Workspace MCP Transport Pool

Overview

McpTransportPool (packages/core/src/tools/mcp-transport-pool.ts) is the F2 (#4175 commit 5) workspace-scoped pool: multiple ACP sessions on one daemon share one transport per unique (serverName + configFingerprint) tuple, instead of each spawning its own MCP child process. The pool lives inside the ACP child (QwenAgent.mcpPool), is constructed once at agent startup with the daemon’s bootstrap Config, and survives session lifecycles. Entries reference-count session attaches and close after a configurable grace period when the reference count reaches zero.

It is the main mechanism that prevents a multi-session daemon from forking one copy of every MCP server per session.

Responsibilities

• Acquire or spawn one MCP transport per (name + fingerprint), deduplicating concurrent acquires via spawnInFlight.
• Release per-session references; arm the entry’s drain timer when the last reference detaches.
• Survive ref-count churn with a hard MAX_IDLE_MS cap so a thrashing client cannot keep an idle transport alive forever.
• Reference-count sessions in a reverse index (sessionToEntries) so releaseSession(sessionId) is O(refs) rather than O(entries).
• Restart entries on demand (restartByName) — single-entry returns {restarted, durationMs}, multi-entry returns {entries: RestartResult[]} (F2 multi-entry contract).
• Drain the entire pool on daemon shutdown with a configurable timeout; refuse new acquires while draining.
• Consult WorkspaceMcpBudget (see 06-mcp-budget-guardrails.md) on acquire to enforce per-name reservation caps; release the slot on entry close when no sibling entry holds the same name.
• Produce per-session filtered tool/prompt snapshots via SessionMcpView so a discovery in one session does not register tools into other sessions.

Architecture

Public surface

class McpTransportPool {
  constructor(cliConfig: Config, options: McpTransportPoolOptions);
  acquire(
    serverName,
    cfg,
    sessionId,
    sessionToolRegistry,
    sessionPromptRegistry,
  ): Promise<PooledConnection>;
  release(id, sessionId): void;
  releaseSession(sessionId): void;
  restartByName(
    name,
    opts?,
  ): Promise<RestartResult | { entries: RestartResult[] }>;
  drainAll(opts?): Promise<void>;
  getBudget(): WorkspaceMcpBudget | undefined;
  getSnapshot(): McpPoolSnapshot;
}

McpTransportPoolOptions:

• workspaceContext: WorkspaceContext (required).
• debugMode: boolean.
• sendSdkMcpMessage? — per-session callback (pool bypasses SDK MCP).
• pooledTransports?: ReadonlySet<McpTransportKind> — default {stdio, websocket}. HTTP/SSE transports stay unpooled by default because their headers can carry session-specific OAuth state, but operators can explicitly opt them into pooling with QWEN_SERVE_MCP_POOL_TRANSPORTS.
• drainDelayMs? — default 30_000.
• entryOptions?: (transport) => PoolEntryOptions.
• budget?: WorkspaceMcpBudget.

Internal state

PoolEntry (per-entry structure, mcp-pool-entry.ts)

State machine: spawning → active ⇄ (active ↔ reconnect) → (active → draining on last detach, draining → active on attach OR draining → closed on timer).

PoolEntryOptions

interface PoolEntryOptions {
  drainDelayMs: number; // default 30_000
  maxIdleMs: number; // default 5 * 60_000
  maxReconnectAttempts: number; // default 3 (stdio/ws) or 5 (http/sse)
  reconnectStrategy:
    | { kind: 'fixed'; delayMs: number }
    | { kind: 'exponential'; baseMs: number; capMs: number };
}

defaultPoolEntryOptions(transport) (mcp-pool-entry.ts) returns stdio/ws defaults {fixed 5s, 3 attempts} and http/sse defaults {exponential 1s → 16s, 5 attempts}. Remote transports get longer retry budgets because their failures are more often transient.

Workflow

acquire

release + drain

hasNameSibling(name) (mcp-transport-pool.ts) iterates both entries.values() and spawnInFlight.keys() parsing the latter with parseConnectionId (server names can legitimately contain ::, so startsWith would false-positive on a sibling name beginning with ${name}::).

releaseSession(sessionId) reads from sessionToEntries, releases all referenced entries in O(refs), then clears the index entry. Used by the bridge’s session-close path so it does not iterate the full entry map.

restartByName

The preflight budget check at the daemon HTTP layer returns {restarted:false, skipped:true, reason:'budget_would_exceed'} (Wave 4 mutation control) when the target’s slot is not already reserved and a restart would push live count over enforce budget.

drainAll

State & Lifecycle

• Pool construction is synchronous; first acquire cold-starts a transport.
• drainDelayMs (default 30s) is reset to cancellation on attach.
• maxIdleMs (default 5 min) is never reset by attach/detach — it starts ticking at the FIRST idle and only stops when the entry actually closes or attaches before the deadline. Defense against thrashing clients.
• nextIndexByName is monotonic. Old entries keep their assigned index even after newer ones appear, so dashboards reading entryIndex do not reshuffle.
• Spawn failure releases the reserved budget slot (V21-4 — without this, a cold spawn that crashed mid-connect would leak the reservation forever).

