diff --git a/packages/bun-types/serve.d.ts b/packages/bun-types/serve.d.ts
index 78b0eea3d4..2558bb5944 100644
--- a/packages/bun-types/serve.d.ts
+++ b/packages/bun-types/serve.d.ts
@@ -1082,6 +1082,15 @@ declare module "bun" {
      */
     readonly hostname: string | undefined;
 
+    /**
+     * The protocol the server is listening on.
+     *
+     * - "http" for normal servers
+     * - "https" when TLS is enabled
+     * - null for unix sockets or when unavailable
+     */
+    readonly protocol: "http" | "https" | null;
+
     /**
      * Is the server running in development mode?
      *
diff --git a/src/bun.js/api/FFIObject.zig b/src/bun.js/api/FFIObject.zig
index f7e2b0a3b8..956900a7d9 100644
--- a/src/bun.js/api/FFIObject.zig
+++ b/src/bun.js/api/FFIObject.zig
@@ -16,11 +16,22 @@ pub const dom_call = DOMCall("FFI", @This(), "ptr", DOMEffect.forRead(.TypedArra
 pub fn toJS(globalObject: *jsc.JSGlobalObject) jsc.JSValue {
     const object = jsc.JSValue.createEmptyObject(globalObject, comptime std.meta.fieldNames(@TypeOf(fields)).len + 2);
     inline for (comptime std.meta.fieldNames(@TypeOf(fields))) |field| {
-        object.put(
-            globalObject,
-            comptime ZigString.static(field),
-            jsc.JSFunction.create(globalObject, field, @field(fields, field), 1, .{}),
-        );
+        if (comptime bun.strings.eqlComptime(field, "CString")) {
+            // CString needs to be callable as a constructor for backward compatibility.
+            // Pass the same function as the constructor so `new CString(ptr)` works.
+            const func = jsc.toJSHostFn(@field(fields, field));
+            object.put(
+                globalObject,
+                comptime ZigString.static(field),
+                jsc.JSFunction.create(globalObject, field, func, 1, .{ .constructor = func }),
+            );
+        } else {
+            object.put(
+                globalObject,
+                comptime ZigString.static(field),
+                jsc.JSFunction.create(globalObject, field, @field(fields, field), 1, .{}),
+            );
+        }
     }
 
     dom_call.put(globalObject, object);
diff --git a/test/regression/issue/25231.test.ts b/test/regression/issue/25231.test.ts
new file mode 100644
index 0000000000..0afabca005
--- /dev/null
+++ b/test/regression/issue/25231.test.ts
@@ -0,0 +1,32 @@
+// https://github.com/oven-sh/bun/issues/25231
+// Bun.FFI.CString should be callable as a constructor (new CString(ptr))
+
+import { expect, test } from "bun:test";
+
+test("Bun.FFI.CString is callable with new", () => {
+  const { CString, ptr } = Bun.FFI;
+
+  // Create a buffer with a null-terminated string
+  const buf = Buffer.from("hello\0");
+  const ptrValue = ptr(buf);
+
+  // CString should be callable with new
+  const result = new CString(ptrValue, 0, 5);
+
+  // The result should be the string "hello"
+  expect(String(result)).toBe("hello");
+});
+
+test("Bun.FFI.CString can be called without new", () => {
+  const { CString, ptr } = Bun.FFI;
+
+  // Create a buffer with a null-terminated string
+  const buf = Buffer.from("hello\0");
+  const ptrValue = ptr(buf);
+
+  // CString should also be callable without new
+  const result = CString(ptrValue, 0, 5);
+
+  // The result should be the string "hello"
+  expect(result).toBe("hello");
+});