diff --git a/src/agent_env/agents/agents/claude-code/claude_bridge.py b/src/agent_env/agents/agents/claude-code/claude_bridge.py
index b3edc851..5e86f05d 100644
--- a/src/agent_env/agents/agents/claude-code/claude_bridge.py
+++ b/src/agent_env/agents/agents/claude-code/claude_bridge.py
@@ -560,6 +560,16 @@ async def feed_stdin() -> None:
         stderr_task.cancel()
 
 
+# Behind a gateway the CLI gives a bare id of these a 200K window, and envs whose tool definitions alone pass that
+# make it exit with "Autocompact is thrashing". Add a model only once the pinned CLI maps its [1m] form.
+_NATIVE_1M_MODELS = frozenset({"claude-opus-5", "claude-opus-5-5", "claude-fable-5-1", "claude-opus-4-8",
+                               "claude-opus-4-7"})
+
+
+def _with_1m_window(model: str) -> str:
+    return f"{model}[1m]" if model in _NATIVE_1M_MODELS else model
+
+
 def _command(session_id: str, resume: bool, *, model: str | None, allowed_tools: list[str] | None,
              mcp_servers: dict[str, dict] | None, system_prompt: str | None, output_format: dict | None,
              role: str | None, effort: str | None, max_turns: int | None) -> list[str]:
@@ -568,7 +578,7 @@ def _command(session_id: str, resume: bool, *, model: str | None, allowed_tools:
            "--setting-sources", "user"]
     cmd += ["--resume", session_id] if resume else ["--session-id", session_id]
     if model:
-        cmd += ["--model", model]
+        cmd += ["--model", _with_1m_window(model)]
     if system_prompt:
         cmd += ["--system-prompt", system_prompt]
     if effort:
diff --git a/tst/unit/agents/claude_code/test_claude_bridge_model.py b/tst/unit/agents/claude_code/test_claude_bridge_model.py
new file mode 100644
index 00000000..702f2208
--- /dev/null
+++ b/tst/unit/agents/claude_code/test_claude_bridge_model.py
@@ -0,0 +1,27 @@
+"""The model the CLI is given: a native-1M model passed without [1m] gets its 1M window."""
+
+import pytest
+
+from tst.unit.agents.claude_code.agent import claude_bridge
+
+
+def _cli_model(model: str | None) -> str | None:
+    cmd = claude_bridge._command("sess-1", False, model=model, allowed_tools=None, mcp_servers=None,
+                                 system_prompt=None, output_format=None, role=None, effort=None, max_turns=None)
+    return cmd[cmd.index("--model") + 1] if "--model" in cmd else None
+
+
+@pytest.mark.parametrize("model", ["claude-opus-5-5", "claude-opus-5", "claude-fable-5-1", "claude-opus-4-8",
+                                   "claude-opus-4-7"])
+def test_a_native_1m_model_gets_its_1m_window(model):
+    assert _cli_model(model) == f"{model}[1m]"
+
+
+@pytest.mark.parametrize("model", ["claude-opus-5-5[1m]", "claude-opus-4-8[1m]", "claude-sonnet-4-6", "gpt-5.6-sol",
+                                   "gemini/gemini-3.1-pro-preview"])
+def test_other_models_pass_through_unchanged(model):
+    assert _cli_model(model) == model
+
+
+def test_no_model_sends_no_model_flag():
+    assert _cli_model(None) is None