fix(codegen): infer backend from sources without metadata

Allow codegen backend resolution to use on-disk source scanning as a fallback
when feature_spec and RPG metadata do not declare a target language. Pass the
repo path through prompt-building call sites so API summaries and TDD prompts
resolve the same backend, while preserving explicit feature_spec/RPG language
metadata precedence.

Add regression coverage for metadata-free Go repos and explicit metadata
overriding source-scan fallback.

Author: QingtaoLi1

CoderMind/scripts/code_gen/batch_prompts.py

@@ -52,6 +52,7 @@
     ToolchainUnavailable,
     get_backend,
     resolve_decoder_language,
+    scan_repo_source_files,
 )
 
 logger = logging.getLogger(__name__)
@@ -387,11 +388,18 @@ def _load_json_if_exists(path: Path) -> Any:
         return None
 
 
-def _resolve_codegen_backend() -> LanguageBackend:
+def _resolve_codegen_backend(repo_path: Path | None = None) -> LanguageBackend:
     """Resolve the target language backend for code generation."""
     feature_spec = _load_json_if_exists(FEATURE_SPEC_FILE)
     rpg_obj = _load_json_if_exists(REPO_RPG_FILE)
-    language = resolve_decoder_language(feature_spec=feature_spec, rpg_obj=rpg_obj)
+    valid_files = None
+    if repo_path is not None:
+        valid_files = scan_repo_source_files(repo_path) or None
+    language = resolve_decoder_language(
+        feature_spec=feature_spec,
+        rpg_obj=rpg_obj,
+        valid_files=valid_files,
+    )
     return get_backend(language)
 
 
@@ -599,7 +607,7 @@ def _build_api_summary(repo_path: Path, source_files: List[str], max_chars: int
     # receive real API context instead of nothing.
     import ast as _ast  # local import; only used for unparse(returns)
 
-    backend = _resolve_codegen_backend()
+    backend = _resolve_codegen_backend(repo_path)
     is_python = backend.name == "python"
     summaries = []
     for filepath in sorted(source_files):
@@ -757,7 +765,7 @@ def build_tdd_prompt(
     Returns:
         Complete prompt string ready for LLMClient.generate().
     """
-    backend = _resolve_codegen_backend()
+    backend = _resolve_codegen_backend(repo_path)
     venv_python = get_dev_python(repo_path) or "python3"
     import_convention = (
         build_import_convention_snippet(repo_path=repo_path)

CoderMind/tests/test_batch_prompts.py

@@ -0,0 +1,36 @@
+import json
+import os
+import sys
+
+# Ensure scripts/ is importable when tests run from the project root.
+_project_root = os.path.join(os.path.dirname(__file__), "..")
+sys.path.insert(0, os.path.join(_project_root, "scripts"))
+
+from code_gen import batch_prompts
+
+
+def _isolate_metadata(monkeypatch, tmp_path):
+    monkeypatch.setattr(batch_prompts, "FEATURE_SPEC_FILE", tmp_path / "missing_feature_spec.json")
+    monkeypatch.setattr(batch_prompts, "REPO_RPG_FILE", tmp_path / "missing_rpg.json")
+
+
+def test_resolve_codegen_backend_uses_repo_source_scan_without_metadata(monkeypatch, tmp_path):
+    _isolate_metadata(monkeypatch, tmp_path)
+    (tmp_path / "cmd" / "app").mkdir(parents=True)
+    (tmp_path / "cmd" / "app" / "main.go").write_text("package main\n", encoding="utf-8")
+
+    assert batch_prompts._resolve_codegen_backend(tmp_path).name == "go"
+
+
+def test_resolve_codegen_backend_prefers_feature_spec_metadata(monkeypatch, tmp_path):
+    feature_spec = tmp_path / "feature_spec.json"
+    feature_spec.write_text(
+        json.dumps({"meta": {"primary_language": "python", "target_languages": ["python"]}}),
+        encoding="utf-8",
+    )
+    monkeypatch.setattr(batch_prompts, "FEATURE_SPEC_FILE", feature_spec)
+    monkeypatch.setattr(batch_prompts, "REPO_RPG_FILE", tmp_path / "missing_rpg.json")
+    (tmp_path / "cmd" / "app").mkdir(parents=True)
+    (tmp_path / "cmd" / "app" / "main.go").write_text("package main\n", encoding="utf-8")
+
+    assert batch_prompts._resolve_codegen_backend(tmp_path).name == "python"