diff --git a/backend/internal/platform/runtime_policy_agent_limits_test.go b/backend/internal/platform/runtime_policy_agent_limits_test.go
new file mode 100644
index 000000000..32f5c604b
--- /dev/null
+++ b/backend/internal/platform/runtime_policy_agent_limits_test.go
@@ -0,0 +1,19 @@
+package platform
+
+import "testing"
+
+func TestAgentStepTimeoutEnvironmentBounds(t *testing.T) {
+	for _, tc := range []struct {
+		value string
+		want  int
+	}{
+		{"-1", 0}, {"0", 0}, {"1", 30}, {"29", 30}, {"30", 30}, {"90", 90}, {"3601", 3600},
+	} {
+		t.Run(tc.value, func(t *testing.T) {
+			t.Setenv("CANVAS_AGENT_STEP_TIMEOUT_SECONDS", tc.value)
+			if got := agentStepTimeoutSecondsDefault(); got != tc.want {
+				t.Fatalf("timeout = %d, want %d", got, tc.want)
+			}
+		})
+	}
+}