thanos-io/thanos · error

bad cached type

Error message

bad cached type

What it means

toResponse unmarshals a cached Any-wrapped protobuf message and asserts it implements the queryrange.Response interface. If the cached entry's type name decodes to a message that is not a known Response type (corrupt or foreign cache entry), it returns "bad cached type".

Solutions

1. Flush the results cache (memcached/redis) after version upgrades
2. Ensure all frontend replicas run the same Cortex version during rollouts
3. Add a cache key version prefix so old entries are ignored

Example fix

// before: shared cache across versions with same keys
// after: bump cache key version
cacheKey := fmt.Sprintf("v2:%s:%s", userID, req.GetQuery())

if resp, ok := msg.(queryrange.Response); !ok {
	return fmt.Errorf("bad cached type %T", msg)
}

resp, err := toResponse(e.Response)
if err != nil && strings.Contains(err.Error(), "bad cached type") {
	cache.Delete(key) // evict poisoned entry and re-fetch upstream
	return upstreamHandler(ctx, req)
}

When it happens

Trigger: Reading a results-cache entry whose stored type URL was written by a different queryrange version/schema, or whose bytes were corrupted, so msg.(Response) fails in newAccumulator.

Common situations: Upgrading/downgrading Cortex while sharing the same memcached/redis cache; cache poisoned by a mixed-version fleet during rolling deploy; manually populated or flushed-incompatible cache backends.

Related errors

• invalid duration
• use of multiple cache storage systems is not supported
• unsupported compression type
• at modifier after end
• negative offset

AI-assisted analysis of thanos-io/thanos@35b8b99117 (2026-09-07). Data as JSON: /api/errors/c57d4c0dc5cb2484. Report an issue: GitHub.

Appendix: source

Thrown at internal/cortex/querier/queryrange/results_cache.go:871

	if ts < start || ts > end {
		return false
	}
	return step <= 0 || (ts-start)%step == 0
}

func (e *Extent) toResponse() (Response, error) {
	msg, err := types.EmptyAny(e.Response)
	if err != nil {
		return nil, err
	}

	if err := types.UnmarshalAny(e.Response, msg); err != nil {
		return nil, err
	}

	resp, ok := msg.(Response)
	if !ok {
		return nil, fmt.Errorf("bad cached type")
	}
	return resp, nil
}

View on GitHub (pinned to 35b8b99117)