sccache "Server startup failed" in CI

sccache could not bring up its background server. That blocks every compile routed through the wrapper. The cause is usually a backend it cannot reach at startup or a port already bound by a prior process.

What this error means

The build fails early with "sccache: error: Server startup failed" or "failed to start server", before any file compiles.

sccache: error: Server startup failed: cache storage failed to read:
Invalid Access Key Id

Common causes

The backend cannot be initialized at startup

sccache validates its storage (S3/GCS/Redis/GHA) when the server starts. Bad or missing credentials make startup fail immediately.

The server port is already in use

A leftover sccache process or a conflicting port binding prevents the new server from starting.

How to fix it

Read the startup error with debug logging

1. Set SCCACHE_LOG and SCCACHE_ERROR_LOG, then start the server.
2. Inspect whether the failure is credentials or a bound port.
3. Fix the backend env or kill the stale server and retry.

export SCCACHE_LOG=debug
export SCCACHE_ERROR_LOG=$RUNNER_TEMP/sccache.log
sccache --stop-server 2>/dev/null || true
sccache --start-server

Fall back to local disk to isolate the cause

Temporarily use the local disk backend; if startup then succeeds, the problem is your remote backend config.

unset SCCACHE_GHA_ENABLED SCCACHE_BUCKET
export SCCACHE_DIR="$RUNNER_TEMP/sccache"
sccache --start-server

How to prevent it

• Validate backend credentials before the build starts.
• Stop any stale server with sccache --stop-server at job start.
• Enable SCCACHE_ERROR_LOG so startup failures are diagnosable.

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