Backport PR jupyterlab#560: Fixes lookup for custom chains

packages/jupyter-ai-magics/jupyter_ai_magics/magics.py

@@ -434,7 +434,11 @@ def _append_exchange_openai(self, prompt: str, output: str):
 
     def _decompose_model_id(self, model_id: str):
         """Breaks down a model ID into a two-tuple (provider_id, local_model_id). Returns (None, None) if indeterminate."""
-        if model_id in self.custom_model_registry:
+        # custom_model_registry maps keys to either a model name (a string) or an LLMChain.
+        # If this is an alias to another model, expand the full name of the model.
+        if model_id in self.custom_model_registry and isinstance(
+            self.custom_model_registry[model_id], str
+        ):
             model_id = self.custom_model_registry[model_id]
 
         return decompose_model_id(model_id, self.providers)
@@ -508,6 +512,17 @@ def run_ai_cell(self, args: CellArgs, prompt: str):
             )
 
         provider_id, local_model_id = self._decompose_model_id(args.model_id)
+
+        # If this is a custom chain, send the message to the custom chain.
+        if args.model_id in self.custom_model_registry and isinstance(
+            self.custom_model_registry[args.model_id], LLMChain
+        ):
+            return self.display_output(
+                self.custom_model_registry[args.model_id].run(prompt),
+                args.format,
+                {"jupyter_ai": {"custom_chain_id": args.model_id}},
+            )
+
         Provider = self._get_provider(provider_id)
         if Provider is None:
             return TextOrMarkdown(