diff --git a/letta/server/rest_api/routers/openai/assistants/threads.py b/letta/server/rest_api/routers/openai/assistants/threads.py
index 1e9be774ae..43d7235faf 100644
--- a/letta/server/rest_api/routers/openai/assistants/threads.py
+++ b/letta/server/rest_api/routers/openai/assistants/threads.py
@@ -1,5 +1,5 @@
 import uuid
-from typing import TYPE_CHECKING, List
+from typing import TYPE_CHECKING, List, Optional
 
 from fastapi import APIRouter, Body, Depends, Header, HTTPException, Path, Query
 
@@ -43,7 +43,7 @@
 def create_thread(
     request: CreateThreadRequest = Body(...),
     server: SyncServer = Depends(get_letta_server),
-    user_id: str = Header(None),  # Extract user_id from header, default to None if not present
+    user_id: Optional[str] = Header(None, alias="user_id"),  # Extract user_id from header, default to None if not present
 ):
     # TODO: use requests.description and requests.metadata fields
     # TODO: handle requests.file_ids and requests.tools
@@ -68,7 +68,7 @@ def create_thread(
 def retrieve_thread(
     thread_id: str = Path(..., description="The unique identifier of the thread."),
     server: SyncServer = Depends(get_letta_server),
-    user_id: str = Header(None),  # Extract user_id from header, default to None if not present
+    user_id: Optional[str] = Header(None, alias="user_id"),  # Extract user_id from header, default to None if not present
 ):
     actor = server.get_user_or_default(user_id=user_id)
     agent = server.get_agent(user_id=actor.id, agent_id=thread_id)
@@ -102,7 +102,7 @@ def create_message(
     thread_id: str = Path(..., description="The unique identifier of the thread."),
     request: CreateMessageRequest = Body(...),
     server: SyncServer = Depends(get_letta_server),
-    user_id: str = Header(None),  # Extract user_id from header, default to None if not present
+    user_id: Optional[str] = Header(None, alias="user_id"),  # Extract user_id from header, default to None if not present
 ):
     actor = server.get_user_or_default(user_id=user_id)
     agent_id = thread_id
@@ -146,7 +146,7 @@ def list_messages(
     after: str = Query(None, description="A cursor for use in pagination. `after` is an object ID that defines your place in the list."),
     before: str = Query(None, description="A cursor for use in pagination. `after` is an object ID that defines your place in the list."),
     server: SyncServer = Depends(get_letta_server),
-    user_id: str = Header(None),  # Extract user_id from header, default to None if not present
+    user_id: Optional[str] = Header(None, alias="user_id"),  # Extract user_id from header, default to None if not present
 ):
     actor = server.get_user_or_default(user_id)
     after_uuid = after if before else None
diff --git a/letta/server/rest_api/routers/openai/chat_completions/chat_completions.py b/letta/server/rest_api/routers/openai/chat_completions/chat_completions.py
index 489abe01c6..3c1afc3938 100644
--- a/letta/server/rest_api/routers/openai/chat_completions/chat_completions.py
+++ b/letta/server/rest_api/routers/openai/chat_completions/chat_completions.py
@@ -1,5 +1,5 @@
 import json
-from typing import TYPE_CHECKING
+from typing import TYPE_CHECKING, Optional
 
