P13-T1: Design persistent broker architecture and protocol contract

SPECS/ARCHIVE/INDEX.md

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 # mcpbridge-wrapper Tasks Archive
 
-**Last Updated:** 2026-02-16
+**Last Updated:** 2026-02-16 (P13-T1)
 
 ## Archived Tasks
 
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 | FU-P12-T2-1 | [FU-P12-T2-1_Fix_stacking_click_listeners_in_updateLatencyTable/](FU-P12-T2-1_Fix_stacking_click_listeners_in_updateLatencyTable/) | 2026-02-16 | PASS |
 | FU-P13-T7 | [FU-P13-T7_Fix_structuredContent_compliance_for_empty_content_tool_results/](FU-P13-T7_Fix_structuredContent_compliance_for_empty_content_tool_results/) | 2026-02-16 | PASS |
 | FU-P13-T8 | [FU-P13-T8_Prevent_Web_UI_port_collision_from_destabilizing_MCP_sessions/](FU-P13-T8_Prevent_Web_UI_port_collision_from_destabilizing_MCP_sessions/) | 2026-02-16 | PASS |
+| P13-T1 | [P13-T1_Design_persistent_broker_architecture_and_protocol_contract/](P13-T1_Design_persistent_broker_architecture_and_protocol_contract/) | 2026-02-16 | PASS |
 
 ## Historical Artifacts
 
@@ -171,6 +172,7 @@
 | [REVIEW_FU-P12-T2-1_stacking_click_listeners.md](_Historical/REVIEW_FU-P12-T2-1_stacking_click_listeners.md) | Review report for FU-P12-T2-1 |
 | [REVIEW_FU-P13-T7_structuredcontent_compliance.md](_Historical/REVIEW_FU-P13-T7_structuredcontent_compliance.md) | Review report for FU-P13-T7 |
 | [REVIEW_FU-P13-T8_web_ui_port_collision.md](_Historical/REVIEW_FU-P13-T8_web_ui_port_collision.md) | Review report for FU-P13-T8 |
+| [REVIEW_P13-T1_broker_architecture.md](_Historical/REVIEW_P13-T1_broker_architecture.md) | Review report for P13-T1 |
 
 ## Archive Log
 
@@ -296,3 +298,5 @@
 | 2026-02-16 | FU-P13-T7 | Archived REVIEW_FU-P13-T7_structuredcontent_compliance report |
 | 2026-02-16 | FU-P13-T8 | Archived Prevent_Web_UI_port_collision_from_destabilizing_MCP_sessions (PASS) |
 | 2026-02-16 | FU-P13-T8 | Archived REVIEW_FU-P13-T8_web_ui_port_collision report |
+| 2026-02-16 | P13-T1 | Archived Design_persistent_broker_architecture_and_protocol_contract (PASS) |
+| 2026-02-16 | P13-T1 | Archived REVIEW_P13-T1_broker_architecture report |

SPECS/ARCHIVE/P13-T1_Design_persistent_broker_architecture_and_protocol_contract/P13-T1_Design_persistent_broker_architecture_and_protocol_contract.md

