What
MCP (Model Context Protocol) is how Claude Code connects to outside tools. A result can carry a block of extra information called _meta.
When a call to an MCP server is rejected, the error result now includes the rejection's own metadata as _meta. Before, the result carried only the rejection text. That text is now also taken from the same rejection object.
Why
Programs that receive the result can now read structured details about the rejection, not just a message.
Names in the bundle_meta
The entry above is what we published on the day. These lines were added later, as Anthropic's own pages caught up, and they sit beside the original rather than replacing it.
Confirmed since
Anthropic's documentation has since written up _meta, on Agent SDK reference - TypeScript.
* An MCP tool whose `structuredContent` serializes to more than 1,048,576 characters of JSON: Claude Code leaves `structuredContent` off `tool_use_result` and sets `structuredContentOmitted: true` in its place, so your application can tell…agent-sdk/typescript see the edit
One source agreesOne thing we can check says the same as this entry.
Anthropic's documentation agrees
Anthropic's documentation has since written up _meta, on Agent SDK reference - TypeScript.