fix(exec): bound buffered session output

tests/tools/test_exec_session_tools.py

@@ -19,6 +19,8 @@ from nanobot.agent.tools.exec_session import (
     ExecSessionManager,
     ListExecSessionsTool,
     WriteStdinTool,
+    _BoundedOutputBuffer,
+    _SessionPoll,
 )
 from nanobot.agent.tools.registry import is_tool_error_result
 from nanobot.agent.tools.shell import ExecTool
@@ -143,6 +145,88 @@ def test_exec_session_accepts_max_output_tokens_alias(tmp_path):
     assert "Exit code: 0" in result
 
 
+def test_bounded_output_buffer_keeps_head_tail_and_exact_drop_count():
+    buffer = _BoundedOutputBuffer(10)
+
+    buffer.append("012345")
+    buffer.append("6789ABCDEF")
+
+    assert buffer.retained_chars == 10
+    assert buffer.drain() == ("01234BCDEF", 6)
+    assert buffer.retained_chars == 0
+
+
+def test_exec_session_bounds_unpolled_stdout_and_stderr(tmp_path):
+    async def run() -> tuple[int, int, str, int]:
+        manager = ExecSessionManager()
+        tool = ExecTool(working_dir=str(tmp_path), timeout=5, session_manager=manager)
+        command = _python_command(
+            "import sys,time; time.sleep(0.05); "
+            "sys.stdout.write('OUT_HEAD' + 'o' * 200000 + 'OUT_TAIL'); "
+            "sys.stderr.write('ERR_HEAD' + 'e' * 200000 + 'ERR_TAIL')"
+        )
+
+        initial = await tool.execute(
+            command=command,
+            yield_time_ms=0,
+            max_output_chars=1000,
+        )
+        sid = _session_id(initial)
+        session = manager._sessions[sid]
+        await asyncio.wait_for(session.process.wait(), timeout=5)
+        await asyncio.wait_for(
+            asyncio.gather(session._stdout_task, session._stderr_task),
+            timeout=5,
+        )
+        retained_stdout = session._stdout.retained_chars
+        retained_stderr = session._stderr.retained_chars
+        poll = await manager.write(
+            session_id=sid,
+            chars=None,
+            close_stdin=False,
+            terminate=False,
+            yield_time_ms=0,
+            max_output_chars=1000,
+        )
+        return retained_stdout, retained_stderr, poll.output, poll.truncated_chars
+
+    retained_stdout, retained_stderr, output, truncated_chars = asyncio.run(run())
+
+    assert retained_stdout == 50000
+    assert retained_stderr == 50000
+    assert output.startswith("OUT_HEAD")
+    assert output.endswith("ERR_TAIL")
+    assert truncated_chars > 390000
+
+
+def test_write_stdin_wait_for_keeps_aggregate_within_output_budget():
+    async def run() -> str:
+        manager = SimpleNamespace(
+            write=AsyncMock(side_effect=[
+                _SessionPoll(output="HEAD" + "a" * 596, done=False, exit_code=None),
+                _SessionPoll(output="b" * 600, done=False, exit_code=None),
+                _SessionPoll(output="c" * 590 + "TARGET", done=False, exit_code=None),
+            ])
+        )
+        tool = WriteStdinTool(manager=manager)
+        return await tool._wait_for_output(
+            session_id="session",
+            chars=None,
+            close_stdin=False,
+            terminate=False,
+            wait_for="TARGET",
+            wait_timeout_ms=1000,
+            max_output_chars=1000,
+        )
+
+    result = asyncio.run(run())
+
+    assert result.startswith("HEAD")
+    assert "TARGET" in result
+    assert "(796 chars truncated from output)" in result
+    assert len(result) < 1100
+
+
 def test_exec_one_shot_accepts_max_output_tokens_alias(tmp_path):
     async def run() -> str:
         tool = ExecTool(working_dir=str(tmp_path), timeout=5)