feat: relax outputSchema to accept non-object JSON Schema types (SEP-2106)#895
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branben wants to merge 4 commits into
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feat: relax outputSchema to accept non-object JSON Schema types (SEP-2106)#895branben wants to merge 4 commits into
branben wants to merge 4 commits into
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added 3 commits
June 6, 2026 13:36
… (SEP-2106) Split validate_and_strip into validate_and_strip_input (keeps type:"object" check for inputSchema) and validate_and_strip_output (accepts any JSON Schema root type per SEP-2106). Update schema_for_output to use the new output variant. Flip test_schema_for_output_rejects_primitive to assert Ok and rename to test_schema_for_output_accepts_primitive. Add test_schema_for_output_accepts_array and test_schema_for_output_strips_title_for_primitive. Update with_output_schema doc comment to remove incorrect "root type object" panic reference.
Add tests verifying schema_for_output accepts non-object types: - test_tool_builder_methods: primitive (i32), array (Vec<String>), option - test_structured_output: tool returning Json<Vec<T>> and Json<i32> - test_json_schema_detection: Json<Vec<T>>, Result<Json<Vec<T>>,E>, Json<String> - tool_traits: ToolBase::output_schema with Vec<AddOutput> output type
Add tests identified during code review: - description stripping for primitive types - composition types (Option<String> with anyOf/oneOf/null) - cache correctness (Arc::ptr_eq for repeated calls) - schema_for_input rejecting array types (not just primitives) - schema_for_output accepting unit type ()
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thanks @branben if you can run a fmt over this to get CI happy - I think this is then good to merge, thanks! |
Author
just checked on this @michaelneale happy I could contribute! |
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DaleSeo
reviewed
Jun 23, 2026
| /// Strip top-level `title`/`description` from a JSON schema for outputSchema. | ||
| /// Unlike inputSchema, outputSchema may have any JSON Schema 2020-12 root type | ||
| /// (objects, arrays, primitives, compositions) per SEP-2106. | ||
| fn validate_and_strip_output(raw: &Arc<JsonObject>) -> Arc<JsonObject> { |
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This performs no validation.
Suggested change
| fn validate_and_strip_output(raw: &Arc<JsonObject>) -> Arc<JsonObject> { | |
| fn strip_output(raw: &Arc<JsonObject>) -> Arc<JsonObject> { |
| /// # Panics | ||
| /// | ||
| /// Panics if the generated schema does not have root type "object" as required by MCP specification. | ||
| /// Panics if output schema generation fails. |
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It won't likely panic because schema_for_output no longer return Err.
| /// Top-level "title" and "description" are always removed. | ||
| pub fn schema_for_output<T: JsonSchema + std::any::Any>() -> Result<Arc<JsonObject>, String> { | ||
| thread_local! { | ||
| static CACHE_FOR_OUTPUT: std::sync::RwLock<HashMap<TypeId, Result<Arc<JsonObject>, String>>> = Default::default(); |
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I think we can cache the success value only.
Suggested change
| static CACHE_FOR_OUTPUT: std::sync::RwLock<HashMap<TypeId, Result<Arc<JsonObject>, String>>> = Default::default(); | |
| static CACHE_FOR_OUTPUT: std::sync::RwLock<HashMap<TypeId, Arc<JsonObject>>> = Default::default(); |
| assert!(tool.output_schema.is_some()); | ||
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| let schema_str = serde_json::to_string(tool.output_schema.as_ref().unwrap()).unwrap(); | ||
| assert!(schema_str.contains("\"type\":\"integer\"")); |
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To prevent false positives
Suggested change
| assert!(schema_str.contains("\"type\":\"integer\"")); | |
| assert_eq!(schema.get("type"), Some(&serde_json::json!("integer"))); |
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Summary
Relax
schema_for_outputto accept any JSON Schema 2020-12 root type (arrays, primitives, compositions) foroutputSchema, while keepingschema_for_inputenforcingtype: "object". This implements SEP-2106, which was accepted May 18 2026.PR #860 (merged Jun 2 2026) added title/desc stripping and input validation. This PR covers the remaining work: decoupling the type gate so output schemas are no longer rejected for non-object types.
Changes
crates/rmcp/src/handler/server/common.rs: Splitvalidate_and_stripinto:validate_and_strip_input— keepstype: "object"check for inputSchemavalidate_and_strip_output— strips title/desc, no type check (accepts any JSON Schema root type)crates/rmcp/src/model/tool.rs: Updatedwith_output_schemadoc comment to remove incorrect "root type object" panic referenceOption<T>), unit type,Json<T>macro path,ToolBasetrait path, cache correctness, and negative tests for input rejectionBackward Compatibility
schema_for_outputreturn type unchanged (Result<Arc<JsonObject>, String>)Verification
cargo test -p rmcp --all-features: 424 passed, 0 failedcargo clippy --all-targets --all-features -- -D warnings: No issues found