Kuberwastaken/claurst · error

MCP server ' ' is configured as ' ' but missing URL

Error message

MCP server '{}' is configured as '{}' but missing URL

What it means

The MCP connection manager rejects connecting to a server whose type is 'sse' or 'http' when the config has no `url` field. These transport types require a remote HTTP endpoint, and the OAuth token acquisition step needs that URL, so a missing URL is a fatal configuration error. The library bails before any network connection is attempted.

Solutions

1. Add the "url" field to the MCP server config entry, e.g. "url": "https://example.com/sse"
2. Verify server_type matches the intended transport; if the server is local, use type "stdio" with a command instead of sse/http
3. Re-run provider/config discovery to ensure the expanded config actually contains the URL

Example fix

// before (.mcp.json)
{ "mcpServers": { "docs": { "type": "sse" } } }
// after
{ "mcpServers": { "docs": { "type": "sse", "url": "https://docs.example.com/sse" } } }

fn ensure_remote_config_ok(cfg: &McpServerConfig) -> anyhow::Result<()> {
    if matches!(cfg.server_type.as_str(), "sse" | "http") && cfg.url.is_none() {
        anyhow::bail!("server '{}' of type '{}' requires a url", cfg.name, cfg.server_type);
    }
    Ok(())
}

When it happens

Trigger: Calling connect (via connect_expanded_config) with an McpServerConfig where server_type is "sse" or "http" and config.url is None.

Common situations: Hand-edited .mcp.json or settings entry omitted the url key; a template was copied and the url placeholder removed; a server was switched from stdio (command-based) to sse/http without adding a URL.

Understand the failure class

Background: "is required", "must be set", "missing required field": configuration validation errors across open-source libraries — this error's family across 36 libraries.

Related errors

• Failed to parse redirect URI
• Redirect URI ' ' is missing host
• Bridge session registration failed: authentication error
• No query string in callback
• invalid legacy SSE base URL

AI-assisted analysis of Kuberwastaken/claurst@b0637c97ec (2026-09-10). Data as JSON: /api/errors/8df03afcad8c35ac. Report an issue: GitHub.

Appendix: source

Thrown at src-rust/crates/mcp/src/connection_manager.rs:124

    }

    /// Connect to all configured servers (env-vars expanded, errors non-fatal).
    pub async fn connect_all(&self) -> anyhow::Result<()> {
        let names: Vec<String> = self.state.iter().map(|e| e.key().clone()).collect();
        for name in names {
            if let Err(e) = self.connect(&name).await {
                error!(server = %name, error = %e, "MCP server failed to connect during startup");
            }
        }
        Ok(())
    }

    async fn connect_expanded_config(name: &str, config: &McpServerConfig) -> anyhow::Result<McpClient> {
        let auth_token = if matches!(config.server_type.as_str(), "sse" | "http") {
            match config.url.as_deref() {
                Some(server_url) => oauth::get_valid_mcp_access_token(name, server_url).await?,
                None => {
                    anyhow::bail!(
                        "MCP server '{}' is configured as '{}' but missing URL",
                        name,
                        config.server_type
                    )
                }
            }
        } else {
            None
        };
        McpClient::connect(config, auth_token).await
    }

    // -----------------------------------------------------------------------
    // Connect / disconnect / restart
    // -----------------------------------------------------------------------

    /// Connect to a single server by name, marking status along the way.
    pub async fn connect(&self, name: &str) -> anyhow::Result<()> {

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