micro/go-micro · error

unsupported Content-Type: %s

Error message

unsupported Content-Type: %s

What it means

Error "unsupported Content-Type: %s" thrown in micro/go-micro.

Source

Thrown at client/grpc/grpc.go:362

		return DefaultMaxSendMsgSize
	}
	return v.(int)
}

func (g *grpcClient) newGRPCCodec(contentType string) (encoding.Codec, error) {
	codecs := make(map[string]encoding.Codec)
	if g.opts.Context != nil {
		if v := g.opts.Context.Value(codecsKey{}); v != nil {
			codecs = v.(map[string]encoding.Codec)
		}
	}
	if c, ok := codecs[contentType]; ok {
		return wrapCodec{c}, nil
	}
	if c, ok := defaultGRPCCodecs[contentType]; ok {
		return wrapCodec{c}, nil
	}
	return nil, fmt.Errorf("unsupported Content-Type: %s", contentType)
}

func (g *grpcClient) Init(opts ...client.Option) error {
	size := g.opts.PoolSize
	ttl := g.opts.PoolTTL

	for _, o := range opts {
		o(&g.opts)
	}

	// update pool configuration if the options changed
	if size != g.opts.PoolSize || ttl != g.opts.PoolTTL {
		g.pool.Lock()
		g.pool.size = g.opts.PoolSize
		g.pool.ttl = int64(g.opts.PoolTTL.Seconds())
		g.pool.Unlock()
	}

View on GitHub (pinned to 24529f1404)

Solutions

1. Use a supported Content-Type: application/grpc+proto, application/grpc+json, or application/grpc+bytes
2. Register a custom codec via codec registration before making calls if you need a custom format
3. Fix typos in the configured Content-Type option
4. Omit the Content-Type option to use the default application/grpc+proto

Example fix

// before
client.ContentType("application/grpc+msgpack")
// after
client.ContentType("application/grpc+json")

var defaultGRPCContentTypes = []string{
    "application/grpc", "application/grpc+proto", "application/grpc+json", "application/grpc+bytes",
}
func contentTypeSupported(ct string) bool {
    return slices.Contains(defaultGRPCContentTypes, ct)
}

err := client.Call(ctx, req, resp)
if err != nil && strings.Contains(err.Error(), "unsupported Content-Type") {
    // fix client.ContentType option at startup
}

When it happens

Trigger: Setting client.ContentType or service Content-Type to something like application/grpc+msgpack, text/plain, or a typo such as application/grpc+protobuff while using the gRPC client.

Common situations: Copy-pasted Content-Type strings with typos; switching clients from mucp (json) to grpc without updating the content type; using a codec name supported elsewhere but not compiled into the gRPC codec map.

Related errors

• unsupported Content-Type
• unsupported Content-Type: %s
• unknown request path
• failed to marshal: %v is not type of *bytes.Frame or proto.M
• failed to unmarshal: %v is not type of proto.Message

AI-assisted analysis of micro/go-micro@24529f1404 (2026-09-01). Data as JSON: /api/errors/97f49f2a730f2fea. Report an issue: GitHub.