anthropics/skills · error · ValueError

URL is required for http transport

Error message

URL is required for http transport

What it means

create_connection() requires a url for transport in ('http', 'streamable_http', 'streamable-http'): streamable-HTTP MCP servers live at a known HTTP endpoint, so constructing MCPConnectionHTTP without a url is always a caller error. The check fires before any connection attempt.

Source

Thrown at skills/mcp-builder/scripts/connections.py:147

    Returns:
        MCPConnection instance
    """
    transport = transport.lower()

    if transport == "stdio":
        if not command:
            raise ValueError("Command is required for stdio transport")
        return MCPConnectionStdio(command=command, args=args, env=env)

    elif transport == "sse":
        if not url:
            raise ValueError("URL is required for sse transport")
        return MCPConnectionSSE(url=url, headers=headers)

    elif transport in ["http", "streamable_http", "streamable-http"]:
        if not url:
            raise ValueError("URL is required for http transport")
        return MCPConnectionHTTP(url=url, headers=headers)

    else:
        raise ValueError(f"Unsupported transport type: {transport}. Use 'stdio', 'sse', or 'http'")

View on GitHub (pinned to f6656c1256)

Solutions

1. Provide the endpoint: create_connection(name='x', transport='http', url='http://localhost:3000/mcp').
2. If the entry has command/args and no url, it is a stdio server — use transport='stdio'.
3. Confirm the url key spelling and that the value includes the scheme and full path (often '/mcp').
4. Add any required headers dict alongside the url.

Example fix

# before
create_connection(name="api", transport="http")

# after
create_connection(name="api", transport="http", url="http://127.0.0.1:3000/mcp", headers={"Authorization": "Bearer tok"})

from urllib.parse import urlparse

def valid_http_entry(entry: dict) -> bool:
    url = entry.get("url") or ""
    return bool(url) and urlparse(url).scheme in ("http", "https")

try:
    conn = create_connection(name=n, transport="http", url=u)
except ValueError as e:
    if "URL is required" in str(e):
        raise ConfigError(f"server {n!r}: http entry missing 'url'") from e
    raise

When it happens

Trigger: Calling create_connection(transport='http') with url omitted — e.g. a config entry that only defines command/args (a stdio server) but declares http transport, or the url field left blank by a config generator.

Common situations: Migrating MCP configs from older sse/stdio examples without adding the url; JSON schema drift between client versions (url vs serverUrl key); empty template values never filled in.

Related errors

• Command is required for stdio transport
• URL is required for sse transport
• Unsupported transport type: {transport}. Use 'stdio', 'sse',
• Expected JSON object with 'reviews' key
• {word} not found (not an unpacked .docx?)

AI-assisted analysis of anthropics/skills@f6656c1256 (2026-08-14). Data as JSON: /api/errors/0ecb2d47193a9344. Report an issue: GitHub.