# Mosaic runtime adapters

An adapter is the entire harness-specific surface of the system. Everything
upstream of an adapter — configuration, contracts, missions, tasks, run
records — is harness-agnostic; everything inside an adapter may assume one
specific agent runtime.

## Contract

An adapter lives at:

```text
/opt/mosaic/adapters/<name>/adapter.sh
```

and must be executable. The dispatcher (`/opt/mosaic/src/run-agent.sh`)
selects it via `MOSAIC_ADAPTER` (default: `pi`) and execs it after the
system prompt has been generated.

**Inputs (environment):**

| Variable | Meaning |
|---|---|
| `MOSAIC_SYSTEM_PROMPT_FILE` | Absolute path to the generated system prompt (contracts + optional mission section). Read it; do not modify it. |
| `MOSAIC_REQUEST` | The exact user request text (may contain newlines). |
| `MOSAIC_PROVIDER` | Configured provider name. |
| `MOSAIC_MODEL` | Configured model id. |

Optional, adapter-specific (documented per adapter):

| Variable | Meaning |
|---|---|
| `MOSAIC_MOCK_RESPONSE` | mock only: the verbatim response to emit |

**Outputs:**

- `stdout`: the model response text — the only channel the orchestrator captures
- `stderr`: diagnostics (never credentials)
- exit `0`: success; nonzero: failure

## Rules

1. Adapters print ONLY the response on stdout. Status lines go to stderr.
2. Adapters never read configuration files; the resolved settings arrive via environment.
3. Adapters never write outside `/var/lib/mosaic`.
4. Adding a worker adapter requires: a new directory, the contract
   implementation, and adding the name to the allowlist in
   `scripts/mosaic-config.mjs`. A managed-session adapter (below) is selected
   by the launch bundle, not by `execution.adapter`, and is not on that
   allowlist unless it also works as a worker adapter.

## Included adapters

- `pi` — the pinned `@earendil-works/pi-coding-agent` CLI in noninteractive
  print mode (`-p`), ambient discovery disabled, stdin detached.
- `mock` — deterministic echo of `MOSAIC_MOCK_RESPONSE`. Test-only: never use
  it where a real model response is required.
- `claude` — the host's `claude` CLI, for managed sessions only (below).
  Not a worker adapter: the image has no `claude`, and the adapter refuses
  without the bundle's hook and MCP files.

## Managed sessions (slice 1 S6)

The session runner (`packages/harness/src/runner.mjs`) runs one adapter
call per turn on the host, as `/bin/sh <adapters/<name>/adapter.sh>` (the
repository keeps adapters 0644; only the image sets the mode), in the
session's workspace and its own process group. Same contract, plus:

| Variable | Meaning |
|---|---|
| `MOSAIC_WORKSPACE` | The session's workspace; the adapter runs there. |
| `MOSAIC_SESSION_DIR` | The session's persistent directory; later turns resume the session kept there. |
| `MOSAIC_TOOLS` | The built-in tool limit (pi names for `pi`, Claude Code names for `claude`), comma-separated. |
| `MOSAIC_POLICY_FILE`, `MOSAIC_TOOLS_FILE` | The bundle's gate policy and typed tools. |
| `MOSAIC_TOOL_SOCKET` | The runner's tool socket, which the typed tools call. |
| `MOSAIC_TURN_MARKER` | `pi`: the extension writes it at `agent_end`; a pi turn that exits 0 without it failed. |
| `MOSAIC_EXTENSIONS` | `pi` only: extension files loaded with `-e`; a missing one exits 2. |
| `MOSAIC_CLAUDE_SETTINGS`, `MOSAIC_CLAUDE_MCP_CONFIG` | `claude` only: the bundle's gate hook and MCP server; required. |

The layers each adapter relies on, and what they don't cover, are in
`packages/harness/README.md`.