diff --git a/tests/runner-e2e/launch.ts b/tests/runner-e2e/launch.ts
index 50e2ef9a39..9d4a739f20 100644
--- a/tests/runner-e2e/launch.ts
+++ b/tests/runner-e2e/launch.ts
@@ -2,7 +2,6 @@ import { randomBytes } from "node:crypto";
 import { spawn } from "node:child_process";
 import { createWriteStream } from "node:fs";
 import { createRequire } from "node:module";
-import { createServer } from "node:net";
 import os from "node:os";
 import path from "node:path";
 import {
@@ -46,6 +45,7 @@ import {
   reapNewDetachedDarwinSharedMemory,
   snapshotDarwinSharedMemory,
 } from "./shared-memory.js";
+import { reserveRunnerE2EServerPort } from "./ports.js";
 
 const repositoryRoot = path.resolve(import.meta.dirname, "../..");
 const localEnvPath = path.join(repositoryRoot, ".env.runner-e2e.local");
@@ -157,21 +157,6 @@ async function loadLocalEnvironment(target: NodeJS.ProcessEnv) {
   }
 }
 
-async function reservePort() {
-  const server = createServer();
-  await new Promise<void>((resolve, reject) => {
-    server.once("error", reject);
-    server.listen(0, "127.0.0.1", resolve);
-  });
-  const address = server.address();
-  if (!address || typeof address === "string")
-    throw new Error("Failed to reserve a loopback port");
-  await new Promise<void>((resolve, reject) =>
-    server.close((error) => (error ? reject(error) : resolve())),
-  );
-  return address.port;
-}
-
 async function prepareProviderPath(
   temporaryRoot: string,
   inheritedPath: string | undefined,
@@ -388,7 +373,7 @@ async function runAttempt(input: {
       instanceId,
       "config.json",
     );
-    const port = await reservePort();
+    const port = await reserveRunnerE2EServerPort();
     await Promise.all([
       mkdir(paperclipHome, { recursive: true }),
       mkdir(workspace, { recursive: true }),
diff --git a/tests/runner-e2e/ports.ts b/tests/runner-e2e/ports.ts
new file mode 100644
index 0000000000..10d0703cfc
--- /dev/null
+++ b/tests/runner-e2e/ports.ts
@@ -0,0 +1,92 @@
+import { createServer } from "node:net";
+import { derivePaperclipViteHmrPort } from "../../packages/shared/src/runtime-exposure/ports.js";
+
+export const RUNNER_E2E_EMBEDDED_POSTGRES_PORT = 54_329;
+
+export interface LoopbackPortReservation {
+  port: number;
+  close(): Promise<void>;
+}
+
+export type OpenLoopbackPort = (
+  requestedPort: number,
+) => Promise<LoopbackPortReservation>;
+
+async function openLoopbackPort(
+  requestedPort: number,
+): Promise<LoopbackPortReservation> {
+  const server = createServer();
+  await new Promise<void>((resolve, reject) => {
+    server.once("error", reject);
+    server.listen(requestedPort, "127.0.0.1", resolve);
+  });
+  const address = server.address();
+  if (!address || typeof address === "string") {
+    server.close();
+    throw new Error("Failed to reserve a loopback port");
+  }
+  return {
+    port: address.port,
+    close: () =>
+      new Promise<void>((resolve, reject) =>
+        server.close((error) => (error ? reject(error) : resolve())),
+      ),
+  };
+}
+
+export function runnerE2EServerPortConflictsWithDatabase(
+  serverPort: number,
+  databasePort = RUNNER_E2E_EMBEDDED_POSTGRES_PORT,
+) {
+  return (
+    serverPort === databasePort ||
+    derivePaperclipViteHmrPort(serverPort) === databasePort
+  );
+}
+
+function isAddressInUse(error: unknown) {
+  return (error as NodeJS.ErrnoException | undefined)?.code === "EADDRINUSE";
+}
+
+export async function reserveRunnerE2EServerPort(
+  options: {
+    databasePort?: number;
+    maxAttempts?: number;
+    openPort?: OpenLoopbackPort;
+  } = {},
+) {
+  const databasePort =
+    options.databasePort ?? RUNNER_E2E_EMBEDDED_POSTGRES_PORT;
+  const maxAttempts = options.maxAttempts ?? 32;
+  const openPort = options.openPort ?? openLoopbackPort;
+
+  for (let attempt = 1; attempt <= maxAttempts; attempt += 1) {
+    const serverReservation = await openPort(0);
+    let hmrReservation: LoopbackPortReservation | undefined;
+    try {
+      if (
+        runnerE2EServerPortConflictsWithDatabase(
+          serverReservation.port,
+          databasePort,
+        )
+      ) {
+        continue;
+      }
+      const hmrPort = derivePaperclipViteHmrPort(serverReservation.port);
+      try {
+        hmrReservation = await openPort(hmrPort);
+      } catch (error) {
+        if (isAddressInUse(error)) continue;
+        throw error;
+      }
+      return serverReservation.port;
+    } finally {
+      await hmrReservation?.close();
+      await serverReservation.close();
+    }
+  }
+
+  throw new Error(
+    `Failed to reserve a conflict-free Paperclip/Vite port pair after ${maxAttempts} attempts`,
+  );
+}
diff --git a/tests/runner-e2e/support.test.ts b/tests/runner-e2e/support.test.ts
index b3894baed2..f950318687 100644
--- a/tests/runner-e2e/support.test.ts
+++ b/tests/runner-e2e/support.test.ts
@@ -26,6 +26,11 @@ import {
   sanitizeJson,
 } from "./redaction.js";
 import { parseDarwinSharedMemory } from "./shared-memory.js";
+import {
+  reserveRunnerE2EServerPort,
+  runnerE2EServerPortConflictsWithDatabase,
+  type LoopbackPortReservation,
+} from "./ports.js";
 import {
   acceptedPlanSessionResetFailures,
   isControlPlaneGovernedResponseWait,
@@ -79,6 +84,36 @@ describe("runner E2E local binary resolution", () => {
   });
 });
 
+describe("runner E2E server port allocation", () => {
+  it("rejects direct and derived embedded-Postgres collisions", () => {
+    expect(runnerE2EServerPortConflictsWithDatabase(44_329)).toBe(true);
+    expect(runnerE2EServerPortConflictsWithDatabase(54_329)).toBe(true);
+    expect(runnerE2EServerPortConflictsWithDatabase(64_329)).toBe(true);
+    expect(runnerE2EServerPortConflictsWithDatabase(43_123)).toBe(false);
+  });
+
+  it("retries a derived collision while closing every reservation", async () => {
+    const closed: number[] = [];
+    const ports = [44_329, 43_123, 53_123];
+    const openPort = vi.fn(async (requestedPort: number) => {
+      const port = requestedPort === 0 ? ports.shift() : requestedPort;
+      if (port === undefined) throw new Error("Missing fake port");
+      return {
+        port,
+        close: async () => {
+          closed.push(port);
+        },
+      } satisfies LoopbackPortReservation;
+    });
+
+    await expect(reserveRunnerE2EServerPort({ openPort })).resolves.toBe(
+      43_123,
+    );
+    expect(openPort.mock.calls.map(([port]) => port)).toEqual([0, 0, 53_123]);
+    expect(closed).toEqual([44_329, 53_123, 43_123]);
+  });
+});
+
 describe("runner E2E sensitive API boundary", () => {
   it("keeps secret request bodies out of Playwright API tracing", async () => {
     vi.stubEnv("PAPERCLIP_RUNNER_E2E_PORT", "43123");