# hive-runtime

The layer an agent's turns are driven through: one `Runtime` interface
(`run`, `compact`, `archive`, `canceller`, `choices`) with a backend per runtime.

- **claude** — `claude --print` through the `hive-claude` crate's
  `InfiniteSession`. A pass-through: same spawn, same session handling, same
  errors.
- **acp** — any [Agent Client Protocol](https://agentclientprotocol.com)
  agent, spawned from a command, args and env handed to it (`RuntimeSpec`,
  read from `HIVE_RUNTIME` / `HIVE_ACP_COMMAND` / `HIVE_ACP_ARGS` /
  `HIVE_ACP_ENV`). It knows no agent by name; which agent runs, and how it is
  configured, is decided in nix (`services.hyperhive.agent.runtime`,
  `services.hyperhive.agent.acp.*`).

Both backends report a turn through `hive_claude::Sink` in claude's
`stream-json` shape. The ACP backend translates `session/update`
notifications into it (text and thought chunks as whole blocks, tool calls as
`tool_use` + `tool_result`, MCP tools named `mcp__<server>__<tool>`), so the
harness's stream consumers read either backend unchanged. Context usage comes
from ACP `usage_update`.

The crate depends on no hyperhive binary crate, so `hive-agent` and
`hive-subagent-mcp` can both drive turns through it.

## ACP backend: what it needs from the agent

- `mcpCapabilities.http` in its `initialize` response. The hyperhive tools
  are only served over HTTP, so an agent without it is refused at startup.
- `loadSession`, to pick its session back up after a harness restart.
  Without it every restart starts a new session.

## ACP backend: stopping a turn

A turn is stopped with `session/cancel`: by the `Canceller` handle, or by the
idle watchdog once no `session/update` has arrived for
`Config::idle_timeout`. An agent that has not answered the prompt 10s later
is killed, and the next turn respawns it. A watchdog stop fails the turn
with `AcpError::IdleTimeout` (`IdleKilled` if the agent was killed). This is
the only way a provider error the agent retries without reporting it, such as
an HTTP 429, ends the turn.

## ACP backend: compaction

The same `CompactionPolicy` as the claude backend decides when: after a turn
past the watermark, or on `compact` (the operator's `/compact`, the agent's
`compact` tool).

- If the agent advertises a `compact` command (`available_commands_update`),
  it is sent as the prompt `/compact` on the same session, as the ACP spec
  runs any advertised command. A proactive compaction runs the policy's
  checkpoint turn first, as on claude.
- Otherwise the policy's checkpoint turn runs, then the session is archived,
  and the next turn starts a new one, carrying the system prompt again.
- A `compact` command that fails, or sends nothing for 180s (the turn's idle
  window if that is shorter), is cancelled and handled as the case above; the
  checkpoint turn is not run a second time. So a compaction always leaves a
  smaller session behind, and a failed one is not retried on the next turn.

## ACP backend: model and effort

Before each prompt, `Config::model` and then `Config::effort` are set on the
session with `session/set_config_option`, as the value of its first `model`
and `thought_level` config option. A value the session does not offer, or
already holds, is not sent, so a claude alias such as `haiku` does nothing
on an ACP agent. Model goes first because the effort levels on offer depend
on it. The `Choices` handle (`Runtime::choices`) reads what the session
offers now, including after a `config_option_update`. It is empty until a
session is attached.