Make all fail loudly when the gpu extra is missing

Step 2 (invisible/SynthID) was skipped with a quiet inline warning and the
run still exited 0, so a missing [gpu] extra was mistaken for a clean result
(recurring #14/#47). Add a prominent end-of-run banner and a non-zero exit.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

README.md

@@ -232,6 +232,10 @@ After installation the `remove-ai-watermarks` command is available system-wide.
 > pip install -e ".[gpu]"   # or: uv pip install -e ".[gpu]"
 > ```
 >
+> Without the `[gpu]` extra, `all` still runs the visible and metadata steps, but
+> it skips the invisible (SynthID) step, prints a clear warning, and exits with a
+> non-zero status so a skipped step is not mistaken for a clean result.
+>
 > To let `identify` decode the open Stable Diffusion / SDXL / FLUX invisible
 > watermarks, install the `detect` extra (adds the `invisible-watermark` decoder):
 >

src/remove_ai_watermarks/cli.py

@@ -918,6 +918,13 @@ def cmd_all(
 
     t0 = time.monotonic()
 
+    # Tracks whether step 2 (invisible / SynthID removal) was skipped because the
+    # GPU extra is missing. A skipped step 2 still produces an output file (visible
+    # mark + metadata stripped), so without a loud end-of-run notice + non-zero exit
+    # the user mistakes it for a clean result and ships an image that still carries
+    # the invisible watermark (recurring reports: #14, #47).
+    synthid_skipped = False
+
     # Use a temp file for intermediate results so the user doesn't see
     # a partial output file during long model downloads.
     import tempfile
@@ -954,6 +961,7 @@ def cmd_all(
         from remove_ai_watermarks.invisible_engine import is_available as invisible_available
 
         if not invisible_available():
+            synthid_skipped = True
             console.print(
                 "    Warning: Skipped - GPU dependencies not installed.\n"
                 "    Install them with: pip install 'remove-ai-watermarks[gpu]'"
@@ -1028,6 +1036,24 @@ def cmd_all(
     size_kb = output.stat().st_size / 1024
     console.print(f"\n  Done: {output}  ({size_kb:.0f} KB, {elapsed:.1f}s total)")
 
+    # A skipped invisible step is the single most common "it didn't work" report:
+    # the output looks processed but still carries the SynthID watermark. Make that
+    # impossible to miss -- a prominent banner plus a non-zero exit so scripts and
+    # batch callers can detect the incomplete run instead of trusting the file.
+    if synthid_skipped:
+        console.print(
+            "\n  =====================================================================\n"
+            "  WARNING: the invisible (SynthID) watermark was NOT removed.\n"
+            "  Step 2 was skipped because the GPU dependencies are not installed,\n"
+            "  so this output still carries the invisible watermark -- only the\n"
+            "  visible mark and metadata were stripped.\n"
+            "\n"
+            "  Install the extra and rerun to remove it:\n"
+            "    pip install 'remove-ai-watermarks[gpu]'\n"
+            "  ====================================================================="
+        )
+        raise SystemExit(1)
+
 
 # -- Batch command ----------------------------------------------------
 

tests/test_cli.py

@@ -389,6 +389,19 @@ class TestAllCommand:
         assert result.exit_code == 0, result.output
         mock_best.assert_called()  # the registry auto-detector drove the visible pass
 
+    def test_all_loud_warning_and_nonzero_exit_when_gpu_missing(self, runner, sample_png, tmp_path):
+        """Regression (#14/#47): when the GPU extra is absent the invisible step is
+        skipped, but the output still looks processed -- the run must fail loudly
+        (prominent banner + non-zero exit) so a skipped SynthID pass is not mistaken
+        for a clean result. The output file is still written (visible + metadata)."""
+        output = tmp_path / "clean.png"
+        with patch("remove_ai_watermarks.invisible_engine.is_available", return_value=False):
+            result = runner.invoke(main, ["all", str(sample_png), "-o", str(output)])
+        assert result.exit_code != 0, result.output
+        assert "NOT removed" in result.output
+        assert "remove-ai-watermarks[gpu]" in result.output
+        assert output.exists()  # visible + metadata still produced a file
+
     def test_all_preserves_rgba_across_invisible_step(self, runner, tmp_path):
         """Regression: ``all`` must keep transparency even when the invisible
         step writes a 3-channel result (as the real diffusion engine does).