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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 v2.1.234 17 Aug 2026

MCP failures are sorted into a cause taxonomy for reporting

Under the hood

Useful2 Signal2

MCP

MCP failures are now sorted into causes like server down, login problem or bad config for reporting.

What

Failures from MCP servers are now classified as downstream_error, downstream_unreachable, user_auth, user_config or other and attached to tool-error telemetry, so it is possible to tell a server that is down from a login or config problem. Nothing you see changes.

Details

Classification uses error classes, JSON-RPC and HTTP codes, and a curated set of network error codes including ECONNREFUSED, ERR_PROXY_TUNNEL and ESOCKETTIMEDOUT, following up to four chained causes.
The classification is also attached to "MCP server is not connected" and MCP tool timeout errors.
MCP connect telemetry gained flags for a suppressed listen and a denylisted negotiation.

Evidence

downstream_unreachable, mcpErrorSource

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

See this entry in the whole of v2.1.234 →