diff --git a/README.md b/README.md
index 3414efc7..5197948d 100644
--- a/README.md
+++ b/README.md
@@ -62,7 +62,7 @@ By default the CLI and MCP server connect to `https://api.runloop.ai`. To use a
 export RUNLOOP_BASE_URL=https://api.runloop.pro
 ```
 
-The URL must be of the form `https://api.<domain>`. The CLI derives other service hostnames from the domain portion:
+The URL must be of the form `https://api.<domain>`, optionally with a `:port` (e.g. `https://api.runloop.pro:8443`). The CLI derives other service hostnames from the domain portion:
 
 | Service  | Host |
 |----------|------|
diff --git a/src/utils/config.ts b/src/utils/config.ts
index 3c29af1a..0e7ff36a 100644
--- a/src/utils/config.ts
+++ b/src/utils/config.ts
@@ -70,15 +70,10 @@ export function checkBaseDomain(): void {
     process.exit(1);
   }
 
-  if (
-    parsed.port ||
-    parsed.pathname.replace(/\/+$/, "") ||
-    parsed.search ||
-    parsed.hash
-  ) {
+  if (parsed.pathname.replace(/\/+$/, "") || parsed.search || parsed.hash) {
     console.error(
-      `Error: RUNLOOP_BASE_URL must not contain port, path, query, or fragment: ${raw}\n` +
-        `Expected format: https://api.<domain>`,
+      `Error: RUNLOOP_BASE_URL must not contain path, query, or fragment: ${raw}\n` +
+        `Expected format: https://api.<domain> (an optional :port is allowed)`,
     );
     process.exit(1);
   }
diff --git a/tests/__tests__/utils/baseUrl.test.ts b/tests/__tests__/utils/baseUrl.test.ts
new file mode 100644
index 00000000..2c1a446c
--- /dev/null
+++ b/tests/__tests__/utils/baseUrl.test.ts
@@ -0,0 +1,88 @@
+/**
+ * Tests for RUNLOOP_BASE_URL handling, including optional port overrides.
+ */
+
+import {
+  describe,
+  it,
+  expect,
+  beforeEach,
+  afterEach,
+  jest,
+} from "@jest/globals";
+import {
+  checkBaseDomain,
+  baseUrl,
+  runloopBaseDomain,
+  _resetBaseDomainCache,
+} from "../../../src/utils/config.js";
+
+describe("RUNLOOP_BASE_URL", () => {
+  const original = process.env.RUNLOOP_BASE_URL;
+  let exitSpy: jest.SpiedFunction<typeof process.exit>;
+  let errorSpy: jest.SpiedFunction<typeof console.error>;
+
+  beforeEach(() => {
+    _resetBaseDomainCache();
+    exitSpy = jest.spyOn(process, "exit").mockImplementation(((
+      code?: number,
+    ) => {
+      throw new Error(`process.exit(${code})`);
+    }) as never);
+    errorSpy = jest.spyOn(console, "error").mockImplementation(() => {});
+  });
+
+  afterEach(() => {
+    if (original === undefined) delete process.env.RUNLOOP_BASE_URL;
+    else process.env.RUNLOOP_BASE_URL = original;
+    exitSpy.mockRestore();
+    errorSpy.mockRestore();
+    _resetBaseDomainCache();
+  });
+
+  describe("checkBaseDomain", () => {
+    it("accepts a bare api.<domain> URL", () => {
+      process.env.RUNLOOP_BASE_URL = "https://api.runloop.pro";
+      expect(() => checkBaseDomain()).not.toThrow();
+      expect(exitSpy).not.toHaveBeenCalled();
+    });
+
+    it("accepts an api.<domain> URL with a port override", () => {
+      process.env.RUNLOOP_BASE_URL = "https://api.runloop.pro:8443";
+      expect(() => checkBaseDomain()).not.toThrow();
+      expect(exitSpy).not.toHaveBeenCalled();
+    });
+
+    it("rejects a URL with a path", () => {
+      process.env.RUNLOOP_BASE_URL = "https://api.runloop.pro/v1";
+      expect(() => checkBaseDomain()).toThrow();
+      expect(exitSpy).toHaveBeenCalledWith(1);
+    });
+
+    it("rejects a URL with a query string", () => {
+      process.env.RUNLOOP_BASE_URL = "https://api.runloop.pro?foo=bar";
+      expect(() => checkBaseDomain()).toThrow();
+      expect(exitSpy).toHaveBeenCalledWith(1);
+    });
+
+    it("rejects a non-https URL", () => {
+      process.env.RUNLOOP_BASE_URL = "http://api.runloop.pro:8443";
+      expect(() => checkBaseDomain()).toThrow();
+      expect(exitSpy).toHaveBeenCalledWith(1);
+    });
+  });
+
+  describe("baseUrl", () => {
+    it("includes the port override in the API base URL", () => {
+      process.env.RUNLOOP_BASE_URL = "https://api.runloop.pro:8443";
+      expect(baseUrl()).toBe("https://api.runloop.pro:8443");
+    });
+  });
+
+  describe("runloopBaseDomain", () => {
+    it("strips the api. prefix and the port", () => {
+      process.env.RUNLOOP_BASE_URL = "https://api.runloop.pro:8443";
+      expect(runloopBaseDomain()).toBe("runloop.pro");
+    });
+  });
+});