diff --git a/integrations/aws-strands/typescript/src/__tests__/disconnect-unhandled-rejection.test.ts b/integrations/aws-strands/typescript/src/__tests__/disconnect-unhandled-rejection.test.ts
index 3424602124..573fd6085a 100644
--- a/integrations/aws-strands/typescript/src/__tests__/disconnect-unhandled-rejection.test.ts
+++ b/integrations/aws-strands/typescript/src/__tests__/disconnect-unhandled-rejection.test.ts
@@ -42,7 +42,7 @@ describe("endpoint disconnect + throwing generator finally", () => {
     app.use(express.json({ limit: "1mb" }));
     addStrandsExpressEndpoint(app, agent, { path: "/" });
     const server = await new Promise<import("http").Server>((resolve) => {
-      const s = app.listen(0, () => resolve(s));
+      const s = app.listen(0, "127.0.0.1", () => resolve(s));
     });
     const port = (server.address() as AddressInfo).port;
 
diff --git a/integrations/aws-strands/typescript/src/__tests__/endpoint-accept.test.ts b/integrations/aws-strands/typescript/src/__tests__/endpoint-accept.test.ts
index 6f5bffb885..2d86bc1ad8 100644
--- a/integrations/aws-strands/typescript/src/__tests__/endpoint-accept.test.ts
+++ b/integrations/aws-strands/typescript/src/__tests__/endpoint-accept.test.ts
@@ -49,7 +49,7 @@ async function startApp(): Promise<{
     { path: "/" },
   );
   const server = await new Promise<import("http").Server>((resolve) => {
-    const s = app.listen(0, () => resolve(s));
+    const s = app.listen(0, "127.0.0.1", () => resolve(s));
   });
   const port = (server.address() as AddressInfo).port;
   return {
diff --git a/integrations/aws-strands/typescript/src/__tests__/endpoint-capabilities.test.ts b/integrations/aws-strands/typescript/src/__tests__/endpoint-capabilities.test.ts
index 0b2092c235..6836570c2d 100644
--- a/integrations/aws-strands/typescript/src/__tests__/endpoint-capabilities.test.ts
+++ b/integrations/aws-strands/typescript/src/__tests__/endpoint-capabilities.test.ts
@@ -17,7 +17,7 @@ async function startApp(configure: (app: express.Express) => void): Promise<{
   app.use(express.json({ limit: "1mb" }));
   configure(app);
   const server = await new Promise<import("http").Server>((resolve) => {
-    const s = app.listen(0, () => resolve(s));
+    const s = app.listen(0, "127.0.0.1", () => resolve(s));
   });
   const port = (server.address() as AddressInfo).port;
   return {
diff --git a/integrations/aws-strands/typescript/src/__tests__/endpoint-disconnect.test.ts b/integrations/aws-strands/typescript/src/__tests__/endpoint-disconnect.test.ts
index 44c86f517c..4e33d48ce0 100644
--- a/integrations/aws-strands/typescript/src/__tests__/endpoint-disconnect.test.ts
+++ b/integrations/aws-strands/typescript/src/__tests__/endpoint-disconnect.test.ts
@@ -59,7 +59,7 @@ async function startApp(agent: StrandsAgent): Promise<{
   app.use(express.json({ limit: "1mb" }));
   addStrandsExpressEndpoint(app, agent, { path: "/" });
   const server = await new Promise<import("http").Server>((resolve) => {
-    const s = app.listen(0, () => resolve(s));
+    const s = app.listen(0, "127.0.0.1", () => resolve(s));
   });
   const port = (server.address() as AddressInfo).port;
   return {
diff --git a/integrations/aws-strands/typescript/src/__tests__/endpoint-validation.test.ts b/integrations/aws-strands/typescript/src/__tests__/endpoint-validation.test.ts
index 1f97946be5..898c615b31 100644
--- a/integrations/aws-strands/typescript/src/__tests__/endpoint-validation.test.ts
+++ b/integrations/aws-strands/typescript/src/__tests__/endpoint-validation.test.ts
@@ -62,7 +62,7 @@ async function startApp(): Promise<{
   const agent = new RecordingStrandsAgent();
   addStrandsExpressEndpoint(app, agent, { path: "/" });
   const server = await new Promise<import("http").Server>((resolve) => {
-    const s = app.listen(0, () => resolve(s));
+    const s = app.listen(0, "127.0.0.1", () => resolve(s));
   });
   const port = (server.address() as AddressInfo).port;
   return {
diff --git a/integrations/aws-strands/typescript/src/__tests__/endpoint.test.ts b/integrations/aws-strands/typescript/src/__tests__/endpoint.test.ts
index 622ff9a6ab..926f4af7c0 100644
--- a/integrations/aws-strands/typescript/src/__tests__/endpoint.test.ts
+++ b/integrations/aws-strands/typescript/src/__tests__/endpoint.test.ts
@@ -50,7 +50,7 @@ async function startApp(agent: StrandsAgent): Promise<{
   addStrandsExpressEndpoint(app, agent, { path: "/" });
   addPing(app, "/ping");
   const server = await new Promise<import("http").Server>((resolve) => {
-    const s = app.listen(0, () => resolve(s));
+    const s = app.listen(0, "127.0.0.1", () => resolve(s));
   });
   const port = (server.address() as AddressInfo).port;
   return {
diff --git a/integrations/aws-strands/typescript/src/__tests__/terminal-error-paths.test.ts b/integrations/aws-strands/typescript/src/__tests__/terminal-error-paths.test.ts
index 822cfc25d4..09dbef02d3 100644
--- a/integrations/aws-strands/typescript/src/__tests__/terminal-error-paths.test.ts
+++ b/integrations/aws-strands/typescript/src/__tests__/terminal-error-paths.test.ts
@@ -439,7 +439,7 @@ describe("terminal error paths", () => {
       app.use(express.json({ limit: "10mb" }));
       addStrandsExpressEndpoint(app, new UnencodableAgent(), { path: "/" });
       const server = await new Promise<import("http").Server>((resolve) => {
-        const s = app.listen(0, () => resolve(s));
+        const s = app.listen(0, "127.0.0.1", () => resolve(s));
       });
       const port = (server.address() as AddressInfo).port;
 
