feat(runtime): cap spawned child agents

docs/advanced/configuration.mdx

@@ -108,6 +108,10 @@ When remote vars are set, Strix dual-writes telemetry to both local JSONL and th
   Maximum size (in MB) of a local directory target that Strix will copy into the sandbox file-by-file. Larger targets exit early with a suggestion to use `--mount` instead. Set to `0` to disable the check.
 </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

@@ -94,6 +94,8 @@ class RuntimeSettings(BaseSettings):
     max_local_copy_mb: int = Field(default=1024, alias="STRIX_MAX_LOCAL_COPY_MB")
     # 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/agents.py

@@ -157,6 +157,34 @@ async def register(
         logger.info("agent.register %s (%s) parent=%s", agent_id, name, parent_id or "-")
         await self._maybe_snapshot()
 
+    async def register_child_if_capacity(
+        self,
+        agent_id: str,
+        name: str,
+        parent_id: str,
+        *,
+        max_child_agents: int,
+        task: str | None = None,
+        skills: list[str] | None = None,
+    ) -> tuple[bool, int]:
+        async with self._lock:
+            child_count = sum(parent is not None for parent in self.parent_of.values())
+            if max_child_agents > 0 and child_count >= max_child_agents:
+                return False, child_count
+            self.statuses[agent_id] = "running"
+            self.parent_of[agent_id] = parent_id
+            self.names[agent_id] = name
+            self.pending_counts.setdefault(agent_id, 0)
+            self.metadata[agent_id] = {
+                "task": task or "",
+                "skills": list(skills or []),
+            }
+            self.runtimes.setdefault(agent_id, AgentRuntime())
+            child_count += 1
+        logger.info("agent.register %s (%s) parent=%s", agent_id, name, parent_id)
+        await self._maybe_snapshot()
+        return True, child_count
+
     async def attach_runtime(
         self,
         agent_id: str,

strix/core/execution.py

@@ -52,6 +52,8 @@
 
 _INPUT_REJECTION_CODES = frozenset({400, 404, 422})
 _MAX_COMPACTIONS_PER_CYCLE = 2
+_UNLIMITED_CHILD_AGENTS = 0
+_CHILD_AGENT_LIMIT_ERROR = "child agent limit reached"
 
 
 def _run_config_model(run_config: RunConfig) -> str | None:
@@ -230,20 +232,39 @@ 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_id = uuid.uuid4().hex[:8]
-    child_agent = factory(name=name, skills=skills)
-    await coordinator.register(
+    registered, child_count = await coordinator.register_child_if_capacity(
         child_id,
         name,
         parent_id,
+        max_child_agents=max_child_agents,
         task=task,
         skills=skills,
     )
+    if not registered:
+        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_agent = factory(name=name, skills=skills)
 
     await _start_child_runner(
         parent_ctx=parent_ctx,

strix/core/runner.py

@@ -318,6 +318,7 @@ async def spawn_child_agent(**kwargs: Any) -> dict[str, Any]:
                 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.fields import FieldInfo
 
 from strix.config import loader
-from strix.config.settings import ContextSettings
+from strix.config.settings import ContextSettings, RuntimeSettings
 
 
 if TYPE_CHECKING:
@@ -34,6 +34,7 @@
     "STRIX_IMAGE",
     "STRIX_RUNTIME_BACKEND",
     "STRIX_MAX_LOCAL_COPY_MB",
+    "STRIX_MAX_CHILD_AGENTS",
     # TelemetrySettings
     "STRIX_TELEMETRY",
 ]
@@ -130,6 +131,10 @@ 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

@@ -5,14 +5,18 @@
 import asyncio
 import contextlib
 import json
-from typing import Any
+from typing import Any, cast
 
 import pytest
 from agents.memory import SQLiteSession
 from agents.tool_context import ToolContext
 
 from strix.core.agents import AgentCoordinator
-from strix.core.execution import _notify_parent_on_terminal, _notify_root_on_budget_reserve
+from strix.core.execution import (
+    _notify_parent_on_terminal,
+    _notify_root_on_budget_reserve,
+    spawn_child_agent,
+)
 from strix.tools.finish.tool import finish_scan
 
 
@@ -56,6 +60,92 @@ async def _record(target_agent_id: str, message: dict[str, Any]) -> bool:
     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_spawn_child_agent_reserves_limit_atomically(
+    monkeypatch: pytest.MonkeyPatch, tmp_path: Any
+) -> None:
+    coordinator = AgentCoordinator()
+    await coordinator.register("root", "strix", parent_id=None)
+
+    async def _noop_start(**_kwargs: Any) -> None:
+        return None
+
+    monkeypatch.setattr("strix.core.execution._start_child_runner", _noop_start)
+
+    def _factory(**_kwargs: Any) -> object:
+        return object()
+
+    async def _spawn(name: str) -> dict[str, Any]:
+        return await spawn_child_agent(
+            coordinator=coordinator,
+            factory=_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=name,
+            task="do more recon",
+            skills=[],
+            parent_history=[],
+            max_child_agents=1,
+        )
+
+    async with coordinator._lock:
+        tasks = [
+            asyncio.create_task(_spawn("extra-a")),
+            asyncio.create_task(_spawn("extra-b")),
+        ]
+        await asyncio.sleep(0)
+
+    results = await asyncio.gather(*tasks)
+
+    assert [result["success"] for result in results].count(True) == 1
+    assert _child_count(coordinator) == 1
+
+
+def _child_count(coordinator: AgentCoordinator) -> int:
+    return sum(parent_id is not None for parent_id in coordinator.parent_of.values())
+
+
 @pytest.mark.asyncio
 async def test_concurrent_reserve_claims_yield_single_root() -> None:
     coordinator = AgentCoordinator()

tests/test_runner_rate_limit.py

@@ -41,7 +41,7 @@ async def test_persistent_rate_limit_stops_gracefully(
             timeout=300,
             prompt_cache=True,
         ),
-        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

@@ -49,7 +49,7 @@ def _patch_engine_scaffold(
             timeout=300,
             prompt_cache=True,
         ),
-        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)