tengu_mcp_server_connection_failed
A Claude Code feature flag, read out of the shipped bundle. No mined inventory carries it: everything below comes from what this site has written about it.
MCP debug output now shows which protocol version and path a server connection used.
Not Anthropic's. This is the line from the earliest changelog entry here that named it, v2.1.221.
In the changelogs
8Releases whose published page names tengu_mcp_server_connection_failed.
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v2.1.248
MCP connection-failure telemetry records where the auth header came from
MCP connection failure reports now note when auth headers came from a credential helper.
gate cited by this entry, found in this entry's text
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v2.1.224
MCP events forwarded to Datadog, with identifying fields removed and a per-server cap
MCP events are forwarded for logging with identifying fields stripped, and skipped entirely if analytics are off.
gate cited by this entry, found in this entry's text
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v2.1.224
MCP OAuth failures name the specific issuer mismatch
MCP OAuth failures now tell you it was an issuer mismatch instead of a generic error.
gate cited by this entry, found in this entry's text
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v2.1.222
Dedicated error when claude.ai rejects a session bearer token
If claude.ai rejects your session token, you now get a clear message telling you to run /login instead of endless retries.
gate cited by this entry, found in this entry's text
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v2.1.221
Connection telemetry gains negotiation and protocol fields
MCP debug output now shows which protocol version and path a server connection used.
gate cited by this entry, found in this entry's text
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v2.1.221
Failure telemetry errorCode is now normalized against a fixed allowlist
MCP connection failure reporting now uses a fixed set of error codes instead of raw values.
gate cited by this entry, found in this entry's text
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v2.1.221
MCP connection-failure telemetry records negotiation outcome
Failed MCP connections now record how the handshake was negotiated and whether it fell back.
gate cited by this entry, found in this entry's text
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v2.1.221
MCP connection-failure telemetry normalizes the error code
MCP failure codes are now cleaned up before reporting, so odd server values don't leak through.
gate cited by this entry, found in this entry's text
Gate readings
0No release here has a gate table behind it for this flag. Most published releases have none at all: the readings exist only for versions the pipeline captured flag state on.
First cited
1The earliest release whose published evidence quoted tengu_mcp_server_connection_failed was v2.1.221, 3 Aug 2026. That is the oldest release this project wrote about it, so it is a floor on the name's age and not the release that introduced it. It is not a presence reading: which builds carried the name is the table below.
Presence across releases
0The miner has never read a build containing this name. It is here because a changelog entry of ours names it, which is evidence that it existed and not evidence of which releases carried it. The inventory starts at v2.1.138 and everything older than that was never mined.
Read out of the published npm bundle release by release, and out of Anthropic's own documentation as this site captured it. Nothing on this page is a description anybody here wrote about what the feature flag does. All feature flags.