feat(runtime): cap spawned child agents

docs/advanced/configuration.mdx

@@ -122,6 +122,10 @@ When remote vars are set, Strix dual-writes telemetry to both local JSONL and th
   Runtime backend for the sandbox environment.
 </ParamField>
 
+<ParamField path="STRIX_MAX_CHILD_AGENTS" default="0" type="integer">
+  Maximum number of child agents a scan may spawn. Set to `0` for no limit.
+</ParamField>
+
 ## Sandbox Configuration
 
 <ParamField path="STRIX_SANDBOX_EXECUTION_TIMEOUT" default="120" type="integer">

strix/config/settings.py

@@ -112,6 +112,8 @@ class RuntimeSettings(BaseSettings):
     backend: str = Field(default="docker", alias="STRIX_RUNTIME_BACKEND")
     # Max screenshot/image tool outputs kept live per agent context (0 = none).
     max_context_images: int = Field(default=3, ge=0, alias="STRIX_MAX_CONTEXT_IMAGES")
+    # Max spawned child agents per scan (0 = unlimited).
+    max_child_agents: int = Field(default=0, ge=0, alias="STRIX_MAX_CHILD_AGENTS")
 
 
 class TelemetrySettings(BaseSettings):

strix/core/execution.py

@@ -54,6 +54,8 @@ StreamEventSink = Callable[[str, Any], None]
 
 _INPUT_REJECTION_CODES = frozenset({400, 404, 422})
 _MAX_COMPACTIONS_PER_CYCLE = 2
+_UNLIMITED_CHILD_AGENTS = 0
+_CHILD_AGENT_LIMIT_ERROR = "child agent limit reached"
 
 
 class ProviderRefusalError(AgentsException):
@@ -302,11 +304,30 @@ async def spawn_child_agent(
     parent_history: list[Any],
     event_sink: StreamEventSink | None = None,
     hooks: RunHooks[dict[str, Any]] | None = None,
+    max_child_agents: int = _UNLIMITED_CHILD_AGENTS,
 ) -> dict[str, Any]:
     parent_id = parent_ctx.get("agent_id")
     if not isinstance(parent_id, str):
         raise TypeError("Parent agent_id missing from context")
 
+    child_count = _child_agent_count(coordinator)
+    if max_child_agents > _UNLIMITED_CHILD_AGENTS and child_count >= max_child_agents:
+        logger.info(
+            "refusing to spawn child agent %r: limit %d already reached",
+            name,
+            max_child_agents,
+        )
+        return {
+            "success": False,
+            "error": _CHILD_AGENT_LIMIT_ERROR,
+            "message": (
+                f"Cannot spawn '{name}': configured child agent limit "
+                f"({max_child_agents}) is already reached."
+            ),
+            "limit": max_child_agents,
+            "current_child_agents": child_count,
+        }
+
     child_id = uuid.uuid4().hex[:8]
     child_agent = factory(name=name, skills=skills)
     await coordinator.register(
@@ -350,6 +371,10 @@ async def spawn_child_agent(
     }
 
 
+def _child_agent_count(coordinator: AgentCoordinator) -> int:
+    return sum(parent_id is not None for parent_id in coordinator.parent_of.values())
+
+
 async def respawn_subagents(
     *,
     coordinator: AgentCoordinator,

strix/core/runner.py

@@ -338,6 +338,7 @@ async def run_strix_scan(
                 interactive=interactive,
                 event_sink=event_sink,
                 hooks=hooks,
+                max_child_agents=settings.runtime.max_child_agents,
                 **kwargs,
             )
 

tests/test_config_loader.py

@@ -10,7 +10,7 @@ from pydantic import AliasChoices, Field, ValidationError
 from pydantic.fields import FieldInfo
 
 from strix.config import loader
-from strix.config.settings import ContextSettings
+from strix.config.settings import ContextSettings, RuntimeSettings
 
 
 if TYPE_CHECKING:
@@ -33,6 +33,7 @@ _LLM_ENV_KEYS = [
     # RuntimeSettings
     "STRIX_IMAGE",
     "STRIX_RUNTIME_BACKEND",
+    "STRIX_MAX_CHILD_AGENTS",
     # TelemetrySettings
     "STRIX_TELEMETRY",
 ]
@@ -129,6 +130,10 @@ def test_tool_output_max_bytes_accepts_floor() -> None:
     assert ContextSettings(STRIX_TOOL_OUTPUT_MAX_BYTES=1024).tool_output_max_bytes == 1024
 
