diff --git a/pycodeloop/core/agent.py b/pycodeloop/core/agent.py
index dd860cb..bf5e443 100644
--- a/pycodeloop/core/agent.py
+++ b/pycodeloop/core/agent.py
@@ -111,7 +111,6 @@ def __init__(
         self.on_turn_end = on_turn_end
         self.on_trace_event = on_trace_event
         self.usage = Usage()
-        self._last_context_tokens = 0
 
     def _trace(self, event_type: str, **fields) -> None:
         if self.on_trace_event:
@@ -325,9 +324,8 @@ def _compact(self, session: Session) -> None:
         provider itself, replacing the older history with one condensed
         message — keeps the conversation going instead of hitting the
         model's context limit."""
-        turn_starts = [
-            i for i, m in enumerate(session.messages) if m.role == "user"
-        ]
+        history = session.history()
+        turn_starts = [i for i, m in enumerate(history) if m.role == "user"]
 
         if len(turn_starts) <= _COMPACT_KEEP_RECENT_TURNS:
             return
@@ -335,9 +333,9 @@ def _compact(self, session: Session) -> None:
         if self.on_compact_start:
             self.on_compact_start()
 
-        before_count = len(session.messages)
+        before_count = len(history)
         cutoff = turn_starts[-_COMPACT_KEEP_RECENT_TURNS]
-        older, recent = session.messages[:cutoff], session.messages[cutoff:]
+        older, recent = history[:cutoff], history[cutoff:]
 
         summary = self._complete(
             system_prompt="Summarize conversations concisely for context compaction.",
@@ -397,7 +395,7 @@ def run(
             context_window = getattr(self.provider, "context_window", None)
             if context_window is None:
                 context_window = context_window_for(self.provider.model)
-            if self.auto_compact and self._last_context_tokens >= (
+            if self.auto_compact and session.get_last_context_tokens() >= (
                 context_window * self.compact_threshold
             ):
                 self._compact(session)
@@ -429,9 +427,9 @@ def run(
                 output_tokens=response.usage.output_tokens,
             )
 
-            self._last_context_tokens = response.usage.input_tokens
+            session.update_last_context_tokens(response.usage.input_tokens)
             if self.on_context:
-                self.on_context(self._last_context_tokens, context_window)
+                self.on_context(response.usage.input_tokens, context_window)
 
             tool_calls = [
                 {
diff --git a/pycodeloop/core/session.py b/pycodeloop/core/session.py
index 9e29227..a2e5ca0 100644
--- a/pycodeloop/core/session.py
+++ b/pycodeloop/core/session.py
@@ -16,6 +16,7 @@ class Session:
     messages: list[Message] = field(default_factory=list)
     cwd: str = "."
     dirty: bool = field(default=False, repr=False, compare=False)
+    last_context_tokens: int = field(default=0, repr=False, compare=False)
     _lock: threading.Lock = field(
         default_factory=threading.Lock, repr=False, compare=False
     )
@@ -42,6 +43,14 @@ def add_tool_result(self, tool_call_id: str, content: str) -> None:
                 )
             )
 
+    def get_last_context_tokens(self) -> int:
+        with self._lock:
+            return self.last_context_tokens
+
+    def update_last_context_tokens(self, value: int) -> None:
+        with self._lock:
+            self.last_context_tokens = value
+
     def history(self) -> list[Message]:
         with self._lock:
             self._repair_dangling_tool_calls()
diff --git a/tests/core/test_agent.py b/tests/core/test_agent.py
index e916d53..7d5ead3 100644
--- a/tests/core/test_agent.py
+++ b/tests/core/test_agent.py
@@ -451,6 +451,32 @@ def test_compacts_when_context_usage_crosses_threshold(self):
         self.assertIn("second", session.messages[0].content)
         self.assertEqual(len(session.messages), 4)
 
+    def test_context_usage_does_not_leak_across_sessions(self):
+        provider = FakeProvider(
+            [
+                ProviderResponse(text="b1", usage=Usage(input_tokens=500)),
+                ProviderResponse(text="b2", usage=Usage(input_tokens=500)),
+                ProviderResponse(text="a1", usage=Usage(input_tokens=190_000)),
+                ProviderResponse(text="b3", usage=Usage(input_tokens=500)),
+            ]
+        )
+        provider.model = "claude-sonnet-5"
+        events = []
+        agent = Agent(
+            provider=provider,
+            on_compact_start=lambda: events.append("start"),
+        )
+        session_a = Session(system_prompt="sys")
+        session_b = Session(system_prompt="sys")
+
+        agent.run("b-first", session=session_b)
+        agent.run("b-second", session=session_b)
+        agent.run("a-first", session=session_a)
+        agent.run("b-third", session=session_b)
+
+        self.assertEqual(events, [])
+        self.assertEqual(len(session_b.messages), 6)
+
     def test_auto_compact_false_never_compacts(self):
         provider = FakeProvider(
             [