 from fastapi import APIRouter, Body, Depends, Header, HTTPException
 
@@ -30,7 +30,7 @@
 async def create_chat_completion(
     completion_request: ChatCompletionRequest = Body(...),
     server: "SyncServer" = Depends(get_letta_server),
-    user_id: str = Header(None),  # Extract user_id from header, default to None if not present
+    user_id: Optional[str] = Header(None, alias="user_id"),  # Extract user_id from header, default to None if not present
 ):
     """Send a message to a Letta agent via a /chat/completions completion_request
     The bearer token will be used to identify the user.
diff --git a/letta/server/rest_api/routers/v1/agents.py b/letta/server/rest_api/routers/v1/agents.py
index 74012db75f..00e1cce98a 100644
--- a/letta/server/rest_api/routers/v1/agents.py
+++ b/letta/server/rest_api/routers/v1/agents.py
@@ -40,7 +40,7 @@
 @router.get("/", response_model=List[AgentState], operation_id="list_agents")
 def list_agents(
     server: "SyncServer" = Depends(get_letta_server),
-    user_id: str = Header(None),  # Extract user_id from header, default to None if not present
+    user_id: Optional[str] = Header(None, alias="user_id"),  # Extract user_id from header, default to None if not present
 ):
     """
     List all agents associated with a given user.
@@ -55,7 +55,7 @@ def list_agents(
 def create_agent(
     agent: CreateAgent = Body(...),
     server: "SyncServer" = Depends(get_letta_server),
-    user_id: str = Header(None),  # Extract user_id from header, default to None if not present
+    user_id: Optional[str] = Header(None, alias="user_id"),  # Extract user_id from header, default to None if not present
 ):
     """
     Create a new agent with the specified configuration.
@@ -76,7 +76,7 @@ def update_agent(
     agent_id: str,
     update_agent: UpdateAgentState = Body(...),
     server: "SyncServer" = Depends(get_letta_server),
-    user_id: str = Header(None),  # Extract user_id from header, default to None if not present
+    user_id: Optional[str] = Header(None, alias="user_id"),  # Extract user_id from header, default to None if not present
 ):
     """Update an exsiting agent"""
     actor = server.get_user_or_default(user_id=user_id)
@@ -89,7 +89,7 @@ def update_agent(
 def get_agent_state(
     agent_id: str,
     server: "SyncServer" = Depends(get_letta_server),
-    user_id: str = Header(None),  # Extract user_id from header, default to None if not present
+    user_id: Optional[str] = Header(None, alias="user_id"),  # Extract user_id from header, default to None if not present
 ):
     """
     Get the state of the agent.
@@ -107,7 +107,7 @@ def get_agent_state(
 def delete_agent(
     agent_id: str,
     server: "SyncServer" = Depends(get_letta_server),
-    user_id: str = Header(None),  # Extract user_id from header, default to None if not present
+    user_id: Optional[str] = Header(None, alias="user_id"),  # Extract user_id from header, default to None if not present
 ):
     """
     Delete an agent.
@@ -159,7 +159,7 @@ def update_agent_memory(
     agent_id: str,
     request: Dict = Body(...),
     server: "SyncServer" = Depends(get_letta_server),
-    user_id: str = Header(None),  # Extract user_id from header, default to None if not present
+    user_id: Optional[str] = Header(None, alias="user_id"),  # Extract user_id from header, default to None if not present
 ):
     """
     Update the core memory of a specific agent.
@@ -202,7 +202,7 @@ def get_agent_archival_memory(
     after: Optional[int] = Query(None, description="Unique ID of the memory to start the query range at."),
     before: Optional[int] = Query(None, description="Unique ID of the memory to end the query range at."),
     limit: Optional[int] = Query(None, description="How many results to include in the response."),
-    user_id: str = Header(None),  # Extract user_id from header, default to None if not present
+    user_id: Optional[str] = Header(None, alias="user_id"),  # Extract user_id from header, default to None if not present
 ):
     """
     Retrieve the memories in an agent's archival memory store (paginated query).
@@ -227,7 +227,7 @@ def insert_agent_archival_memory(
     agent_id: str,
     request: CreateArchivalMemory = Body(...),
     server: "SyncServer" = Depends(get_letta_server),
-    user_id: str = Header(None),  # Extract user_id from header, default to None if not present
+    user_id: Optional[str] = Header(None, alias="user_id"),  # Extract user_id from header, default to None if not present
 ):
     """
     Insert a memory into an agent's archival memory store.
@@ -245,7 +245,7 @@ def delete_agent_archival_memory(
     memory_id: str,
     # memory_id: str = Query(..., description="Unique ID of the memory to be deleted."),
     server: "SyncServer" = Depends(get_letta_server),
-    user_id: str = Header(None),  # Extract user_id from header, default to None if not present
+    user_id: Optional[str] = Header(None, alias="user_id"),  # Extract user_id from header, default to None if not present
 ):
     """
     Delete a memory from an agent's archival memory store.
@@ -276,7 +276,7 @@ def get_agent_messages(
         DEFAULT_MESSAGE_TOOL_KWARG,
         description="[Only applicable if use_assistant_message is True] The name of the message argument in the designated message tool.",
     ),
-    user_id: str = Header(None),  # Extract user_id from header, default to None if not present
+    user_id: Optional[str] = Header(None, alias="user_id"),  # Extract user_id from header, default to None if not present
 ):
     """
     Retrieve message history for an agent.
@@ -315,7 +315,7 @@ async def send_message(
     agent_id: str,
     server: SyncServer = Depends(get_letta_server),
     request: LettaRequest = Body(...),
-    user_id: str = Header(None),  # Extract user_id from header, default to None if not present
+    user_id: Optional[str] = Header(None, alias="user_id"),  # Extract user_id from header, default to None if not present
 ):
     """
     Process a user message and return the agent's response.
diff --git a/letta/server/rest_api/routers/v1/blocks.py b/letta/server/rest_api/routers/v1/blocks.py
index df9c86cc85..74dc76dad5 100644
--- a/letta/server/rest_api/routers/v1/blocks.py
+++ b/letta/server/rest_api/routers/v1/blocks.py
@@ -19,7 +19,7 @@ def list_blocks(
     templates_only: bool = Query(True, description="Whether to include only templates"),
     name: Optional[str] = Query(None, description="Name of the block"),
     server: SyncServer = Depends(get_letta_server),
-    user_id: str = Header(None),  # Extract user_id from header, default to None if not present
+    user_id: Optional[str] = Header(None, alias="user_id"),  # Extract user_id from header, default to None if not present
 ):
     actor = server.get_user_or_default(user_id=user_id)
 
