hashicorp/nomad · warning

Duplicate key %q in passed metadata

Error message

Duplicate key %q in passed metadata

What it means

Dispatch metadata (req.Meta) is converted to a map, so duplicate keys cannot be represented; validation iterates the map to assert it is a proper set. In practice this guards against upstream decoders that may surface duplicated keys; identical keys in the metadata map are rejected.

Source

Thrown at nomad/job_endpoint.go:2150

func validateDispatchRequest(req *structs.JobDispatchRequest, job *structs.Job, config *Config) error {
	// Check the payload constraint is met
	hasInputData := len(req.Payload) != 0
	if job.ParameterizedJob.Payload == structs.DispatchPayloadRequired && !hasInputData {
		return fmt.Errorf("Payload is not provided but required by parameterized job")
	} else if job.ParameterizedJob.Payload == structs.DispatchPayloadForbidden && hasInputData {
		return fmt.Errorf("Payload provided but forbidden by parameterized job")
	}

	// Check the payload doesn't exceed the size limit
	if l := len(req.Payload); l > DispatchPayloadSizeLimit {
		return fmt.Errorf("Payload exceeds maximum size; %d > %d", l, DispatchPayloadSizeLimit)
	}

	// Check if the metadata is a set
	keys := make(map[string]struct{}, len(req.Meta))
	for k := range req.Meta {
		if _, ok := keys[k]; ok {
			return fmt.Errorf("Duplicate key %q in passed metadata", k)
		}
		keys[k] = struct{}{}
	}

	required := set.From(job.ParameterizedJob.MetaRequired)
	optional := set.From(job.ParameterizedJob.MetaOptional)

	// Check the metadata key constraints are met
	unpermitted := make(map[string]struct{})
	for k := range req.Meta {
		req := required.Contains(k)
		opt := optional.Contains(k)
		if !req && !opt {
			unpermitted[k] = struct{}{}
		}
	}

	if len(unpermitted) != 0 {

View on GitHub (pinned to 482b49bf1a)

Solutions

1. Deduplicate meta keys before building JobDispatchRequest.Meta
2. Ensure a single source of truth for meta (flags should override config, not append)
3. Check for merge/annotation code that can inject duplicate keys

Example fix

// before
meta := map[string]string{cfg.Meta}
for k, v := range flagMeta { meta[k] = meta[k] + v } // duplication risk
// after
meta := map[string]string{}
for k, v := range cfg.Meta { meta[k] = v }
for k, v := range flagMeta { meta[k] = v } // flags override, never duplicate

seen := map[string]struct{}{}
for k := range meta {
    if _, dup := seen[k]; dup { return fmt.Errorf("duplicate meta key %q", k) }
    seen[k] = struct{}{}
}

When it happens

Trigger: Submitting a JobDispatchRequest whose Meta map contains duplicate keys (possible when request data is merged/decoded from multiple sources before reaching the RPC).

Common situations: Merging meta from config files and CLI flags where the same key appears twice; JSON/HCL decoding merging duplicate keys into conflicting entries; middleware injecting meta that collides with user-provided meta.

Related errors

• Dispatch request included unpermitted metadata keys: %v
• unknown task name %q
• must specify at least one healthy/unhealthy allocation ID
• missing job for registration
• missing job ID for revert

AI-assisted analysis of hashicorp/nomad@482b49bf1a (2026-09-04). Data as JSON: /api/errors/d8d00408c7e0db6c. Report an issue: GitHub.