on stdout (conversation-text
  mentions don't count), sets the `rate_limited` sentinel, parks for
  `HIVE_RATE_LIMIT_SLEEP_SECS` (default 300), then retries. The UI shows
  a `⊘ rate limited` badge while parked.
- **Auth failure (401)** — `drive_turn` retries once (transient
  token-refresh races clear on retry); a second `AuthFailed` writes
  `{state_dir}/hyperhive-needs-login`, requeues the message, and parks in
  `wait_for_login` — the same path as a cold boot with no session. The
  operator re-auths via the per-agent web UI; the queued message then
  drives the next turn.
- **Login detection** — both boot (`login::has_session`, Online vs
  NeedsLogin) and `wait_for_login`'s resume check key off the credential
  files in `login::CRED_FILE_NAMES` (the set `/logout` deletes).
  `wait_for_login` takes a `since: SystemTime` baseline (the instant of
  the 401 that parked it, or `login::NO_PRIOR_FAILURE` at cold boot) and
  resumes only once a credential file's mtime postdates it — so stale
  credentials already on disk at the 401 don't trigger an instant
  false-resume, and a login that lands *before* `wait_for_login` even
  starts polling still resumes correctly (baselining on a fixed instant
  rather than an entry-time directory snapshot is what closes that race).
  Leftover session-history files still don't read as a live session after
  a logout + container recreate (`has_session`/`is_cred_file` scope to
  `CRED_FILE_NAMES` either way).

## Harness binary shape

Two sibling binaries out of the one `hive-ag3nt` crate, all
role-agnostic. (The earlier split into `hive-ag3nt` + `hive-m1nd`
was collapsed because the privilege boundary lives server-side at
the broker socket (`/run/hive/mcp.sock`): `ManagerRequest` calls are
refused by the standard agent socket regardless of who sends them.)

- `hive-agent` — long-running harness loop (the inbox poll +
  claude-pump + ack/requeue cycle described above).
- `hive-agent-mcp` — MCP server for the built-in `hyperhive` surface.
  Run with `--http <addr>` as a persistent streamable-HTTP daemon (the
  `hive-mcp-http` systemd unit, on `hyperhive.mcp.httpPort`, default
  8790); claude connects to its URL via `--mcp-config`. HTTP is the sole
  transport — no per-turn stdio child (eliminates the re-registration
  race).

### `Surface` trait + zero-sized type tags

`AgentRequest` / `AgentResponse` (= `ManagerRequest` / `ManagerResponse` —
type aliases) are the wire types. There is one role: agent.
`bin/hive-agent.rs` factors the turn loop through a `Surface` trait
with one zero-sized impl (`AgentSurface`) wrapping:

- One async method per wire op: `ack_turn`, `requeue_inflight`,
  `inbox_unread`, `post_turn_counts`, `send_to_parent`, `recv_next`.

`main()` calls `serve_main::<AgentSurface>` for all roles. The turn
loop (`serve_loop` / `handle_turn`) has no per-role branches.

### Boot wiring

`serve_main` reads `HIVE_PORT` (default `DEFAULT_WEB_PORT`) +
`HIVE_LABEL` (default `"hive"` for standalone runs; the meta
flake sets it unconditionally for any container-deployed agent;
see `docs/conventions.md::Hive identity` for the env stack),
opens turn-stats sqlite, prepares the on-boot files (see
[claude-invocation](claude-invocation.md#on-boot-files)),
installs claude plugins, spawns `web_ui::serve` + `vacuum::run`,
and either drops into `serve_loop` directly (`Online`) or parks on
the login flow first (`NeedsLogin`). (The forge notification poller
used to be spawned here too; it is its own process now —
`hive-forge-notify`, see [`forge.md`](../forge.md).)

`spawn_todo_socket` opens the todos store and the socket the in-container
producers dial. Matrix / bash / forge-notify daemons and the in-process
`disk_watch` todo producer (low state-disk space) are the built-in
producers, but the socket accepts any `subsystem` marker — a
user-configured MCP server can push its own todos the same way. See
[`docs/persistence.md`](../persistence.md#state-dirs-per-agent) for
what each built-in todo producer watches and how the store + `get_loose_ends`
merge work.

Plugin install failures are not fatal: each entry comes back as a
human-readable failure string that gets routed via
`Surface::send_to_parent` to the agent's topology parent (the
broker resolves `<parent>` per `topology::parent_of`; root agents
and the manager fall through to operator).

### Turn outcomes

`turn::TurnOutcome` (`Result<bool, TurnError>` — `Ok(compacted)` on success,
else a `TurnError`) drives the post-claude branch:

| Outcome | Action |
| --- | --- |
| `Ok(_)` (`false` normal / `true` compacted) | `ack_turn` |
| `Err(PromptTooLong)` | `drive_turn` archived the session (the lib already compacted + retried and it still overflowed); requeue inflight so the message redelivers into a fresh session that fits — no status park |
| `Err(RateLimited)` | sleep `HIVE_RATE_LIMIT_SLEEP_SECS` (default 300), requeue inflight, status back to `online` |
| `Err(AuthFailed)` | emit `needs_login_idle` sentinel, requeue inflight, park in `wait_for_login` |
| `Err(SessionNotFound)` | resume + create self-heal both missed ("shouldn't happen"); requeue inflight so the next turn creates fresh — no status park, message not dropped |
| `Err(ApiStall)` | idle watchdog killed claude after `HIVE_TURN_IDLE_SECS` (default 600) of output silence; sleep `HIVE_STALL_SLEEP_SECS` (default 60), requeue inflight, status back to `online` |
| `Err(Failed(err))` | route `[system] \`<qualified-label>\` claude turn failed:\n<err>` to `<parent>` via `send_to_parent` |

`ApiStall` catches an Anthropic API stall — a multi-retry connection storm where
the stream goes silent for minutes. The idle watchdog lives in `hive-claude`'s
driver (`Config::idle_timeout`, enforced around `child.wait()`): the timer
resets on every stdout line, so a large but still-streaming turn is never cut —
only complete output silence for the window trips it. The harness sets the
window from `HIVE_TURN_IDLE_SECS` (`0` disables) and maps the driver's
`Error::IdleTimeout` onto `TurnError::ApiStall`.

After the outcome handler, the stats sink records a row. `handle_turn`
reports the result to `serve_loop` via `TurnControl { auth_failed }` —
on auth failure the loop parks in `wait_for_login`; otherwise it loops
straight back to the idle wait (step 1). There is no same-turn
self-continue mechanism: every multi-step continuation rides an
external wake instead — a new inbox message, a `remind`, or an
in-container todo wake (bash-task completion, forge notification,
matrix activity). Ending the turn and letting one of
those drive the next one is strictly better than parking in-process:
it checkpoints the session and observes wakes that only reach the
harness between turns.

## Sub-pages

The rest lives alongside this page, in three topic files:

- **[claude-invocation.md](claude-invocation.md)** — how the harness
  spawns `claude --print` each turn, the two-pronged compaction (reactive +
  proactive), and the on-boot files it materialises (`--mcp-config`,
  `--system-prompt-file`).
- **[config.md](config.md)** — the optional per-agent knobs the meta
  flake wires in (reference docs, icon, passwordless sudo, dashboard links,
  custom static files, connectivity overrides, claude plugins, cargo message
  filtering).
- **[mcp.md](mcp.md)** — the MCP tool surface claude sees: core tools,
  privileged tool groups, self-wake, authoritative state, the tool envelope,
  and the built-in tool whitelist.

Per-subsystem impl detail lives in each module's `//!` doc-comment; these pages
describe present-state behaviour + wiring, not line-level mechanics.