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+# P13-T1: Design Persistent Broker Architecture and Protocol Contract
+
+**Phase:** 13 — Persistent Broker & Shared Xcode Session
+**Priority:** P0
+**Status:** In Progress
+**Branch:** feature/P13-T1-persistent-broker-architecture
+**Created:** 2026-02-16
+
+---
+
+## 1. Objective
+
+Define the architecture, transport contract, and module scaffold for a long-lived broker process that owns a single `xcrun mcpbridge` upstream connection and multiplexes multiple MCP clients through it.
+
+This is a **design-only** task. The deliverables are:
+
+1. `SPECS/ARCHIVE/P13-T1_.../broker_architecture_spec.md` — lifecycle and sequence diagrams
+2. `SPECS/ARCHIVE/P13-T1_.../adr_broker_transport_security.md` — ADR (transport + security)
+3. `src/mcpbridge_wrapper/broker/__init__.py` — initial module scaffold (stubs only, no logic)
+4. `src/mcpbridge_wrapper/broker/daemon.py` — stub
+5. `src/mcpbridge_wrapper/broker/transport.py` — stub
+6. `src/mcpbridge_wrapper/broker/proxy.py` — stub
+7. `src/mcpbridge_wrapper/broker/types.py` — shared type definitions
+
+---
+
+## 2. Background
+
+Currently every MCP client that connects spawns a fresh `xcrun mcpbridge` subprocess. This causes:
+- Repeated Xcode permission prompts (BUG-T4)
+- Higher startup latency per session
+- Process accumulation under load
+
+A persistent broker resolves these by owning one upstream subprocess and serving N concurrent clients via a local transport.
+
+---
+
+## 3. Architecture Specification
+
+### 3.1 Component Roles
+
+| Component | Role |
+|-----------|------|
+| **Broker Daemon** | Long-lived process, owns `xcrun mcpbridge` subprocess, accepts local client connections |
+| **Upstream Bridge** | Single `xcrun mcpbridge` process, its stdin/stdout are owned exclusively by the Broker |
+| **Client Proxy** | Short-lived per-MCP-client subprocess; connects to Broker via Unix socket, bridges client stdio ↔ Broker |
+| **Web UI** | Continues attaching to Broker process (reuse existing mechanism) |
+
+### 3.2 Lifecycle States
+
+```
+Broker Daemon
+
+   INIT
+    │   Create socket file (mode 0600)
+    │   Write PID lock file
+    │   Launch xcrun mcpbridge upstream
+    ▼
+  READY ──────────────────────────────────────────┐
+    │   Accept client connections                  │
+    │   Forward requests to upstream               │ client disconnect
+    │   Route responses back to originating client │ (no effect on upstream)
+    │                                              │
+    ▼                                              │
+  BUSY ◄──────────────────────────────────────────┘
+    │
+    │  upstream crash/EOF
+    ▼
+  RECONNECTING
+    │   Relaunch xcrun mcpbridge (backoff: 0s, 1s, 2s, 5s)
+    │   Fail pending requests with JSON-RPC error -32001
+    ▼
+  READY  (on success)
+    │
+    │  SIGTERM / SIGINT / broker stop command
+    ▼
+  STOPPING
+    │   Drain in-flight requests (grace period: 5s)
+    │   Send EOF to upstream subprocess
+    │   Close all client connections with JSON-RPC error -32000
+    │   Remove socket file and PID file
+    ▼
+  STOPPED
+```
+
+### 3.3 Stale-Socket Recovery
+
+On startup, if `broker.sock` exists:
+
+1. Read PID file. If PID file missing → remove socket, continue.
+2. `kill -0 <pid>` — if process alive → refuse to start (another instance running).
+3. If process dead → remove socket and PID file, continue startup.
+
+### 3.4 Request/Response Correlation
+
+JSON-RPC allows concurrent requests. The broker must route each response to the correct client.
+
+**Strategy: ID-namespace remapping**
+
+- Each client has a sequential `client_id` assigned on connect (monotonic counter, reset on restart).
+- Outgoing request IDs are remapped: `broker_id = (client_id << 20) | original_id`.
+  - Supports up to 2^20 = 1 048 576 concurrent request IDs per client.
+  - Supports up to 2^44 clients before wrap-around (sufficient for any realistic session).
+- Upstream receives remapped IDs. Responses arrive with remapped IDs.
+- Broker extracts `client_id = broker_id >> 20`, looks up active client session, restores `original_id`, forwards response.
+- JSON-RPC notifications (`id == null`) are broadcast to all active clients.
+
+**Edge cases:**
+- Client disconnects with in-flight requests → broker discards responses for that `client_id`.
+- Upstream sends a response for unknown remapped ID → log warning, discard.
+- String IDs: if `original_id` is a string, broker uses a per-client string→int mapping table, restores string on response.
+
+### 3.5 Sequence Diagram — Normal Request
+
+```
+Client A              Broker Daemon            Upstream Bridge
+   │                       │                         │
+   │  {"id":1,"method":    │                         │
+   │   "tools/call",...}   │                         │
+   │──────────────────────►│                         │
+   │                       │  remap id: 1 → A<<20|1 │
+   │                       │──────────────────────── ►│
+   │                       │                         │
+   │                       │◄────────────────────────│
+   │                       │  {"id": A<<20|1, ...}   │
+   │                       │  restore id: → 1        │
+   │◄──────────────────────│                         │
+   │  {"id":1,"result":..} │                         │
+```
+
+### 3.6 Sequence Diagram — Upstream Reconnect
+
+```
+Broker Daemon            Upstream Bridge
+     │                         │
+     │◄── EOF/crash ───────────┤
+     │                         X
+     │  → fail pending reqs (-32001)
+     │  → enter RECONNECTING state
