UPinar/contrastapi

Summary

Hotfix for a latent MCP spec violation in the mcp_tool_safe decorator that affected error envelopes from all 49 MCP tools. On AppException, the decorator returned an ErrorResponse body — which FastMCP wraps into result.structuredContent with isError: false — instead of raising ToolError, which sets isError: true on the wire as the spec requires.

Agent clients that switch on result.isError (the spec-conformant path) treated every error as success and parsed the error payload as data.

Bug fix

mcp_tool_safe decorator now raises ToolError

Before:

except AppException as e:
    return ErrorResponse(error=e.to_error_detail())  # isError=false, payload in structuredContent

After:

except AppException as e:
    payload = ErrorResponse(error=e.to_error_detail()).model_dump_json(exclude_none=True)
    raise ToolError(payload) from None  # isError=true, payload embedded in content[0].text

FastMCP catches ToolError, prepends "Error executing tool {name}: ", and emits the payload inside result.content[0].text with isError: true.

Test helper for the new error envelope shape

mcp_error_payload(response) extracts the JSON error envelope from the new wire format:

• Asserts result.isError is True
• Strips the FastMCP "Error executing tool {name}: " prefix
• Returns parsed {"error": {...}} payload

11 error-path MCP tests migrated to this helper.

Hardening fold-in

AppException.__init__ now truncates self.message to 500 chars, matching ErrorDetail.message Pydantic max_length=500. This eliminates a latent path where a long message would cause to_error_detail() itself to raise ValidationError inside the decorator's except AppException block (which would escape unhandled). Two regression tests added.

Production verification

• cve_lookup with bad input → isError: true + code: invalid_argument
• sigma_rule_lookup with bad UUID → isError: true + code: invalid_argument
• domain_report with malformed domain → isError: true + code: invalid_argument
• Full suite: 2376 passed (was 2374, +2 regression)
• Ruff: clean

No catalogue / schema change

• MCP_TOOL_COUNT stays at 52
• Tool list, schema, and PivotHint Literal unchanged
• No MCP Registry republish required (no contract surface change — only error envelope encoding)