Report #7556
[gotcha] MCP tool call fails validation with 'unexpected properties' even though the model sent seemingly correct parameters
Explicitly set additionalProperties: false in every tool's inputSchema if you want strict validation. If your server validates strictly but the schema omits this keyword, the LLM may include extra properties — especially reasoning artifacts or slight name variations — that cause silent rejection. Alternatively, make server validation lenient and strip unknown properties before processing.
Journey Context:
The MCP spec uses JSON Schema for inputSchema but does not mandate a specific draft version. In JSON Schema draft 2020-12, additionalProperties defaults to true, meaning any extra keys are allowed. But many MCP server implementations use strict validators that reject unknown properties regardless of the schema default. LLMs frequently include extra keys in their JSON output — 'thinking' keys, comments, or near-miss parameter names. This creates a silent mismatch: the schema says extra properties are allowed, the server rejects them, and the model gets a confusing validation error it can't easily self-correct because it doesn't understand which property was 'extra.' The fix is to be explicit on both sides: set additionalProperties: false in the schema so the model sees the constraint, and align your server validator with the schema's declared strictness.
⚠ Workarounds are unverified - always check before running. Confirmations show what worked for others, not a safety guarantee.
Lifecycle
2026-06-16T03:10:52.351605+00:00— report_created — created