feat(a4): ACP stdio JSON-RPC dispatcher (Zed/JetBrains/Neovim agent server)

[ravituringworks]

Summary

Wave 5 / A4 of v13 fitgap §16.1 — VibeCLI as an Agent Client Protocol server speaking Zed's ACP v0.11+ (JSON-RPC 2.0 over stdio). Once Phase 54 ships and the daemon registers in the live ACP Registry (built into Zed + JetBrains in Q1 2026), Zed/JetBrains/Neovim users discover VibeCLI natively alongside Claude Code, Codex CLI, GitHub Copilot CLI, OpenCode, Gemini CLI.

This PR is distinct from the existing acp.rs HTTP surface — that's an older HTTP/SSE shape. Zed's ACP is JSON-RPC over stdio, similar in spirit to LSP.

Scope

In: the dispatcher half — JSON-RPC envelope handling + the five always-on methods that hosts call during initialization and session lifecycle.

Out (intentional, sized for one PR):

• prompt + sessionUpdate notifications — full agent loop integration with provider / agent_runtime. The dispatcher recognises prompt and rejects it with METHOD_NOT_IMPLEMENTED (-32001) rather than METHOD_NOT_FOUND, so hosts see partial-impl honestly.
• Stdin/stdout plumbing — a thin shell over dispatch that lives in the CLI subcommand or a separate binary. Isolated from the dispatcher logic; trivial follow-up.
• Live registration in the ACP Registry — external step once the wiring above lands.

Implementation

stdin (line-delimited JSON)
    ↓
parse_request → AcpRequest | AcpResponse(parse_error)
    ↓                                ↓
AcpServer::dispatch                stdout
    ↓
run_method ─┬─ "initialize"      → handle_initialize
            ├─ "authenticate"    → handle_authenticate
            ├─ "newSession"      → handle_new_session
            ├─ "loadSession"     → handle_load_session   (or SESSION_NOT_FOUND)
            ├─ "cancel"          → handle_cancel
            ├─ "prompt"          → MethodNotImplemented (recognised, not yet shipped)
            ├─ "setSessionMode"  → MethodNotImplemented
            └─ ...               → MethodNotFound
    ↓
AcpResponse (Ok | Err) → stdout

AcpServer holds session state in Arc<Mutex<...>> so handlers can be cloned across threads without lifetime gymnastics. Sessions are in-memory for this slice; persistence ties into crate::session_store in a follow-up.

JSON-RPC error code mapping

Variant	Code	When
MethodNotFound	-32601	Unknown method name
MethodNotImplemented	-32001 (ACP ext)	Recognised method that's not yet shipped
SessionNotFound	-32002 (ACP ext)	Well-formed sessionId that doesn't exist
InvalidParams	-32602	Method called with wrong shape
Internal	-32603	Lock poisoned, etc.
ParseError	-32700	Invalid JSON; carries id: null
InvalidRequest	-32600	jsonrpc field != "2.0"

The two ACP extensions (-32001, -32002) are in the JSON-RPC implementation-defined range (-32000 to -32099) and don't conflict with any standard JSON-RPC error.

Tests (red → green)

6 BDD scenarios in acp_stdio::tests, all committed before any impl in the red commit:

#	Scenario	What it pins
1	initialize returns protocolVersion + agentCapabilities + serverInfo.name = "vibecli"	Handshake shape
2	Two newSession calls produce unique ids; session_count grows	Sessions are tracked
3	loadSession on unknown id → SESSION_NOT_FOUND (-32002)	ACP extension code, error message
4	Unknown method → METHOD_NOT_FOUND (-32601)	Standard JSON-RPC error
5	Malformed JSON via parse_request → response with code: -32700, id: null	Parse-error envelope
6	jsonrpc: "1.0" → INVALID_REQUEST (-32600)	Version validation

Tests run in milliseconds — no Rust workspace cold-compile required.

Test plan

• cargo test -p vibecli --lib acp_stdio::tests — all 6 scenarios pass
• cargo check -p vibecli clean
• Manual: pipe a JSON-RPC initialize request to the eventual vibecli acp serve subcommand, observe the v0.11.0 handshake response

Patent-distance posture

A4 implements the public Zed Agent Client Protocol specification (open, on agent-client-protocol.org). No UX surface, no overlap with Cursor / Copilot patents. JSON-RPC 2.0 itself is decades old and unencumbered.

Cross-cutting invariants

• ✅ Module declaration — pub mod acp_stdio; in lib.rs, mod acp_stdio; in main.rs per CLAUDE.md.
• ✅ No new deps — workspace serde + serde_json + anyhow are already pulled.
• ✅ Distinct from existing acp.rs — that's the HTTP/SSE ACP surface; this is the stdio JSON-RPC one. Both names start with acp_ to keep them adjacent in the module list.

🤖 Generated with Claude Code