fix: friendly error messages for optional backend dependencies

AGENTS.md

@@ -39,7 +39,9 @@ uv run ruff format . && uv run ruff check .  # Lint & format
 - **Google-style docstrings**
 - **Ruff** for linting/formatting
 - Use `...` in `@generative` function bodies
+- Use `...` in `@generative` function bodies
 - Prefer primitives over classes
+- **Friendly Dependency Errors**: Wraps optional backend imports in `try/except ImportError` with a helpful message (e.g., "Please pip install mellea[hf]"). See `mellea/stdlib/session.py` for examples.
 
 ## 5. Commits & Hooks
 [Angular format](https://github.com/angular/angular/blob/main/CONTRIBUTING.md#commit): `feat:`, `fix:`, `docs:`, `test:`, `refactor:`, `release:`

mellea/stdlib/session.py

@@ -58,21 +58,39 @@ def backend_name_to_class(name: str) -> Any:
 
         return OllamaModelBackend
     elif name == "hf" or name == "huggingface":
-        from mellea.backends.huggingface import LocalHFBackend
-
-        return LocalHFBackend
+        try:
+            from mellea.backends.huggingface import LocalHFBackend
+
+            return LocalHFBackend
+        except ImportError as e:
+            raise ImportError(
+                "The 'hf' backend requires extra dependencies. "
+                "Please install them with: pip install 'mellea[hf]'"
+            ) from e
     elif name == "openai":
         from ..backends.openai import OpenAIBackend
 
         return OpenAIBackend
     elif name == "watsonx":
-        from ..backends.watsonx import WatsonxAIBackend
-
-        return WatsonxAIBackend
+        try:
+            from ..backends.watsonx import WatsonxAIBackend
+
+            return WatsonxAIBackend
+        except ImportError as e:
+            raise ImportError(
+                "The 'watsonx' backend requires extra dependencies. "
+                "Please install them with: pip install 'mellea[watsonx]'"
+            ) from e
     elif name == "litellm":
-        from ..backends.litellm import LiteLLMBackend
-
-        return LiteLLMBackend
+        try:
+            from ..backends.litellm import LiteLLMBackend
+
+            return LiteLLMBackend
+        except ImportError as e:
+            raise ImportError(
+                "The 'litellm' backend requires extra dependencies. "
+                "Please install them with: pip install 'mellea[litellm]'"
+            ) from e
     else:
         return None