fix sse stream

[matheusfrancisco]

No description provided.

[qodo-code-review]

PR Summary by Qodo

Fix SSE GET-stream 405 messaging and MCP revision classification

🐞 Bug fix 🧪 Tests 🕐 20-40 Minutes

AI Description

• Return an actionable error when SSE GET stream is rejected with HTTP 405.
• Classify MCP Legacy vs Modern using _meta (not just the version header).
• Add regression tests for streamable-http hinting and protocol-version edge cases.

Diagram

graph TD
  Client["MCP client"] --> Gateway["Gateway classify()"] --> Meta{{"_meta modern?"}}
  Meta -- "no" --> Legacy["Legacy path"]
  Meta -- "yes" --> HeaderCheck{{"Header matches _meta?"}} --> Modern["Modern path"]
  Modern --> SSE["SSEBackend Start"] --> Endpoint[("Backend endpoint")]
  Endpoint -- "GET stream 405" --> ActionErr["Error: set transport=streamable-http"]
  subgraph Legend
    direction LR
    _comp["Component/handler"] ~~~ _dec{{"Decision"}} ~~~ _ext[("Endpoint")]
  end

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High-Level Assessment

The following are alternative approaches to this PR:

1. Auto-detect and fallback to streamable-http on 405

• ➕ Zero-config recovery when an endpoint is actually streamable-http
• ➕ Avoids operator intervention for a common misconfiguration
• ➖ Surprising behavior change (transport silently changes semantics)
• ➖ Harder to debug if both transports are reachable but behave differently
• ➖ May mask genuine server-side routing/method issues

2. Preflight transport detection (probe capabilities)

• ➕ Can provide a clearer error before starting the long-lived stream
• ➕ Could validate config at startup and fail fast
• ➖ Adds extra round-trips and complexity
• ➖ Capability probing is not always reliable without a spec-defined endpoint

Recommendation: Keep the PR’s explicit-config approach: improving the 405 error message and tightening revision classification yields correct behavior without hidden fallbacks. Consider an opt-in auto-fallback later if operator friction remains high.

Files changed (4) +106 / -15

Bug fix (2) +36 / -15

sse.goHint streamable-http when SSE GET stream returns 405 +10/-0

Hint streamable-http when SSE GET stream returns 405

• Adds a special-case for HTTP 405 during SSE stream establishment. The returned error now explicitly indicates the endpoint is likely streamable-http and instructs setting 'transport: streamable-http', preventing confusing downstream 502-style failures.

backend/sse.go

revision.goDecide Modern vs Legacy from _meta, not version header alone +26/-15

Decide Modern vs Legacy from _meta, not version header alone

• Adjusts protocol-era classification so 'MCP-Protocol-Version' is not treated as a Modern signal by itself; only '_meta' controls Modern routing. Adds validation for Legacy-shaped requests echoing unknown versions, and simplifies the header mismatch check to compare against '_meta' when present.

gateway/revision.go

Tests (2) +70 / -0

sse_test.goTest 405 GET stream produces streamable-http hint +21/-0

Test 405 GET stream produces streamable-http hint

• Introduces a regression test that simulates a backend returning HTTP 405 with a 'use POST' JSON-RPC error body. Verifies the surfaced error includes guidance to switch to streamable-http.

backend/sse_test.go

revision_test.goAdd tests for Legacy echoed version header and modern-meta requirements +49/-0

Add tests for Legacy echoed version header and modern-meta requirements

• Adds coverage for Legacy clients that echo negotiated versions (including the claude-code scenario) to ensure they are not misclassified as Modern. Also tests that unknown echoed versions are rejected for the right reason, and that Modern routing requires '_meta' rather than a header-only claim.

gateway/revision_test.go

[qodo-code-review]

Code Review by Qodo

🐞 Bugs (0) 📘 Rule violations (0) 📎 Requirement gaps (0)

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