[Oxygen56]

Fixes #1916

Summary

Tools wrapped in DelegatingMcpServerTool now advertise an output schema matching their structured content to clients on the 2025-06-18 and 2025-11-25 protocols.

Unwrap the delegation chain only to identify SDK-created tools, then apply the existing legacy schema transform to the outer tool's ProtocolTool. Wrapper descriptions and schema constraints are preserved. Custom tools and 2026-07-28 clients retain their existing behavior. No public API is added.

Verification

Validated with the original .NET project/MSBuild and xUnit/VSTest entries, using the prepared local dependencies without restore or downloads. Builds retained warnings-as-errors and included the referenced analyzers, hosting/task/app libraries, and test-server prerequisites.

• Built tests/ModelContextProtocol.Tests/ModelContextProtocol.Tests.csproj with --no-restore --framework net10.0 --configuration Debug: passed with zero warnings/errors on both baseline and repaired sources.
• Ran the same focused regression through dotnet test against the selected baseline (3338e88) and repaired sources. The baseline produced 10 expected schema failures and 28 passes; the repair passed all 38 cases. The actual baseline production files were used with the added tests backported temporarily; repaired source bytes were restored before the after-fix build.
• Ran the broader server/protocol suites through the project test entry: 1,333 passed, zero failed/skipped.
• git diff --check: passed; the contribution worktree is clean.

Focused test filter:
FullyQualifiedName~DelegatingToolOutputSchemaTests|FullyQualifiedName~DelegatingMcpServerToolTests|FullyQualifiedName~Sep2106ListToolsBackCompatTests

Tests used --no-build --no-restore --framework net10.0 --configuration Debug, the existing blame/crash handling and 7-minute hang budget, TRX output, and XPlat coverage collection. Coverage includes nested wrappers, wrapper overrides, nullable object and absent schemas, custom tools, both legacy protocols, and modern-client controls.

The complete platform/framework/configuration matrix, npm-backed integrations, packaging/API validation, AOT, documentation, and combined coverage workflows remain for CI. They were not run in this offline validation. A separate independent review is still required before publication.

AI assistance was used in preparing this contribution.

[jeffhandley]

On first glance, this looks good. We will take a closer look before merging, but thanks for getting this prep'd, @Oxygen56!

[tarekgh]

The production fix looks good and addresses a real compatibility issue for delegated tools with older clients.

One issue to fix before merging: the new tests use JsonSerializer.SerializeToElement(...) with anonymous objects, but the test project intentionally disables reflection-based serialization on .NET 9. This causes 15 test failures.

Could you replace those calls in DelegatingToolOutputSchemaTests.cs at lines 94-95, 161, and 172-173 with reflection-free JSON parsing? For example:

using var document = JsonDocument.Parse("""{"type":"integer","minimum":0}""");
var schema = document.RootElement.Clone();

Please keep the reflection-disabled setting unchanged. The focused tests pass on the other three target frameworks, and all 1,333 server/protocol tests pass on .NET 10.

[tarekgh]

@Oxygen56 do you have a chance to address my feedback so we can proceed? Thanks!

[Copilot]

🟡 Changes recommended

The new tests retain a short discovery timeout instead of the documented controlled-server defaults, risking avoidable CI failures.

1 open issue

• Use shared MCP test options to avoid discovery timeout failures · New

What changed in this PR

Fixes #1916 by applying legacy output-schema conversion to SDK-created tools behind delegating wrappers, keeping advertised schemas consistent with structured content for older clients.

Changes:

• Unwraps delegation chains to identify SDK-created tools while preserving the outer wrapper’s metadata and schema constraints.
• Leaves custom tools and modern-protocol clients unchanged, without adding public API.
• Adds regression coverage for nested wrappers, schema overrides, optional schemas, and protocol versions.

File	Description
tests/​ModelContextProtocol.Tests/​Server/​DelegatingToolOutputSchemaTests.cs	Adds schema and structured-content regression tests.
src/​ModelContextProtocol.Core/​Server/​McpServerImpl.cs	Recognizes wrapped SDK tools for legacy schema conversion.
src/​ModelContextProtocol.Core/​Server/​DelegatingMcpServerTool.cs	Adds internal access to the wrapped tool.
src/​ModelContextProtocol.Core/​Server/​AIFunctionMcpServerTool.cs	Allows legacy conversion of the outer wrapper’s protocol tool.

_{Review effort: Balanced}

[Copilot]

🔵 Needs a closer look

Composition-based wrapper schemas can still cause legacy listings to advertise a result envelope that invocation does not return.

No open issues 🎉

1 previously missed issue

In code that hasn't changed since last review

Align advertised and invoked schemas for allOf wrapper overrides

src/​ModelContextProtocol.Core/​Server/​McpServerImpl.cs:1555

For an SDK tool with an object output schema, a delegating wrapper that expresses the equivalent schema as allOf: [innerSchema] now advertises a required result envelope to legacy clients. Invocation still uses the inner schema in CreateStructuredResponse and returns the bare object, so the content fails the advertised schema. Align the listing and invocation wrapping decisions while preserving wrapper constraints, and add a regression test for this override on both legacy protocols.

1 resolved since last review

• Use shared MCP test options to avoid discovery timeout failures

_{Review effort: Balanced}

[Copilot]

🟡 Changes recommended

Wrapped tools returning an explicit CallToolResult can advertise a required result envelope while passing through unwrapped structured content, causing a schema mismatch for legacy clients.

1 open issue

• Preserve explicit CallToolResult pass-through in wrapped schemas · New

_{Review effort: Balanced}

[Copilot]

🟡 Changes recommended

Raw CallToolResult pass-throughs can return unwrapped objects while legacy tool listings require a result wrapper, breaking schema–response compatibility.

1 open issue

• Preserve raw CallToolResult pass-through in AIFunction creation · New

1 resolved since last review

• Preserve explicit CallToolResult pass-through in wrapped schemas

_{Review effort: Balanced}

[Copilot]

🟡 Changes recommended

Output-schema normalization mishandles nullable tool results and recursive nullable children, causing advertised schemas to reject responses sent to legacy clients.

2 open issues

• Nullable callbacks bypass schema normalization and emit null results · New
• Root object normalization narrows recursive nullable schemas · New

1 resolved since last review

• Preserve raw CallToolResult pass-through in AIFunction creation

_{Review effort: Balanced}

[Copilot]

🔵 Needs a closer look

Expanded schema normalization and reflection-based delegation require independent runtime validation of recursive references, Native AOT behavior, and legacy-client compatibility.

No open issues 🎉

2 resolved since last review

• Root object normalization narrows recursive nullable schemas
• Nullable callbacks bypass schema normalization and emit null results

_{Review effort: Balanced}