diff --git a/tests/tools/test_shared_container_task_id.py b/tests/tools/test_shared_container_task_id.py
new file mode 100644
index 0000000000..ab599fa855
--- /dev/null
+++ b/tests/tools/test_shared_container_task_id.py
@@ -0,0 +1,107 @@
+"""
+Regression tests for the shared-container task_id mapping.
+
+The top-level agent and all delegate_task subagents share a single
+terminal sandbox keyed by ``"default"``.  ``_resolve_container_task_id``
+is the sole gatekeeper for which tool-call task_ids go to the shared
+container vs. get their own isolated sandbox.  RL / benchmark
+environments opt in to isolation by calling
+``register_task_env_overrides(task_id, {...})`` before the agent loop;
+every other task_id collapses back to ``"default"``.
+
+If you change the collapse logic, update both the helper and these
+tests -- see `hermes-agent-dev` skill, "Why do subagents get their own
+containers?" section, and the Container lifecycle paragraph under
+Docker Backend in ``website/docs/user-guide/configuration.md``.
+"""
+
+import pytest
+
+from tools import terminal_tool
+
+
+@pytest.fixture(autouse=True)
+def _clean_overrides():
+    """Ensure no stray overrides from other tests leak in."""
+    before = dict(terminal_tool._task_env_overrides)
+    terminal_tool._task_env_overrides.clear()
+    yield
+    terminal_tool._task_env_overrides.clear()
+    terminal_tool._task_env_overrides.update(before)
+
+
+def test_none_task_id_maps_to_default():
+    assert terminal_tool._resolve_container_task_id(None) == "default"
+
+
+def test_empty_task_id_maps_to_default():
+    assert terminal_tool._resolve_container_task_id("") == "default"
+
+
+def test_literal_default_stays_default():
+    assert terminal_tool._resolve_container_task_id("default") == "default"
+
+
+def test_subagent_task_id_collapses_to_default():
+    # delegate_task constructs IDs like "subagent-<N>-<uuid_hex>"; these
+    # should share the parent's container, not spin up their own.
+    assert terminal_tool._resolve_container_task_id("subagent-0-deadbeef") == "default"
+    assert terminal_tool._resolve_container_task_id("subagent-42-cafef00d") == "default"
+
+
+def test_arbitrary_session_id_collapses_to_default():
+    # Session UUIDs or anything else without an override still collapse.
+    assert terminal_tool._resolve_container_task_id("sess-123e4567-e89b-12d3") == "default"
+
+
+def test_rl_task_with_override_keeps_its_own_id():
+    # RL / benchmark pattern: register a per-task image, then the task_id
+    # must survive ``_resolve_container_task_id`` so the rollout lands in
+    # its own sandbox.
+    terminal_tool.register_task_env_overrides(
+        "tb2-task-fix-git", {"docker_image": "tb2:fix-git", "cwd": "/app"}
+    )
+    try:
+        assert (
+            terminal_tool._resolve_container_task_id("tb2-task-fix-git")
+            == "tb2-task-fix-git"
+        )
+    finally:
+        terminal_tool.clear_task_env_overrides("tb2-task-fix-git")
+
+
+def test_cleared_override_collapses_again():
+    terminal_tool.register_task_env_overrides("tb2-x", {"docker_image": "x:y"})
+    assert terminal_tool._resolve_container_task_id("tb2-x") == "tb2-x"
+    terminal_tool.clear_task_env_overrides("tb2-x")
+    assert terminal_tool._resolve_container_task_id("tb2-x") == "default"
+
+
+def test_get_active_env_reads_shared_container_from_subagent_id():
+    """``get_active_env`` must see the shared ``"default"`` sandbox when
+    called with a subagent's task_id, so the agent loop's turn-budget
+    enforcement reads the real env (not None) during delegation."""
+    sentinel = object()
+    terminal_tool._active_environments["default"] = sentinel
+    try:
+        assert terminal_tool.get_active_env("subagent-7-cafe") is sentinel
+        assert terminal_tool.get_active_env(None) is sentinel
+        assert terminal_tool.get_active_env("default") is sentinel
+    finally:
+        terminal_tool._active_environments.pop("default", None)
+
+
+def test_get_active_env_honours_rl_override():
+    rl_env = object()
+    default_env = object()
+    terminal_tool._active_environments["default"] = default_env
+    terminal_tool._active_environments["rl-42"] = rl_env
+    terminal_tool.register_task_env_overrides("rl-42", {"docker_image": "x"})
+    try:
+        # With an override registered, lookup returns the task's own env,
+        # not the shared "default" one.
+        assert terminal_tool.get_active_env("rl-42") is rl_env
+    finally:
+        terminal_tool.clear_task_env_overrides("rl-42")
+        terminal_tool._active_environments.pop("default", None)
+        terminal_tool._active_environments.pop("rl-42", None)
diff --git a/tools/code_execution_tool.py b/tools/code_execution_tool.py
index 96e21d0cb1..db706e6a4c 100644
--- a/tools/code_execution_tool.py
+++ b/tools/code_execution_tool.py
@@ -440,9 +440,10 @@ def _get_or_create_env(task_id: str):
         _active_environments, _env_lock, _create_environment,
         _get_env_config, _last_activity, _start_cleanup_thread,
         _creation_locks, _creation_locks_lock, _task_env_overrides,
+        _resolve_container_task_id,
     )
 
