diff --git a/src/agents/tatlock.py b/src/agents/tatlock.py
index 4d04d4b..b3331be 100644
--- a/src/agents/tatlock.py
+++ b/src/agents/tatlock.py
@@ -157,7 +157,10 @@ class TatlockAgent(AgentInterface):
 
     def __init__(self):
         """Initialize Tatlock (lazy agent creation)."""
-        self._agent = None  # Lazy initialization
+        # Deps are a ToolCallTracker: every registered tool takes
+        # RunContext[ToolCallTracker], and run() is called with one. Saying so
+        # is what lets the tool registrations below type-check at all.
+        self._agent: Agent[ToolCallTracker, str] | None = None  # Lazy initialization
 
     def _ensure_agent(self):
         """Ensure the PydanticAI agent is initialized (lazy initialization)."""
@@ -180,13 +183,19 @@ class TatlockAgent(AgentInterface):
         self._agent = Agent(
             model,
             system_prompt=TATLOCK_SYSTEM_PROMPT,
+            deps_type=ToolCallTracker,
         )
 
         # Register tools with the agent
         self._register_tools()
 
-    def _register_tools(self):
-        """Register permanent tools with the PydanticAI agent."""
+    def _register_tools(self) -> None:
+        """Register permanent tools with the PydanticAI agent.
+
+        Called only from _ensure_agent, immediately after the agent is built, so
+        the assert documents an invariant rather than guarding a real case.
+        """
+        assert self._agent is not None, "_register_tools called before the agent exists"
 
         # Calculator tool
         @self._agent.tool
@@ -324,9 +333,15 @@ class TatlockAgent(AgentInterface):
 
             # Build message history (all messages except the last user message)
             # PydanticAI expects history as list of ModelRequest/ModelResponse objects
-            from pydantic_ai.messages import ModelRequest, ModelResponse, TextPart, UserPromptPart
+            from pydantic_ai.messages import (
+                ModelMessage,
+                ModelRequest,
+                ModelResponse,
+                TextPart,
+                UserPromptPart,
+            )
 
-            message_history = []
+            message_history: list[ModelMessage] = []
             for i, msg in enumerate(messages[:-1]):  # All messages except the last one
                 role = msg.get("role")
                 content = msg.get("content", "")
@@ -498,9 +513,15 @@ class TatlockAgent(AgentInterface):
         enriched_message = f"{steward_note}\n\n{user_message}"
 
         # Convert message history to PydanticAI format
-        from pydantic_ai.messages import ModelRequest, ModelResponse, TextPart, UserPromptPart
+        from pydantic_ai.messages import (
+            ModelMessage,
+            ModelRequest,
+            ModelResponse,
+            TextPart,
+            UserPromptPart,
+        )
 
-        pydantic_history = []
+        pydantic_history: list[ModelMessage] = []
         for msg in message_history:
             role = msg.get("role")
             content = msg.get("content", "")
@@ -580,9 +601,15 @@ class TatlockAgent(AgentInterface):
         enriched_message = f"{steward_note}\n\n{user_message}"
 
         # Convert message history to PydanticAI format
-        from pydantic_ai.messages import ModelRequest, ModelResponse, TextPart, UserPromptPart
+        from pydantic_ai.messages import (
+            ModelMessage,
+            ModelRequest,
+            ModelResponse,
+            TextPart,
+            UserPromptPart,
+        )
 
-        pydantic_history = []
+        pydantic_history: list[ModelMessage] = []
         for msg in message_history:
             role = msg.get("role")
             content = msg.get("content", "")
@@ -642,6 +669,7 @@ class TatlockAgent(AgentInterface):
                 - raw_output: The agent's raw text output
         """
         from pydantic_ai.messages import (
+            ModelMessage,
             ModelRequest,
             ModelResponse,
             TextPart,
@@ -683,7 +711,7 @@ class TatlockAgent(AgentInterface):
         enriched_message = f"{steward_note}\n\n{user_message}"
 
         # Convert message history to PydanticAI format
-        pydantic_history = []
+        pydantic_history: list[ModelMessage] = []
         for msg in message_history:
             role = msg.get("role")
             content = msg.get("content", "")
@@ -781,7 +809,13 @@ class TatlockAgent(AgentInterface):
         Returns:
             str: Butler-toned response synthesized from all results
         """
-        from pydantic_ai.messages import ModelRequest, ModelResponse, TextPart, UserPromptPart
+        from pydantic_ai.messages import (
+            ModelMessage,
+            ModelRequest,
+            ModelResponse,
+            TextPart,
+            UserPromptPart,
+        )
 
         from src.anthropic.model_selector import get_model
 
@@ -840,7 +874,7 @@ class TatlockAgent(AgentInterface):
         )
 
         # Convert message history to PydanticAI format
-        pydantic_history = []
+        pydantic_history: list[ModelMessage] = []
         for msg in message_history:
             role = msg.get("role")
             content = msg.get("content", "")