[jmagar]

Summary

Custom extension responses with _meta can match CallToolResult and lose their extension fields during ServerResult decoding.

Preserve those responses as CustomResult when unknown fields accompany _meta and no tool-specific field identifies the response. Keep legacy _meta-only tool results compatible with #1103.

_meta only                              -> CallToolResult
_meta + custom fields, no tool fields    -> CustomResult
content, structuredContent, or isError   -> CallToolResult

These rules also apply when resultType: complete is present. A present structuredContent: null still identifies a tool result.

Merged current upstream main into this branch. The resulting diff changes only the classifier and regression tests.

Evidence

• Before: The original patch failed the existing legacy _meta-only test and the new compatibility test: 13 passed, 2 failed.
  After: All 15 wire-compatibility tests passed.
• Added coverage for legacy and complete results, custom fields with null or false values, and genuine tool results with unknown fields.
• Rust 1.96.1, all public features except local: 1,434 unit/integration tests and 59 doctests passed; 18 ignored.
• Rust 1.96.1, all features including local: 1,024 tests and doctests passed; 15 ignored.
• Strict Clippy passed for all rmcp targets with and without local.
• Stable formatting and git diff --check passed. Nightly formatting was not verified locally.

The non-local doctest run passed separately after a temporary-storage quota failure. These are local results; GitHub CI is separate.

Merge Danger

Door: Two-way

The change can be reverted. No public types or method signatures change.

Blast Radius: Decoding

Responses that contain _meta and unknown fields now fall through to CustomResult when they lack tool-specific fields. Legacy _meta-only responses remain tool results. Tool-specific fields retain priority, so this change does not preserve custom fields in responses that also match a genuine tool result.

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[jmagar]

We hit this too: custom results carrying _meta were being decoded as CallToolResult, and this change fixes the core bug for us. One catch: on current main it trips test_result_type_wire::server_result_should_accept_legacy_call_tool_result_when_only_meta_present (added in #1103), which keeps a _meta-only response as a tool result. We have a version that keeps that case and only falls through to a custom result when unrecognized fields come with _meta, with regression tests. Happy to push it here or open it as a separate PR crediting this one, whichever you prefer.

Good catch, no that wasn't intentional. Definitely feel free to push it here. Appreciate your contributions!