diff --git a/src/lib/upgrade.ts b/src/lib/upgrade.ts
index 0862f7db..13ad8a5d 100644
--- a/src/lib/upgrade.ts
+++ b/src/lib/upgrade.ts
@@ -117,12 +117,22 @@ export function cleanupOldBinary(): void {
 
 /**
  * Check if a process with the given PID is still running.
+ *
+ * On Unix, process.kill(pid, 0) throws:
+ * - ESRCH: Process does not exist (not running)
+ * - EPERM: Process exists but we lack permission to signal it (IS running)
  */
 export function isProcessRunning(pid: number): boolean {
   try {
     process.kill(pid, 0); // Signal 0 just checks if process exists
     return true;
-  } catch {
+  } catch (error) {
+    // EPERM means process exists but we can't signal it (different user)
+    // This is still a running process, so return true
+    if ((error as NodeJS.ErrnoException).code === "EPERM") {
+      return true;
+    }
+    // ESRCH or other errors mean process is not running
     return false;
   }
 }
diff --git a/test/lib/upgrade.test.ts b/test/lib/upgrade.test.ts
index 9a4f5576..1da25de5 100644
--- a/test/lib/upgrade.test.ts
+++ b/test/lib/upgrade.test.ts
@@ -4,7 +4,7 @@
  * Tests for upgrade detection and logic.
  */
 
-import { afterEach, beforeEach, describe, expect, test } from "bun:test";
+import { afterEach, beforeEach, describe, expect, spyOn, test } from "bun:test";
 import { chmodSync, mkdirSync, readFileSync, writeFileSync } from "node:fs";
 import { unlink } from "node:fs/promises";
 import { homedir, platform } from "node:os";
@@ -482,6 +482,36 @@ describe("isProcessRunning", () => {
     // PID 4194304 is above typical max PID on most systems
     expect(isProcessRunning(4_194_304)).toBe(false);
   });
+
+  test("returns true on EPERM (process exists but owned by different user)", () => {
+    // Mock process.kill to throw EPERM
+    const epermError = Object.assign(new Error("EPERM"), { code: "EPERM" });
+    const spy = spyOn(process, "kill").mockImplementation(() => {
+      throw epermError;
+    });
+
+    try {
+      // EPERM means the process exists, we just can't signal it
+      expect(isProcessRunning(12_345)).toBe(true);
+    } finally {
+      spy.mockRestore();
+    }
+  });
+
+  test("returns false on ESRCH (process does not exist)", () => {
+    // Mock process.kill to throw ESRCH
+    const esrchError = Object.assign(new Error("ESRCH"), { code: "ESRCH" });
+    const spy = spyOn(process, "kill").mockImplementation(() => {
+      throw esrchError;
+    });
+
+    try {
+      // ESRCH means the process does not exist
+      expect(isProcessRunning(12_345)).toBe(false);
+    } finally {
+      spy.mockRestore();
+    }
+  });
 });
 
 describe("acquireUpgradeLock", () => {