Implement default-server support

- Auto-select server when --config is used without --server
- Use 'default-server' if it exists in config
- Use single server if only one is defined
- Show helpful error if multiple servers exist without default
- Add tests for all default-server scenarios

Author: felixweinberger

cli/scripts/cli-tests.js

@@ -780,6 +780,102 @@ async function runTests() {
     "tools/list",
   );
 
+  console.log(
+    `\n${colors.YELLOW}=== Running Default Server Tests ===${colors.NC}`,
+  );
+
+  // Create config with single server for auto-selection
+  const singleServerConfigPath = path.join(TEMP_DIR, "single-server-config.json");
+  fs.writeFileSync(
+    singleServerConfigPath,
+    JSON.stringify(
+      {
+        mcpServers: {
+          "only-server": {
+            command: "npx",
+            args: ["@modelcontextprotocol/server-everything"],
+          },
+        },
+      },
+      null,
+      2,
+    ),
+  );
+
+  // Create config with default-server
+  const defaultServerConfigPath = path.join(TEMP_DIR, "default-server-config.json");
+  fs.writeFileSync(
+    defaultServerConfigPath,
+    JSON.stringify(
+      {
+        mcpServers: {
+          "default-server": {
+            command: "npx",
+            args: ["@modelcontextprotocol/server-everything"],
+          },
+          "other-server": {
+            command: "node",
+            args: ["other.js"],
+          },
+        },
+      },
+      null,
+      2,
+    ),
+  );
+
+  // Create config with multiple servers (no default)
+  const multiServerConfigPath = path.join(TEMP_DIR, "multi-server-config.json");
+  fs.writeFileSync(
+    multiServerConfigPath,
+    JSON.stringify(
+      {
+        mcpServers: {
+          "server1": {
+            command: "npx",
+            args: ["@modelcontextprotocol/server-everything"],
+          },
+          "server2": {
+            command: "node",
+            args: ["other.js"],
+          },
+        },
+      },
+      null,
+      2,
+    ),
+  );
+
+  // Test 29: Config with single server auto-selection
+  await runBasicTest(
+    "single_server_auto_select",
+    "--config",
+    singleServerConfigPath,
+    "--cli",
+    "--method",
+    "tools/list",
+  );
+
+  // Test 30: Config with default-server auto-selection
+  await runBasicTest(
+    "default_server_auto_select",
+    "--config",
+    defaultServerConfigPath,
+    "--cli",
+    "--method",
+    "tools/list",
+  );
+
+  // Test 31: Config with multiple servers and no default (should fail)
+  await runErrorTest(
+    "multi_server_no_default",
+    "--config",
+    multiServerConfigPath,
+    "--cli",
+    "--method",
+    "tools/list",
+  );
+
   console.log(
     `\n${colors.YELLOW}=== Running HTTP Transport Tests ===${colors.NC}`,
   );
@@ -799,7 +895,7 @@ async function runTests() {
 
   await new Promise((resolve) => setTimeout(resolve, 3000));
 
-  // Test 29: HTTP transport inferred from URL ending with /mcp
+  // Test 32: HTTP transport inferred from URL ending with /mcp
   await runBasicTest(
     "http_transport_inferred",
     "http://127.0.0.1:3001/mcp",
@@ -808,7 +904,7 @@ async function runTests() {
     "tools/list",
   );
 
-  // Test 30: HTTP transport with explicit --transport http flag
+  // Test 33: HTTP transport with explicit --transport http flag
   await runBasicTest(
     "http_transport_with_explicit_flag",
     "http://127.0.0.1:3001/mcp",
@@ -819,7 +915,7 @@ async function runTests() {
     "tools/list",
   );
 
-  // Test 31: HTTP transport with suffix and --transport http flag
+  // Test 34: HTTP transport with suffix and --transport http flag
   await runBasicTest(
     "http_transport_with_explicit_flag_and_suffix",
     "http://127.0.0.1:3001/mcp",
@@ -830,7 +926,7 @@ async function runTests() {
     "tools/list",
   );
 
-  // Test 32: SSE transport given to HTTP server (should fail)
+  // Test 35: SSE transport given to HTTP server (should fail)
   await runErrorTest(
     "sse_transport_given_to_http_server",
     "http://127.0.0.1:3001",
@@ -841,7 +937,7 @@ async function runTests() {
     "tools/list",
   );
 
-  // Test 33: HTTP transport without URL (should fail)
+  // Test 36: HTTP transport without URL (should fail)
   await runErrorTest(
     "http_transport_without_url",
     "--transport",
@@ -851,7 +947,7 @@ async function runTests() {
     "tools/list",
   );
 
-  // Test 34: SSE transport without URL (should fail)
+  // Test 37: SSE transport without URL (should fail)
   await runErrorTest(
     "sse_transport_without_url",
     "--transport",

cli/src/cli.ts

@@ -220,16 +220,40 @@ function parseArgs(): Args {
   // Add back any arguments that came after --
   const finalArgs = [...remainingArgs, ...postArgs];
 
-  // Validate that config and server are provided together
-  if (
-    (options.config && !options.server) ||
-    (!options.config && options.server)
-  ) {
+  // Validate config and server options
+  if (!options.config && options.server) {
     throw new Error(
-      "Both --config and --server must be provided together. If you specify one, you must specify the other.",
+      "--server requires --config to be specified",
     );
   }
 
+  // If config is provided without server, try to auto-select
+  if (options.config && !options.server) {
+    const configContent = fs.readFileSync(
+      path.isAbsolute(options.config)
+        ? options.config
+        : path.resolve(process.cwd(), options.config),
+      "utf8"
+    );
+    const parsedConfig = JSON.parse(configContent);
+    const servers = Object.keys(parsedConfig.mcpServers || {});
+    
+    if (servers.includes("default-server")) {
+      // Use default-server if it exists
+      options.server = "default-server";
+    } else if (servers.length === 1) {
+      // Use the only server if there's just one
+      options.server = servers[0];
+    } else if (servers.length === 0) {
+      throw new Error("No servers found in config file");
+    } else {
+      // Multiple servers, no default-server
+      throw new Error(
+        `Multiple servers found in config file. Please specify one with --server.\nAvailable servers: ${servers.join(", ")}`
+      );
+    }
+  }
+
   // If config file is specified, load and use the options from the file. We must merge the args
   // from the command line and the file together, or we will miss the method options (--method,
   // etc.)