thanos-io/thanos · error · api.ApiError

' ' bad engine

Error message

'%s' bad engine

What it means

parseEngineParam validates the 'engine' query parameter against the two supported engines (PromQL 'prometheus' and Thanos 'thanos'); any other value is rejected with ErrorBadData. The message echoes the offending engine name.

Solutions

1. Use exactly 'prometheus' or 'thanos' (lowercase) for the engine parameter
2. Omit the parameter to use the server's default engine (-query.promql-engine flag)
3. Check the Thanos version's supported engine constants (PromqlEnginePrometheus/PromqlEngineThanos)
4. Fix casing in generated clients

Example fix

// before
GET /api/v1/query?query=up&engine=Thanos
// after
GET /api/v1/query?query=up&engine=thanos

const ENGINES = new Set(['prometheus','thanos']);
if (engine !== undefined && !ENGINES.has(engine)) throw new Error(`bad engine: ${engine}`);

function isKnownEngine(e) { return e === 'prometheus' || e === 'thanos'; }

try {
  const res = await fetch(urlWithEngine);
  const body = await res.json();
  if (body.errorType === 'bad_data' && /bad engine/.test(body.error)) throw new Error(body.error);
} catch (e) {
  if (/bad engine/.test(e.message)) { /* drop the engine param (use server default) and retry */ }
  throw e;
}

When it happens

Trigger: Passing engine=<anything else> on /api/v1/query, /api/v1/query_range or their explain endpoints, e.g. engine=thanos-impl2, engine=PROMETHEUS (case mismatch), or engine=jq.

Common situations: Copying engine options from other engines/tools; attempting to select experimental engines not present in the deployed Thanos version; case-sensitivity mistakes.

Understand the failure class

Background: Invalid enum value errors: "Unknown type", "Invalid scope", "must be one of" — when a string is not on the library's allowed list — this error's family across 23 libraries.

Related errors

• ' ' parameter
• negative ' ' is not accepted. Try a positive integer
• invalid reload method
• ULID is not valid
• not supported marker

AI-assisted analysis of thanos-io/thanos@35b8b99117 (2026-09-07). Data as JSON: /api/errors/eca5fd3174113ed5. Report an issue: GitHub.

Appendix: source

Thrown at pkg/api/query/v1.go:318

			return false, &api.ApiError{Typ: api.ErrorBadData, Err: errors.Wrapf(err, "'%s' parameter", DedupParam)}
		}
	}
	return enableDeduplication, nil
}

func (qapi *QueryAPI) parseQueryParam(r *http.Request) string {
	return r.FormValue(QueryParam)
}

func (qapi *QueryAPI) parseEngineParam(r *http.Request) (e PromqlEngineType, _ *api.ApiError) {
	param := PromqlEngineType(r.FormValue(EngineParam))
	if param == "" {
		param = qapi.defaultEngine
	}
	switch param {
	case PromqlEnginePrometheus, PromqlEngineThanos:
	default:
		return param, &api.ApiError{Typ: api.ErrorBadData, Err: errors.Errorf("'%s' bad engine", param)}
	}

	return param, nil
}

func (qapi *QueryAPI) parseReplicaLabelsParam(r *http.Request) (replicaLabels []string, _ *api.ApiError) {
	if err := r.ParseForm(); err != nil {
		return nil, &api.ApiError{Typ: api.ErrorInternal, Err: errors.Wrap(err, "parse form")}
	}

	replicaLabels = qapi.replicaLabels
	// Overwrite the cli flag when provided as a query parameter.
	if len(r.Form[ReplicaLabelsParam]) > 0 {
		replicaLabels = r.Form[ReplicaLabelsParam]
	}
	return replicaLabels, nil
}

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