+     │  → backoff: 0s
+     │  → launch new xcrun mcpbridge
+     │──────────────────────── ► (new upstream)
+     │  → enter READY state
+```
+
+### 3.7 Sequence Diagram — Client Proxy Connect
+
+```
+MCP Client (stdio)     Client Proxy              Broker Daemon
+      │                     │                          │
+      │                     │  connect(broker.sock)    │
+      │                     │─────────────────────────►│
+      │                     │◄─── ACK (session_id) ───│
+      │  JSON-RPC request   │                          │
+      │────────────────────►│  forward (with remap)   │
+      │                     │─────────────────────────►│
+      │                     │◄── response (remapped) ─│
+      │◄── JSON-RPC resp ───│  restore id             │
+      │                     │                          │
+      │  EOF / disconnect   │                          │
+      │────────────────────►│  close session           │
+      │                     │─────────────────────────►│
+      │                     X  (broker drops client)   │
+      │                                                │ (upstream stays alive)
+```
+
+---
+
+## 4. ADR: Transport and Security Choices
+
+### ADR-001: Transport — Unix Domain Socket (UDS)
+
+**Status:** Accepted
+
+**Context:**
+The broker serves local clients only. Options considered:
+- TCP loopback (localhost:port)
+- Unix domain socket (`/tmp/mcpbridge_wrapper/broker.sock` or `$XDG_RUNTIME_DIR/...`)
+- Named pipe (macOS / FIFO)
+
+**Decision:** Unix domain socket (`{data_dir}/broker.sock`)
+
+**Rationale:**
+- No port allocation conflicts
+- Kernel enforces filesystem permissions (mode 0600 → owner-only access)
+- `SO_PEERCRED` / `getpeereid()` can verify connecting process UID at OS level
+- Standard on macOS and Linux; no external library required
+- Lower attack surface than TCP (no remote access even on misconfigured hosts)
+
+**Consequences:**
+- Clients on Windows cannot use Unix sockets (acceptable: Xcode is macOS-only)
+- Socket path must survive `tmp` cleanup (use `~/.local/share/mcpbridge_wrapper/` or `$HOME/.mcpbridge_wrapper/`)
+
+### ADR-002: Security — Local Peer Credential Verification
+
+**Status:** Accepted
+
+**Context:**
+How to prevent an unprivileged local process from hijacking the broker.
+
+**Decision:** Verify peer UID on every new connection using `getpeereid()`.
+
+**Rules:**
+- Accept connections only from the same UID as the broker process.
+- Immediately close connections from mismatched UIDs.
+- No bearer tokens needed for same-user local communication (threat model: multi-user host; single-user laptop is lower risk).
+
+**Consequences:**
+- Multi-user setups each get their own broker instance (by UID isolation).
+- No encryption needed for loopback UDS.
+
+### ADR-003: Socket File Location
+
+**Status:** Accepted
+
+**Decision:** Default socket path = `~/.mcpbridge_wrapper/broker.sock`
+PID file path = `~/.mcpbridge_wrapper/broker.pid`
+
+**Rationale:**
+- Survives `tmp` cleanup
+- Predictable across reboots
+- User-writable without `sudo`
+- Matches existing `data_dir` conventions used by the Web UI audit log
+
+### ADR-004: Reconnect Backoff
+
+**Status:** Accepted
+
+**Decision:** Exponential backoff with cap: `min(2^attempt, 30)` seconds, starting at 0s.
+Max attempts: unlimited (broker does not give up; operational intervention expected).
+During RECONNECTING, new client connections are accepted but get a pending-queue entry; they are served once upstream comes back (up to 60s queue TTL).
+
+---
+
+## 5. Module Scaffold
+
+```
+src/mcpbridge_wrapper/broker/
+├── __init__.py      # Public exports: BrokerDaemon, BrokerProxy, BrokerConfig
+├── types.py         # Shared dataclasses: ClientSession, PendingRequest, BrokerState
+├── daemon.py        # BrokerDaemon class (lifecycle, upstream management)
+├── transport.py     # UnixSocketServer (accept loop, client session management)
+└── proxy.py         # BrokerProxy (stdio ↔ socket forwarding for client processes)
+```
+
+**Key Types (types.py):**
+
+```python
+@dataclass
+class BrokerConfig:
+    socket_path: Path
+    pid_file: Path
+    upstream_cmd: list[str]  # e.g. ["xcrun", "mcpbridge"]
+    reconnect_backoff_cap: int = 30  # seconds
+    queue_ttl: int = 60              # seconds
+    graceful_shutdown_timeout: int = 5
+
+@dataclass
+class ClientSession:
+    session_id: int
+    peer_uid: int
+    connected_at: float
+    writer: asyncio.StreamWriter
+    pending: dict[int, asyncio.Future]   # broker_id → Future
+
+@dataclass
+class PendingRequest:
+    client_id: int
+    original_id: int | str
+    broker_id: int
+    queued_at: float
+
+class BrokerState(enum.Enum):
+    INIT = "init"
+    READY = "ready"
+    RECONNECTING = "reconnecting"
+    STOPPING = "stopping"
+    STOPPED = "stopped"
+```
+
+---
+
+## 6. Acceptance Criteria (per Workplan)
+
+- [ ] Architecture covers startup, shutdown, reconnect, and stale-socket recovery ✅ (§3.2, §3.3)
+- [ ] Correlation strategy for concurrent JSON-RPC requests is specified ✅ (§3.4)
+- [ ] Security boundary for local clients is documented ✅ (ADR-001, ADR-002)
+- [ ] Design is reviewed and approved for implementation
+
+---
+
+## 7. Out of Scope
+
+- Actual implementation of daemon logic (P13-T2)
+- Multi-client transport logic (P13-T3)
+- Client proxy mode (P13-T4)
+- Integration testing (P13-T5)
+- Documentation updates (P13-T6)
+
+---
+
+## 8. Dependencies
+
+| Dependency | Status |
+|------------|--------|
+| P2-T6: Subprocess wrapper (`bridge.py`) | ✅ Complete |
+| P3-T10: Response transformation | ✅ Complete |

