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DisclaimerUnofficial, and not affiliated with Anthropic. Nearly all of this is read straight out of what ships: npm bundles, captured prompts, published docs. Anthropic's own notes go in verbatim, marked as theirs. The rest is my reading, and every entry carries the strings behind it. If one looks wrong, vote it down and say why.

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Claude Code v2.1.246

MCP calls made over the control channel report client cancellation

You'll notice
Useful2 Signal1
MCP

An interrupted MCP call over the SDK control channel now returns a real error naming the server.

What

When a tool call issued through the SDK control channel is interrupted, the handler now returns an error naming the MCP server instead of a normal, empty-looking content response.

Details
  • The message is built at runtime from the fixed text "mcp_call cancelled by client: " followed by the server name.
  • Triggered by an interrupted field on the tool result; unconditional for control-channel MCP calls.
Evidence

mcp_call cancelled by client:

Strings lifted out of the shipped bundle, so the claim above can be checked against them.

See this entry in the whole of v2.1.246 →