[cli] Don't force specific endpoints for remote servers

At the moment, the CLI tool expects that remote MCP servers, either
using SSE or streamable HTTP transport, have certain endpoints or
fragments -- `/sse` and `/mcp`, respectively. This is not part of the
MCP spec, nor should it be expected.

I also added a check to make sure a URL is provided if STDIO isn't
picked, erroring out if not.

cli/scripts/cli-tests.js

@@ -710,6 +710,26 @@ async function runTests() {
     "tools/list",
   );
 
+  // Test 29: HTTP transport without URL (should fail)
+  await runErrorTest(
+    "http_transport_without_url",
+    "--transport",
+    "http",
+    "--cli",
+    "--method",
+    "tools/list",
+  );
+
+  // Test 30: SSE transport without URL (should fail)
+  await runErrorTest(
+    "sse_transport_without_url",
+    "--transport",
+    "sse",
+    "--cli",
+    "--method",
+    "tools/list",
+  );
+
   // Kill HTTP server
   try {
     process.kill(-httpServer.pid);

cli/src/transport.ts

@@ -14,24 +14,6 @@ export type TransportOptions = {
   url?: string;
 };
 
-function createSSETransport(options: TransportOptions): Transport {
-  const baseUrl = new URL(options.url ?? "");
-  const sseUrl = baseUrl.pathname.endsWith("/sse")
-    ? baseUrl
-    : new URL("/sse", baseUrl);
-
-  return new SSEClientTransport(sseUrl);
-}
-
-function createHTTPTransport(options: TransportOptions): Transport {
-  const baseUrl = new URL(options.url ?? "");
-  const mcpUrl = baseUrl.pathname.endsWith("/mcp")
-    ? baseUrl
-    : new URL("/mcp", baseUrl);
-
-  return new StreamableHTTPClientTransport(mcpUrl);
-}
-
 function createStdioTransport(options: TransportOptions): Transport {
   let args: string[] = [];
 
@@ -75,12 +57,18 @@ export function createTransport(options: TransportOptions): Transport {
       return createStdioTransport(options);
     }
 
+    // If not STDIO, then it must be either SSE or HTTP.
+    if (!options.url) {
+      throw new Error("URL must be provided for SSE or HTTP transport types.");
+    }
+    const url = new URL(options.url);
+
     if (transportType === "sse") {
-      return createSSETransport(options);
+      return new SSEClientTransport(url);
     }
 
     if (transportType === "http") {
-      return createHTTPTransport(options);
+      return new StreamableHTTPClientTransport(url);
     }
 
     throw new Error(`Unsupported transport type: ${transportType}`);