diff --git a/src/media/image-ops.helpers.test.ts b/src/media/image-ops.helpers.test.ts
index 09e908362e6..e90f4a13d4e 100644
--- a/src/media/image-ops.helpers.test.ts
+++ b/src/media/image-ops.helpers.test.ts
@@ -1,5 +1,5 @@
-import { describe, expect, it } from "vitest";
-import { buildImageResizeSideGrid, IMAGE_REDUCE_QUALITY_STEPS } from "./image-ops.js";
+import { afterEach, beforeEach, describe, expect, it, vi } from "vitest";
+import { buildImageResizeSideGrid, IMAGE_REDUCE_QUALITY_STEPS, prefersSips } from "./image-ops.js";
 
 describe("buildImageResizeSideGrid", () => {
   it("returns descending unique sides capped by maxSide", () => {
@@ -16,3 +16,136 @@ describe("IMAGE_REDUCE_QUALITY_STEPS", () => {
     expect([...IMAGE_REDUCE_QUALITY_STEPS]).toEqual([85, 75, 65, 55, 45, 35]);
   });
 });
+
+// ---------------------------------------------------------------------------
+// prefersSips()
+//
+// Logic:
+//   OPENCLAW_IMAGE_BACKEND === "sips"                         → true  (any platform)
+//   OPENCLAW_IMAGE_BACKEND === "sharp"                        → false (any platform)
+//   OPENCLAW_IMAGE_BACKEND unset / empty + darwin             → true  (← fix: was Node.js-broken)
+//   OPENCLAW_IMAGE_BACKEND unset / empty + linux | win32      → false
+// ---------------------------------------------------------------------------
+describe("prefersSips", () => {
+  let platformSpy: ReturnType<typeof vi.spyOn>;
+
+  beforeEach(() => {
+    // Allow overriding process.platform per test
+    platformSpy = vi.spyOn(process, "platform", "get");
+    // Restore env vars automatically after each test
+    vi.stubEnv("OPENCLAW_IMAGE_BACKEND", "");
+  });
+
+  afterEach(() => {
+    vi.unstubAllEnvs();
+    platformSpy.mockRestore();
+  });
+
+  // Branch 1 — explicit sips, any platform
+  describe("OPENCLAW_IMAGE_BACKEND=sips", () => {
+    it("returns true on darwin", () => {
+      vi.stubEnv("OPENCLAW_IMAGE_BACKEND", "sips");
+      platformSpy.mockReturnValue("darwin" as NodeJS.Platform);
+      expect(prefersSips()).toBe(true);
+    });
+
+    it("returns true on linux", () => {
+      vi.stubEnv("OPENCLAW_IMAGE_BACKEND", "sips");
+      platformSpy.mockReturnValue("linux" as NodeJS.Platform);
+      expect(prefersSips()).toBe(true);
+    });
+
+    it("returns true on win32", () => {
+      vi.stubEnv("OPENCLAW_IMAGE_BACKEND", "sips");
+      platformSpy.mockReturnValue("win32" as NodeJS.Platform);
+      expect(prefersSips()).toBe(true);
+    });
+  });
+
+  // Branch 2 — explicit sharp wins even on darwin
+  describe("OPENCLAW_IMAGE_BACKEND=sharp", () => {
+    it("returns false on darwin (sharp override takes priority)", () => {
+      vi.stubEnv("OPENCLAW_IMAGE_BACKEND", "sharp");
+      platformSpy.mockReturnValue("darwin" as NodeJS.Platform);
+      expect(prefersSips()).toBe(false);
+    });
+
+    it("returns false on linux", () => {
+      vi.stubEnv("OPENCLAW_IMAGE_BACKEND", "sharp");
+      platformSpy.mockReturnValue("linux" as NodeJS.Platform);
+      expect(prefersSips()).toBe(false);
+    });
+
+    // Branch 5 — explicit sharp on darwin even if runtime resembles Bun
+    it("returns false on darwin when env=sharp (Bun-like runtime, regression guard)", () => {
+      // Simulates: OPENCLAW_IMAGE_BACKEND=sharp + darwin + Bun process
+      vi.stubEnv("OPENCLAW_IMAGE_BACKEND", "sharp");
+      // Fake a Bun version string to confirm sharp env still wins
