fix: round-trip safety for input_provider, memory scopes, embedder class

- input_provider: enforce InputProvider protocol via dedicated
  validator/serializer; reject non-class dotted paths in
  _dotted_path_to_instance
- MemoryScope/MemorySlice: allow restore without live Memory; expose
  bind() to reattach the dependency post-restore
- Knowledge.embedder: add BeforeValidator that resolves provider_class
  dotted paths back to a BaseEmbeddingsProvider subclass

lib/crewai/src/crewai/flow/flow.py

@@ -120,7 +120,6 @@ from crewai.state.checkpoint_config import (
     _coerce_checkpoint,
     apply_checkpoint,
 )
-from crewai.types.callback import SerializableInstance
 
 
 if TYPE_CHECKING:
@@ -168,6 +167,28 @@ def _serialize_persistence(value: Any) -> dict[str, Any] | None:
     return None
 
 
+def _validate_input_provider(value: Any) -> Any:
+    if value is None or isinstance(value, InputProvider):
+        return value
+    from crewai.types.callback import _dotted_path_to_instance
+
+    resolved = _dotted_path_to_instance(value)
+    if resolved is None or isinstance(resolved, InputProvider):
+        return resolved
+    raise ValueError(
+        f"Resolved input_provider {resolved!r} does not implement the "
+        "InputProvider protocol (missing request_input)."
+    )
+
+
+def _serialize_input_provider(value: Any) -> str | None:
+    if value is None:
+        return None
+    from crewai.types.callback import _instance_to_dotted_path
+
+    return _instance_to_dotted_path(value)
+
+
 _INITIAL_STATE_CLASS_MARKER = "__crewai_pydantic_class_schema__"
 
 
@@ -964,7 +985,13 @@ class Flow(BaseModel, Generic[T], metaclass=FlowMeta):
         ]
         | None
     ) = Field(default=None)
-    input_provider: SerializableInstance | None = Field(default=None)
+    input_provider: Annotated[
+        InputProvider | None,
+        BeforeValidator(_validate_input_provider),
+        PlainSerializer(
+            _serialize_input_provider, return_type=str | None, when_used="json"
+        ),
+    ] = Field(default=None)
     suppress_flow_events: bool = Field(default=False)
     human_feedback_history: list[HumanFeedbackResult] = Field(default_factory=list)
     last_human_feedback: HumanFeedbackResult | None = Field(default=None)
@@ -3189,7 +3216,7 @@ class Flow(BaseModel, Generic[T], metaclass=FlowMeta):
         from crewai.flow.flow_config import flow_config
 
         if self.input_provider is not None:
-            return cast(InputProvider, self.input_provider)
+            return self.input_provider
         if flow_config.input_provider is not None:
             return flow_config.input_provider
         return ConsoleProvider()

lib/crewai/src/crewai/knowledge/knowledge.py

@@ -75,6 +75,21 @@ def _serialize_embedder_spec(value: Any) -> dict[str, Any] | None:
     )
 
 
+def _validate_embedder_spec(value: Any) -> Any:
+    """Resolve provider_class dotted-path dicts back to a class on restore."""
+    if isinstance(value, dict) and set(value.keys()) == {"provider_class"}:
+        from crewai.types.callback import _resolve_dotted_path
+
+        cls = _resolve_dotted_path(value["provider_class"])
+        if not isinstance(cls, type) or not issubclass(cls, BaseEmbeddingsProvider):
+            raise ValueError(
+                f"provider_class {value['provider_class']!r} did not resolve to a "
+                "BaseEmbeddingsProvider subclass."
+            )
+        return cls
+    return value
+
+
 class Knowledge(BaseModel):
     """
     Knowledge is a collection of sources and setup for the vector store to save and query relevant context.
@@ -92,6 +107,7 @@ class Knowledge(BaseModel):
     storage: KnowledgeStorage | None = Field(default=None)
     embedder: Annotated[
         EmbedderConfig | None,
+        BeforeValidator(_validate_embedder_spec),
         PlainSerializer(
             _serialize_embedder_spec, return_type=dict | None, when_used="json"
         ),

lib/crewai/src/crewai/memory/memory_scope.py

@@ -6,6 +6,7 @@ from datetime import datetime
 from typing import Any, Literal
 
 from pydantic import BaseModel, ConfigDict, Field, PrivateAttr, model_validator
+from typing_extensions import Self
 
 from crewai.memory.types import (
     _RECALL_OVERSAMPLE_FACTOR,
@@ -36,17 +37,25 @@ class MemoryScope(BaseModel):
             return data
         if not isinstance(data, dict):
             raise ValueError(f"Expected dict or MemoryScope, got {type(data).__name__}")
-        if "memory" not in data:
-            raise ValueError("MemoryScope requires a 'memory' key")
-        memory = data.pop("memory")
+        memory = data.pop("memory", None)
         instance: MemoryScope = handler(data)
-        instance._memory = memory
+        if memory is not None:
+            instance._memory = memory
         root = instance.root_path.rstrip("/") or ""
         if root and not root.startswith("/"):
             root = "/" + root
         instance._root = root
         return instance
 
+    def bind(self, memory: Memory) -> Self:
+        """Rebind the runtime ``Memory`` dependency after restore.
+
+        Required after deserializing from a checkpoint, since the live
+        ``Memory`` cannot be serialized.
+        """
+        self._memory = memory
+        return self
+
     @property
     def read_only(self) -> bool:
         """Whether the underlying memory is read-only."""
@@ -209,14 +218,18 @@ class MemorySlice(BaseModel):
             return data
         if not isinstance(data, dict):
             raise ValueError(f"Expected dict or MemorySlice, got {type(data).__name__}")
-        if "memory" not in data:
-            raise ValueError("MemorySlice requires a 'memory' key")
-        memory = data.pop("memory")
+        memory = data.pop("memory", None)
         data["scopes"] = [s.rstrip("/") or "/" for s in data.get("scopes", [])]
         instance: MemorySlice = handler(data)
-        instance._memory = memory
+        if memory is not None:
+            instance._memory = memory
         return instance
 
+    def bind(self, memory: Memory) -> Self:
+        """Rebind the runtime ``Memory`` dependency after restore."""
+        self._memory = memory
+        return self
+
     def remember(
         self,
         content: str,

lib/crewai/src/crewai/types/callback.py

@@ -188,6 +188,11 @@ def _dotted_path_to_instance(value: Any) -> Any:
             "Only enable this for trusted checkpoint data."
         )
     cls = _resolve_dotted_path(value)
+    if not inspect.isclass(cls):
+        raise ValueError(
+            f"Invalid provider path {value!r}: expected a class, got "
+            f"{type(cls).__name__}"
+        )
     return cls()