feat: relax outputSchema to accept non-object JSON Schema types (SEP-2106) by branben · Pull Request #895 · modelcontextprotocol/rust-sdk

branben · 2026-06-06T18:39:51Z

Summary

Relax schema_for_output to accept any JSON Schema 2020-12 root type (arrays, primitives, compositions) for outputSchema, while keeping schema_for_input enforcing type: "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: Split validate_and_strip into:
- validate_and_strip_input — keeps type: "object" check for inputSchema
- validate_and_strip_output — strips title/desc, no type check (accepts any JSON Schema root type)
crates/rmcp/src/model/tool.rs: Updated with_output_schema doc comment to remove incorrect "root type object" panic reference
Tests: 17 new tests across 4 test files covering primitives, arrays, compositions (Option<T>), unit type, Json<T> macro path, ToolBase trait path, cache correctness, and negative tests for input rejection

Backward Compatibility

schema_for_output return type unchanged (Result<Arc<JsonObject>, String>)
All existing tests for object output types still pass
Change is strictly relaxing: types that were previously rejected are now accepted

Verification

cargo test -p rmcp --all-features: 424 passed, 0 failed
cargo clippy --all-targets --all-features -- -D warnings: No issues found

michaelneale · 2026-06-09T06:03:26Z

thanks @branben if you can run a fmt over this to get CI happy - I think this is then good to merge, thanks!

branben · 2026-06-10T12:51:18Z

thanks @branben if you can run a fmt over this to get CI happy - I think this is then good to merge, thanks!

just checked on this @michaelneale happy I could contribute!

DaleSeo

Thanks for implementing the sep, @branben! This looks good overall. I have some minor comments.

DaleSeo · 2026-06-23T15:22:26Z

+    assert!(tool.output_schema.is_some());
+
+    let schema_str = serde_json::to_string(tool.output_schema.as_ref().unwrap()).unwrap();
+    assert!(schema_str.contains("\"type\":\"integer\""));


To prevent false positives

Suggested change

assert!(schema_str.contains("\"type\":\"integer\""));

assert_eq!(schema.get("type"), Some(&serde_json::json!("integer")));

Fixed in latest push. ✅

@branben You marked this as resolved but it still doesn't seem to be fixed!

branben · 2026-06-29T01:35:52Z

Thanks for the review @DaleSeo! All 4 comments addressed in the latest push:

strip_output naming (line 78) — Added strip_output() that strips title/description without validation, per your suggestion.
Dead panic path (line 321) — schema_for_output now returns Arc<JsonObject> directly (no Result). The unwrap_or_else panic path is removed from with_output_schema, ToolBase::output_schema(), and the macros.
Cache success only (line 125) — Cache changed to HashMap<TypeId, Arc<JsonObject>> — success values only, as suggested.
Test false positives (line 72) — Tightened to assert_eq!(schema.get("type"), Some(&serde_json::json!("object"))) to prevent false positives.

The test_schema_for_output_rejects_primitive test was also updated to test_schema_for_output_accepts_primitive since non-object output schemas are now valid per SEP-2106.

Please let me know where else I can make meaningful contributions if there's a specific pain point that's bugging the team!

DaleSeo · 2026-06-29T17:39:33Z

@branben You might've forgotten to push those fixes? I'm still seeing the original code. 😅
Could you double-check it? Happy to re-review as soon as it shows up.

…urns Result - Add strip_output() that strips title/description without validating type (Dale modelcontextprotocol#1) - Change schema_for_output to return Arc<JsonObject> instead of Result (Dale modelcontextprotocol#2) - Cache only Arc<JsonObject> success values, not Result (Dale modelcontextprotocol#3) - Remove dead unwrap_or_else panic paths in with_output_schema, ToolBase, and macros - Tighten test assertions from contains to assert_eq on type field (Dale modelcontextprotocol#4) - Update test_schema_for_output_rejects_primitive to accept_primitive (SEP-2106) Co-authored-by: Orca <help@stably.ai>

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 ()

branben · 2026-06-30T03:46:44Z

@branben You might've forgotten to push those fixes? I'm still seeing the original code. 😅 Could you double-check it? Happy to re-review as soon as it shows up.

sorry about this @DaleSeo, could you check now and let me know if it's showing the fixes? Thank you again!

DaleSeo · 2026-06-30T15:32:38Z

+    assert!(tool.output_schema.is_some());
+
+    let schema_str = serde_json::to_string(tool.output_schema.as_ref().unwrap()).unwrap();
+    assert!(schema_str.contains("\"type\":\"integer\""));


@branben You marked this as resolved but it still doesn't seem to be fixed!

