diff --git a/internal/campaign/campaign.go b/internal/campaign/campaign.go
index 783264f9..8cf0dc82 100644
--- a/internal/campaign/campaign.go
+++ b/internal/campaign/campaign.go
@@ -123,6 +123,15 @@ func ParseYAML(data []byte) (*Campaign, error) {
 		if s.DependsOn == s.ID {
 			return nil, fmt.Errorf("campaign %q step %q: self-dependency", c.Name, s.ID)
 		}
+		// A when clause is evaluated against the predecessor's output, so it is
+		// only meaningful with a depends_on. Without one, the Runner never
+		// reaches the when check and the step runs UNCONDITIONALLY — an operator
+		// gating a destructive tool with `when:` would get a silently-inert
+		// guard (fail-open). Reject at load so the misconfiguration surfaces
+		// loudly instead of running the tool.
+		if s.When != "" && s.DependsOn == "" {
+			return nil, fmt.Errorf("campaign %q step %q: when clause requires depends_on (a when is evaluated against the predecessor's output; without one the step would run unconditionally)", c.Name, s.ID)
+		}
 	}
 
 	// Third pass: declaration-order check. The Runner iterates steps in
diff --git a/internal/campaign/campaign_test.go b/internal/campaign/campaign_test.go
index bd72858a..e27ea86c 100644
--- a/internal/campaign/campaign_test.go
+++ b/internal/campaign/campaign_test.go
@@ -513,3 +513,23 @@ func (r *recordingExecutor) Run(ctx context.Context, tool string, _ map[string]i
 	}
 	return "ok", nil
 }
+
+// TestParseYAML_RejectsWhenWithoutDependsOn pins the fail-closed fix for the
+// inert-gate bug: a `when` clause is evaluated against the predecessor's output,
+// so without a depends_on the Runner never reaches the when check and the step
+// runs UNCONDITIONALLY. An operator gating a destructive tool with `when:` would
+// otherwise get a silently-inert guard. Such a campaign must be rejected at load.
+func TestParseYAML_RejectsWhenWithoutDependsOn(t *testing.T) {
+	yamlDoc := `campaign: ungated-when
+steps:
+  - id: scan
+    tool: wifi_scan_ap
+  - id: gated
+    tool: wifi_sniff_pmkid
+    when: contains "AUTHORIZED"
+`
+	_, err := ParseYAML([]byte(yamlDoc))
+	if err == nil || !strings.Contains(err.Error(), "when clause requires depends_on") {
+		t.Errorf("a when without depends_on must be rejected (else the gate runs unconditionally): %v", err)
+	}
+}