7 MCP Gateway Bugs: Session Leaks, Dead SSE, and OAuth in Gateway Mode

After the happy path demos, a Reddit user hit seven specific bugs when putting an MCP gateway between real clients and servers. The fixes were not prompt engineering — they were explicit session boundaries, per-tool timeouts, idempotency, structured action logs, gateway-level traces, and tests against concurrent tool calls. The result was a large reduction in parallel tool wall time, but the bigger win was knowing where failure lived.

The seven bugs that actually mattered

• Session state leaking across clients — shared state between sessions caused data contamination.
• SSE connections dying silently — no error surfaced when a server-sent event connection dropped.
• OAuth flows working in local tests but breaking in gateway mode — redirect URIs or token validation failed behind the proxy.
• Discovery probes returning stale server metadata — cached capabilities didn't reflect server updates.
• SQLite writes blocking parallel tool calls — database locks serialized concurrent requests.
• Retry logic duplicating tool side effects — retries re-executed mutations like writes or API calls.
• Tool latency hiding inside the gateway instead of the model call — monitoring attributed time to the wrong layer.

The fix: boring infra, not better prompts

The author's approach to each bug:

• Explicit session boundaries — separate state per client, no shared objects.
• Per-tool timeout policy — individual timeouts to prevent one slow tool holding up others.
• Idempotency where possible — deduplication keys or transactional behavior to make retries safe.
• Structured action logs — detailed, parseable logs of every gateway action for debugging.
• Gateway-level traces — distributed tracing to attribute latency correctly across layers.
• Tests against concurrent tool calls — integration tests that fire parallel requests to surface race conditions.

These are specific, practical patterns for anyone running an MCP gateway in production. The post's key insight: the hard problems are state isolation, silent failures, and observability — not model prompts.