 
+def test_max_child_agents_env_alias() -> None:
+    assert RuntimeSettings(STRIX_MAX_CHILD_AGENTS=7).max_child_agents == 7
+
+
 # --------------------------------------------------------------------------- #
 # _aliases_for
 # --------------------------------------------------------------------------- #

tests/test_execution.py

@@ -20,6 +20,7 @@ from strix.core.agents import AgentCoordinator
 from strix.core.execution import (
     _notify_root_on_budget_reserve,
     notify_parent_on_terminal,
+    spawn_child_agent,
 )
 from strix.core.sessions import seed_initial_input
 from strix.tools.agents_graph.tools import agent_finish, stop_agent
@@ -130,6 +131,43 @@ async def test_reserve_stop_notifies_root_once(monkeypatch: pytest.MonkeyPatch)
     assert "finish_scan" in str(message["content"])
 
 
+@pytest.mark.asyncio
+async def test_spawn_child_agent_respects_child_limit(
+    monkeypatch: pytest.MonkeyPatch, tmp_path: Any
+) -> None:
+    coordinator = AgentCoordinator()
+    await coordinator.register("root", "strix", parent_id=None)
+    await coordinator.register("child-1", "recon", parent_id="root")
+
+    def _unexpected_factory(**_kwargs: Any) -> object:
+        raise AssertionError("child factory should not be called at the child limit")
+
+    async def _unexpected_start(**_kwargs: Any) -> None:
+        raise AssertionError("child runner should not start at the child limit")
+
+    monkeypatch.setattr("strix.core.execution._start_child_runner", _unexpected_start)
+
+    result = await spawn_child_agent(
+        coordinator=coordinator,
+        factory=_unexpected_factory,
+        agents_db_path=tmp_path / "agents.db",
+        sessions_to_close=[],
+        run_config=cast("Any", object()),
+        max_turns=1,
+        interactive=False,
+        parent_ctx={"agent_id": "root"},
+        name="extra",
+        task="do more recon",
+        skills=[],
+        parent_history=[],
+        max_child_agents=1,
+    )
+
+    assert result["success"] is False
+    assert "child agent limit" in result["error"]
+    assert set(coordinator.parent_of) == {"root", "child-1"}
+
+
 @pytest.mark.asyncio
 async def test_concurrent_reserve_claims_yield_single_root() -> None:
     coordinator = AgentCoordinator()

tests/test_runner_rate_limit.py

@@ -43,7 +43,7 @@ async def test_persistent_rate_limit_stops_gracefully(
             prompt_cache=True,
             extra_headers=None,
         ),
-        runtime=types.SimpleNamespace(max_context_images=3),
+        runtime=types.SimpleNamespace(max_context_images=3, max_child_agents=0),
     )
     monkeypatch.setattr(runner, "load_settings", lambda: settings)
     monkeypatch.setattr(runner, "configure_sdk_model_defaults", lambda _settings: None)

tests/test_runner_root_prompt.py

@@ -51,7 +51,7 @@ def _patch_engine_scaffold(
             prompt_cache=True,
             extra_headers=None,
         ),
-        runtime=types.SimpleNamespace(max_context_images=3),
+        runtime=types.SimpleNamespace(max_context_images=3, max_child_agents=0),
     )
     monkeypatch.setattr(runner, "load_settings", lambda: settings)
     monkeypatch.setattr(runner, "configure_sdk_model_defaults", lambda _settings: None)