What causes "MCP server command cannot be empty" in alibaba/open-code-review?

Pass the actual executable path or name, e.g. npx or /usr/local/bin/myserver

MCP server command cannot be empty

Error message

MCP server command cannot be empty

What it means

When setting mcp_servers.<name>.command, an empty string is rejected because a stdio MCP server needs an executable command to launch. This error guards against writing a config entry that could never start.

Source

Thrown at cmd/opencodereview/config_cmd.go:864

	if len(parts) != 3 || parts[1] == "" || parts[2] == "" {
		return fmt.Errorf("invalid MCP server key %q: expected mcp_servers.<name>.<field>", key)
	}
	name, field := parts[1], parts[2]

	if cfg.MCPServers == nil {
		cfg.MCPServers = make(map[string]MCPServerConfig)
	}
	entry := cfg.MCPServers[name]

	switch field {
	case "type":
		if value != "stdio" && value != "remote" {
			return fmt.Errorf("invalid MCP server type %q: must be \"stdio\" or \"remote\"", value)
		}
		entry.Type = value
	case "command":
		if value == "" {
			return fmt.Errorf("MCP server command cannot be empty")
		}
		entry.Command = value
	case "args":
		var args []string
		if err := json.Unmarshal([]byte(value), &args); err != nil {
			return fmt.Errorf("invalid JSON array for %s: %w", key, err)
		}
		entry.Args = args
	case "env":
		var env []string
		if err := json.Unmarshal([]byte(value), &env); err != nil {
			return fmt.Errorf("invalid JSON array for %s: %w", key, err)
		}
		for _, e := range env {
			idx := strings.Index(e, "=")
			if idx <= 0 {
				return fmt.Errorf("invalid env entry %q: must be in KEY=VALUE format", e)
			}

View on GitHub (pinned to 5cf97d0d15)

Solutions

Pass the actual executable path or name, e.g. npx or /usr/local/bin/myserver
If the server is remote instead, set mcp_servers.<name>.type remote and a url rather than command
Remove the half-configured entry entirely if it is no longer needed

Example fix

// before
ocr config set mcp_servers.filesystem.command ""
// after
ocr config set mcp_servers.filesystem.command npx

Defensive patterns

Strategy: validation

Validate before calling

if command == "" {
	return fmt.Errorf("command is required for stdio servers")
}

Try / catch

if err := setMCPServerValue(cfg, key, value); err != nil {
	fmt.Fprintf(os.Stderr, "config set failed: %v\n", err)
	os.Exit(1)
}

Prevention

Always quote the command value so empty strings are not silently dropped
For remote servers set url, not command
Verify the command exists on PATH before configuring it

When it happens

Trigger: Calling setMCPServerValue with field "command" and value "".

Common situations: Running `ocr config set mcp_servers.myserver.command` with an empty argument (shell consumes the empty string), or scripting config generation that skips the value.

Understand the failure class

Background: "must not be empty", "cannot be empty" — required-field validation errors across open-source libraries — this error's family across 41 libraries.

Related errors

AI-assisted analysis of alibaba/open-code-review@5cf97d0d15 (2026-09-02). Data as JSON: /api/errors/52f6d37a6989e1b7. Report an issue: GitHub.