fix(mcp-apps): update ui/message to use ContentBlock[] for content#6546
Merged
angiejones merged 3 commits intomainfrom Jan 17, 2026
Merged
fix(mcp-apps): update ui/message to use ContentBlock[] for content#6546angiejones merged 3 commits intomainfrom
angiejones merged 3 commits intomainfrom
Conversation
The MCP Apps draft spec (SEP-1865) has been in flux regarding the
ui/message params.content type. After discussion with the spec authors,
the agreed format is ContentBlock[] (array of content blocks) rather
than a single ContentBlock object.
This change:
- Updates McpMethodParams['ui/message'] to expect content as ContentBlock[]
- Adds ContentBlock type supporting text, image, and resource blocks
- Updates the handler to extract text from the first text block in the array
- Returns empty object {} on success per the spec
Related spec discussions:
- modelcontextprotocol/ext-apps#48
- modelcontextprotocol/ext-apps#119
- MCP-UI-Org/mcp-ui#166
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Pull request overview
This PR fixes a bug in the MCP Apps ui/message handler where it was crashing when trying to access content.text on an array. The spec has been clarified to use ContentBlock[] instead of a single content object, and this PR updates both the types and handler implementation accordingly.
Changes:
- Updated
ui/messagetype definitions to useContentBlock[]array with support for text, image, and resource blocks - Modified handler to validate array format and extract text from the first text block
- Changed response type from status object to empty object per spec
Reviewed changes
Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.
| File | Description |
|---|---|
| ui/desktop/src/components/McpApps/types.ts | Added ContentBlock type union and updated ui/message params/response types |
| ui/desktop/src/components/McpApps/McpAppRenderer.tsx | Updated handler to process ContentBlock array and extract text content |
Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>
Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>
angiejones
approved these changes
Jan 17, 2026
fbalicchia
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Jan 23, 2026
…lock#6546) Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com> Signed-off-by: fbalicchia <fbalicchia@cuebiq.com>
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Summary
Fixes a bug where the
ui/messagehandler was crashing withTypeError: Cannot read properties of undefined (reading 'trim')when MCP Apps sent messages.Problem
The MCP Apps draft spec (SEP-1865) has been in flux regarding the
ui/messageparams.contenttype. The written documentation showedcontentas a single object:But MCP Apps were actually sending
contentas an array (this will the standard going forward):Our code was trying to access
content.textdirectly, which wasundefinedbecausecontentwas an array.Resolution
After discussion with the spec authors, the agreed format going forward is
ContentBlock[](array of content blocks):Changes
types.ts:ContentBlocktype supportingtext,image, andresourceblock typesMcpMethodParams['ui/message']to use{ role: 'user'; content: ContentBlock[] }McpMethodResponse['ui/message']to return empty object{}per specMcpAppRenderer.tsx:contentis an arrayTesting