fix(mcp): serialize non-object tool outputs as JSON instead of throwing in mcpToModelOutput - #23830

Background

#23825 reports that mcpToModelOutput crashes on any tool output that is not an object. The guard meant to fall back to JSON serialization runs 'content' in result first, and the in operator throws a TypeError on scalars (string, number, boolean, null) before the fallback can run. Same defect as #22630, reached through a different producer: a subagent tool result rather than a schema transform.

Opening with the reproduction and regression test the report points at, plus the minimal guard.

Summary

• mcpToModelOutput now checks typeof result !== 'object' || result === null before the in check, so scalar outputs take the JSON fallback ({ type: 'json', value }) instead of throwing.
• Regression test covering 7, 'plain', null, and true outputs.
• Reproduction examples/ai-functions/src/reproduction/issue-23825-mcp-scalar-tool-output.ts: an MCP tool whose outputSchema transforms its structured payload into the scalar 7. Before the fix it crashes with TypeError: Cannot use 'in' operator to search for 'content' in 7; after the fix it returns {"type":"json","value":7}.

Contributor Credit

• @Dilergore for the report and stack trace in mcpToModelOutput throws on any non-object output — second independent trigger, from subagent tool results #23825
• @brennanbutler01 for the earlier MCP typed tool outputs are misread as raw envelopes, dropping fields or throwing for scalars #22630 report of the same defect

End-to-End Verification

• Reproduction script crashes on main and prints {"type":"json","value":7} with the fix: pnpm tsx src/reproduction/issue-23825-mcp-scalar-tool-output.ts from examples/ai-functions
• pnpm test:node in packages/mcp: 495 tests pass, including the new regression test

Checklist

• All commits are signed (PRs with unsigned commits cannot be merged)
• Tests have been added / updated (for bug fixes / features)
• Documentation has been added / updated (for bug fixes / features)
• A patch changeset for relevant packages has been added (for bug fixes / features - run pnpm changeset in the project root)
• I have reviewed this pull request (self-review)

Fixes #23825