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Improved Tool Schema Validation for Broader MCP Server Compatibility#2342

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kevalmahajan merged 1 commit intomainfrom
2322_json_validation_failures_gateways
Jan 23, 2026
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Improved Tool Schema Validation for Broader MCP Server Compatibility#2342
kevalmahajan merged 1 commit intomainfrom
2322_json_validation_failures_gateways

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@kevalmahajan kevalmahajan commented Jan 23, 2026 •
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🐛 Bug-fix PR

📌 Summary

Closes #2322

This PR fixes an issue with the validation of JSON Schemas attached to tools during insert/update operations causing failure of registration of some MCP servers in Context Forge Gateway.

Previously, only Draft 7 schemas were checked, and invalid schemas would raise a ValueError. The new implementation:

  • Detects the correct schema draft automatically.
  • Supports multiple drafts (Draft 4, 6, 7, 2019-09, 2020-12).
  • Logs warnings for unsupported drafts or invalid schemas instead of raising exceptions.

This improves stability and provides better visibility into schema issues.

🔁 Reproduction Steps

  1. Create or update a tool with an Draft4 (or any other than draft7) JSON Schema.
  2. Trigger an insert or update operation.
  3. Observe a runtime jsonschema.SchemaError or unclear failure behavior.

🐞 Root Cause

  • Always used Draft 7 validator, ignoring other schema drafts.
  • Raised a ValueError for invalid schemas, potentially crashing the operation.

💡 Fix Description

  1. Use jsonschema.validators.validator_for to detect the appropriate validator.
  2. Validate schemas explicitly via check_schema.
  3. Supported draft check: Verifies the detected validator is one of the explicitly supported drafts:
    1. Draft 4
    2. Draft 6
    3. Draft 7
    4. Draft 2019-09
    5. Draft 2020-12
  4. Logs a warning if the schema uses an unsupported or unknown draft.

🧪 Verification

Check Command Status
Lint suite make lint
Unit tests make test
Coverage ≥ 90 % make coverage
Manual regression no longer fails steps / screenshots

📐 MCP Compliance (if relevant)

  • Matches current MCP spec
  • No breaking change to MCP clients

✅ Checklist

  • Code formatted (make black isort pre-commit)
  • No secrets/credentials committed

@kevalmahajan kevalmahajan added wxo wxo integration bug Something isn't working labels Jan 23, 2026
madhav165
madhav165 approved these changes Jan 23, 2026
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Changes look fine

@crivetimihai crivetimihai force-pushed the 2322_json_validation_failures_gateways branch from 56ce7bf to ef295a3 Compare January 23, 2026 12:36
@crivetimihai crivetimihai mentioned this pull request Jan 23, 2026
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@kevalmahajan @crivetimihai
fix(validation): Improve schema validation for broader MCP server com…
579597a 
…patibility

- Use jsonschema.validators.validator_for to detect schema draft automatically
- Support multiple JSON Schema drafts (Draft 4, 6, 7, 2019-09, 2020-12)
- Log warnings for unsupported drafts or invalid schemas instead of raising errors
- Handle None schemas gracefully
- Apply consistent validation behavior to both tool and prompt schemas
- Add comprehensive tests for different schema drafts
- Add fallback validator logic in tool_service.py for runtime validation
- Disable MCP SDK's built-in input validation which uses strict Draft 2020-12

Closes #2322

Signed-off-by: Keval Mahajan <mahajankeval23@gmail.com>
Signed-off-by: Mihai Criveti <crivetimihai@gmail.com>
@crivetimihai crivetimihai force-pushed the 2322_json_validation_failures_gateways branch from ef295a3 to 579597a Compare January 23, 2026 13:45
@crivetimihai
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crivetimihai commented Jan 23, 2026

Additional Fixes Added

During integration testing against real MCP servers, discovered and fixed additional issues:

Changes

  1. mcpgateway/services/tool_service.py - Added fallback validator logic

    • When auto-detected validator fails check_schema(), tries older drafts (Draft7 → Draft6 → Draft4)
    • Handles schemas using legacy features like Draft 4 style exclusiveMinimum: true

  2. mcpgateway/transports/streamablehttp_transport.py - Disabled MCP SDK's built-in input validation

    • Added validate_input=False to @mcp_app.call_tool() decorator
    • The SDK internally uses jsonschema.validate() which calls check_schema() with strict Draft 2020-12
    • Gateway handles schema validation separately in tool_service.py with multi-draft support

Testing

Verified against:

  • https://custom-registration-enum-val.onrender.com/mcp/
  • https://complex-tools-openapi-1.onrender.com/mcp/

Both servers now register and invoke tools successfully.

Related

Created issue #2348 to track feedback about behavioral changes (invalid schemas now persist with warnings rather than errors).

@crivetimihai crivetimihai self-assigned this Jan 23, 2026
@kevalmahajan kevalmahajan merged commit 579d677 into main Jan 23, 2026
51 checks passed
@kevalmahajan kevalmahajan deleted the 2322_json_validation_failures_gateways branch January 23, 2026 14:22
kcostell06 pushed a commit to kcostell06/mcp-context-forge that referenced this pull request Feb 24, 2026
@kevalmahajan
fix(validation): Improve schema validation for broader MCP server com…
421741f 
…patibility (IBM#2342)

- Use jsonschema.validators.validator_for to detect schema draft automatically
- Support multiple JSON Schema drafts (Draft 4, 6, 7, 2019-09, 2020-12)
- Log warnings for unsupported drafts or invalid schemas instead of raising errors
- Handle None schemas gracefully
- Apply consistent validation behavior to both tool and prompt schemas
- Add comprehensive tests for different schema drafts
- Add fallback validator logic in tool_service.py for runtime validation
- Disable MCP SDK's built-in input validation which uses strict Draft 2020-12

Closes IBM#2322

Signed-off-by: Keval Mahajan <mahajankeval23@gmail.com>
Signed-off-by: Mihai Criveti <crivetimihai@gmail.com>
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@crivetimihai crivetimihai crivetimihai approved these changes

@madhav165 madhav165 madhav165 approved these changes

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[BUG]: Few MCP servers are not supported - Error when adding gateway

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