diff --git a/src/ell/providers/anthropic.py b/src/ell/providers/anthropic.py
index 4a3d8ba2..ba379b09 100644
--- a/src/ell/providers/anthropic.py
+++ b/src/ell/providers/anthropic.py
@@ -197,7 +197,7 @@ def _content_block_to_anthropic_format(content_block: ContentBlock):
                 type="tool_use",
                 id=tool_call.tool_call_id,
                 name=tool_call.tool.__name__,
-                input=tool_call.params.model_dump() if isinstance(tool_call.params, BaseModel) else tool_call.params,
+                input=tool_call.serialize_params(),
             )
         elif (tool_result := content_block.tool_result):
             return dict(
diff --git a/src/ell/providers/bedrock.py b/src/ell/providers/bedrock.py
index 8c69948b..fc0a0cd6 100644
--- a/src/ell/providers/bedrock.py
+++ b/src/ell/providers/bedrock.py
@@ -202,7 +202,7 @@ def content_block_to_bedrock_format(content_block: ContentBlock) -> Dict[str, An
             "toolUse": {
                 "toolUseId": content_block.tool_call.tool_call_id,
                 "name": content_block.tool_call.tool.__name__,
-                "input": content_block.tool_call.params.model_dump() if isinstance(content_block.tool_call.params, BaseModel) else content_block.tool_call.params,
+                "input": content_block.tool_call.serialize_params(),
             }
         }
     elif content_block.tool_result:
diff --git a/src/ell/providers/openai.py b/src/ell/providers/openai.py
index b3cca688..ef85ff41 100644
--- a/src/ell/providers/openai.py
+++ b/src/ell/providers/openai.py
@@ -64,7 +64,7 @@ def translate_to_provider(self, ell_call : EllCallParams) -> Dict[str, Any]:
                                 type="function",
                                 function=dict(
                                     name=tool_call.tool.__name__,
-                                    arguments=tool_call.params.model_dump_json() if isinstance(tool_call.params,BaseModel) else json.dumps(tool_call.params, ensure_ascii=False)
+                                    arguments=json.dumps(tool_call.serialize_params(), ensure_ascii=False)
                                 )
                             ) for tool_call in tool_calls ],
                         role="assistant",
diff --git a/src/ell/types/message.py b/src/ell/types/message.py
index 14e39841..852b4d6f 100644
--- a/src/ell/types/message.py
+++ b/src/ell/types/message.py
@@ -40,10 +40,15 @@ def text_only(self) -> str:
     def __repr__(self):
         return f"{self.__class__.__name__}(tool_call_id={self.tool_call_id}, result={_content_to_text(self.result)})"
 
+class ToolReference(BaseModel):
+    """A reference to an invocable tool"""
+    fqn: str = Field(description="The fully qualified name of the tool")
+    hash: str = Field(description="The hash of the tool and its dependencies")
+    
 class ToolCall(BaseModel):
-    tool: Union[InvocableTool, str] = Field(description="The tool function to call or a reference to it when serialized")
+    tool: Union[InvocableTool, ToolReference] = Field(description="The tool function to call or a reference to it when serialized")
     tool_call_id: Optional[_lstr_generic] = Field(default=None)
-    params: Union[Dict[str, Any], BaseModel]
+    params: Union[Dict[str, Any], BaseModel] = Field(description="Arguments for the tool call provided by the model.")
 
     def __init__(self, tool, params: Optional[Union[BaseModel, Dict[str, Any]]], tool_call_id: Optional[_lstr_generic]=None):
         if (not isinstance(params, BaseModel)) and isinstance(tool, FunctionType) and hasattr(tool, '__ell_params_model__'):
@@ -53,18 +58,25 @@ def __init__(self, tool, params: Optional[Union[BaseModel, Dict[str, Any]]], too
 
         super().__init__(tool=tool, tool_call_id=tool_call_id, params=params)
 