Dependencies

• packages/core/src/tools/mcp-client.ts — McpClient, status enum, SendSdkMcpMessage.
• packages/core/src/tools/mcp-pool-entry.ts — PoolEntry, PoolEntryOptions, defaultPoolEntryOptions.
• packages/core/src/tools/mcp-pool-key.ts — connectionIdOf, parseConnectionId, isPoolable, mcpTransportOf, POOLED_TRANSPORTS_DEFAULT.
• packages/core/src/tools/mcp-pool-events.ts — ConnectionId, PoolEntryState, PoolEvent.
• packages/core/src/tools/session-mcp-view.ts — per-session view that filters pool snapshots.
• packages/core/src/tools/mcp-workspace-budget.ts — WorkspaceMcpBudget (see 06-mcp-budget-guardrails.md).
• packages/core/src/tools/mcp-discovery-timeout.ts — discoveryTimeoutFor, runWithTimeout.

Configuration

Unpooled entries (HTTP / SSE / SDK-MCP)

Transports outside the configured pooledTransports allowlist (HTTP, SSE, and SDK-MCP by default) take a separate path: createUnpooledConnection(name, cfg, sessionId, ...) (mcp-transport-pool.ts) creates a per-session entry with id ${name}::unpooled-${entryIndex}. Differences from pooled entries:

• Stored in entries AND tracked in unpooledIds: Set<ConnectionId> so release / releaseSession can fast-path the close-on-detach behavior (refs always max out at 1).
• McpClient.discover() is used directly instead of pool replay; applyTools / applyPrompts are no-ops because the session’s registries already hold what was registered (W77 / skipReplay: true in attach()).
• Workspace budget still gates them — the F2 budget follow-up closed the prior loophole where unpooled connections bypassed tryReserve; the same WorkspaceMcpBudget slot is reserved and released on entry close (whether pooled or unpooled).

The W77 race (cb206da36): createUnpooledConnection stores the entry in this.entries BEFORE awaiting client.connect() / client.discover(), but only indexes sessionToEntries[sessionId] AFTER attach() succeeds. A concurrent closeStoredSession() / releaseSession(sessionId) during the connect/discover window saw an empty index, let the unpooled spawn finish, and attach() then registered tools/prompts into an already-closed session. The fix:

• mcp-pool-entry.ts: public isTerminated(): boolean probe (state === 'closed' || state === 'failed').
• mcp-pool-entry.ts: markActive() short-circuits if isTerminated() so a torn-down entry cannot be resurrected to 'active'.
• Callers (the pool’s unpooled path) probe isTerminated() between the awaits and abort the attach if the parent session went away.

This race was latent at the time (the W61/W71 per-session releaseSession hooks land in F4), but would become live the moment that hook arrived. The fix was applied early in the F2 series.

GET /workspace/mcp pool-aware snapshot fields

When the pool is active, each ServeWorkspaceMcpStatus server cell (packages/acp-bridge/src/status.ts) includes three additional fields:

(entryCount, entrySummary) are always broadcast as a pair. The mcp_workspace_pool capability tag implies both fields. Older SDK clients ignore them under the additive protocol contract.

Pool snapshots also expose subprocessCount. It counts only the 'stdio' family. WebSocket, HTTP, and SSE transports connect to remote servers and do not spawn local child processes. Early versions counted WebSocket transports as local subprocesses, which inflated resource dashboards.

Drain runs from both shutdown paths

Pool drain is not limited to the SIGTERM handler. The normal IDE shutdown path (await connection.closed) also calls drainAll via packages/cli/src/acp-integration/acpAgent.ts’s drainPoolBeforeExit. Whether the daemon receives a process signal or the IDE closes its connection cleanly, the pool enters draining, refuses new acquires, and waits for entries to close.

/mcp refresh shares the boot discovery path

discoverAllMcpTools (boot discovery) and discoverAllMcpToolsIncremental (/mcp refresh / hot reload) both consult the pool first in pool mode (packages/core/src/tools/mcp-client-manager.ts). The shared gate prevents hot reload from accidentally creating a per-session client, double-counting budget, or leaving an orphan transport behind.

