Add docs

Author: jlowin

docs/patterns/cli.mdx

@@ -53,10 +53,11 @@ This command runs the server directly in your current Python environment. You ar
 #### Server Specification
 <VersionBadge version="2.3.5" />
 
-The server can be specified in three ways:
+The server can be specified in four ways:
 1. `server.py` - imports the module and looks for a FastMCP object named `mcp`, `server`, or `app`. Errors if no such object is found.
 2. `server.py:custom_name` - imports and uses the specified server object
 3. `http://server-url/path` or `https://server-url/path` - connects to a remote server and creates a proxy
+4. `mcp.json` - runs servers defined in a standard MCP configuration file
 
 <Tip>
 When using `fastmcp run` with a local file, it **ignores** the `if __name__ == "__main__"` block entirely. Instead, it finds your server object and calls its `run()` method directly with the transport options you specify. This means you can use `fastmcp run` to override the transport specified in your code.
@@ -98,6 +99,44 @@ fastmcp run https://example.com/mcp-server
 fastmcp run https://example.com/mcp-server --log-level DEBUG
 ```
 
+#### Running MCP Configuration Files
+
+FastMCP can run servers defined in standard MCP configuration files (typically named `mcp.json`). When you run an mcp.json file, FastMCP creates a proxy server that runs all the servers referenced in the configuration.
+
+**Example mcp.json:**
+```json
+{
+    "mcpServers": {
+        "fetch": {
+            "command": "uvx",
+            "args": [
+                "mcp-server-fetch"
+            ]
+        },
+        "filesystem": {
+            "command": "npx",
+            "args": [
+                "-y",
+                "@modelcontextprotocol/server-filesystem",
+                "/Users/username/Documents"
+            ]
+        }
+    }
+}
+```
+
+**Run the configuration:**
+```bash
+# Run with default stdio transport
+fastmcp run mcp.json
+
+# Run with HTTP transport on custom port
+fastmcp run mcp.json --transport http --port 8080
+
+# Run with SSE transport
+fastmcp run mcp.json --transport sse
+```
+
 ### `dev`
 
 Run a MCP server with the [MCP Inspector](https://github.com/modelcontextprotocol/inspector) for testing.

src/fastmcp/mcp_config.py

@@ -270,7 +270,7 @@ def from_file(cls, file_path: Path) -> Self:
             if content := file_path.read_text().strip():
                 return cls.model_validate_json(content)
 
-        return cls(mcpServers={})
+        raise ValueError(f"No MCP servers defined in the config: {file_path}")
 
 
 class CanonicalMCPConfig(MCPConfig):