fix(checkpoint): rebuild custom BaseLLM as concrete LLM on restore

A custom BaseLLM subclass serializes with the inherited llm_type "base",
which the registry maps to the abstract BaseLLM. Restore then crashed on
cls(**value). Rebuild a concrete LLM from the saved config when the
resolved class is abstract.

lib/crewai/src/crewai/agents/agent_builder/base_agent.py

@@ -82,6 +82,7 @@ _LLM_TYPE_REGISTRY: dict[str, str] = {
 def _validate_llm_ref(value: Any) -> Any:
     if isinstance(value, dict):
         import importlib
+        import inspect
 
         llm_type = value.get("llm_type")
         if not llm_type or llm_type not in _LLM_TYPE_REGISTRY:
@@ -92,6 +93,12 @@ def _validate_llm_ref(value: Any) -> Any:
         dotted = _LLM_TYPE_REGISTRY[llm_type]
         mod_path, cls_name = dotted.rsplit(".", 1)
         cls = getattr(importlib.import_module(mod_path), cls_name)
+        if inspect.isabstract(cls):
+            from crewai.llm import LLM
+
+            return LLM(
+                **{k: v for k, v in value.items() if v is not None and k != "llm_type"}
+            )
         return cls(**value)
     return value
 

lib/crewai/tests/test_checkpoint.py

@@ -2,6 +2,7 @@
 
 from __future__ import annotations
 
+import inspect
 import json
 import os
 import sqlite3
@@ -742,3 +743,26 @@ class TestCheckpointReusedExecutor:
 
         assert len(result.tasks_output) == 2
         assert result.tasks_output[1].raw
+
+
+class TestCustomLLMCheckpointRestore:
+    """A custom BaseLLM subclass serializes with the inherited llm_type "base".
+
+    Restoring it must not try to instantiate the abstract BaseLLM; it is rebuilt
+    as a concrete LLM from the saved config instead.
+    """
+
+    def test_restore_does_not_instantiate_abstract_base_llm(self) -> None:
+        agent = Agent(role="r", goal="g", backstory="b", llm=_FinalAnswerLLM())
+        task = Task(description="d", expected_output="e", agent=agent)
+        crew = Crew(agents=[agent], tasks=[task], verbose=False)
+
+        raw = RuntimeState(root=[crew]).model_dump_json()
+        restored = RuntimeState.model_validate_json(
+            raw, context={"from_checkpoint": True}
+        )
+
+        llm = restored.root[0].agents[0].llm
+        assert isinstance(llm, BaseLLM)
+        assert not inspect.isabstract(type(llm))
+        assert llm.model == "stub"