diff --git a/packages/core/test/api-request.test.ts b/packages/core/test/api-request.test.ts
index e2a94b27..82393bd6 100644
--- a/packages/core/test/api-request.test.ts
+++ b/packages/core/test/api-request.test.ts
@@ -1,6 +1,5 @@
 import { expect, test } from "bun:test";
 import { buildApiRequest } from "../src/api-request.ts";
-import { REACTIVE_RUNTIME } from "../../csr/src/reactive-runtime.ts";
 
 test("GET builds a query string", () => {
   expect(buildApiRequest("/api/users", "GET", { name: "Ajay" }).url).toBe("/api/users?name=Ajay");
@@ -47,9 +46,7 @@ test("values are encoded", () => {
   );
 });
 
-test("the browser runtime and the shared builder agree", () => {
-  // Cheap structural guard: the runtime must apply the same omission rule.
-  // If someone changes one side's rules, this fails.
-  expect(REACTIVE_RUNTIME).toContain('value === ""');
-  expect(REACTIVE_RUNTIME).toContain("application/json");
-});
+// The behavioural agreement test between this builder and the browser
+// runtime's wrnexusCallApi lives in packages/csr/test/api-request-agreement.test.ts,
+// which can host the happy-dom harness needed to run the runtime and compare
+// outputs. This file has no DOM available.
diff --git a/packages/csr/test/api-request-agreement.test.ts b/packages/csr/test/api-request-agreement.test.ts
new file mode 100644
index 00000000..26a1f8d4
--- /dev/null
+++ b/packages/csr/test/api-request-agreement.test.ts
@@ -0,0 +1,128 @@
+import { expect, test, beforeEach } from "bun:test";
+import { Window } from "happy-dom";
+import { buildApiRequest } from "@wrnexus/core";
+import { REACTIVE_RUNTIME } from "../src/reactive-runtime.ts";
+import { restoreGlobalsAfterAll } from "./global-restore.ts";
+
+// This test compares the two transports by OUTPUT, not by text. A substring
+// search over REACTIVE_RUNTIME cannot detect a logic change on either side
+// (it never touches buildApiRequest at all, and it isn't anchored to the
+// right line on the runtime side). Only running both implementations on the
+// same input and diffing what they actually send can catch drift.
+
+const REPLACED_GLOBALS = ["window", "document", "location", "fetch", "NodeFilter"];
+restoreGlobalsAfterAll(REPLACED_GLOBALS);
+
+beforeEach(() => {
+  for (const name of REPLACED_GLOBALS) delete (globalThis as Record<string, unknown>)[name];
+});
+
+interface RecordedCall {
+  url: string;
+  method: string;
+  body: string | undefined;
+  contentType: string | undefined;
+}
+
+/** Mount the runtime with a recording fetch and run one call through it. */
+async function callThroughRuntime(
+  path: string,
+  method: string,
+  input: Record<string, unknown> | undefined,
+): Promise<RecordedCall> {
+  const win = new Window() as unknown as Window & Record<string, unknown>;
+  win.document.body.innerHTML = `<div data-scope="x: 1"></div>`;
+  let recorded: RecordedCall | undefined;
+
+  (globalThis as Record<string, unknown>).window = win;
+  (globalThis as Record<string, unknown>).document = win.document;
+  (globalThis as Record<string, unknown>).location = win.location;
+  (globalThis as Record<string, unknown>).NodeFilter = (
+    win as unknown as { NodeFilter: unknown }
+  ).NodeFilter;
+  (globalThis as Record<string, unknown>).fetch = (url: string, init: RequestInit) => {
+    const headers = init.headers as Record<string, string>;
+    recorded = {
+      url,
+      method: init.method as string,
+      body: init.body as string | undefined,
+      contentType: headers ? headers["content-type"] : undefined,
+    };
+    return Promise.resolve({
+      ok: true,
+      status: 200,
+      json: () => Promise.resolve({}),
+    });
+  };
+
+  (0, eval)(REACTIVE_RUNTIME);
+  const callApi = (win as unknown as { __wrnexusCallApi: (...args: any[]) => Promise<any> })
+    .__wrnexusCallApi;
+
+  await callApi(path, method, input);
+  if (!recorded) throw new Error("runtime did not call fetch");
+  return recorded;
+}
+
+function callThroughBuilder(
+  path: string,
+  method: string,
+  input: Record<string, unknown> | undefined,
+): RecordedCall {
+  const built = buildApiRequest(path, method, input);
+  return {
+    url: built.url,
+    method: String(method || "GET").toUpperCase(),
+    body: built.body,
+    contentType: built.contentType,
+  };
+}
+
+const CASES: Array<{
+  name: string;
+  path: string;
+  method: string;
+  input: Record<string, unknown> | undefined;
+}> = [
+  {
+    name: "GET with a mix of present and omitted values",
+    path: "/api/users",
+    method: "GET",
+    input: { name: "Ajay", age: undefined, team: null, note: "" },
+  },
+  {
+    name: "GET carrying 0 and false",
+    path: "/api/users",
+    method: "GET",
+    input: { count: 0, active: false },
+  },
+  {
+    name: "GET whose values need encoding",
+    path: "/api/users",
+    method: "GET",
+    input: { name: "a b&c" },
+  },
+  {
+    name: "POST with a body",
+    path: "/api/users",
+    method: "POST",
+    input: { name: "Ajay" },
+  },
+  {
+    name: "POST with no input",
+    path: "/api/users",
+    method: "POST",
+    input: undefined,
+  },
+];
+
+for (const { name, path, method, input } of CASES) {
+  test(`the browser runtime and buildApiRequest agree: ${name}`, async () => {
+    const fromRuntime = await callThroughRuntime(path, method, input);
+    const fromBuilder = callThroughBuilder(path, method, input);
+
+    expect(fromRuntime.url).toBe(fromBuilder.url);
+    expect(fromRuntime.body).toBe(fromBuilder.body);
+    expect(fromRuntime.contentType).toBe(fromBuilder.contentType);
+  });
+}