diff --git a/extensions/matrix/src/matrix/probe.runtime.ts b/extensions/matrix/src/matrix/probe.runtime.ts
new file mode 100644
index 00000000000..d247a59d2a9
--- /dev/null
+++ b/extensions/matrix/src/matrix/probe.runtime.ts
@@ -0,0 +1,4 @@
+import { createMatrixClient } from "./client.js";
+
+// Keep probe's runtime seam narrow so tests can mock it without loading the full client barrel.
+export { createMatrixClient };
diff --git a/extensions/matrix/src/matrix/probe.test.ts b/extensions/matrix/src/matrix/probe.test.ts
index 9d6f3e3c1ff..b01d78caee9 100644
--- a/extensions/matrix/src/matrix/probe.test.ts
+++ b/extensions/matrix/src/matrix/probe.test.ts
@@ -3,8 +3,11 @@ import { beforeEach, describe, expect, it, vi } from "vitest";
 const createMatrixClientMock = vi.fn();
 const isBunRuntimeMock = vi.fn(() => false);
 
-vi.mock("./client.js", () => ({
+vi.mock("./probe.runtime.js", () => ({
   createMatrixClient: (...args: unknown[]) => createMatrixClientMock(...args),
+}));
+
+vi.mock("./client/runtime.js", () => ({
   isBunRuntime: () => isBunRuntimeMock(),
 }));
 
diff --git a/extensions/matrix/src/matrix/probe.ts b/extensions/matrix/src/matrix/probe.ts
index 9795a601563..f90cf5a8074 100644
--- a/extensions/matrix/src/matrix/probe.ts
+++ b/extensions/matrix/src/matrix/probe.ts
@@ -3,13 +3,13 @@ import type { SsrFPolicy } from "../runtime-api.js";
 import type { BaseProbeResult } from "../runtime-api.js";
 import { isBunRuntime } from "./client/runtime.js";
 
-type MatrixProbeRuntimeDeps = Pick<typeof import("./client.js"), "createMatrixClient">;
+type MatrixProbeRuntimeDeps = Pick<typeof import("./probe.runtime.js"), "createMatrixClient">;
 
 let matrixProbeRuntimeDepsPromise: Promise<MatrixProbeRuntimeDeps> | undefined;
 
 async function loadMatrixProbeRuntimeDeps(): Promise<MatrixProbeRuntimeDeps> {
-  matrixProbeRuntimeDepsPromise ??= import("./client.js").then((clientModule) => ({
-    createMatrixClient: clientModule.createMatrixClient,
+  matrixProbeRuntimeDepsPromise ??= import("./probe.runtime.js").then((runtimeModule) => ({
+    createMatrixClient: runtimeModule.createMatrixClient,
   }));
   return await matrixProbeRuntimeDepsPromise;
 }