test(agent): characterize active runner provider drift

tests/agent/test_runner_runtime_identity.py

@@ -0,0 +1,51 @@
+from unittest.mock import AsyncMock, MagicMock
+
+import pytest
+
+from nanobot.agent.runner import AgentRunner, AgentRunSpec
+from nanobot.config.schema import AgentDefaults
+from nanobot.providers.base import LLMProvider, LLMResponse, ToolCallRequest
+
+
+@pytest.mark.asyncio
+@pytest.mark.xfail(
+    strict=True,
+    reason="AgentRunner reads its mutable provider again between model iterations",
+)
+async def test_active_run_keeps_provider_captured_at_admission() -> None:
+    first_provider = MagicMock(spec=LLMProvider)
+    second_provider = MagicMock(spec=LLMProvider)
+    first_calls = 0
+    second_calls = 0
+    runner = AgentRunner(first_provider)
+
+    async def first_chat(**_kwargs):
+        nonlocal first_calls
+        first_calls += 1
+        runner.provider = second_provider
+        return LLMResponse(
+            content="working",
+            tool_calls=[ToolCallRequest(id="call-1", name="read_file", arguments={})],
+        )
+
+    async def second_chat(**_kwargs):
+        nonlocal second_calls
+        second_calls += 1
+        return LLMResponse(content="done")
+
+    first_provider.chat_with_retry = first_chat
+    second_provider.chat_with_retry = second_chat
+    tools = MagicMock()
+    tools.get_definitions.return_value = []
+    tools.execute = AsyncMock(return_value="contents")
+
+    await runner.run(AgentRunSpec(
+        initial_messages=[{"role": "user", "content": "read it"}],
+        tools=tools,
+        model="captured-model",
+        max_iterations=2,
+        max_tool_result_chars=AgentDefaults().max_tool_result_chars,
+    ))
+
+    assert first_calls == 2
+    assert second_calls == 0