In-flight tool calls during reconnect (MCPCallInterruptedError)

When the underlying MCP transport silently disconnects (the connection jumps from 'active' / 'draining' to localStatus === DISCONNECTED without an explicit close), the pool marks the entry 'failed', evicts it from pool.entries, and emits the failed event before detaching subscriber views. That emit-before-detach order matters: subscribers receive the failed event soon enough to route pending callTool promises to MCPCallInterruptedError, so a stuck await client.callTool(...) rejects cleanly instead of hanging. forceShutdown uses the same emit-then-detach ordering.

Fingerprint and canonicalOAuth normalization

The pool key comes from fingerprint(cfg) in mcp-pool-key.ts. The hash covers all transport-defining fields:

transport, command, args, cwd, env, url, httpUrl, tcp, headers, timeout, oauth

Per-session filtering and metadata fields (includeTools, excludeTools, trust, description, extensionName, discoveryTimeoutMs) are excluded, so sessions with different filters can share one entry.

For the OAuth cell, canonicalOAuth(o) hashes every MCPOAuthConfig field: clientId, clientSecret, sorted scopes, sorted audiences, authorizationUrl, tokenUrl, redirectUri, tokenParamName, and registrationUrl. This is the credential-isolation contract: two session configs that differ only by clientSecret, audiences, or redirectUri get different fingerprints and cannot share one entry. Confidential clients and multi-audience token deployments depend on this.

Sorting scopes and audiences makes callsite order irrelevant. Explicit null is normalized so undefined fields hash the same as explicit null. The key does not include discoveryTimeoutMs; concurrent acquire calls with the same key but different timeouts are “first wins”, matching the pre-F2 per-session manager behavior.

PoolEntry keeps cfg: MCPServerConfig private. External code must use the entry.transportKind getter when it needs the transport family. That prevents env, header auth, and OAuth fields from leaking to consumers by accident.

Extension unloads rely on MAX_IDLE_MS

There is intentionally no active cleanup path for unloading an MCP extension at runtime. Orphan entries whose MCPServerConfig no longer appears in the merged workspace settings are reclaimed naturally by the MAX_IDLE_MS hard cap after the last subscriber detaches. A synchronous unload-cleanup path would add complexity for a rare operator edge case; the hard cap limits orphan process lifetime past the unload point to 5 minutes by default.

Operators who need faster cleanup can restart the daemon or call POST /workspace/mcp/:server/restart for the now-unconfigured name, which goes through the disabled-server path and tears the entry down.

Self-heal observability

The pool emits two structured diagnostics on the self-heal path:

McpClient.lastTransportError: Error | undefined (packages/core/src/tools/mcp-client.ts) — McpClient.onerror stores the most recent transport exception in a private field and clears it at connect() entry. The PoolEntry silent-drop path reads client.getLastTransportError() and includes it in emit({kind:'failed', lastError}), so subscribers and dashboards do not have to grep stderr for root cause.

SweepResult (internal interface, not exported; packages/core/src/tools/mcp-pool-entry.ts) — sweepAndDisconnect(reason) returns Promise<SweepResult>:

interface SweepResult {
  pidSweepError?: Error; // listDescendantPids itself threw
  descendantsFound?: number; // descendant pid count found
  descendantsSignaled?: number; // successfully SIGTERM'd count
}

The only consumer is the silent-drop block in statusChangeListener. It uses descendantsFound / descendantsSignaled to detect partial-signal cases (fewer processes signaled than found, usually because a process exited or EPERM occurred between listDescendantPids and sigtermPids) and sweep errors, then logs a structured warning. forceShutdown and doRestart ignore this return value because their catch paths already carry richer failure signals.

Subprocess cleanup: the pid-descendants snapshot path

When McpTransportPool shuts down stdio subprocesses, it has to enumerate their descendant processes; npx wrappers and shell wrappers can create multiple fork levels. packages/core/src/tools/pid-descendants.ts exposes listDescendantPids(rootPid) → Promise<number[]> and sigtermPids(pids) for sweepAndDisconnect.

Linux / macOS primary path

A single ps -A -o pid=,ppid= snapshot reads the process table, parses it into Map<ppid, pid[]>, then walkDescendants(tree, root) performs BFS to extract the subtree. Any depth requires only one ps fork.

walkDescendants maintains visited: Set<number> and includes root in the set to defend against PID-reuse cycles. Under fast process churn, the snapshot can theoretically contain A→B / B→A loops. Without visited, the walker could fill the MAX_DESCENDANTS quota with bogus data and crowd out real descendants.

