diff --git a/backend/packages/harness/deerflow/config/acp_config.py b/backend/packages/harness/deerflow/config/acp_config.py
index 736ca67f4c..7b2021e6f5 100644
--- a/backend/packages/harness/deerflow/config/acp_config.py
+++ b/backend/packages/harness/deerflow/config/acp_config.py
@@ -3,7 +3,9 @@
 import logging
 from collections.abc import Mapping
 
-from pydantic import BaseModel, Field
+from pydantic import BaseModel, Field, ValidationInfo, field_validator
+
+from deerflow.config._boolean_guards import reject_boolean
 
 logger = logging.getLogger(__name__)
 
@@ -36,6 +38,11 @@ class ACPAgentConfig(BaseModel):
         ),
     )
 
+    @field_validator("timeout_seconds", mode="before")
+    @classmethod
+    def _reject_boolean_timeout(cls, value: object, info: ValidationInfo) -> object:
+        return reject_boolean(value, info, kind="an integer")
+
 
 _acp_agents: dict[str, ACPAgentConfig] = {}
 
diff --git a/backend/packages/harness/deerflow/config/subagents_config.py b/backend/packages/harness/deerflow/config/subagents_config.py
index ff46d5347a..8265a27a66 100644
--- a/backend/packages/harness/deerflow/config/subagents_config.py
+++ b/backend/packages/harness/deerflow/config/subagents_config.py
@@ -2,8 +2,9 @@
 
 import logging
 
-from pydantic import BaseModel, Field
+from pydantic import BaseModel, Field, ValidationInfo, field_validator
 
+from deerflow.config._boolean_guards import reject_boolean
 from deerflow.config.prompt_overlay import PromptOverlay
 from deerflow.config.token_budget_config import TokenBudgetConfig
 
@@ -114,6 +115,11 @@ class SubagentOverrideConfig(BaseModel):
         description="Per-run token budget override for this subagent (None = use the global subagents.token_budget default). Symmetric with timeout_seconds/max_turns.",
     )
 
+    @field_validator("timeout_seconds", "max_turns", mode="before")
+    @classmethod
+    def _reject_boolean_override_backstops(cls, value: object, info: ValidationInfo) -> object:
+        return reject_boolean(value, info, kind="an integer")
+
 
 class CustomSubagentConfig(BaseModel):
     """User-defined subagent type declared in config.yaml."""
@@ -151,6 +157,11 @@ class CustomSubagentConfig(BaseModel):
         description="Maximum execution time in seconds",
     )
 
+    @field_validator("timeout_seconds", "max_turns", mode="before")
+    @classmethod
+    def _reject_boolean_custom_backstops(cls, value: object, info: ValidationInfo) -> object:
+        return reject_boolean(value, info, kind="an integer")
+
 
