Summary

PR #2342 changed schema validation behavior in validate_tool_schema and validate_prompt_schema to improve MCP server compatibility. However, this introduces potential issues that should be addressed.

Issues Identified

High: Invalid schemas can now be persisted

Location: mcpgateway/db.py:5252, mcpgateway/db.py:5312

Previously, invalid schemas raised ValueError and were rejected at registration time. Now they only log a warning and are persisted. When these tools/prompts are later invoked, tool_service.py:175 calls check_schema() which will raise SchemaError, potentially causing 500 errors.

Impact: Runtime failures instead of upfront validation failures.

Medium: Default draft changed from Draft7 to Draft2020-12

Location: mcpgateway/db.py:5252, mcpgateway/db.py:5312

When a schema lacks $schema, validator_for() defaults to Draft202012Validator. The previous code always used Draft7Validator.

Impact:

• Keywords valid in Draft7 but changed in 2020-12 may behave differently
• Example: exclusiveMinimum is boolean in Draft4/6/7 but must be a number in 2019-09/2020-12

Low: Hardcoded validator names may cause false warnings

Location: mcpgateway/db.py:5239, mcpgateway/db.py:5299

The allowed_validator_names set is hardcoded. Future jsonschema versions may add new validators that work correctly but will trigger "unsupported" warnings.

Recommendations

1. Consider making validation strictness configurable via environment variable (e.g., SCHEMA_VALIDATION_STRICT=true)
2. Consider explicit default draft when $schema is missing (e.g., default to Draft7 for backwards compatibility)
3. Document the behavior change so users understand schemas are validated at registration (warning) and runtime (error)

Related

• Closes: N/A (tracking issue)
• Related PR: Improved Tool Schema Validation for Broader MCP Server Compatibility #2342
• Original issue: [BUG]: Few MCP servers are not supported - Error when adding gateway #2322