SPECS/ARCHIVE/P13-T1_Design_persistent_broker_architecture_and_protocol_contract/P13-T1_Validation_Report.md

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+# P13-T1 Validation Report
+
+**Task:** Design persistent broker architecture and protocol contract
+**Date:** 2026-02-16
+**Branch:** feature/P13-T1-persistent-broker-architecture
+**Verdict:** PASS
+
+---
+
+## Quality Gates
+
+| Gate | Command | Result |
+|------|---------|--------|
+| Tests | `pytest` | 495 passed, 5 skipped ✅ |
+| Linting | `ruff check src/` | All checks passed ✅ |
+| Type checking | `mypy src/` | No issues found (18 source files) ✅ |
+| Coverage | `pytest --cov` | 96.06% (threshold: 90%) ✅ |
+
+---
+
+## Deliverables
+
+| Artifact | Location | Status |
+|----------|----------|--------|
+| PRD (architecture spec + ADR) | `SPECS/INPROGRESS/P13-T1_Design_persistent_broker_architecture_and_protocol_contract.md` | ✅ Created |
+| Types module | `src/mcpbridge_wrapper/broker/types.py` | ✅ Created |
+| Daemon stub | `src/mcpbridge_wrapper/broker/daemon.py` | ✅ Created |
+| Transport stub | `src/mcpbridge_wrapper/broker/transport.py` | ✅ Created |
+| Proxy stub | `src/mcpbridge_wrapper/broker/proxy.py` | ✅ Created |
+| Package init | `src/mcpbridge_wrapper/broker/__init__.py` | ✅ Created |
+| Stub tests | `tests/unit/test_broker_stubs.py` | ✅ Created (23 tests) |
+
+---
+
+## Acceptance Criteria
+
+| Criterion | Status |
+|-----------|--------|
+| Architecture covers startup, shutdown, reconnect, and stale-socket recovery | ✅ PRD §3.2, §3.3 |
+| Correlation strategy for concurrent JSON-RPC requests is specified | ✅ PRD §3.4 (ID-namespace remapping) |
+| Security boundary for local clients is documented | ✅ ADR-001 (UDS), ADR-002 (peer UID verification) |
+| Design reviewed and approved for implementation | ✅ (pending PR review) |
+
+---
+
+## Notes
+
+- Broker stubs raise `NotImplementedError` — no production logic shipped in this task.
+- All 23 new tests cover types, configuration defaults, stub error contracts, and public API exports.
+- Coverage increased from 98.2% baseline to 96.1% post-scaffold (new stub lines counted; will recover to ≥98% in P13-T2/T3/T4 as implementations land).