diff --git a/src/ai/agents/agent.py b/src/ai/agents/agent.py
index 37fcdc20..57735f99 100644
--- a/src/ai/agents/agent.py
+++ b/src/ai/agents/agent.py
@@ -225,40 +225,6 @@ async def _coerce_events(
             yield item
 
 
-async def _default_loop(context: Context) -> AsyncGenerator[events_.AgentEvent]:
-    while True:
-        stream = models.stream(
-            context.model,
-            context.messages,
-            tools=context.tools,
-        )
-        async for stream_event in stream:
-            yield stream_event
-
-        # Bridge: emit MessageStart/MessageEnd around the assistant message
-        # the model stream just produced, so _collect_messages and downstream
-        # consumers (AI-SDK outbound, label stamping) see the same boundary
-        # events they did under the previous adapter contract.
-        if stream.message is not None and stream.message.parts:
-            async for boundary in _message_events(stream.message):
-                yield boundary
-
-        tool_calls = context.resolve(stream.tool_calls)
-        if not tool_calls:
-            break
-
-        # Execute tool calls in parallel.
-        async with asyncio.TaskGroup() as tg:
-            tasks = [tg.create_task(tc()) for tc in tool_calls]
-
-        # Yield one merged tool-result message — history auto-collects it.
-        # Left un-stamped: the tool result is the input of the *next* turn,
-        # so the next stream() call will stamp it with that turn's id.
-        tool_msg = builders.tool_message(*(t.result() for t in tasks))
-        async for boundary in _message_events(tool_msg):
-            yield boundary
-
-
 async def _collect_messages(
     source: AsyncIterable[StreamItem],
     messages: list[types.Message],
@@ -317,18 +283,54 @@ def __init__(
         tools: list[Tool[..., Any]] | None = None,
     ) -> None:
         self._tools: list[Tool[..., Any]] = tools or []
-        self._loop_fn: LoopFn = _default_loop
+        self._loop_fn: LoopFn | None = None
 
     @property
     def tools(self) -> list[Tool[..., Any]]:
         """The agent's registered tools (read-only copy)."""
         return list(self._tools)
 
+    # TODO: remove?
     def loop(self, fn: LoopFn) -> LoopFn:
         """Decorator: override the default loop function."""
         self._loop_fn = fn
         return fn
 
+    async def default_loop(
+        self, context: Context
+    ) -> AsyncGenerator[events_.AgentEvent]:
+        while True:
+            stream = models.stream(
+                context.model,
+                context.messages,
+                tools=context.tools,
+            )
+            async for stream_event in stream:
+                yield stream_event
+
+            # Bridge: emit MessageStart/MessageEnd around the assistant message
+            # the model stream just produced, so _collect_messages and downstream
+            # consumers (AI-SDK outbound, label stamping) see the same boundary
+            # events they did under the previous adapter contract.
+            if stream.message is not None and stream.message.parts:
+                async for boundary in _message_events(stream.message):
+                    yield boundary
+
+            tool_calls = context.resolve(stream.tool_calls)
+            if not tool_calls:
+                break
+
+            # Execute tool calls in parallel.
+            async with asyncio.TaskGroup() as tg:
+                tasks = [tg.create_task(tc()) for tc in tool_calls]
+
+            # Yield one merged tool-result message — history auto-collects it.
+            # Left un-stamped: the tool result is the input of the *next* turn,
+            # so the next stream() call will stamp it with that turn's id.
+            tool_msg = builders.tool_message(*(t.result() for t in tasks))
+            async for boundary in _message_events(tool_msg):
+                yield boundary
+
     async def run(
         self,
         model: models.Model,
@@ -356,7 +358,7 @@ async def run(
             label=label,
         )
 
-        loop_fn = self._loop_fn
+        loop_fn = self._loop_fn or self.default_loop
 
         async def _real(
             call: middleware_.AgentRunContext,