core/agent+cli: treat last-request prompt_tokens as context size

context_tokens prefers API usage from the last model call; char est.
is only a pre-call fallback. Clarify billed turn/session totals in UI.

tests/test_agent/test_loop.py

@@ -273,6 +273,21 @@ async def test_chat_agent_turn_usage_sums_tool_rounds() -> None:
     assert agent.last_request_usage.prompt_tokens == 200
     assert agent.last_turn_usage.prompt_tokens == 300
     assert agent.last_turn_usage.total_tokens == 308
+    # Context size is last request prompt, not billed sum
+    assert agent.context_tokens == 200
+    assert agent.context_tokens_source == "api"
+
+
+async def test_context_tokens_falls_back_to_message_estimate() -> None:
+    agent = ChatAgent(
+        ScriptedClient([]),
+        model="m",
+        stream=False,
+        messages=[UserChatMessage(content="12345678")],
+    )
+    assert agent.last_request_usage.is_zero()
+    assert agent.context_tokens_source == "estimate"
+    assert agent.context_tokens >= 1
 
 
 async def test_stream_yields_deltas_incrementally() -> None:

tests/test_cli/test_repl_commands.py

@@ -145,12 +145,25 @@ async def test_rounds(state: ReplState) -> None:
 async def test_status_shows_context_tokens(
     state: ReplState, capsys: pytest.CaptureFixture[str]
 ) -> None:
+    from plyngent.agent.usage import TokenUsage
     from plyngent.lmproto.openai_compatible.model import UserChatMessage
 
     state.agent.messages = [UserChatMessage(content="hello")]
     assert await handle_slash(state, "/status") is True
     out = capsys.readouterr().out
-    assert "context_tokens~=" in out
+    assert "context_tokens=" in out
+    assert "(est)" in out  # no API usage yet
     assert "context_chars=" in out
     assert "tool_result_max=" in out
     assert str(state.workspace) in out
+
+    state.agent.last_request_usage = TokenUsage(
+        prompt_tokens=1234,
+        completion_tokens=10,
+        total_tokens=1244,
+        source="api",
+    )
+    assert await handle_slash(state, "/status") is True
+    out2 = capsys.readouterr().out
+    assert "context_tokens=1234/" in out2
+    assert "(api)" in out2