Windows primary path

A single Get-CimInstance Win32_Process | ConvertTo-Csv -Delimiter "," snapshot emits all (ProcessId, ParentProcessId) rows, then the same Map and walkDescendants path runs.

The explicit -Delimiter "," is required. PowerShell 5.1, which ships with Windows, defaults ConvertTo-Csv to the system locale list separator; DE, FR, NL, IT, and similar locales use ;, so the pre-fix parser ^"(\d+)","(\d+)"$ never matched and every daemon shutdown fell back to the per-pid CIM filter path, adding roughly 0.5-1s of PowerShell startup cost per child.

Fallback path

BusyBox <v1.28 lacks ps -o, distroless containers might not include ps, and some Windows environments truncate CIM output via ACLs. When the primary path parses zero rows or throws, the code falls back to per-pid BFS: Linux / macOS use pgrep -P <pid>, and Windows uses Get-CimInstance -Filter "ParentProcessId=$p" where $p is a PowerShell variable binding rather than string concatenation. The current Number.isInteger guard is sufficient for the entry point; the binding is defense-in-depth.

Shared constraints

Both paths are bounded by MAX_DESCENDANTS = 256 and MAX_DEPTH = 8 to keep a malicious or degenerate process tree from dragging down sweep.

The snapshot path uses maxBuffer: 8MB, enough for pathological hosts with about 250k processes. Node’s default 1MB buffer can truncate child-process output around 30k processes.

The performance gain is intentionally modest (typical 200-500 process dev machines parse in under 10ms, around 2x faster than per-pid pgrep). The main benefit is fork hygiene and snapshot consistency: BFS sees the full subtree at once, while the previous per-pid query path could miss a grandchild forked between two queries.

Embedder note: McpClientManager constructor

McpClientManager is constructed as (config, toolRegistry, options?: McpClientManagerOptions). Embedders that import the class directly should pass:

new McpClientManager(config, toolRegistry, {
  eventEmitter,
  sendSdkMcpMessage,
  healthConfig,
  budgetConfig,
  pool,
});

Tests should prefer an mkManager(overrides?) factory so cases that care about one or two fields stay one line.

Implementation notes

These helpers are internal, but source readers may see them:

• McpTransportPool.acquire() uses attachPooledSession and rollbackReservationOnSpawnFailure to share fast-path attach, post-spawn attach, and pooled spawn-in-flight catch behavior. Runtime behavior is unchanged; race-window invariants still live at the call sites.
• SessionMcpView.applyTools / applyPrompts compile includeTools / excludeTools once via compileNameFilter(cfg) and check each tool with compiledFilterAccepts(compiled, name). Exported passesSessionFilter / passesSessionPromptFilter use the same compiled path. excludeTools is exact-match; includeTools strips the first (...) suffix so toolName(args) matches toolName.

Design document: ../../design/f2-mcp-transport-pool.md §6 covers the transport pool state machine, reconnect, drain, and descendant sweep paths.

Caveats & Known Limits

• HTTP / SSE transports are unpooled by default — unless operators explicitly include them in QWEN_SERVE_MCP_POOL_TRANSPORTS, each acquire mints a fresh entry that lives only as long as its session. Their headers may carry session-specific OAuth state, so pooling them by default would risk leaking credentials across sessions.
• maxIdleMs is a hard cap that survives attach/detach churn. A 5-minute idle hard cap means even an aggressively attaching/detaching client cannot keep an idle transport pinned past 5 minutes. Operators who want pinned long-lived transports should increase maxIdleMs or run the server outside the pool.
• Per-server-name budget slots mean two pool entries that share a name but differ by fingerprint consume ONE slot together, not two. Subprocess accounting is exposed separately via pool.getSnapshot().subprocessCount.
• startsWith regression was avoided in hasNameSibling because MCP server names can legitimately contain :: (mcp-pool-key.test.ts). Always use parseConnectionId’s lastIndexOf('::') split, never string-prefix matching.
• Pool draining is one-way — drainAll sets draining = true permanently; a fresh pool is required for further work.

References

• packages/core/src/tools/mcp-transport-pool.ts (entire file)
• packages/core/src/tools/mcp-pool-entry.ts (entry lifecycle)
• packages/core/src/tools/mcp-pool-key.ts (connectionIdOf, parseConnectionId)
• packages/core/src/tools/mcp-pool-events.ts (event types)
• packages/core/src/tools/session-mcp-view.ts (per-session filtered view)
• F2 design document (v2.2, with the 32-item review fold-in changelog): ../../design/f2-mcp-transport-pool.md. Treat the design contract as authoritative; this page is the developer deep dive.
• F2 design notes: issue #4175  (commits 4-6 of the F2 series).