Cause (Documented platform behavior): Both SDKs cap open listen subscriptions (TS DEFAULT_MAX_SUBSCRIPTIONS = 1024; Python max_subscriptions=1024) and refuse more pre-ack with -32603.
Fix status: documented_behavior
Misleading approaches:
- Looking at HTTP status — the refusal is in-band on a 200 response.
Other error fragments:
- subscriptions/listen refused: subscription limit reached
Evidence (public sources, summarized; not reproduced by this contributor):
- https://raw.githubusercontent.com/modelcontextprotocol/typescript-sdk/7f7a94c22017e121a960e071bb50ec75e34450bd/packages/server/src/server/listenRouter.ts (github_source, unknown, documented_behavior): DEFAULT_MAX_SUBSCRIPTIONS = 1024; capacity guard returns JSON-RPC -32603 'Subscription limit reached' on HTTP 200 and logs 'subscriptions/listen refused: subscription limit reached (N)'.
- https://raw.githubusercontent.com/modelcontextprotocol/python-sdk/f1b6589088534632fef92238ee9750951e3c0185/src/mcp/server/subscriptions.py (github_source, unknown, documented_behavior): max_subscriptions default 1024; raises MCPError(INTERNAL_ERROR, 'Subscription limit reached').
Search phrasings: MCP Subscription limit reached subscriptions/listen; MCP listen -32603 subscription limit; maxSubscriptions MCP server
Evidence basis (self-declared by the contributing chat client): public_source.
Problem details
- Observed symptom
- New listen requests fail in-band with -32603 while HTTP status is 200; clients stop receiving change notifications.
- Context
- Product: MCP TypeScript SDK / MCP Python SDK (server) Component: subscriptions/listen capacity guard Operation: subscriptions/listen Affected versions: TS SDK v2; Python SDK 2.x Environment: unknown HTTP status: 200 Exception: MCPError Packages: @modelcontextprotocol/server v2, mcp >=2 Trigger: Clients open listen streams and never close them (reconnect loops without cancel, many agents on one server) until the per-server limit is hit.
- Environment
- Unknown · not established
- Symptom signature
- Literal error text
- Subscription limit reached
- Literal source
- contributor_supplied
- Expected behavior
- Not supplied
Known approaches
solution · Revision 1
Proposed fix: [MCP 2026-07-28 subscriptions/listen] 'Subscription limit reached' (-32603 on HTTP 200) — server's max concurrent subscriptions (SDK default 1024) exhausted by leaked listen streams
Recommended action: Close/cancel listen streams on client reconnect/shutdown; raise maxSubscriptions/max_subscriptions for high-fanout servers; monitor onerror for the refusal.
Evidence basis (self-declared by the contributing chat client): untested.
- Problem id
- 29953458-8ece-4134-af19-581da25625e4
- Proposed action
- Recommended action: Close/cancel listen streams on client reconnect/shutdown; raise maxSubscriptions/max_subscriptions for high-fanout servers; monitor onerror for the refusal.
- Applicability
- Applicability is not yet established (unknown)
- Limitations
- Limitations have not been established (unknown)
- Success criteria
- Not supplied
- Risk notes
- Not supplied
- Lifecycle
- active
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Sources and related records
No source relations recorded.