@@ -33,7 +33,7 @@ def list_blocks(
 def create_block(
     create_block: CreateBlock = Body(...),
     server: SyncServer = Depends(get_letta_server),
-    user_id: str = Header(None),  # Extract user_id from header, default to None if not present
+    user_id: Optional[str] = Header(None, alias="user_id"),  # Extract user_id from header, default to None if not present
 ):
     actor = server.get_user_or_default(user_id=user_id)
 
diff --git a/letta/server/rest_api/routers/v1/jobs.py b/letta/server/rest_api/routers/v1/jobs.py
index 113ea81d66..bd581a98ef 100644
--- a/letta/server/rest_api/routers/v1/jobs.py
+++ b/letta/server/rest_api/routers/v1/jobs.py
@@ -13,7 +13,7 @@
 def list_jobs(
     server: "SyncServer" = Depends(get_letta_server),
     source_id: Optional[str] = Query(None, description="Only list jobs associated with the source."),
-    user_id: str = Header(None),  # Extract user_id from header, default to None if not present
+    user_id: Optional[str] = Header(None, alias="user_id"),  # Extract user_id from header, default to None if not present
 ):
     """
     List all jobs.
@@ -34,7 +34,7 @@ def list_jobs(
 @router.get("/active", response_model=List[Job], operation_id="list_active_jobs")
 def list_active_jobs(
     server: "SyncServer" = Depends(get_letta_server),
-    user_id: str = Header(None),  # Extract user_id from header, default to None if not present
+    user_id: Optional[str] = Header(None, alias="user_id"),  # Extract user_id from header, default to None if not present
 ):
     """
     List all active jobs.
diff --git a/letta/server/rest_api/routers/v1/sources.py b/letta/server/rest_api/routers/v1/sources.py
index 2feae2df9b..a59abd31bb 100644
--- a/letta/server/rest_api/routers/v1/sources.py
+++ b/letta/server/rest_api/routers/v1/sources.py
@@ -1,6 +1,6 @@
 import os
 import tempfile
-from typing import List
+from typing import List, Optional
 
 from fastapi import APIRouter, BackgroundTasks, Depends, Header, Query, UploadFile
 