DaleSeo · 2026-06-30T15:34:47Z

+    let schema_str = serde_json::to_string(schema).unwrap();
+    assert!(schema_str.contains("integer"));


Suggested change

let schema_str = serde_json::to_string(schema).unwrap();

assert!(schema_str.contains("integer"));

assert_eq!(schema.get("type"), Some(&serde_json::json!("integer")));

DaleSeo · 2026-06-30T15:36:34Z

+        let schema_value: serde_json::Value = serde_json::from_str(
+            &serde_json::to_string(&*schema).expect("failed to serialize schema"),
+        )
+        .expect("failed to parse schema JSON");
+        assert_eq!(schema_value["type"], "array");


schema is already an Arc<JsonObject> so we can read the key directly.

Suggested change

let schema_value: serde_json::Value = serde_json::from_str(

&serde_json::to_string(&*schema).expect("failed to serialize schema"),

)

.expect("failed to parse schema JSON");

assert_eq!(schema_value["type"], "array");

assert_eq!(schema.get("type"), Some(&serde_json::json!("array")));

DaleSeo · 2026-06-30T15:38:37Z

+    impl SyncTool<TraitBasedToolServer> for ArrayTool {
+        fn invoke(
+            _service: &TraitBasedToolServer,
+            _param: Self::Parameter,
+        ) -> Result<Self::Output, Self::Error> {
+            Ok(vec![])
+        }
+    }
+    impl AsyncTool<TraitBasedToolServer> for ArrayTool {
+        async fn invoke(
+            _service: &TraitBasedToolServer,
+            _param: Self::Parameter,
+        ) -> Result<Self::Output, Self::Error> {
+            Ok(vec![])
+        }
+    }


What are these for?

DaleSeo · 2026-06-30T15:39:43Z

+
+/// Generate and strip a JSON schema for outputSchema (top-level "title" and
+/// "description" are removed; output schemas are not restricted to root type "object").
+pub fn schema_for_output<T: JsonSchema + std::any::Any>() -> Arc<JsonObject> {


Public API Check is red in CI because the schema_for_output return-type change is a breaking change:

-pub fn ...::schema_for_output<...>() -> Result<Arc<JsonObject>, String> +pub fn ...::schema_for_output<...>() -> Arc<JsonObject> Error: The API diff is not allowed as per --deny: Changed items not allowed

That's expected and fine, and the plan is to batch all 2026-07-28 spec breaking changes into v3.0. So this SEP-2106 implementation belongs in that major.

We need to mark the introducing commit breaking though replacing feat: with feat!: so the CI gate computes major and Public API Check passes.

branben requested a review from a team as a code owner June 6, 2026 18:39

github-actions Bot added T-test Testing related changes T-core Core library changes T-handler Handler implementation changes T-model Model/data structure changes labels Jun 6, 2026

DaleSeo added the 2026-07-28 label Jun 23, 2026

DaleSeo linked an issue Jun 23, 2026 that may be closed by this pull request

Implement SEP-2106: Tool schemas conform to JSON Schema 2020-12 #874

Open

4 tasks

DaleSeo reviewed Jun 23, 2026

View reviewed changes

This was referenced Jun 23, 2026

feat!: relax tool result structuredContent type (SEP-2106) #919

Merged

feat!: relax tool result structuredContent type (SEP-2106) #933

Draft

branben force-pushed the feat/sep-2106-output-schema-non-object branch from 47e5d23 to 6588978 Compare June 29, 2026 21:19

github-actions Bot added the T-macros Macro changes label Jun 29, 2026

Brandon Bennett and others added 3 commits June 29, 2026 15:19

branben force-pushed the feat/sep-2106-output-schema-non-object branch from 6588978 to 28c2bcc Compare June 29, 2026 22:34

DaleSeo reviewed Jun 30, 2026

View reviewed changes

	assert!(schema_str.contains("\"type\":\"integer\""));
	assert_eq!(schema.get("type"), Some(&serde_json::json!("integer")));

		let schema_str = serde_json::to_string(schema).unwrap();
		assert!(schema_str.contains("integer"));

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branben commented Jun 6, 2026

Summary

Changes

Backward Compatibility

Verification

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michaelneale commented Jun 9, 2026

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branben commented Jun 10, 2026

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branben commented Jun 29, 2026 • edited Loading Uh oh! There was an error while loading. Please reload this page.

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DaleSeo commented Jun 29, 2026

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branben commented Jun 29, 2026 •

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