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Client revalidates tool results with one-shot jsonschema.validate(), recompiling the validator on every call

Status: Open.

#3133 In modelcontextprotocol/python-sdk;

· jlowin opened on Jul 19, 2026

Fix case-insensitive handling of text MIME types for FileResource

Status: Open.

#3131 In modelcontextprotocol/python-sdk;

· dchou1618 opened on Jul 19, 2026

Dispatcher mints a request id already used by a completed caller-supplied request (spec: ids MUST NOT be reused in a session)

Status: Open.

#3126 In modelcontextprotocol/python-sdk;

· ayaangazali opened on Jul 18, 2026

Image content blocks >99 bytes fail with "illegal base64 at byte 0"

Status: Open.

#3123 In modelcontextprotocol/python-sdk;

· PeeperFrog opened on Jul 18, 2026

Server._handle_message can loop forever re-logging warnings when a log handler itself emits a warning (v1.x)

Status: Open.

#3122 In modelcontextprotocol/python-sdk;

· fas89 opened on Jul 17, 2026

Recommended deployment pattern for FastMCP on AWS Lambda using Mangum

questionFurther information is requestedFurther information is requested

Status: Open.

#3121 In modelcontextprotocol/python-sdk;

· shoaib1207k opened on Jul 17, 2026

RequireAuthMiddleware omits RFC 6750 scope in WWW-Authenticate on 401/403

Status: Open.

#3103 In modelcontextprotocol/python-sdk;

· velias opened on Jul 16, 2026

Streamable HTTP server never returns the spec-mandated 405 for GET requests it won't serve as SSE (406/400 instead) — breaks client SSE-probe fallback

Status: Open.

#3102 In modelcontextprotocol/python-sdk;

· fgranata opened on Jul 16, 2026

Generated output schema uses Pydantic's validation shape while structured output uses its serialization shape

Status: Open.

#3100 In modelcontextprotocol/python-sdk;

· destire-mio opened on Jul 16, 2026

Field alias is published in the tool schema but forwarded as an invalid Python keyword

Status: Open.

#3099 In modelcontextprotocol/python-sdk;

· destire-mio opened on Jul 16, 2026

streamable_http: HTTP status errors (401/403/5xx) on tools/list, tools/call never reach the caller — indistinguishable from a timeout

Status: Open.

#3091 In modelcontextprotocol/python-sdk;

· afterrburn opened on Jul 13, 2026

Heavy sync init work (e.g. local ML models) starves stdio initialize/tool calls even with threadpool offload -- Windows GIL contention

Status: Open.

#3089 In modelcontextprotocol/python-sdk;

· scotthibbs opened on Jul 12, 2026