fix: prefer checkout venv for SynthID scorer subprocess (#115) (#123)

service/scripts/image_meta.py

@@ -1240,6 +1240,17 @@ def _synthid_score_http(
     return payload
 
 
+def _synthid_python(upstream: Path) -> str:
+    """Prefer the checkout venv so the scorer deps (cv2, sklearn) are importable."""
+    if os.name == "nt":
+        venv = upstream / ".venv" / "Scripts" / "python.exe"
+    else:
+        venv = upstream / ".venv" / "bin" / "python"
+    if venv.is_file():
+        return str(venv)
+    return sys.executable
+
+
 def run_synthid_score(
     path: Path,
     upstream_dir: str | None = None,
@@ -1248,8 +1259,9 @@ def run_synthid_score(
 
     Uses the HTTP sidecar when WATERMARKS_SYNTHID_SCORER_URL is set,
     otherwise a subprocess against a local checkout. Returns None when the
-    scorer is not configured or unavailable (exit 3), so callers can keep
-    the default "no SynthID score" behavior.
+    scorer is not configured; a dict with "available": False and an "error"
+    when it is configured but unavailable (e.g. exit 3), so callers can
+    distinguish "not scored" from "scored and clean".
     """
     scorer_url = os.environ.get("WATERMARKS_SYNTHID_SCORER_URL", "").strip()
     if scorer_url:
@@ -1266,7 +1278,7 @@ def run_synthid_score(
 
     script = SCRIPTS_DIR / "score_synthid.py"
     cmd = [
-        sys.executable,
+        _synthid_python(Path(upstream_dir)),
         str(script),
         str(path),
         "--upstream-dir",
@@ -1286,7 +1298,10 @@ def run_synthid_score(
         return {"available": False, "error": str(e)}
 
     if r.returncode == 3:
-        return None
+        return {
+            "available": False,
+            "error": (r.stderr or "SynthID scorer unavailable (exit 3)").strip()[:2000],
+        }
     if r.returncode != 0:
         return {"available": False, "error": (r.stderr or "").strip()[:2000]}
     try:

tests/test_synthid_score.py

@@ -3,6 +3,7 @@
 from __future__ import annotations
 
 import json
+import os
 import subprocess
 import sys
 from pathlib import Path
@@ -45,7 +46,7 @@ def test_run_synthid_score_unconfigured_returns_none(
     assert run_synthid_score(Path("x.png")) is None
 
 
-def test_run_synthid_score_unavailable_returns_none(
+def test_run_synthid_score_unavailable_returns_error(
     tmp_path: Path,
     monkeypatch: pytest.MonkeyPatch,
 ):
@@ -53,7 +54,48 @@ def test_run_synthid_score_unavailable_returns_none(
         return SimpleNamespace(returncode=3, stdout="", stderr="unavailable")
 
     monkeypatch.setattr(image_meta.subprocess, "run", fake_run)
-    assert run_synthid_score(Path("x.png"), upstream_dir=str(tmp_path / "upstream")) is None
+    result = run_synthid_score(Path("x.png"), upstream_dir=str(tmp_path / "upstream"))
+
+    assert result is not None
+    assert result.get("available") is False
+    assert "unavailable" in result.get("error", "")
+
+
+def test_run_synthid_score_prefers_checkout_venv_python(
+    tmp_path: Path,
+    monkeypatch: pytest.MonkeyPatch,
+):
+    upstream = tmp_path / "upstream"
+    venv_python = upstream / ".venv" / ("Scripts/python.exe" if os.name == "nt" else "bin/python")
+    venv_python.parent.mkdir(parents=True)
+    venv_python.write_text("#!/bin/sh\n")
+    captured: dict = {}
+
+    def fake_run(cmd, **kwargs):
+        captured["cmd"] = cmd
+        return SimpleNamespace(returncode=0, stdout="{}", stderr="")
+
+    monkeypatch.setattr(image_meta.subprocess, "run", fake_run)
+    run_synthid_score(Path("img.png"), upstream_dir=str(upstream))
+
+    assert captured["cmd"][0] == str(venv_python)
+
+
+def test_run_synthid_score_falls_back_to_sys_executable(
+    tmp_path: Path,
+    monkeypatch: pytest.MonkeyPatch,
+):
+    upstream = tmp_path / "upstream"  # no .venv present
+    captured: dict = {}
+
+    def fake_run(cmd, **kwargs):
+        captured["cmd"] = cmd
+        return SimpleNamespace(returncode=0, stdout="{}", stderr="")
+
+    monkeypatch.setattr(image_meta.subprocess, "run", fake_run)
+    run_synthid_score(Path("img.png"), upstream_dir=str(upstream))
+
+    assert captured["cmd"][0] == sys.executable
 
 
 def test_run_synthid_score_parses_json(