 class SubagentsAppConfig(BaseModel):
     """Configuration for the subagent system."""
@@ -171,6 +182,12 @@ class SubagentsAppConfig(BaseModel):
         le=MAX_TOTAL_SUBAGENTS_PER_RUN,
         description="Default total number of subagent delegations allowed in one lead-agent run. This is a deterministic backstop against repeated legal-sized task batches. Valid range: 1-50.",
     )
+
+    @field_validator("timeout_seconds", "max_turns", "max_total_per_run", mode="before")
+    @classmethod
+    def _reject_boolean_global_backstops(cls, value: object, info: ValidationInfo) -> object:
+        return reject_boolean(value, info, kind="an integer")
+
     token_budget: TokenBudgetConfig = Field(
         default_factory=default_subagent_token_budget,
         description="Default per-run token budget for subagents — a cost-ceiling backstop that engages by default (#3875 Phase 2). Set enabled: false to disable, or override per agent via agents.<name>.token_budget.",
diff --git a/backend/tests/test_subagents_acp_boolean_backstops.py b/backend/tests/test_subagents_acp_boolean_backstops.py
new file mode 100644
index 0000000000..b4a2d8a13e
--- /dev/null
+++ b/backend/tests/test_subagents_acp_boolean_backstops.py
@@ -0,0 +1,73 @@
+"""YAML booleans must not coerce into the subagent and ACP invocation backstops.
+
+`config.yaml` legitimately mixes booleans (`enabled: true`) with integer knobs on
+neighbouring lines, so a typo like `subagents.timeout_seconds: true` loads cleanly —
+and Pydantic coerces it to `1`. The subagent executor bounds every run with
+`future.result(timeout=self.config.timeout_seconds)`, so 1800 seconds becomes a
+1-second kill, and `resolve_recursion_limit(self.config.max_turns)` turns a
+`max_turns: true` typo into a LangGraph budget of a single super-step. The ACP
+agent timeout mirrors `subagents.timeout_seconds` and has the same hole. Mirrors
+the app-config, loop-detection, and tool-output boolean guards already in the repo.
+"""
+
+import pytest
+from pydantic import ValidationError
+
+from deerflow.config.acp_config import ACPAgentConfig
+from deerflow.config.subagents_config import CustomSubagentConfig, SubagentOverrideConfig, SubagentsAppConfig
+
+BACKSTOP_FIELDS = ("timeout_seconds", "max_turns")
+GLOBAL_INT_FIELDS = ("timeout_seconds", "max_turns", "max_total_per_run")
+
+
+@pytest.mark.parametrize("value", [True, False])
+@pytest.mark.parametrize("field", BACKSTOP_FIELDS)
+def test_override_backstops_reject_booleans(field: str, value: bool) -> None:
+    with pytest.raises(ValidationError, match="must be an integer, not a boolean"):
+        SubagentOverrideConfig(**{field: value})
+
+
+@pytest.mark.parametrize("value", [True, False])
+@pytest.mark.parametrize("field", BACKSTOP_FIELDS)
+def test_custom_backstops_reject_booleans(field: str, value: bool) -> None:
+    with pytest.raises(ValidationError, match="must be an integer, not a boolean"):
+        CustomSubagentConfig(description="d", system_prompt="s", **{field: value})
+
+
+@pytest.mark.parametrize("value", [True, False])
+@pytest.mark.parametrize("field", GLOBAL_INT_FIELDS)
+def test_global_backstops_reject_booleans(field: str, value: bool) -> None:
+    with pytest.raises(ValidationError, match="must be an integer, not a boolean"):
+        SubagentsAppConfig(**{field: value})
+
+
+@pytest.mark.parametrize("field", BACKSTOP_FIELDS)
+def test_override_backstops_keep_none_and_numeric_inputs(field: str) -> None:
+    config = SubagentOverrideConfig(**{field: None})
+    assert getattr(config, field) is None
+    config = SubagentOverrideConfig(**{field: "30"})
+    assert getattr(config, field) == 30
+
+
+def test_custom_backstops_keep_numeric_inputs() -> None:
+    config = CustomSubagentConfig(description="d", system_prompt="s", max_turns="25", timeout_seconds=600)
+    assert config.max_turns == 25
+    assert config.timeout_seconds == 600
+
+
+def test_global_backstops_keep_numeric_inputs() -> None:
+    config = SubagentsAppConfig(timeout_seconds="900", max_turns="40", max_total_per_run=8)
+    assert config.timeout_seconds == 900
+    assert config.max_turns == 40
+    assert config.max_total_per_run == 8
+
+
+@pytest.mark.parametrize("value", [True, False])
+def test_acp_timeout_rejects_booleans(value: bool) -> None:
+    with pytest.raises(ValidationError, match="must be an integer, not a boolean"):
+        ACPAgentConfig(command="cmd", description="d", timeout_seconds=value)
+
+
+def test_acp_timeout_keeps_numeric_input() -> None:
+    config = ACPAgentConfig(command="cmd", description="d", timeout_seconds="600")
+    assert config.timeout_seconds == 600