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Timoteo 725b0a3330 feat: expose Fusion custom tools to ACP agents via loopback MCP bridge (#3476) 
## Summary

- ACP runtimes can expose Fusion custom tools (`fn_*`) to external
agents such as Hermes ACP and Prime.
- When the engine passes `customTools`, `AcpRuntimeAdapter` starts a
per-session loopback tool bridge and registers it as a stdio MCP server
in `session/new.mcpServers`.
- The bridge uses a per-session bearer token, binds only to loopback,
preserves the MCP request ID as `toolCallId`, propagates tool `isError`
results, and exposes only runnable non-built-in tools.
- The MCP shim supports `initialize`, `ping`, `tools/list`, and
`tools/call`, reporting transport, authentication, malformed-request,
and unknown-tool failures correctly.
- Bridge startup failures degrade gracefully: the ACP session remains
usable without custom tools and exposes a fixed `fusionToolBridgeError`
reason code for engine auditing.
- Startup, request, and teardown paths clean temporary schemas and
listeners; disposal aborts cooperative tools, bounds non-cooperative
drains, prevents post-dispose execution, and preserves the existing
synchronous session contract through an awaitable `disposePromise`.
- The packaged CLI stages `mcp-schema-server.cjs` beside the ACP runtime
distribution.

## Verification

Validated at source commit `2fa6fd2e26923ac6241d1b9379b3819f1832c079`:

- `pnpm --filter @fusion-plugin-examples/acp-runtime test` — 21 files /
251 tests passed
- `pnpm --filter @fusion-plugin-examples/acp-runtime typecheck` — passed
- Plugin build with `.cjs` asset copy — passed
- `pnpm check:changesets` — passed
- `git diff --check` — passed
- MCP shim smoke test — passed
- Greptile Review — passed
- CodeRabbit — passed
- Devin review — 0 bugs, 6 analyses, `errored_tasks=[]`, quality `4/10`

## Scope

ACP runtime plugin only. The Hermes CLI runtime remains untouched;
Hermes ACP is enabled by selecting the generic ACP runtime with
`acpBinaryPath: hermes` and `acpArgs: ["acp"]`.

---------

Co-authored-by: gsxdsm <gsxdsm@users.noreply.github.com>
2026-08-18 00:11:06 -07:00
..
src
feat: expose Fusion custom tools to ACP agents via loopback MCP bridge (#3476)
2026-08-18 00:11:06 -07:00
AGENTS.md
FN-6457: add ACP ask runner and bundled Claude bridge setup
2026-06-15 02:30:41 -07:00
CHANGELOG.md
chore(release): v0.77.0-beta.1
2026-08-16 08:39:57 -07:00
manifest.json
feat(acp): scaffold fusion-plugin-acp-runtime (U1)
2026-06-03 08:35:51 -07:00
package.json
feat: expose Fusion custom tools to ACP agents via loopback MCP bridge (#3476)
2026-08-18 00:11:06 -07:00
README.md
FN-6457: add ACP ask runner and bundled Claude bridge setup
2026-06-15 02:30:41 -07:00
tsconfig.json
feat(acp): scaffold fusion-plugin-acp-runtime (U1)
2026-06-03 08:35:51 -07:00
vitest.config.ts
FN-6042: raise dependency security floors
2026-06-08 15:19:27 -07:00

README.md

@fusion-plugin-examples/acp-runtime

A Fusion runtime plugin that drives any external Agent Client Protocol (ACP) agent over JSON-RPC/stdio. One integration unlocks every ACP-compatible agent (Gemini CLI, the Claude Code ACP adapter, and any future agent that speaks the protocol) through the standard protocol instead of a bespoke per-CLI integration.

Selected via runtimeId: "acp". Installed on demand (experimental) — see the Fusion plugin catalog (fn plugin install fusion-plugin-acp-runtime).

Security posture

The ACP agent is an untrusted subprocess that calls back into Fusion for permissions and filesystem access. This plugin enforces a defense-in-depth floor:

  • Per-category permission gating. Each session/request_permission is classified by tool kind into a Fusion action category and checked against the live permission policy — never a preset shortcut. allow_once only (never a persisted blanket grant). Unmappable kinds and missing policy default-deny.
  • Unrestricted-risk acknowledgement (acpAllowUnrestricted). Because the shipped default policy is unrestricted (allow-all), a blanket allow on a sensitive category is escalated to approval unless the user explicitly sets acpAllowUnrestricted: true. Prefer running the ACP runtime under an approval-required policy.
  • Filesystem jail. fs/read_text_file / fs/write_text_file are opt-in (acpFsRead / acpFsWrite, writes default OFF), confined to the session cwd by a real symlink-resolving jail (realpath + O_NOFOLLOW), with a deny-list for secrets (.env, *.pem, …) and git internals (.git/**). Writes are gated through the file_write_delete permission category.
  • Untrusted-input bounds. Streamed output is sanitized (ANSI/control strip) and bounded (per-turn + per-chunk caps; bounded tool-call correlation map).
  • Subprocess isolation. The agent env is built from an allow-list (acpEnvAllowList) — inherited process.env is not forwarded.

Not sandboxed in v1: the agent's own process/network syscalls run with Fusion's user privileges (OS-level sandboxing is recommended future work).

Settings

Key Default Meaning
acpBinaryPath acp-agent Agent binary to spawn
acpArgs [] Args that launch the agent in ACP/stdio mode (e.g. ["--acp"])
acpModel — Optional model identifier reported via describeModel
acpFsRead false Advertise/register fs/read_text_file
acpFsWrite false Advertise/register fs/write_text_file (gated)
acpEnvAllowList [] Env var names forwarded to the agent subprocess
acpAllowUnrestricted false Acknowledge the untrusted-agent risk under an allow-all policy

Upstream / third-party integration evidence

Per AGENTS.md (External-integration evidence):

Bundled Claude ACP bridge evidence

  • Canonical upstream repo URL: https://github.com/moabualruz/claude-code-cli-acp
  • Docs / homepage URL: https://github.com/moabualruz/claude-code-cli-acp#readme
  • Release / download URL: npm package claude-code-cli-acp (version 0.1.1) — https://www.npmjs.com/package/claude-code-cli-acp
  • Binary / CLI name: claude-code-cli-acp
  • Checksum: sha512-qpfRGOXkOs9mqI7oumsGistWisyXcCC0r7ng7wdLvGMIORdzHjmUUa+94Jftgr/NYAVnAUe6N7kimD8PaO3D5g== (from pnpm-lock.yaml for claude-code-cli-acp@0.1.1)
  • Pinned-commit spot-review: tag v0.1.1 points to commit c93f4f4ca449f451d9f3b7db536caf4060883da9 (annotated tag ca33404fc1128d6a88a55b248f042f70b4bc9f9a, unsigned). License: Apache-2.0. Behavior reviewed for this integration: runs claude through a PTY, reads transcript JSONL, exposes an ACP server over stdio, and requires @anthropic-ai/claude-code installed + authenticated.

See docs/acp-contract.md for the launch/readiness contract and failure taxonomy.

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