fusion/docs/solutions/architecture-patterns/acp-persistent-jsonrpc-agent-runtime-integration.md

category, module, date, problem_type, component, severity, applies_when, tags, related_components

category	module	date	problem_type	component	severity	applies_when	tags	related_components
architecture-patterns	fusion-plugin-acp-runtime	2026-06-03	architecture_pattern	tooling	high	Integrating a new external coding agent or agent protocol into Fusion Building anything that holds a long-lived bidirectional JSON-RPC peer over stdio Running an untrusted subprocess that can call back into Fusion (permissions, filesystem)	acp agent-client-protocol runtime-plugin json-rpc untrusted-subprocess security-floor path-jail	development_workflow testing_framework

Integrating a persistent bidirectional JSON-RPC agent (the ACP runtime pattern)

Context

plugins/fusion-plugin-acp-runtime (PR #1354, 2026-06) was the first integration in this codebase that holds a persistent, bidirectional JSON-RPC peer over stdio. Every prior agent integration (droid, cursor, hermes, openclaw, pi-claude-cli) is one-shot: write one NDJSON turn, read stdout, force-kill. ACP inverts part of the relationship — Fusion spawns the agent, but the agent calls back into Fusion mid-turn (session/request_permission, fs/read_text_file, fs/write_text_file). That makes the agent untrusted input on every channel, and several hard-won rules from this build will apply to any future integration with the same shape.

Guidance

1. The canonical integration shape is a runtime plugin, not a packages/*-cli package. packages/pi-claude-cli / droid-cli are legacy shims. New agent integrations live in plugins/fusion-plugin-<name>-runtime using @fusion/plugin-sdk's definePlugin with a runtime: { metadata: { runtimeId }, factory } block, implementing the AgentRuntime interface (createSession / promptWithFallback / describeModel / dispose). The engine resolves it via getRuntimeById(runtimeId) once installed.

2. Engine lifecycle truths (verified against the engine, not docs):

• The engine's AgentRuntimeOptions (packages/engine/src/agent-runtime.ts) already carries actionGateContext — populated at every run call site and funneled through createResolvedAgentSession. Consume it structurally via a narrow plugin-local interface; never import @fusion/engine and never add a parallel permission channel to the shared contract.
• There is no AbortSignal in the runtime contract. Teardown enters via an unawaited synchronous dispose() (StuckTaskDetector, executor timeout) plus the process-registry kill. Design teardown so the registry SIGKILL is the authoritative no-orphan/no-deadlock guarantee; any graceful protocol cancel (session/cancel + pending-request drain) is opportunistic. Register process.on("exit", killAllProcesses).
• Bundling is not automatic: a new runtime plugin must be added to RUNTIME_PLUGIN_IDS in packages/cli/tsup.config.ts (or it silently never ships) and to an install list (BUILTIN_PLUGINS for on-demand, BUNDLED_PLUGIN_IDS for auto-install), plus pnpm-workspace.yaml.

3. Security floor for an untrusted callback-capable subprocess:

• Per-category permission gating, never per-preset. The shipped default policy preset is unrestricted (every category → allow). A preset-level shortcut auto-approves everything the moment the runtime is selected. Classify each call's kind into a category and read permissionPolicy.rules[category]; add an explicit acknowledgement setting before honoring blanket allows on sensitive categories.
• Select allow_once only — never allow_always/reject_always (a persisted grant inside untrusted code loses per-call interception). Unmappable/missing kinds, missing gate/policy, and HITL-without-a-readable-decision all default-deny. Require both pauseForApproval AND findApprovalByDedupeKey before creating an approval request — otherwise a human approval is silently discarded and a pending record is orphaned.
• Filesystem jail = realpath, not string checks. project-root-guard.ts is a suffix check, not a jail. Use realpath-within-realpath(cwd), lstat the final component for new files, O_NOFOLLOW open, and truncate only after post-open re-validation (passing O_TRUNC into open() truncates an escaped target before validation — write-path TOCTOU). Deny-list secrets and .git/** by basename regardless of cwd membership. Stat-gate reads (a full readFile before a byte ceiling is an OOM vector).
• Bound everything the agent emits, including the channels that don't look like output: per-turn + per-chunk caps on text/thinking, ANSI/control stripping, bounded identifier lengths and correlation maps, and plan/structured events (entry size was bounded but entry count wasn't — 1,000 × 64KB entries bypassed the per-turn budget). Redact stderr across chunk boundaries, not per-chunk (secrets split across data events evade per-chunk regexes). Build the subprocess env from an allow-list, never inherited process.env.