diff --git a/integrations/aws-strands/typescript/src/__tests__/transport-harness.test.ts b/integrations/aws-strands/typescript/src/__tests__/transport-harness.test.ts
index f51de76b80..228d854033 100644
--- a/integrations/aws-strands/typescript/src/__tests__/transport-harness.test.ts
+++ b/integrations/aws-strands/typescript/src/__tests__/transport-harness.test.ts
@@ -27,16 +27,32 @@ function appThatFailsToBind(failure: Error): Express {
   } as unknown as Express;
 }
 
-/** An app that binds successfully, exposing the emitter the harness wires up. */
-function appThatBinds(): { app: Express; emitter: EventEmitter } {
+/**
+ * An app that binds successfully, exposing the emitter the harness wires up
+ * and the arguments it was asked to bind with.
+ */
+function appThatBinds(): {
+  app: Express;
+  emitter: EventEmitter;
+  boundTo: () => { port: number; host: string };
+} {
   const emitter = new EventEmitter();
+  let bound: { port: number; host: string } | undefined;
   const app = {
-    listen: (_port: number, onListening: () => void) => {
+    listen: (port: number, host: string, onListening: () => void) => {
+      bound = { port, host };
       setImmediate(onListening);
       return emitter;
     },
   } as unknown as Express;
-  return { app, emitter };
+  return {
+    app,
+    emitter,
+    boundTo: () => {
+      if (!bound) throw new Error("listen was never called");
+      return bound;
+    },
+  };
 }
 
 describe("listen reports a bind failure instead of hanging", () => {
@@ -61,6 +77,17 @@ describe("listen reports a bind failure instead of hanging", () => {
     expect(settled).toBe(failure);
   });
 
+  // The probes all dial `127.0.0.1`, and an unspecified host binds the IPv6
+  // wildcard instead. A process already holding the IPv4 wildcard on the port
+  // the kernel hands out then answers those probes in this server's place, so
+  // dropping the host does not fail here or anywhere obvious: it hands a
+  // stranger's reply to whichever suite drew the colliding number.
+  it("binds the loopback address the probes dial, not the wildcard", async () => {
+    const { app, boundTo } = appThatBinds();
+    await listen(app);
+    expect(boundTo()).toEqual({ port: 0, host: "127.0.0.1" });
+  });
+
   it("does not absorb a post-bind server error into the settled promise", async () => {
     const { app, emitter } = appThatBinds();
     const server = await listen(app);
diff --git a/integrations/aws-strands/typescript/src/__tests__/transport-harness.ts b/integrations/aws-strands/typescript/src/__tests__/transport-harness.ts
index 6a10c18cd6..de0ef0055c 100644
--- a/integrations/aws-strands/typescript/src/__tests__/transport-harness.ts
+++ b/integrations/aws-strands/typescript/src/__tests__/transport-harness.ts
@@ -53,7 +53,17 @@ export interface StartedApp {
 }
 
 /**
- * Bind an app to an ephemeral port.
+ * Bind an app to an ephemeral port on the IPv4 loopback.
+ *
+ * The address is pinned because every probe below dials `127.0.0.1`, and an
+ * unspecified host binds the IPv6 wildcard instead. That pair is not the same
+ * port: an unrelated process already holding the IPv4 wildcard on the number
+ * the kernel hands out keeps receiving the loopback traffic, so the probe is
+ * answered by that process while this server never sees a connection. It
+ * reaches the suite as whatever that stranger happens to reply -- a foreign
+ * status and body, an HTTP parse error, or a bare close -- on whichever test
+ * drew the colliding number. Binding the loopback explicitly takes precedence
+ * over a wildcard holder, so the probe reaches this app or nothing at all.
  *
  * Two failure modes, and each needs its own handler. A bind failure
  * (EADDRINUSE, a permission denial) arrives before the promise settles, so it
@@ -68,7 +78,7 @@ export function listen(
   app: import("express").Express,
 ): Promise<import("http").Server> {
   return new Promise((resolve, reject) => {
-    const server = app.listen(0, () => {
+    const server = app.listen(0, "127.0.0.1", () => {
       server.removeListener("error", reject);
       // Thrown synchronously out of `emit`, which surfaces it as an uncaught
       // exception and fails the run. Loud beats lost for a server fault the