Omit icon sizes field for backward compatibility with older MCP clients #1645
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- Remove Sizes field from octicons.Icons() to fix compatibility with older MCP clients - Older clients like Cursor expect sizes to be a string, not an array - The 2025-11-25 MCP spec changed sizes from string to array - Omitting the optional Sizes field makes icons compatible with all clients - Update tests and toolsnaps to reflect the change Fixes #1644 Co-authored-by: SamMorrowDrums <[email protected]>
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[WIP] Add SDK version recording in initialize process
Omit icon sizes field for backward compatibility with older MCP clients
Dec 18, 2025
SamMorrowDrums
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Dec 18, 2025
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Pull request overview
This PR removes the sizes field from icon definitions to maintain backward compatibility with older MCP clients. The MCP spec changed on 2025-11-25 from requiring Icon.sizes as a string to an array format. Since the SDK now uses the array format but older clients like Cursor v2.2.36 still expect a string, omitting this optional field prevents schema validation errors during initialization while maintaining icon functionality through the source and mimeType fields.
Key Changes
- Removed
Sizesfield assignment inocticons.Icons()function - Updated tests to verify the
Sizesfield remains empty - Regenerated toolsnaps for the 5 tools with explicit icons
Reviewed changes
Copilot reviewed 7 out of 7 changed files in this pull request and generated no comments.
Show a summary per file
| File | Description |
|---|---|
| pkg/octicons/octicons.go | Removed Sizes field assignments from both light and dark icon variants in the Icons() function; added documentation explaining the backward compatibility rationale |
| pkg/octicons/octicons_test.go | Updated test assertions to verify Sizes field is empty using assert.Empty() with clarifying comments |
| pkg/github/toolsnaps/assign_copilot_to_issue.snap | Removed sizes array from icon definitions in tool schema snapshot |
| pkg/github/toolsnaps/fork_repository.snap | Removed sizes array from icon definitions in tool schema snapshot |
| pkg/github/toolsnaps/merge_pull_request.snap | Removed sizes array from icon definitions in tool schema snapshot |
| pkg/github/toolsnaps/request_copilot_review.snap | Removed sizes array from icon definitions in tool schema snapshot |
| pkg/github/toolsnaps/star_repository.snap | Removed sizes array from icon definitions in tool schema snapshot |
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Summary
Removes the
sizesfield from icon definitions to fix connection failures with older MCP clients that expect a string instead of an array.Why
The 2025-11-25 MCP spec changed
Icon.sizesfrom a string to an array. The server uses SDK v1.2.0-pre.1 which implements the array format, but clients like Cursor v2.2.36 still expect a string, causing schema validation errors during initialization.What changed
Sizesfield inocticons.Icons()functionMCP impact
Tools with icons now omit the
sizesfield. Icons remain functional viasource(data URI) andmimeTypefields.Prompts tested (tool changes only)
Not applicable - change affects icon metadata only, not tool functionality.
Security / limits
Lint & tests
./script/lint./script/testDocs
Icon field is internal metadata; no user-facing documentation required.
Original prompt
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