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

Pass the full server URL, e.g. https://mcp.example.com/sse

MCP server URL cannot be empty

Error message

MCP server URL cannot be empty

What it means

Validation in setMCPServerValue's url case: a remote MCP server entry requires a non-empty URL. Setting mcp_servers.<name>.url to an empty string would leave the remote transport with no endpoint, so the setter rejects it.

Source

Thrown at cmd/opencodereview/config_cmd.go:887

		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)
			}
		}
		entry.Env = env
	case "url":
		if value == "" {
			return fmt.Errorf("MCP server URL cannot be empty")
		}
		parsed, err := url.Parse(value)
		if err != nil {
			return fmt.Errorf("invalid MCP server URL %q: %w", value, err)
		}
		if parsed.Scheme != "http" && parsed.Scheme != "https" {
			return fmt.Errorf("MCP server URL must use http or https scheme, got %q", parsed.Scheme)
		}
		if parsed.Host == "" {
			return fmt.Errorf("MCP server URL %q must include a host", value)
		}
		entry.URL = value
	case "headers":
		parsed, err := parseMCPHeaders(value)
		if err != nil {
			return fmt.Errorf("invalid headers for %s: %w", key, err)
		}
		entry.Headers = parsed

View on GitHub (pinned to 5cf97d0d15)

Solutions

Pass the full server URL, e.g. https://mcp.example.com/sse
If the server is local, use type stdio with command/args instead of url
Check that any script variable holding the URL is actually set

Example fix

// before
ocr config set mcp_servers.api.url ""
// after
ocr config set mcp_servers.api.url https://mcp.example.com/sse

Defensive patterns

Strategy: validation

Validate before calling

if url == "" {
	return fmt.Errorf("url is required for remote 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 pass the URL as a quoted argument
In scripts, fail fast if the URL variable is empty (set -u / : "${URL:?}")
Use stdio type with command for local servers instead

When it happens

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

Common situations: Running `ocr config set mcp_servers.api.url` without the value argument (shell passes empty string), or templated scripts where the URL variable is unset.

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/9717825c02b069a2. Report an issue: GitHub.