Reject sd-cli batch runs and clear stale output targets before a run

studio/backend/core/inference/sd_cpp_args.py

@@ -269,7 +269,12 @@ def build_sd_cpp_command(
     if params.seed is not None:
         cmd += ["--seed", str(int(params.seed))]
     if params.batch_count and params.batch_count != 1:
-        cmd += ["--batch-count", str(int(params.batch_count))]
+        # sd-cli names the extra batch images itself (output_2.png, ...) and the runner
+        # collects only the literal --output path, so a CLI batch would silently drop
+        # every image after the first. Batches go through the sdcpp server API instead.
+        raise ValueError(
+            "sd-cli runs are single-image; use the sdcpp server API for batch generation."
+        )
 
     cmd += ["--output", output_path]
     if threads is not None:

studio/backend/core/inference/sd_cpp_engine.py

@@ -364,6 +364,9 @@ class SdCppEngine:
     def _prepare_out(output_path: str) -> Path:
         out = Path(output_path)
         out.parent.mkdir(parents = True, exist_ok = True)
+        # Drop a stale file at the target so the post-run is_file() check proves THIS
+        # run produced the image, not a leftover from an earlier run at the same path.
+        out.unlink(missing_ok = True)
         return out
 
     def _run(

studio/backend/tests/test_sd_cpp_args.py

@@ -166,10 +166,18 @@ def test_build_appends_offload_and_extra_args_last():
 
 def test_build_negative_prompt_and_batch():
     files = SdCppModelFiles(diffusion_model = "/m/z.gguf")
-    params = SdCppGenParams(prompt = "x", negative_prompt = "blurry", batch_count = 3)
+    params = SdCppGenParams(prompt = "x", negative_prompt = "blurry")
     cmd = build_sd_cpp_command("/bin/sd-cli", files, params, output_path = "/o.png")
     assert _pair(cmd, "--negative-prompt") == "blurry"
-    assert _pair(cmd, "--batch-count") == "3"
+    # A CLI batch would silently drop every image after the first (the runner only
+    # collects the literal --output path), so the builder rejects it outright.
+    with pytest.raises(ValueError, match = "single-image"):
+        build_sd_cpp_command(
+            "/bin/sd-cli",
+            files,
+            SdCppGenParams(prompt = "x", batch_count = 3),
+            output_path = "/o.png",
+        )
 
 
 def test_build_omits_unset_optional_params():

studio/backend/tests/test_sd_cpp_engine.py

@@ -323,6 +323,22 @@ def test_generate_raises_when_no_output_despite_success(tmp_path, monkeypatch):
         )
 
 
+def test_generate_does_not_return_stale_preexisting_output(tmp_path, monkeypatch):
+    # A leftover file at the target path must not satisfy the post-run output check
+    # when the run itself produced nothing: the target is cleared before the run.
+    e = _engine(tmp_path)
+    out = tmp_path / "img.png"
+    out.write_bytes(b"stale")
+    _patch_popen(monkeypatch, lines = ["ok"], returncode = 0, out_file = out, write = False)
+    with pytest.raises(RuntimeError, match = "no image"):
+        e.generate(
+            SdCppModelFiles(diffusion_model = "/m/z.gguf"),
+            SdCppGenParams(prompt = "x"),
+            output_path = str(out),
+        )
+    assert not out.exists()
+
+
 def test_generate_raises_when_binary_missing():
     e = SdCppEngine(binary = None)
     with pytest.raises(RuntimeError, match = "not found"):