+      const origBun = (process.versions as Record<string, unknown>).bun;
+      (process.versions as Record<string, unknown>).bun = "1.0.0";
+      platformSpy.mockReturnValue("darwin" as NodeJS.Platform);
+      try {
+        expect(prefersSips()).toBe(false);
+      } finally {
+        if (origBun === undefined) {
+          delete (process.versions as Record<string, unknown>).bun;
+        } else {
+          (process.versions as Record<string, unknown>).bun = origBun;
+        }
+      }
+    });
+  });
+
+  // Branch 3 — THE FIX: no env var + darwin → must be true for both Node.js and Bun
+  describe("no OPENCLAW_IMAGE_BACKEND env var + darwin", () => {
+    it("returns true on darwin (Node.js runtime — core fix, was false before)", () => {
+      vi.stubEnv("OPENCLAW_IMAGE_BACKEND", "");
+      platformSpy.mockReturnValue("darwin" as NodeJS.Platform);
+      expect(prefersSips()).toBe(true);
+    });
+
+    it("returns true on darwin when Bun is detected (no regression)", () => {
+      vi.stubEnv("OPENCLAW_IMAGE_BACKEND", "");
+      const origBun = (process.versions as Record<string, unknown>).bun;
+      (process.versions as Record<string, unknown>).bun = "1.0.0";
+      platformSpy.mockReturnValue("darwin" as NodeJS.Platform);
+      try {
+        expect(prefersSips()).toBe(true);
+      } finally {
+        if (origBun === undefined) {
+          delete (process.versions as Record<string, unknown>).bun;
+        } else {
+          (process.versions as Record<string, unknown>).bun = origBun;
+        }
+      }
+    });
+
+    // Empty string is distinct from "sips" — falls through to platform check
+    it("returns true on darwin when env var is empty string", () => {
+      vi.stubEnv("OPENCLAW_IMAGE_BACKEND", "");
+      platformSpy.mockReturnValue("darwin" as NodeJS.Platform);
+      expect(prefersSips()).toBe(true);
+    });
+  });
+
+  // Branch 4 — no env var + non-darwin → false
+  describe("no OPENCLAW_IMAGE_BACKEND env var + non-darwin", () => {
+    it("returns false on linux", () => {
+      vi.stubEnv("OPENCLAW_IMAGE_BACKEND", "");
+      platformSpy.mockReturnValue("linux" as NodeJS.Platform);
+      expect(prefersSips()).toBe(false);
+    });
+
+    it("returns false on win32", () => {
+      vi.stubEnv("OPENCLAW_IMAGE_BACKEND", "");
+      platformSpy.mockReturnValue("win32" as NodeJS.Platform);
+      expect(prefersSips()).toBe(false);
+    });
+
+    it("returns false on linux when env var is empty string", () => {
+      vi.stubEnv("OPENCLAW_IMAGE_BACKEND", "");
+      platformSpy.mockReturnValue("linux" as NodeJS.Platform);
+      expect(prefersSips()).toBe(false);
+    });
+  });
+});
diff --git a/src/media/image-ops.ts b/src/media/image-ops.ts
index cbf0ed20ffc..8467e9dadaf 100644
--- a/src/media/image-ops.ts
+++ b/src/media/image-ops.ts
@@ -19,14 +19,10 @@ export function buildImageResizeSideGrid(maxSide: number, sideStart: number): nu
     .toSorted((a, b) => b - a);
 }
 
-function isBun(): boolean {
-  return typeof (process.versions as { bun?: unknown }).bun === "string";
-}
-
-function prefersSips(): boolean {
+export function prefersSips(): boolean {
   return (
     process.env.OPENCLAW_IMAGE_BACKEND === "sips" ||
-    (process.env.OPENCLAW_IMAGE_BACKEND !== "sharp" && isBun() && process.platform === "darwin")
+    (process.env.OPENCLAW_IMAGE_BACKEND !== "sharp" && process.platform === "darwin")
   );
 }