mcp: tolerate 404 on subscriptions/listen in stateless mode - #1193
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When connecting to remote stateless servers (such as GitHub's MCP server), optional streams like subscriptions/listen may be rejected by middleware with a plain-text HTTP 404 before reaching a JSON-RPC handler. Previously, checkResponse misclassified these 404s as lost stateful sessions (ErrSessionMissing) and tore down the transport connection because the error didn't wrap jsonrpc2.ErrRejected. This change aligns with the MCP 2026-07-28 Streamable HTTP specification: 1. Conditionally check for an active session ID before returning ErrSessionMissing, since a 404 on a stateless connection cannot mean a session was lost. 2. Wrap subscriptions/listen failures with ErrRejected (matching server/discover) so the jsonrpc2 layer doesn't permanently break the transport when encountering non-compliant plain-text 404s.
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When connecting to remote stateless servers (such as GitHub's MCP server), optional streams like subscriptions/listen may be rejected by middleware with a plain-text HTTP 404 before reaching a JSON-RPC handler.
Previously, checkResponse misclassified these 404s as lost stateful sessions (ErrSessionMissing) and tore down the transport connection because the error didn't wrap jsonrpc2.ErrRejected.
This change aligns with the MCP 2026-07-28 Streamable HTTP specification:
Conditionally check for an active session ID before returning ErrSessionMissing, since a 404 on a stateless connection cannot mean a session was lost.
Wrap subscriptions/listen failures with ErrRejected (matching server/discover) so the jsonrpc2 layer doesn't permanently break the transport when encountering non-compliant plain-text 404s.
I have manually tested this change against GitHub's hosted MCP server and confirmed that it fixes github/github-mcp-server#3129