fix(filesystem): use valid MCP content type for read_media_file#4044
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MukundaKatta wants to merge 1 commit into
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fix(filesystem): use valid MCP content type for read_media_file#4044MukundaKatta wants to merge 1 commit into
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read_media_file returned content with type: "blob", which is not a
valid MCP content type per the 2025-11-25 spec (only text, image,
audio, resource_link, and resource are allowed). Strict clients
rejected the response at the transport layer with
"MCP error -32602: Invalid tools/call result".
Map the resolved MIME type to a valid content type:
- image/* -> { type: "image", data, mimeType }
- audio/* -> { type: "audio", data, mimeType }
- other -> { type: "resource", resource: { uri, mimeType, blob } }
Update the tool's outputSchema to match the new union shape and add
integration tests covering both the image and arbitrary-binary paths.
Closes modelcontextprotocol#4029.
LuuOW
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Jun 13, 2026
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Technical audit: Verified MCP server implementation for consistency with current SDK patterns.
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Can it be fixed that it reads images as base64 thus overflowing any context window, while it should instead send it as image to the vision model? Otherwise it pretty much impossible to use if you have to worry about image sizes. |
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Closes #4029.
read_media_filereturned content withtype: "blob", which is not a valid MCP content type, so strict clients drop the response with "MCP error -32602: Invalid tools/call result". Mapped the file's MIME type to the correct MCP content (imagefor images,audiofor audio,resourcewith embedded blob for other binaries) and updatedoutputSchemato match.Tests added in
src/filesystem/__tests__/structured-content.test.ts.