diff --git a/.github/workflows/macos-release.yml b/.github/workflows/macos-release.yml
index 7a4921a..6045a9a 100644
--- a/.github/workflows/macos-release.yml
+++ b/.github/workflows/macos-release.yml
@@ -39,6 +39,9 @@ jobs:
       - name: Install frontend dependencies
         run: npm ci
 
+      - name: Build frontend assets for Rust tests
+        run: npm run build
+
       - name: Smoke test HEIC and HEIF conversion
         run: cargo test --manifest-path src-tauri/Cargo.toml --lib heic_and_heif_complete_the_jpeg_and_pdf_pipeline
 
diff --git a/src/lib/types.test.ts b/src/lib/types.test.ts
new file mode 100644
index 0000000..b00dc6e
--- /dev/null
+++ b/src/lib/types.test.ts
@@ -0,0 +1,121 @@
+import { describe, expect, it } from "vitest";
+import { isAppError, toAppError, type AppError } from "./types";
+
+describe("isAppError", () => {
+  it("accepts valid AppError objects with only required fields", () => {
+    const validMinimal: AppError = {
+      code: "FILE_NOT_FOUND",
+      title: "File Not Found",
+      message: "The requested file could not be found.",
+    };
+    expect(isAppError(validMinimal)).toBe(true);
+  });
+
+  it("accepts valid AppError objects with string details and suggestion", () => {
+    const validFull: AppError = {
+      code: "INVALID_PDF",
+      title: "Corrupt PDF",
+      message: "The PDF header is invalid.",
+      details: "qpdf exited with code 2",
+      suggestion: "Try opening the file in a PDF repair tool.",
+    };
+    expect(isAppError(validFull)).toBe(true);
+  });
+
+  it("accepts valid AppError objects with null or undefined details and suggestion", () => {
+    const withNulls: AppError = {
+      code: "PERMISSION_DENIED",
+      title: "Access Denied",
+      message: "Cannot write to destination.",
+      details: null,
+      suggestion: null,
+    };
+    const withUndefined: AppError = {
+      code: "PERMISSION_DENIED",
+      title: "Access Denied",
+      message: "Cannot write to destination.",
+      details: undefined,
+      suggestion: undefined,
+    };
+    expect(isAppError(withNulls)).toBe(true);
+    expect(isAppError(withUndefined)).toBe(true);
+  });
+
+  it("rejects non-object and null values", () => {
+    expect(isAppError(null)).toBe(false);
+    expect(isAppError(undefined)).toBe(false);
+    expect(isAppError("error string")).toBe(false);
+    expect(isAppError(12345)).toBe(false);
+    expect(isAppError(true)).toBe(false);
+    expect(isAppError(Symbol("err"))).toBe(false);
+    expect(isAppError(() => {})).toBe(false);
+  });
+
+  it("rejects objects missing required fields", () => {
+    expect(isAppError({ title: "Error", message: "Failed" })).toBe(false);
+    expect(isAppError({ code: "ERR", message: "Failed" })).toBe(false);
+    expect(isAppError({ code: "ERR", title: "Error" })).toBe(false);
+    expect(isAppError({})).toBe(false);
+  });
+
+  it("rejects objects with non-string code, title, or message", () => {
+    expect(isAppError({ code: 500, title: "Error", message: "Failed" })).toBe(false);
+    expect(isAppError({ code: "ERR", title: 404, message: "Failed" })).toBe(false);
+    expect(isAppError({ code: "ERR", title: "Error", message: null })).toBe(false);
+    expect(isAppError({ code: "ERR", title: "Error", message: ["Failed"] })).toBe(false);
+    expect(isAppError({ code: null, title: "Error", message: "Failed" })).toBe(false);
+    expect(isAppError({ code: "ERR", title: {}, message: "Failed" })).toBe(false);
+  });
+
+  it("rejects objects with invalid details or suggestion types", () => {
+    const base = { code: "ERR", title: "Error", message: "Failed" };
+    expect(isAppError({ ...base, details: 123 })).toBe(false);
+    expect(isAppError({ ...base, details: true })).toBe(false);
+    expect(isAppError({ ...base, details: ["detail"] })).toBe(false);
+    expect(isAppError({ ...base, details: { nested: "error" } })).toBe(false);
+
+    expect(isAppError({ ...base, suggestion: 456 })).toBe(false);
+    expect(isAppError({ ...base, suggestion: false })).toBe(false);
+    expect(isAppError({ ...base, suggestion: ["suggestion"] })).toBe(false);
+    expect(isAppError({ ...base, suggestion: { tip: "help" } })).toBe(false);
+  });
+});
+
+describe("toAppError", () => {
+  it("passes valid AppError objects through unchanged", () => {
+    const error: AppError = {
+      code: "DISK_FULL",
+      title: "Disk Full",
+      message: "Not enough space available.",
+      details: "Required: 100MB, Available: 10MB",
+      suggestion: "Free up disk space and retry.",
+    };
+    expect(toAppError(error)).toBe(error);
+  });
+
+  it("converts standard Error instances into an UNKNOWN AppError", () => {
+    const jsError = new Error("Network timeout");
+    const appError = toAppError(jsError);
+    expect(appError.code).toBe("UNKNOWN");
+    expect(appError.title).toBe("Something went wrong");
+    expect(appError.message).toBe("Network timeout");
+    expect(appError.details).toBe(jsError.stack);
+  });
+
+  it("converts malformed objects into UNKNOWN AppError safely", () => {
+    const malformed = { code: 123, title: null, message: false };
+    const appError = toAppError(malformed);
+    expect(appError.code).toBe("UNKNOWN");
+    expect(appError.title).toBe("Something went wrong");
+    expect(appError.message).toBe("An unexpected error occurred.");
+    expect(appError.details).toBe(JSON.stringify(malformed));
+  });
+
+  it("converts string values into UNKNOWN AppError", () => {
+    const appError = toAppError("Custom failure message");
+    expect(appError.code).toBe("UNKNOWN");
+    expect(appError.title).toBe("Something went wrong");
+    expect(appError.message).toBe("Custom failure message");
+    expect(appError.details).toBe(JSON.stringify("Custom failure message"));
+  });
+});
diff --git a/src/lib/types.ts b/src/lib/types.ts
index e4fe6e1..db71f25 100644
--- a/src/lib/types.ts
+++ b/src/lib/types.ts
@@ -171,15 +171,35 @@ export interface RecentJob {
 
 /** Type guard: is this value an AppError coming back from `invoke`? */
 export function isAppError(value: unknown): value is AppError {
-  return (
-    typeof value === "object" &&
-    value !== null &&
-    "code" in value &&
-    "title" in value &&
-    "message" in value
-  );
+  if (typeof value !== "object" || value === null) {
+    return false;
+  }
+  const candidate = value as Record<string, unknown>;
+  if (
+    typeof candidate.code !== "string" ||
+    typeof candidate.title !== "string" ||
+    typeof candidate.message !== "string"
+  ) {
+    return false;
+  }
+  if (
+    candidate.details !== undefined &&
+    candidate.details !== null &&
+    typeof candidate.details !== "string"
+  ) {
+    return false;
+  }
+  if (
+    candidate.suggestion !== undefined &&
+    candidate.suggestion !== null &&
+    typeof candidate.suggestion !== "string"
+  ) {
+    return false;
+  }
+  return true;
 }
 
+
 /** Coerce anything thrown from a command into an AppError for the UI. */
 export function toAppError(value: unknown): AppError {
   if (isAppError(value)) return value;