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 · 2026-06-30T15:39:43Z

+
+/// 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.

This introduces SEP-2106: schema_for_output no longer validates or returns Result. The public signature changed, so bump major.

…ArrayTool - Replace loose schema_str.contains(...) assertions with direct schema.get("type") equality checks in test_tool_builder_methods.rs and test_structured_output.rs - Remove redundant ArrayTool fixture and its round-trip serde_json::from_str test from tool_traits.rs since schema is already Arc<JsonObject> - Drop dead schema_str variable in test_structured_output.rs

branben · 2026-07-01T20:44:26Z

Hey again @DaleSeo ! context on this commit ended up getting malformed and rotted from a memory mcp tool as I was working on this, so I apologize for a bit of the run around!

Let me know if I'm on the right path here!

DaleSeo

@branben This all looks good now. We'll hold off for now to keep the v2 migration guide focused on the 2025-11-25 alignment. So, this is approved from my side, and I'll merge it as part of the 2026-07-28 batch.

branben · 2026-07-01T23:14:23Z

@branben This all looks good now. We'll hold off for now to keep the v2 migration guide focused on the 2025-11-25 alignment. So, this is approved from my side, and I'll merge it as part of the 2026-07-28 batch.

Thanks @DaleSeo you're the GOAT!, are there any other specific pain points I could contribute to that might make the next migration easier?

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

Open

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

Brandon Bennett added 2 commits July 1, 2026 13:03

feat!: mark schema_for_output return-type change as breaking

20f981f

This introduces SEP-2106: schema_for_output no longer validates or returns Result. The public signature changed, so bump major.

DaleSeo approved these changes Jul 1, 2026

View reviewed changes

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

Uh oh!

Conversation

branben commented Jun 6, 2026

Summary

Changes

Backward Compatibility

Verification

Uh oh!

michaelneale commented Jun 9, 2026

Uh oh!

branben commented Jun 10, 2026

Uh oh!

DaleSeo left a comment

Choose a reason for hiding this comment

Uh oh!

Uh oh!

Uh oh!

Uh oh!

DaleSeo Jun 23, 2026

Choose a reason for hiding this comment

Uh oh!

branben Jun 29, 2026

Choose a reason for hiding this comment

Uh oh!

DaleSeo Jun 30, 2026

Choose a reason for hiding this comment

Uh oh!

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

Uh oh!

Uh oh!

DaleSeo commented Jun 29, 2026

Uh oh!

branben commented Jun 30, 2026

Uh oh!

DaleSeo Jun 30, 2026

Choose a reason for hiding this comment

Uh oh!

Uh oh!

Uh oh!

Uh oh!

DaleSeo Jun 30, 2026

Choose a reason for hiding this comment

Uh oh!

branben commented Jul 1, 2026

Uh oh!

DaleSeo left a comment

Choose a reason for hiding this comment

Uh oh!

branben commented Jul 1, 2026 • edited Loading Uh oh! There was an error while loading. Please reload this page.

Uh oh!

Uh oh!

Reviewers

Assignees

Labels

Projects

Milestone

Development

Uh oh!

3 participants

branben commented Jun 29, 2026 •

edited

Loading

branben commented Jul 1, 2026 •

edited

Loading