Handle message.Contents named type in agentResultToMCPCallToolResult

[PratikDhanave (PratikDhanave)]

What

agentResultToMCPCallToolResult in tool/mcptool/mcp.go type-switched on case message.Content: and case []message.Content: but had no case for message.Contents — the named slice type (type Contents []Content) that Message.Contents is actually declared as.

A Go type switch matches on dynamic type identity, so a message.Contents value does not match case []message.Content:, nor does it satisfy the Content interface. It fell through to default -> structuredResultToMCPCallToolResult, which json.Marshaled the whole slice into a single TextContent. That:

• collapsed multiple content blocks into one,
• encoded image/audio DataContent as base64 JSON strings instead of mcp.ImageContent/mcp.AudioContent, and
• never set IsError for *message.ErrorContent blocks.

The same defect affected the FunctionResultContent.Result path via functionResultToMCPCallToolResult.

Fix

Add case message.Contents: that converts to []message.Content and delegates to the existing correct branch. That branch already iterates each block, calls agentContentToMCPContent per item (mapping image/audio/resource types), and sets IsError for error content. Minimal and surgical.

Parity

This restores the .NET/Python semantics of surfacing each content block individually to MCP (distinct image/audio/text blocks and an error flag), rather than one opaque JSON text block.

Testing

Added TestAddToolReturnsNamedContentsSlice in tool/mcptool/mcp_test.go: a stub tool returns message.Contents{TextContent, image DataContent, ErrorContent} and the test asserts the resulting mcp.CallToolResult has three blocks — distinct *mcp.TextContent and *mcp.ImageContent (not one JSON text) — with IsError == true. The test fails before the fix (one collapsed block) and passes after. go build ./..., go vet ./tool/mcptool/..., and go test ./tool/mcptool/... are green.

[Copilot]

Pull request overview

Fixes MCP tool-result conversion so message.Contents (the named slice type used by Message.Contents) is handled explicitly in agentResultToMCPCallToolResult, preserving per-block content semantics (text/image/audio blocks) and proper error signaling.

Changes:

• Add a case message.Contents: branch that converts to []message.Content and reuses the existing multi-content handling.
• Add a regression test ensuring a tool returning message.Contents yields multiple MCP content blocks and sets IsError when an error block is present.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
tool/mcptool/mcp.go	Adds explicit handling for message.Contents so type-switching doesn’t fall back to JSON collapsing behavior.
tool/mcptool/mcp_test.go	Adds a regression test covering named-slice (message.Contents) tool results, including image conversion and error-flag propagation.

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[github-actions]

Parity Review — Approved

This PR fixes a bug in the unexported agentResultToMCPCallToolResult function where message.Contents (the named slice type type Contents []Content) fell through to the default branch instead of matching case []message.Content:. No exported Go APIs were added or changed.

Cross-repo parity: The fix aligns Go with .NET and Python semantics — surfacing each content block individually as distinct TextContent, ImageContent/AudioContent, and ErrorContent (with IsError set) rather than collapsing them into a single opaque JSON text block.

Labels: public-api-change label is not needed; no exported API surface changed. The existing parity-approved label is correct.

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