grpc/grpc-go · error

error parsing config for policy %q: %v

Error message

error parsing config for policy %q: %v

What it means

Returned by gracefulswitch.ParseConfig when the child balancer's ConfigParser.ParseConfig rejects the config JSON. The gracefulswitch parser found a registered builder for the policy name (config.go:64-68) and confirmed it implements balancer.ConfigParser (config.go:70-74), but the builder's own parser returned an error. The %q is the policy name; %v is the underlying parse error from the child.

Source

Thrown at internal/balancer/gracefulswitch/config.go:78

		var jsonCfg json.RawMessage
		for name, jsonCfg = range e {
		}

		builder := balancer.Get(name)
		if builder == nil {
			// Skip unregistered balancer names.
			continue
		}

		parser, ok := builder.(balancer.ConfigParser)
		if !ok {
			// This is a valid child with no config.
			return &lbConfig{childBuilder: builder}, nil
		}

		cfg, err := parser.ParseConfig(jsonCfg)
		if err != nil {
			return nil, fmt.Errorf("error parsing config for policy %q: %v", name, err)
		}
		return &lbConfig{childBuilder: builder, childConfig: cfg}, nil
	}

	return nil, fmt.Errorf("no supported policies found in config: %v", string(cfg))
}

View on GitHub (pinned to 0c51461d27)

Solutions

1. Inspect the wrapped error (%v) to identify which field or value the child parser rejected
2. Compare your config JSON against the child balancer's expected config struct definition for your gRPC version
3. Ensure the gRPC version that generated the service config matches the version running in the client
4. Test the child config in isolation by calling the specific balancer's ParseConfig directly

Example fix

// before: weighted_round_robin with invalid child config
{"loadBalancingConfig": [{"weighted_round_robin": {"children": {"child-0": {"weight": "not-a-number"}}}}]}

// after: correct numeric weight
{"loadBalancingConfig": [{"weighted_round_robin": {"children": {"child-0": {"weight": 1}}}}]}

// Validate child config by testing the specific balancer's parser in isolation
func validateChildConfig(policyName string, childCfg json.RawMessage) error {
    builder := balancer.Get(policyName)
    if builder == nil {
        return fmt.Errorf("balancer %q not registered", policyName)
    }
    parser, ok := builder.(balancer.ConfigParser)
    if !ok {
        return nil // no config to validate
    }
    _, err := parser.ParseConfig(childCfg)
    return err
}

// When calling ParseConfig, inspect the wrapped error for child-specific details
cfg, err := gracefulswitch.ParseConfig(rawCfg)
if err != nil {
    // Check if this is a child parse error (contains 'error parsing config for policy')
    if strings.Contains(err.Error(), "error parsing config for policy") {
        // Extract the policy name and underlying cause for targeted fix
        log.Printf("child balancer config invalid: %v", err)
    }
    return err
}

When it happens

Trigger: Passing a loadBalancingConfig entry whose inner config JSON is invalid for the named policy. For example, a weighted_round_robin config with invalid child field types, or a priority config referencing unknown child policies. The error surfaces through gracefulswitch.ParseConfig -> parser.ParseConfig at config.go:76-78.

Common situations: Version mismatch between the service config JSON schema and the installed gRPC version. A config generated for a newer balancer that expects different field names or types. Typos in config field names that the child parser strictly validates.

Related errors

• expected a JSON struct with one entry; received entry %v at
• no supported policies found in config: %v
• least-request: unable to unmarshal LBConfig: %v
• least-request: lbConfig.choiceCount: %v, must be >= 2
• pickfirst: unable to unmarshal LB policy config: %s, error:

AI-assisted analysis of grpc/grpc-go@0c51461d27 (2026-08-11). Data as JSON: /api/errors/1de96bf3f9abf1e9. Report an issue: GitHub.