micro/go-micro · error

unsupported Content-Type: %s

Error message

unsupported Content-Type: %s

What it means

The generic rpcClient's newCodec looked up the request Content-Type in DefaultCodecs and found no match, so it cannot encode the request. This happens for the default (mucp/rpc) client, independent of the gRPC-specific check, and fails the call/stream/Publish before any network I/O.

Source

Thrown at client/rpc_client.go:76

	// wrap in reverse
	for i := len(opts.Wrappers); i > 0; i-- {
		c = opts.Wrappers[i-1](c)
	}

	return c
}

func (r *rpcClient) newCodec(contentType string) (codec.NewCodec, error) {
	if c, ok := r.opts.Codecs[contentType]; ok {
		return c, nil
	}

	if cf, ok := DefaultCodecs[contentType]; ok {
		return cf, nil
	}

	return nil, fmt.Errorf("unsupported Content-Type: %s", contentType)
}

func (r *rpcClient) call(
	ctx context.Context,
	node *registry.Node,
	req Request,
	resp interface{},
	opts CallOptions,
) error {
	// In-process fast-path: if the callee runs in this process and both bodies
	// are raw frames, dispatch directly and skip the network entirely.
	if handled, err := r.localCall(ctx, req, resp); handled {
		return err
	}

	address := node.Address
	logger := r.Options().Logger

View on GitHub (pinned to 24529f1404)

Solutions

1. Use a supported Content-Type such as application/json, application/protobuf, or application/octet-stream (see DefaultCodecs)
2. Fix the typo in the ContentType client option
3. Register a custom codec in DefaultCodecs at startup if the format is genuinely needed
4. Log/inspect the configured ContentType at boot to catch drift

Example fix

// before
client.ContentType("application/xml")
// after
client.ContentType("application/json")

var supported = []string{"application/json", "application/protobuf", "application/octet-stream"}
if !slices.Contains(supported, clientOptsContentType) {
    return fmt.Errorf("content type %q not in DefaultCodecs", clientOptsContentType)
}

err := client.Publish(ctx, topic, msg)
if err != nil && strings.Contains(err.Error(), "unsupported Content-Type") {
    // correct client.ContentType to a DefaultCodecs entry
}

When it happens

Trigger: Configuring client.ContentType (or a service's advertised content type) to a value absent from DefaultCodecs — e.g. "application/xml", "text/plain", or a misspelled "applcation/json" — then calling call, Stream, or Publish.

Common situations: Typos in content-type strings; copying options between transports where the codec sets differ; middleware or plugins overriding ContentType with an unsupported value.

Related errors

• unsupported Content-Type: %s
• unsupported Content-Type
• rpc.Handle: handler has no name
• Unable to encode body
• agent: ResumeStreamAsk requires a checkpoint

AI-assisted analysis of micro/go-micro@24529f1404 (2026-09-01). Data as JSON: /api/errors/65c2d6ba1193a56e. Report an issue: GitHub.