Treat empty env vars as unset in config expansion

CHANGELOG.md

@@ -1,6 +1,10 @@
 # Changelog
 ## [Unreleased]
 
+### Changed
+
+- A bare `${VAR}` in YAML config now raises `MissingEnvVarError` when the variable is set but empty, matching the unset case. Use `${VAR:-default}` to allow an empty/absent value.
+
 ## [0.61.2] - 2026-06-24
 
 ### Fixed

haiku_rag_slim/haiku/rag/config/loader.py

@@ -12,7 +12,7 @@ _ENV_VAR_PATTERN = re.compile(r"\$\$|\$\{([A-Za-z_][A-Za-z0-9_]*)(?::-([^}]*))?\
 
 
 class MissingEnvVarError(ValueError):
-    """A ${VAR} in the config references an unset environment variable."""
+    """A ${VAR} in the config references an unset or empty environment variable."""
 
 
 def _expand_str(value: str) -> str:
@@ -20,12 +20,12 @@ def _expand_str(value: str) -> str:
         if match.group(0) == "$$":
             return "$"
         name, default = match.group(1), match.group(2)
-        if name in os.environ and (default is None or os.environ[name] != ""):
+        if os.environ.get(name, "") != "":
             return os.environ[name]
         if default is not None:
             return default
         raise MissingEnvVarError(
-            f"Config references unset environment variable ${{{name}}}. "
+            f"Config references unset or empty environment variable ${{{name}}}. "
             f"Set it, or use ${{{name}:-default}} to provide a fallback."
         )
 

tests/test_config.py

@@ -419,6 +419,18 @@ def test_expand_env_var_unset_raises(tmp_path, monkeypatch):
         load_yaml_config(config_file)
 
 
+def test_expand_env_var_empty_raises(tmp_path, monkeypatch):
+    """A bare ${VAR} set to an empty string is treated as unset and raises."""
+    from haiku.rag.config.loader import MissingEnvVarError
+
+    monkeypatch.setenv("HAIKU_TEST_EMPTY", "")
+    config_file = tmp_path / "test.yaml"
+    config_file.write_text("api_key: ${HAIKU_TEST_EMPTY}")
+
+    with pytest.raises(MissingEnvVarError, match="HAIKU_TEST_EMPTY"):
+        load_yaml_config(config_file)
+
+
 def test_expand_env_var_default_when_unset(tmp_path, monkeypatch):
     """${VAR:-default} falls back to the default when VAR is unset."""
     monkeypatch.delenv("HAIKU_TEST_MISSING", raising=False)