diff --git a/internal/campaign/edge_case_detector_test.go b/internal/campaign/edge_case_detector_test.go
index 30bceb09..ebf5e648 100644
--- a/internal/campaign/edge_case_detector_test.go
+++ b/internal/campaign/edge_case_detector_test.go
@@ -4,6 +4,7 @@ import (
 	"context"
 	"strings"
 	"testing"
+	"math"
 	"time"
 )
 
@@ -326,6 +327,19 @@ func TestEdgeCaseAnalysis_FileCategories(t *testing.T) {
 // Expected behavior: The serial O(N) iteration over facts per file causes an O(N * M) performance cliff.
 // Test should define boundaries of acceptable latency.
 
-// TODO: Missing Edge Case - Extreme Values: Max file size boundaries.
-// Test determineAction with `LineCount = math.MaxInt32`.
-// Expected behavior: Should cleanly suggest ActionModularize without overflow in heuristics (e.g., complexity calc).
+func TestEdgeCaseDetector_DetermineAction_MaxLineCount(t *testing.T) {
+	detector := NewEdgeCaseDetector(nil, nil)
+	decision := FileDecision{
+		LineCount: math.MaxInt32,
+	}
+
+	action, reasoning := detector.determineAction(decision)
+
+	if action != ActionModularize {
+		t.Errorf("expected %v, got %v", ActionModularize, action)
+	}
+
+	if reasoning == "" {
+		t.Errorf("expected reasoning to not be empty")
+	}
+}
diff --git a/test_plan.txt b/test_plan.txt
new file mode 100644
index 00000000..932b6789
--- /dev/null
+++ b/test_plan.txt
@@ -0,0 +1,6 @@
+1. Read the TODO at the bottom of `internal/campaign/edge_case_detector_test.go` related to `Max file size boundaries`.
+2. Add a test function `TestEdgeCaseDetector_DetermineAction_MaxLineCount` to `internal/campaign/edge_case_detector_test.go`.
+3. In the test, instantiate `EdgeCaseDetector`.
+4. Create a `FileDecision` with `LineCount = math.MaxInt32`.
+5. Call `determineAction(decision)` on the detector.
+6. Assert that the returned action is `ActionModularize`.