fix: stabilize system prompt for better cache reuse

tests/test_context_prompt_cache.py

@@ -0,0 +1,63 @@
+"""Tests for cache-friendly prompt construction."""
+
+from __future__ import annotations
+
+from datetime import datetime as real_datetime
+from pathlib import Path
+import datetime as datetime_module
+
+from nanobot.agent.context import ContextBuilder
+
+
+class _FakeDatetime(real_datetime):
+    current = real_datetime(2026, 2, 24, 13, 59)
+
+    @classmethod
+    def now(cls, tz=None):  # type: ignore[override]
+        return cls.current
+
+
+def _make_workspace(tmp_path: Path) -> Path:
+    workspace = tmp_path / "workspace"
+    workspace.mkdir(parents=True)
+    return workspace
+
+
+def test_system_prompt_stays_stable_when_clock_changes(tmp_path, monkeypatch) -> None:
+    """System prompt should not change just because wall clock minute changes."""
+    monkeypatch.setattr(datetime_module, "datetime", _FakeDatetime)
+
+    workspace = _make_workspace(tmp_path)
+    builder = ContextBuilder(workspace)
+
+    _FakeDatetime.current = real_datetime(2026, 2, 24, 13, 59)
+    prompt1 = builder.build_system_prompt()
+
+    _FakeDatetime.current = real_datetime(2026, 2, 24, 14, 0)
+    prompt2 = builder.build_system_prompt()
+
+    assert prompt1 == prompt2
+
+
+def test_runtime_context_is_appended_to_current_user_message(tmp_path) -> None:
+    """Dynamic runtime details should be added at the tail user message, not system."""
+    workspace = _make_workspace(tmp_path)
+    builder = ContextBuilder(workspace)
+
+    messages = builder.build_messages(
+        history=[],
+        current_message="Return exactly: OK",
+        channel="cli",
+        chat_id="direct",
+    )
+
+    assert messages[0]["role"] == "system"
+    assert "## Current Session" not in messages[0]["content"]
+
+    assert messages[-1]["role"] == "user"
+    user_content = messages[-1]["content"]
+    assert isinstance(user_content, str)
+    assert "Return exactly: OK" in user_content
+    assert "Current Time:" in user_content
+    assert "Channel: cli" in user_content
+    assert "Chat ID: direct" in user_content