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Sweep 22 Oct 2026 · 21:04Z Build v2.1.309 528 read Stable v2.1.308 Latest v2.1.309 Next v2.1.309 Feeds RSS JSON llms.txt llms-full.txt Unofficial
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Claude Code v2.1.265 ·

MCP connections fall back to legacy HTTP+SSE when Streamable-HTTP initialize fails

MCP servers that reject the modern initialize request now get retried over legacy HTTP+SSE instead of failing, recorded in telemetry

Group of 3 You'll notice Internal Changes
JSON All of v2.1.265
You'll noticeTier: how much it should matter to you
3Useful: my rating, 1 to 5
2Signal: worth watching, 1 to 5
MCPArea: what it touches
Internal ChangesKind: in v2.1.265,
Bug FixesSection of the release

What

  • When a Streamable-HTTP MCP server's initialize POST request fails (for example, a 405 Method Not Allowed or similar transport error), Claude Code now retries the connection using the legacy HTTP+SSE transport instead of failing immediately — closing the failed transport and re-dialing with mode: "legacy".
  • This fallback is gated behind tengu_mcp_legacy_sse_fallback, dials with an abort controller, and uses a capped retry budget.
  • The mcp_connect success telemetry event gained a new negotiatedLegacySse field recording whether a connection ended up using the legacy SSE transport.

Why Some MCP servers only support the older HTTP+SSE transport and reject the newer Streamable-HTTP handshake. This fallback lets Claude Code still connect to them automatically instead of the connection simply failing, and the telemetry field makes it possible to see how often the older transport is still needed.

See this entry in the whole of v2.1.265 →

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