-    # TODO. This should reference a tool fqn + version if possible
-    # ell should have an InvocableTool with __ properties that have this info at serialization time
     @field_serializer('tool')
-    def serialize_tool(self, tool: InvocableTool, _info):
-        return tool.__name__ if hasattr(tool, '__name__') else str(tool)
+    def serialize_tool(self, tool: Union[InvocableTool, ToolReference], _info):
+        if isinstance(tool, ToolReference):
+            return tool
+        return ToolReference(
+            # todo(alex). add the value of fqn we want to standardize on to all lmps so we don't keep using qualname
+            fqn=tool.__qualname__,
+            hash=getattr(tool, '__ell_hash__', 'unknown')
+        )
 
     @field_serializer('params')
-    def serialize_params(self, params: Union[Dict[str,Any],BaseModel], _info):
+    def _serialize_params(self, params: Union[Dict[str, Any], BaseModel]) -> Dict[str, Any]:
         if isinstance(params, dict):
             return params
         return params.model_dump(exclude_none=True, exclude_unset=True)
 
+    def serialize_params(self) -> Dict[str, Any]:
+        return self._serialize_params(self.params)
+
     @field_serializer('tool_call_id')
     def serialize_tool_call_id(self, tool_call_id: _lstr_generic):
         if tool_call_id is None:
@@ -76,19 +88,19 @@ def serialize_tool_call_id(self, tool_call_id: _lstr_generic):
 
     def __call__(self, **kwargs):
         assert not kwargs, "Unexpected arguments provided. Calling a tool uses the params provided in the ToolCall."
-        assert not isinstance(self.tool, str), "ToolCall.tool is a string. Tools are not invocable once serialized."
+        assert not isinstance(self.tool, ToolReference), f"Tools are not invocable once serialized. ToolCall.tool is a ToolReference: {self.tool}"
 
         # XXX: TODO: MOVE TRACKING CODE TO _TRACK AND OUT OF HERE AND API.
-        return self.tool(**self.params.model_dump())
+        return self.tool(**self.serialize_params())
     
     # XXX: Deprecate in 0.1.0
     def call_and_collect_as_message_block(self):
         raise DeprecationWarning("call_and_collect_as_message_block is deprecated. Use collect_as_content_block instead.")
     
     def call_and_collect_as_content_block(self):
-        if isinstance(self.tool, str):
-            raise ValueError("Cannot call a tool that is a string reference.")
-        res = self.tool(**(self.params.model_dump() if isinstance(self.params, BaseModel) else self.params), 
+        if isinstance(self.tool, ToolReference):
+            raise ValueError(f"Cannot call a tool that is a ToolReference: {self.tool}")
+        res = self.tool(**self.serialize_params(),
                         _tool_call_id=self.tool_call_id)
         return ContentBlock(tool_result=res)
 
@@ -203,7 +215,7 @@ def type(self):
     
     @property
     def content(self):
-        return getattr(self, self.type)
+        return getattr(self, self.type) # type: ignore
 
     @classmethod
     def coerce(cls, content: AnyContent) -> "ContentBlock":
diff --git a/tests/api/test_api.py b/tests/api/test_api.py
index 421d6aeb..d478c176 100644
--- a/tests/api/test_api.py
+++ b/tests/api/test_api.py
@@ -218,6 +218,8 @@ def my_sample_tool(args: MySampleToolInput = Field(
 
 
 def test_invocation_json_round_trip():
+    # pretend it's being tracked
+    my_sample_tool.__ell_hash__ = "lmp-123"
     invocation_id = "invocation-" + uuid4().hex
     tool_call = ToolCall(
         tool=my_sample_tool,
@@ -281,6 +283,8 @@ def test_write_invocation_tool_call(async_sqlite_serializer: AsyncSQLiteSerializ
         print(response.json())
         raise e
 
+    # pretend it's being tracked
+    my_sample_tool.__ell_hash__ = "lmp-123"
     invocation_id = "invocation-" + uuid4().hex
     tool_call = ToolCall(
         tool=my_sample_tool,