@@ -21,7 +21,7 @@
 def get_source(
     source_id: str,
     server: "SyncServer" = Depends(get_letta_server),
-    user_id: str = Header(None),  # Extract user_id from header, default to None if not present
+    user_id: Optional[str] = Header(None, alias="user_id"),  # Extract user_id from header, default to None if not present
 ):
     """
     Get all sources
@@ -35,7 +35,7 @@ def get_source(
 def get_source_id_by_name(
     source_name: str,
     server: "SyncServer" = Depends(get_letta_server),
-    user_id: str = Header(None),  # Extract user_id from header, default to None if not present
+    user_id: Optional[str] = Header(None, alias="user_id"),  # Extract user_id from header, default to None if not present
 ):
     """
     Get a source by name
@@ -49,7 +49,7 @@ def get_source_id_by_name(
 @router.get("/", response_model=List[Source], operation_id="list_sources")
 def list_sources(
     server: "SyncServer" = Depends(get_letta_server),
-    user_id: str = Header(None),  # Extract user_id from header, default to None if not present
+    user_id: Optional[str] = Header(None, alias="user_id"),  # Extract user_id from header, default to None if not present
 ):
     """
     List all data sources created by a user.
@@ -63,7 +63,7 @@ def list_sources(
 def create_source(
     source: SourceCreate,
     server: "SyncServer" = Depends(get_letta_server),
-    user_id: str = Header(None),  # Extract user_id from header, default to None if not present
+    user_id: Optional[str] = Header(None, alias="user_id"),  # Extract user_id from header, default to None if not present
 ):
     """
     Create a new data source.
@@ -78,7 +78,7 @@ def update_source(
     source_id: str,
     source: SourceUpdate,
     server: "SyncServer" = Depends(get_letta_server),
-    user_id: str = Header(None),  # Extract user_id from header, default to None if not present
+    user_id: Optional[str] = Header(None, alias="user_id"),  # Extract user_id from header, default to None if not present
 ):
     """
     Update the name or documentation of an existing data source.
@@ -94,7 +94,7 @@ def update_source(
 def delete_source(
     source_id: str,
     server: "SyncServer" = Depends(get_letta_server),
-    user_id: str = Header(None),  # Extract user_id from header, default to None if not present
+    user_id: Optional[str] = Header(None, alias="user_id"),  # Extract user_id from header, default to None if not present
 ):
     """
     Delete a data source.
@@ -109,7 +109,7 @@ def attach_source_to_agent(
     source_id: str,
     agent_id: str = Query(..., description="The unique identifier of the agent to attach the source to."),
     server: "SyncServer" = Depends(get_letta_server),
-    user_id: str = Header(None),  # Extract user_id from header, default to None if not present
+    user_id: Optional[str] = Header(None, alias="user_id"),  # Extract user_id from header, default to None if not present
 ):
     """
     Attach a data source to an existing agent.
@@ -127,7 +127,7 @@ def detach_source_from_agent(
     source_id: str,
     agent_id: str = Query(..., description="The unique identifier of the agent to detach the source from."),
     server: "SyncServer" = Depends(get_letta_server),
-    user_id: str = Header(None),  # Extract user_id from header, default to None if not present
+    user_id: Optional[str] = Header(None, alias="user_id"),  # Extract user_id from header, default to None if not present
 ) -> None:
     """
     Detach a data source from an existing agent.
@@ -143,7 +143,7 @@ def upload_file_to_source(
     source_id: str,
     background_tasks: BackgroundTasks,
     server: "SyncServer" = Depends(get_letta_server),
-    user_id: str = Header(None),  # Extract user_id from header, default to None if not present
+    user_id: Optional[str] = Header(None, alias="user_id"),  # Extract user_id from header, default to None if not present
 ):
     """
     Upload a file to a data source.
@@ -176,7 +176,7 @@ def upload_file_to_source(
 def list_passages(
     source_id: str,
     server: SyncServer = Depends(get_letta_server),
-    user_id: str = Header(None),  # Extract user_id from header, default to None if not present
+    user_id: Optional[str] = Header(None, alias="user_id"),  # Extract user_id from header, default to None if not present
 ):
     """
     List all passages associated with a data source.
@@ -190,7 +190,7 @@ def list_passages(
 def list_documents(
     source_id: str,
     server: "SyncServer" = Depends(get_letta_server),
-    user_id: str = Header(None),  # Extract user_id from header, default to None if not present
+    user_id: Optional[str] = Header(None, alias="user_id"),  # Extract user_id from header, default to None if not present
 ):
     """
     List all documents associated with a data source.
diff --git a/letta/server/rest_api/routers/v1/tools.py b/letta/server/rest_api/routers/v1/tools.py
index 9b4d58a561..404fabfdd8 100644
--- a/letta/server/rest_api/routers/v1/tools.py
+++ b/letta/server/rest_api/routers/v1/tools.py
@@ -1,4 +1,4 @@
-from typing import List
+from typing import List, Optional
 
