SEP-2106: Tools inputSchema & outputSchema Conform to JSON Schema 2020-12#2106
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olaservo wants to merge 20 commits intomodelcontextprotocol:mainfrom
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SEP-2106: Tools inputSchema & outputSchema Conform to JSON Schema 2020-12#2106olaservo wants to merge 20 commits intomodelcontextprotocol:mainfrom
inputSchema & outputSchema Conform to JSON Schema 2020-12#2106olaservo wants to merge 20 commits intomodelcontextprotocol:mainfrom
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Add SEP proposing to loosen inputSchema, outputSchema, and structuredContent restrictions to enable full JSON Schema 2020-12 compliance. This allows array responses and schema composition keywords (anyOf, oneOf, allOf). Relates to modelcontextprotocol#834 Co-Authored-By: John McBride <[email protected]> Co-Authored-By: Claude Opus 4.5 <[email protected]>
… support - inputSchema: Allow any valid JSON Schema (enables oneOf/anyOf/allOf at root) - outputSchema: Allow any valid JSON Schema (enables array schemas) - structuredContent: Allow any JSON value (enables array responses) - Update tools.mdx documentation - Add example files demonstrating new capabilities Reference: modelcontextprotocol#834 Co-Authored-By: John McBride <[email protected]> Co-Authored-By: Claude Opus 4.5 <[email protected]>
- Updated inputSchema spec to allow any valid JSON Schema (full 2020-12 compliance) - Replaced get-array-content with get-weather-forecast tool - Weather forecast matches the exact example from Motivation section - Updated npm package references to latest versions: - SDK: @olaservo/[email protected] - Server: @olaservo/[email protected] Co-Authored-By: Claude Opus 4.5 <[email protected]>
- Renamed 0000-json-schema-2020-12.md to 2106-json-schema-2020-12.md - Updated SEP number to 2106 - Updated PR link to modelcontextprotocol#2106 Co-Authored-By: Claude Opus 4.5 <[email protected]>
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- Update title to match original: "Tools inputSchema & outputSchema Conform to JSON Schema 2020-12" - inputSchema: Keep type: "object" required, but allow additional JSON Schema properties - outputSchema: Fully flexible (any valid JSON Schema) - structuredContent: Any JSON value (unknown) - Update examples to show composition with type: "object" Co-Authored-By: John McBride <[email protected]> Co-Authored-By: Claude Opus 4.5 <[email protected]>
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inputSchema & outputSchema Conform to JSON Schema 2020-12
Aligns schema.ts with John's original PR modelcontextprotocol#881: - inputSchema: Keep type: "object" required, allow additional properties - outputSchema: Any valid JSON Schema (with $schema field) - Regenerate schema.json and schema.mdx Co-Authored-By: John McBride <[email protected]> Co-Authored-By: Claude Opus 4.5 <[email protected]>
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SamMorrowDrums
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Feb 5, 2026
Maintainer Activity CheckHi @olaservo! You're assigned to this SEP but there hasn't been any activity from you in 14 days. Please provide an update on:
If you're no longer able to sponsor this SEP, please let us know so we can find another maintainer. This is an automated message from the SEP lifecycle bot. |
- Replace "SEP-834" references with "SEP-2106" throughout - Update JSON-serialized content examples to model-friendly summaries - Add cross-references to modelcontextprotocol#1906, TS SDK modelcontextprotocol#1149, and SEP-2200 - Create docs/community/seps/2106-json-schema-2020-12.mdx Co-Authored-By: Claude Opus 4.6 <[email protected]>
Run `npm run generate:seps` to update index, docs.json, and .mdx to match the CI-expected output. Co-Authored-By: Claude Opus 4.6 <[email protected]>
Co-Authored-By: Claude Opus 4.6 <[email protected]>
olaservo
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Feb 16, 2026
| "content": [ | ||
| { | ||
| "type": "text", | ||
| "text": "Found 2 users: Alice ([email protected]) and Bob ([email protected])." |
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Note - example is deliberately aligned with this clarification of content vs structured content audience and format.
Maintainer Activity CheckHi @olaservo! You're assigned to this SEP but there hasn't been any activity from you in 20 days. Please provide an update on:
If you're no longer able to sponsor this SEP, please let us know so we can find another maintainer. This is an automated message from the SEP lifecycle bot. |
Resolve merge conflicts keeping the PR's open JSON Schema 2020-12 approach for inputSchema/outputSchema (index signatures instead of explicit properties/required fields). Updated SEP index to reflect 25 Final SEPs + 1 Draft. Co-Authored-By: Claude Opus 4.6 <[email protected]>
Co-Authored-By: Claude Opus 4.6 <[email protected]>
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Summary
Note: This PR converts John McBride's original SEP proposal (PR #881) from issue #834 to the new SEP format established by SEP-1850. The original proposal and community discussion remain in PR #881—this PR formalizes it under the new process with a reference implementation.
inputSchema: Keeptype: "object"required (since tool arguments are objects), but allow any additional JSON Schema properties to support composition keywords (anyOf,oneOf,allOf, etc.)outputSchema: Fully support JSON Schema 2020-12 (arrays, primitives, objects, compositions)structuredContent: Accept any JSON value validated byoutputSchemaMotivation
The current MCP specification restricts tool schemas in ways that conflict with full JSON Schema support and create real friction in the ecosystem.
The Problem: A weather API tool that returns hourly forecasts cannot return the natural response:
[ { "hour": "09:00", "temp": 68, "conditions": "sunny" }, { "hour": "10:00", "temp": 72, "conditions": "partly cloudy" } ]Instead, developers must wrap arrays in unnecessary container objects—conflicting with common REST API patterns (GitHub Events API, AccuWeather, standard collection endpoints).
SDK Evidence: This isn't theoretical friction. FastMCP, one of the most popular Python SDKs, has implemented extensive workarounds with error messages that explicitly cite "MCP spec limitations." These workarounds have caused real bugs:
$refschemas withouttype: objectbroke ALL tools on a server{"result": ...}wrapping confused usersIndustry Precedent: OpenAPI went through this same evolution. OpenAPI 3.0's "extended subset" of JSON Schema caused years of tooling confusion. OpenAPI 3.1 made the strategic decision to fully align with JSON Schema 2020-12—MCP can learn from this rather than repeating the same journey.
Backward Compatible: All existing schemas continue to work. No migration required.
Related issue: #834
Original proposal: #881
Reference Implementation
@olaservo/[email protected](source)@olaservo/[email protected](source)Testing
Reference implementation tested with demo tools demonstrating:
get-weather-forecast: Raw array output (matches SEP motivation example)get-count: Raw primitive outputfind-by-id-or-name: Flexible input patterns usingoneOfwithtype: "object"AI disclosure
This PR was written with assistance from Claude. The SEP content, reference implementations, and rationale were developed collaboratively.