@@ -134,9 +134,10 @@ permanent home in the tooling, not a gate.
 
 - **`dominatorSummary`** — the dominator tree of one entry's eager-load closure over VALUE edges
   only (dynamic imports are a deliberate cold-start seam, so they never join the closure).
-  `reachableFiles` is the eager closure size; `bottlenecks` ranks every reachable file by how many
-  files become unreachable if it disappeared — the size of the branch moving it behind `import()`
-  would cut. Defaults to `src/daemon.ts`; override with `pnpm depgraph --dominator-entry <path>`.
+  `reachableFiles` is the eager closure size; `bottlenecks` ranks every reachable non-entry file
+  by how many files become unreachable if it disappeared — the size of the branch moving it
+  behind `import()` would cut. Defaults to `src/daemon.ts`; override with
+  `pnpm depgraph --dominator-entry <path>`.
 - **`zoneSccSummary`** — strongly connected components of the zone graph, built from VALUE zone
   pairs only (`zoneEdges` entries with `valueCount > 0`), the same edge kind R4 keeps acyclic at
   file level. R4 is file-level; nothing stops a loop from closing once files collapse into zones —
 
@@ -187,10 +187,16 @@ function formatSummary(payload: Payload, jsonPath: string): string {
   );
 }
 
-/** A flag's value, or `undefined` when the flag is absent or trailing with nothing after it. */
+/**
+ * A flag's value, or `undefined` when the flag is absent, trailing with nothing after it, or
+ * given an empty string — the last case matters because `?? <default>` only catches nullish
+ * values, and an empty `--out ""` must fall back the same way a missing one does, not resolve to
+ * the current directory and crash the subsequent write with EISDIR.
+ */
 function flagValue(argv: readonly string[], flag: string): string | undefined {
   const index = argv.indexOf(flag);
-  return index >= 0 ? argv[index + 1] : undefined;
+  const value = index >= 0 ? argv[index + 1] : undefined;
+  return value ? value : undefined;
 }
 
 async function main(argv: readonly string[]): Promise<number> {
 
@@ -16,10 +16,11 @@ export const VALUE_EDGES: ReadonlySet<EdgeKind> = new Set(['value']);
 export const EXECUTABLE_EDGES: ReadonlySet<EdgeKind> = new Set(['value', 'dynamic']);
 
 /**
- * Every collapsed edge kind, for the structural questions ("does anything reference this file
- * at all") that community detection and zone-level cycle reporting ask. Community algorithms
- * treat the graph as undirected regardless of kind, so mixing kinds in is the right input, not
- * a loosening of a rule — there is no gate riding on this set.
+ * Every collapsed edge kind, for the structural question ("does anything reference this file at
+ * all") community detection asks (`computeCohesionSummary`). Community algorithms treat the graph
+ * as undirected regardless of kind, so mixing kinds in is the right input, not a loosening of a
+ * rule — there is no gate riding on this set. Zone-level cycle reporting (`computeZoneSccSummary`)
+ * does not use this set: it builds its own zone graph from VALUE zone pairs only, to mirror R4.
  */
 export const ALL_EDGES: ReadonlySet<EdgeKind> = new Set(['value', 'type', 'dynamic']);
 
 
@@ -504,10 +504,21 @@ test('build.ts writes the default path and a summary consistent with the JSON',
       `dominator summary for src/daemon\\.ts: ${payload.dominatorSummary.reachableFiles} of ${payload.dominatorSummary.totalFiles} files loaded eagerly`,
     ),
   );
-  assert.match(stdout, /zone-level SCCs: \d+ cyclic group\(s\) across \d+ zones/);
-  assert.match(
+  // Tied to the exact payload numbers (not a shape-only `\d+` regex) so a CLI summary that
+  // drifts from the JSON it was derived from — wrong count, wrong rounding — fails this test.
+  assert.ok(
+    stdout.includes(
+      `zone-level SCCs: ${payload.zoneSccSummary.components.length} cyclic group(s) across ` +
+        `${payload.zoneSccSummary.zones} zones`,
+    ),
+    stdout,
+  );
+  assert.ok(
+    stdout.includes(
+      `modularity: declared zones ${payload.cohesionSummary.modularity.declaredZones.toFixed(3)}, ` +
+        `detected communities ${payload.cohesionSummary.modularity.detectedCommunities.toFixed(3)}`,
+    ),
     stdout,
-    /modularity: declared zones -?\d+\.\d{3}, detected communities -?\d+\.\d{3}/,
   );
 });
 
@@ -543,3 +554,18 @@ test('build.ts falls back to the default path when --out has no value', () => {
   assert.equal(status, 0, stdout);
   assert.ok(stdout.includes('.tmp/depgraph/graph.json'), stdout);
 });
+
+test('build.ts falls back to the default path when --out is given an empty string', () => {
+  // `--out ""` must fall back the same way a missing value does. `flagValue` returning `''`
+  // instead of `undefined` here would skip the `??` default and resolve to the current
+  // directory, crashing the write with EISDIR instead of writing the default report.
+  const { status, stdout } = runBuild(['--out', '']);
+  assert.equal(status, 0, stdout);
+  assert.ok(stdout.includes('.tmp/depgraph/graph.json'), stdout);
+});
+
+test('build.ts falls back to the default dominator entry when --dominator-entry is given an empty string', () => {
+  const { status, stdout } = runBuild(['--dominator-entry', '']);
+  assert.equal(status, 0, stdout);
+  assert.ok(stdout.includes('dominator summary for src/daemon.ts:'), stdout);
+});