Go 'context deadline exceeded' from http.Client or gRPC (DEADLINE_EXCEEDED, 'Client.Timeout exceeded while awaiting headers'): what is timing out and how do I fix it?

Cause: A context or client timeout expired before the call finished. context.DeadlineExceeded is the error returned by Context.Err once a deadline passes. For net/http, Client.Timeout covers the whole exchange: connecting, redirects and reading the response body, and its timer keeps running after Do returns. For gRPC, the client fails the call with DEADLINE_EXCEEDED when the server runs past the deadline; gRPC has no deadline by default, so any deadline came from your code or a wrapper. Causes are a genuinely slow or overloaded server, a deadline set too short, a parent context that was already nearly expired (a child can never outlive its parent), or work happening after the deadline because a handler ignores cancellation. Fix: 1. Find which deadline fired. Check errors.Is(err, context.DeadlineExceeded); for HTTP, a *url.Error with Timeout() true. Log how long the call took. 2. Set realistic values. For gRPC, the docs advise an explicit deadline on every call, based on measured latency and confirmed by load testing. Use ctx, cancel := context.WithTimeout(parent, d) and always defer cancel(); go vet checks that cancel is used. 3. Pass the request context through: http.NewRequestWithContext(ctx, ...) and the same ctx into downstream calls, so budgets propagate. In Go and Java gRPC servers the remaining deadline is forwarded to downstream calls by default. 4. Make long server work check ctx.Done() and stop when cancelled, so you don't waste resources after the client gave up. 5. If the server is the problem, profile and fix the slow dependency; do not simply raise the timeout to a very large value or remove it. 6. If only large downloads fail, don't use a tiny Client.Timeout; use per-request contexts sized to the transfer. Verify: The call succeeds under normal load within its deadline, and under a deliberately slowed dependency it fails fast with the expected error rather than hanging. Sources: https://pkg.go.dev/context https://pkg.go.dev/net/http https://grpc.io/docs/guides/deadlines/