Narrow fragment spreads to their type condition in MCP tool schemas

[glen-84]

Summary

• OperationToolFactory visited a named fragment's selection set without narrowing to the fragment's type condition, so a fragment on a concrete type selecting an implementation-declared field failed the field lookup on the abstract parent type (KeyNotFoundException from CreateTool). Fragment spreads now push a narrowed frame for the duration of the fragment, exactly like inline fragments.
• A spread's type condition now also feeds the output schema's requiredness: a field reached through a fragment whose type condition does not match the parent type is optional, matching the existing inline-fragment rule. This changes emitted output schemas for such operations — previously those fields were marked required.

Test plan

• New snapshot tests: a fragment on an implementation selecting an implementation-declared field resolves and is optional (previously threw), and a fragment on the parent type itself keeps its fields required.
• One existing snapshot updated deliberately: fields selected only through fragments on concrete types are no longer required, aligning spread and inline-fragment semantics.
• Full Adapters.Mcp.Tests suite passes (246 tests).

[Copilot]

Pull request overview

This PR fixes MCP tool output-schema generation for GraphQL operations that use named fragment spreads by narrowing field resolution to the fragment’s type condition (matching inline-fragment behavior) and aligning requiredness rules so fields selected only through a type-conditional fragment are not marked required.

Changes:

• Narrow the OperationToolFactory walker frame to a fragment spread’s type condition while visiting that fragment’s selection set.
• Use a fragment spread’s type condition when computing SelectionState so requiredness matches inline-fragment semantics.
• Add snapshot coverage for fragment spreads selecting implementation-declared fields and for spreads on the parent type; update one existing snapshot to reflect new requiredness.

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
src/HotChocolate/Adapters/src/Adapters.Mcp.Core/OperationToolFactory.cs	Narrows walker type during fragment spread visitation and passes fragment type condition into selection-state computation.
src/HotChocolate/Adapters/src/Adapters.Mcp.Core/Extensions/FieldNodeExtensions.cs	Extends selection-state computation to treat fragment spreads as type-conditional when their type condition differs from the parent type.
src/HotChocolate/Adapters/test/Adapters.Mcp.Tests/OperationToolFactoryTests.cs	Adds tests covering spread narrowing to an implementation type and requiredness when spreading on the parent type.
src/HotChocolate/Adapters/test/Adapters.Mcp.Tests/snapshots/OperationToolFactoryTests.CreateTool_FragmentSpreadsSelectSameField_MergesIntoSingleProperty.json	Updates requiredness snapshot to reflect optional fields when reached only via concrete-type fragment spreads.
src/HotChocolate/Adapters/test/Adapters.Mcp.Tests/snapshots/OperationToolFactoryTests.CreateTool_FragmentSpreadSelectsImplementationDeclaredField_NarrowsToTypeCondition.json	New snapshot for successful schema generation when a concrete-type fragment selects an implementation-declared field.
src/HotChocolate/Adapters/test/Adapters.Mcp.Tests/snapshots/OperationToolFactoryTests.CreateTool_FragmentSpreadOnParentType_KeepsFieldsRequired.json	New snapshot ensuring spreads on the parent type keep fields required.

---

💡 Add a code-review agent skill or configure MCP servers for context-aware, tailored reviews. Learn more in the docs.

[github-actions]

Patch coverage

100.0% of changed lines covered (20/20)

File	Covered	Changed	Patch %
…/src/Adapters.Mcp.Core/Extensions/FieldNodeExtensions.cs	5	5	100.0% 🟢
…/Adapters/src/Adapters.Mcp.Core/OperationToolFactory.cs	15	15	100.0% 🟢

---

Project coverage: 57.7% (286089/495867 lines)