 from fastapi import APIRouter, Body, Depends, Header, HTTPException
 
@@ -13,7 +13,7 @@
 def delete_tool(
     tool_id: str,
     server: SyncServer = Depends(get_letta_server),
-    user_id: str = Header(None),  # Extract user_id from header, default to None if not present
+    user_id: Optional[str] = Header(None, alias="user_id"),  # Extract user_id from header, default to None if not present
 ):
     """
     Delete a tool by name
@@ -43,7 +43,7 @@ def get_tool(
 def get_tool_id(
     tool_name: str,
     server: SyncServer = Depends(get_letta_server),
-    user_id: str = Header(None),  # Extract user_id from header, default to None if not present
+    user_id: Optional[str] = Header(None, alias="user_id"),  # Extract user_id from header, default to None if not present
 ):
     """
     Get a tool ID by name
@@ -60,7 +60,7 @@ def get_tool_id(
 @router.get("/", response_model=List[Tool], operation_id="list_tools")
 def list_all_tools(
     server: SyncServer = Depends(get_letta_server),
-    user_id: str = Header(None),  # Extract user_id from header, default to None if not present
+    user_id: Optional[str] = Header(None, alias="user_id"),  # Extract user_id from header, default to None if not present
 ):
     """
     Get a list of all tools available to agents created by a user
@@ -78,7 +78,7 @@ def create_tool(
     tool: ToolCreate = Body(...),
     update: bool = False,
     server: SyncServer = Depends(get_letta_server),
-    user_id: str = Header(None),  # Extract user_id from header, default to None if not present
+    user_id: Optional[str] = Header(None, alias="user_id"),  # Extract user_id from header, default to None if not present
 ):
     """
     Create a new tool
@@ -98,7 +98,7 @@ def update_tool(
     tool_id: str,
     request: ToolUpdate = Body(...),
     server: SyncServer = Depends(get_letta_server),
-    user_id: str = Header(None),  # Extract user_id from header, default to None if not present
+    user_id: Optional[str] = Header(None, alias="user_id"),  # Extract user_id from header, default to None if not present
 ):
     """
     Update an existing tool
diff --git a/letta/server/server.py b/letta/server/server.py
index 9c7d721ed5..fae55f30b1 100644
--- a/letta/server/server.py
+++ b/letta/server/server.py
@@ -1921,7 +1921,10 @@ def get_user_or_default(self, user_id: Optional[str]) -> User:
         if user_id is None:
             return self.get_default_user()
         else:
-            return self.get_user(user_id=user_id)
+            try:
+                return self.get_user(user_id=user_id)
+            except ValueError:
+                raise HTTPException(status_code=404, detail=f"User with id {user_id} not found")
 
     def list_llm_models(self) -> List[LLMConfig]:
         """List available models"""