diff --git a/src/infra/stale-lock-file.test.ts b/src/infra/stale-lock-file.test.ts
index 56c257de6f2..b792895fe7c 100644
--- a/src/infra/stale-lock-file.test.ts
+++ b/src/infra/stale-lock-file.test.ts
@@ -1,5 +1,5 @@
 import { describe, expect, it } from "vitest";
-import { isPidDefinitelyDead, shouldRemoveDeadOwnerOrExpiredLock } from "./stale-lock-file.js";
+import { shouldRemoveDeadOwnerOrExpiredLock } from "./stale-lock-file.js";
 
 describe("stale lock file ownership", () => {
   it("treats permission-denied process probes as not definitely dead", () => {
@@ -27,8 +27,4 @@ describe("stale lock file ownership", () => {
       }),
     ).toBe(true);
   });
-
-  it("treats invalid pids as definitely dead", () => {
-    expect(isPidDefinitelyDead(-1)).toBe(true);
-  });
 });
diff --git a/src/infra/stale-lock-file.ts b/src/infra/stale-lock-file.ts
index 7d4ef534cbe..d59f3025309 100644
--- a/src/infra/stale-lock-file.ts
+++ b/src/infra/stale-lock-file.ts
@@ -1,4 +1,5 @@
 import fs from "node:fs/promises";
+import { isPidDefinitelyDead as defaultIsPidDefinitelyDead } from "../shared/pid-alive.js";
 
 export type LockFileSnapshot = {
   raw: string;
@@ -55,7 +56,7 @@ export function shouldRemoveDeadOwnerOrExpiredLock(params: {
 }): boolean {
   const payload = readLockFileOwnerPayload(params.payload);
   if (payload?.pid) {
-    return (params.isPidDefinitelyDead ?? isPidDefinitelyDead)(payload.pid);
+    return (params.isPidDefinitelyDead ?? defaultIsPidDefinitelyDead)(payload.pid);
   }
   if (payload?.createdAt) {
     const createdAt = Date.parse(payload.createdAt);
@@ -64,18 +65,6 @@ export function shouldRemoveDeadOwnerOrExpiredLock(params: {
   return false;
 }
 
-export function isPidDefinitelyDead(pid: number): boolean {
-  if (!Number.isInteger(pid) || pid <= 0) {
-    return true;
-  }
-  try {
-    process.kill(pid, 0);
-    return false;
-  } catch (err) {
-    return (err as NodeJS.ErrnoException).code === "ESRCH";
-  }
-}
-
 export async function removeLockFileIfSnapshotMatches(params: {
   lockPath: string;
   snapshot: LockFileSnapshot;
diff --git a/src/shared/pid-alive.test.ts b/src/shared/pid-alive.test.ts
index 36df219e2c3..f26cd7f5b6c 100644
--- a/src/shared/pid-alive.test.ts
+++ b/src/shared/pid-alive.test.ts
@@ -1,6 +1,10 @@
 import fsSync from "node:fs";
-import { describe, expect, it, vi } from "vitest";
-import { getProcessStartTime, isPidAlive } from "./pid-alive.js";
+import { afterEach, describe, expect, it, vi } from "vitest";
+import { getProcessStartTime, isPidAlive, isPidDefinitelyDead } from "./pid-alive.js";
+
+afterEach(() => {
+  vi.restoreAllMocks();
+});
 
 function mockProcReads(entries: Record<string, string>) {
   const originalReadFileSync = fsSync.readFileSync;
@@ -80,6 +84,60 @@ describe("isPidAlive", () => {
   });
 });
 
+describe("isPidDefinitelyDead", () => {
+  it("returns true for invalid PIDs", () => {
+    expect(isPidDefinitelyDead(0)).toBe(true);
+    expect(isPidDefinitelyDead(-1)).toBe(true);
+    expect(isPidDefinitelyDead(1.5)).toBe(true);
+    expect(isPidDefinitelyDead(Number.NaN)).toBe(true);
+    expect(isPidDefinitelyDead(Number.POSITIVE_INFINITY)).toBe(true);
+  });
+
+  it("returns true when process probing reports ESRCH", () => {
+    const error = Object.assign(new Error("missing process"), { code: "ESRCH" });
+    vi.spyOn(process, "kill").mockImplementation(() => {
+      throw error;
+    });
+
+    expect(isPidDefinitelyDead(42)).toBe(true);
+    expect(process.kill).toHaveBeenCalledWith(42, 0);
+  });
+
+  it("returns false when process probing reports EPERM", () => {
+    const error = Object.assign(new Error("permission denied"), { code: "EPERM" });
+    vi.spyOn(process, "kill").mockImplementation(() => {
+      throw error;
+    });
+
+    expect(isPidDefinitelyDead(42)).toBe(false);
+    expect(process.kill).toHaveBeenCalledWith(42, 0);
+  });
+
+  it("returns true for zombie processes on Linux", async () => {
+    const zombiePid = process.pid;
+    vi.spyOn(process, "kill").mockImplementation(() => true);
+    mockProcReads({
+      [`/proc/${zombiePid}/status`]: `Name:\tnode\nUmask:\t0022\nState:\tZ (zombie)\nTgid:\t${zombiePid}\nPid:\t${zombiePid}\n`,
+    });
+
+    await withLinuxProcessPlatform(async () => {
+      expect(isPidDefinitelyDead(zombiePid)).toBe(true);
+    });
+  });
+
+  it("returns false for live non-zombie processes", async () => {
+    const livePid = process.pid;
+    vi.spyOn(process, "kill").mockImplementation(() => true);
+    mockProcReads({
+      [`/proc/${livePid}/status`]: `Name:\tnode\nUmask:\t0022\nState:\tS (sleeping)\nTgid:\t${livePid}\nPid:\t${livePid}\n`,
+    });
+
+    await withLinuxProcessPlatform(async () => {
+      expect(isPidDefinitelyDead(livePid)).toBe(false);
+    });
+  });
+});
+
 describe("getProcessStartTime", () => {
   it("parses linux /proc stat start times and rejects malformed variants", async () => {
     const fakeStatPrefix = "42 (node) S 1 42 42 0 -1 4194304 12345 0 0 0 100 50 0 0 20 0 8 0 ";
diff --git a/src/shared/pid-alive.ts b/src/shared/pid-alive.ts
index 522566fb3fd..826ba2eebeb 100644
--- a/src/shared/pid-alive.ts
+++ b/src/shared/pid-alive.ts
@@ -36,6 +36,18 @@ export function isPidAlive(pid: number): boolean {
   return true;
 }
 
+export function isPidDefinitelyDead(pid: number): boolean {
+  if (!isValidPid(pid)) {
+    return true;
+  }
+  try {
+    process.kill(pid, 0);
+  } catch (err) {
+    return (err as NodeJS.ErrnoException).code === "ESRCH";
+  }
+  return isZombieProcess(pid);
+}
+
 /**
  * Read the process start time (field 22 "starttime") from /proc/<pid>/stat.
  * Returns the value in clock ticks since system boot, or null on non-Linux