-    effective_task_id = task_id or "default"
+    effective_task_id = _resolve_container_task_id(task_id)
 
     # Fast path: environment already exists
     with _env_lock:
diff --git a/tools/file_tools.py b/tools/file_tools.py
index 609506c05e..2e1d3875c2 100644
--- a/tools/file_tools.py
+++ b/tools/file_tools.py
@@ -88,8 +88,14 @@ def _resolve_path(filepath: str, task_id: str = "default") -> Path:
 
 def _get_live_tracking_cwd(task_id: str = "default") -> str | None:
     """Return the task's live terminal cwd for bookkeeping when available."""
+    try:
+        from tools.terminal_tool import _resolve_container_task_id
+        container_key = _resolve_container_task_id(task_id)
+    except Exception:
+        container_key = task_id
+
     with _file_ops_lock:
-        cached = _file_ops_cache.get(task_id)
+        cached = _file_ops_cache.get(container_key) or _file_ops_cache.get(task_id)
     if cached is not None:
         live_cwd = getattr(getattr(cached, "env", None), "cwd", None) or getattr(
             cached, "cwd", None
@@ -101,7 +107,7 @@ def _get_live_tracking_cwd(task_id: str = "default") -> str | None:
         from tools.terminal_tool import _active_environments, _env_lock
 
         with _env_lock:
-            env = _active_environments.get(task_id)
+            env = _active_environments.get(container_key) or _active_environments.get(task_id)
             live_cwd = getattr(env, "cwd", None) if env is not None else None
         if live_cwd:
             return live_cwd
@@ -261,15 +267,23 @@ def _get_file_ops(task_id: str = "default") -> ShellFileOperations:
 
     Thread-safe: uses the same per-task creation locks as terminal_tool to
     prevent duplicate sandbox creation from concurrent tool calls.
+
+    Note: subagent task_ids are collapsed to "default" via
+    ``_resolve_container_task_id`` so delegate_task children share the
+    parent's container and its cached file_ops. RL/benchmark task_ids with
+    a registered env override keep their isolation.
     """
     from tools.terminal_tool import (
         _active_environments, _env_lock, _create_environment,
         _get_env_config, _last_activity, _start_cleanup_thread,
         _creation_locks,
         _creation_locks_lock,
+        _resolve_container_task_id,
     )
     import time
 
+    task_id = _resolve_container_task_id(task_id)
+
     # Fast path: check cache -- but also verify the underlying environment
     # is still alive (it may have been killed by the cleanup thread).
     with _file_ops_lock:
diff --git a/tools/terminal_tool.py b/tools/terminal_tool.py
index b0f81b8868..a2e8a21898 100644
--- a/tools/terminal_tool.py
+++ b/tools/terminal_tool.py
@@ -803,6 +803,31 @@ def clear_task_env_overrides(task_id: str):
     """
     _task_env_overrides.pop(task_id, None)
 