4. Per-turn bridge state must actually reset per turn. Anything accumulated per "turn" (output budgets, cap-flag latches, tool-call correlation maps) needs an explicit reset() invoked at the top of each prompt — a latch that never resets silently suppresses all output for the rest of the session after one flood. Write a two-turns-through-the-same-handler test; single-turn tests cannot catch it.

5. The installed SDK's types are authoritative over docs/research. Verify a young SDK's exports with a smoke-import test at scaffold time (a missing export is a day-one blocker, not a late surprise), and read the generated .d.ts for shapes: research/docs said session/update used content_chunk/tool_call_started; the real SDK uses agent_message_chunk/tool_call/tool_call_update, and plan_update carries a plan field, not entries (the wrong-shape cast silently no-op'd).

6. Test fixtures for bidirectional protocols must be race-proof. JSON-RPC notifications are dispatched concurrently with suspended request handlers. A fixture that registers a cancellable hang after an awaited write loses the race on loaded CI runners (cancel lands first, no-ops, prompt hangs forever). Record a pending-cancel flag so resolution is order-independent — never rely on a setImmediate tick for ordering.

Why This Matters

The one-shot integrations never needed any of this: they hold no server→client channel, no long-lived session state, and their kill-after-turn lifecycle hides teardown bugs. A bidirectional peer fails in new ways — permission deadlocks on cancel, latched per-session state, TOCTOU in callback-served filesystem access, budget bypasses through structured events — and three of those shipped as P1s caught only by adversarial review, not by 170+ passing unit tests. The next protocol-shaped integration (MCP-server hosting, a future agent protocol) inherits this checklist instead of rediscovering it.

When to Apply

• Adding any new agent runtime to Fusion (use the plugin shape + wiring checklist in §1–2).
• Any subprocess that can call back into Fusion — apply the full §3 security floor, not just spawn hardening.
• Any streaming bridge with per-turn accounting (§4) or any young/pinned SDK dependency (§5).
• Writing test fixtures for request/notification protocols (§6).

Examples

Truncate-after-validate (write-path TOCTOU, path-jail.ts / fs-capabilities.ts):

// WRONG: O_TRUNC truncates an escaped target BEFORE re-validation
const h = await open(p, O_WRONLY | O_CREAT | O_TRUNC | O_NOFOLLOW);

// RIGHT: open without truncate, re-validate realpath, then truncate via the fd
const h = await openWithinCwd(p, cwd, O_WRONLY | O_CREAT); // re-validates inside
await h.truncate(0);

Per-category floor, never preset (control-handler.ts):

// WRONG: preset shortcut — default preset is `unrestricted` ⇒ auto-approve everything
if (policy.preset === "unrestricted") return allow();

// RIGHT: classify the call, read the category rule, escalate blanket allows
const category = classifyToolKind(toolCall.kind);            // unmappable → DENY
const disposition = effectiveDisposition(category, gate, {   // allow on sensitive
  allowUnrestricted,                                         // category escalates to
});                                                          // approval unless acked

Reference implementation: plugins/fusion-plugin-acp-runtime/ (184 tests), plan with full rationale at docs/plans/2026-06-02-002-feat-acp-client-integration-plan.md, contract at docs/acp-contract.md.