stack/adapters

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:

/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): MOSAIC_CLAUDE_BIN when set, else claude on PATH. 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. The pi module names two: the gate and typed tools extension, then the unslop check's tools/unslop-hook/extension.ts (row 66).
MOSAIC_UNSLOP_LOG	The unslop check's JSONL log, <run>/unslop.jsonl, which the runner sets for both adapters and reads after each turn (row 66).
MOSAIC_CLAUDE_SETTINGS, MOSAIC_CLAUDE_MCP_CONFIG	claude only: the bundle's gate hook and MCP server; required.
MOSAIC_THINKING	The route's thinking level (row 65), which the harness module's levels() already checked. pi passes --thinking (off, minimal, low, medium, high, xhigh, max) and omits it when unset, as for run-task.sh workers. claude passes --effort (low, medium, high, xhigh, max) and requires it; unset exits 2.
MOSAIC_CLAUDE_BIN	claude only: absolute path of the CLI to run, from the host's environment (the bus unit pins it). Unset or empty runs claude from PATH; a relative path or a non-executable file exits 2.

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

A role's skills (row 64)

A managed session's skills reach both adapters as text in the bundle's prompt.md, which they pass as a file: pi with --system-prompt <path> and claude with --system-prompt-file <path> (row 213). Neither adapter loads a skill of its own: pi keeps --no-skills, since role sessions never get MOSAIC_SKILLS, and claude keeps --disable-slash-commands and --restricted. Sage chose this on 2026-10-19 from a probe, recorded in agents/sage/work/queue-64/probe.md:

• Pi's --skill puts only a skill's name and description in the prompt, and the model has to read the SKILL.md itself. The gate refuses every file-tool path outside the workspace, so the session couldn't open its own skills. Opening the gate to the bundle would change a security boundary.
• Claude Code loads a bundle-local --plugin-dir skill under --restricted only with its Skill tool on, and then it also offers its own bundled skills. Which ones it offers changes with the Claude Code version, and the Skill tool would pass through the gate hook.

The cost is prompt size: a role's skill text is in every turn, where the other routes load a skill when it's used. The bundle caps a role's skills at 64 KiB together. The cap dates from when claude passed prompt.md as one argument, which Linux caps at 128 KiB; row 213 passes the file instead, so no local user reads the prompt in /proc/<pid>/cmdline. Pi 0.85.1 uses the path string itself as the prompt when the file is missing or can't be read, so the pi adapter exits 2 unless the path is a readable regular file.

The unslop check (row 66)

Both adapters run the mechanical writing check from tools/unslop-hook, driven by one lists.json. pi loads it as its second -e extension. claude runs claude-hook.js from the bundle's settings.json: the Stop hook checks the turn's replies, and the UserPromptSubmit hook adds the correction notice as context on the next turn when the last reply had tells. Each hook command ends ; exit 0, so neither blocks. Both log to MOSAIC_UNSLOP_LOG, and the runner adds a note for the operator to the next RESULT it sends (packages/harness/README.md, the runner). The harness modules mount the three files read-only at their own paths, and the manifest records their digests. MOSAIC_UNSLOP_HOOK and UNSLOP_LISTS aren't in the session environment's allowlist, so a managed session can't switch the check off or swap its lists.