+
+def _resolve_container_task_id(task_id: Optional[str]) -> str:
+    """
+    Map a tool-call ``task_id`` to the container/sandbox key used by
+    ``_active_environments``.
+
+    The top-level agent passes ``task_id=None`` and lands on ``"default"``.
+    ``delegate_task`` children pass their own subagent ID so that
+    file-state tracking, the active-subagents registry, and TUI events stay
+    distinct per child -- but we deliberately collapse that ID back to
+    ``"default"`` here so subagents share the parent's long-lived container
+    (one bash, one /workspace, one set of installed packages).
+
+    Exception: RL / benchmark environments (TerminalBench2, HermesSweEnv, ...)
+    call ``register_task_env_overrides(task_id, {...})`` to request a
+    per-task Docker/Modal image. When an override is registered for a
+    task_id, we honour it by returning the task_id unchanged -- those
+    rollouts need their own isolated sandbox, which is the whole point of
+    the override.
+    """
+    if task_id and task_id in _task_env_overrides:
+        return task_id
+    return "default"
+
+
 # Configuration from environment variables
 
 def _parse_env_var(name: str, default: str, converter=int, type_label: str = "integer"):
@@ -1139,8 +1164,9 @@ def _stop_cleanup_thread():
 
 def get_active_env(task_id: str):
     """Return the active BaseEnvironment for *task_id*, or None."""
+    lookup = _resolve_container_task_id(task_id)
     with _env_lock:
-        return _active_environments.get(task_id)
+        return _active_environments.get(lookup) or _active_environments.get(task_id)
 
 
 def is_persistent_env(task_id: str) -> bool:
@@ -1473,8 +1499,11 @@ def terminal_tool(
         config = _get_env_config()
         env_type = config["env_type"]
 
-        # Use task_id for environment isolation
-        effective_task_id = task_id or "default"
+        # Use task_id for environment isolation. By default all subagent
+        # task_ids collapse back to "default" so the top-level agent and
+        # every delegate_task child share one container; only task_ids with
+        # a registered env override (RL benchmarks) get isolated sandboxes.
+        effective_task_id = _resolve_container_task_id(task_id)
 
         # Check per-task overrides (set by environments like TerminalBench2Env)
         # before falling back to global env var config
diff --git a/website/docs/user-guide/configuration.md b/website/docs/user-guide/configuration.md
index ac48e9f884..61eed114e0 100644
--- a/website/docs/user-guide/configuration.md
+++ b/website/docs/user-guide/configuration.md
@@ -146,9 +146,9 @@ terminal:
 
 **Requirements:** Docker Desktop or Docker Engine installed and running. Hermes probes `$PATH` plus common macOS install locations (`/usr/local/bin/docker`, `/opt/homebrew/bin/docker`, Docker Desktop app bundle).
 
-**Container lifecycle:** Hermes reuses a single long-lived container (`docker run -d ... sleep 2h`) for every terminal and file-tool call made by the top-level agent, across sessions, `/new`, and `/reset`, for the lifetime of the Hermes process. Commands run via `docker exec` with a login shell, so working-directory changes, installed packages, and files in `/workspace` all persist from one tool call to the next. The container is stopped and removed on Hermes shutdown (or when the idle-sweep reclaims it).
+**Container lifecycle:** Hermes reuses a single long-lived container (`docker run -d ... sleep 2h`) for every terminal and file-tool call, across sessions, `/new`, `/reset`, and `delegate_task` subagents, for the lifetime of the Hermes process. Commands run via `docker exec` with a login shell, so working-directory changes, installed packages, and files in `/workspace` all persist from one tool call to the next. The container is stopped and removed on Hermes shutdown (or when the idle-sweep reclaims it).
 
-Subagents (`delegate_task`) and RL rollouts get their own isolated containers keyed by `task_id` — only the top-level agent shares the `default` container.
+Parallel subagents spawned via `delegate_task(tasks=[...])` share this one container — concurrent `cd`, env mutations, and writes to the same path will collide. If a subagent needs an isolated sandbox, it must register a per-task image override via `register_task_env_overrides()`, which RL and benchmark environments (TerminalBench2, HermesSweEnv, etc.) do automatically for their per-task Docker images.
 
 **Security hardening:**
 - `--cap-drop ALL` with only `DAC_OVERRIDE`, `CHOWN`, `FOWNER` added back