diff --git a/tests/integration/idle-costs-nothing.test.ts b/tests/integration/idle-costs-nothing.test.ts
index 2374627e..7e664a28 100644
--- a/tests/integration/idle-costs-nothing.test.ts
+++ b/tests/integration/idle-costs-nothing.test.ts
@@ -70,8 +70,22 @@ describe('an idle brain costs nothing', () => {
     await brain.flush()
 
     const logged: string[] = []
+    // The STACK behind each narration, kept beside the line it belongs to.
+    // vitest tags a stdout block with the test that is RUNNING, not the brain
+    // that wrote it, so teeing these lines through would only ever name this
+    // test. The call stack does name the driver: `kickBackgroundFlush('idle')`
+    // under `armIdleFlushTimer` is a cadence flush on some brain, the deferred-
+    // embed worker's commit path is a brain still landing vectors, and a bare
+    // `flush()` is an explicit caller. That distinction is the whole question.
+    const stacks: string[] = []
     const origLog = console.log
-    console.log = ((...a: unknown[]) => { logged.push(a.map(String).join(' ')) }) as typeof console.log
+    console.log = ((...a: unknown[]) => {
+      const line = a.map(String).join(' ')
+      logged.push(line)
+      if (/All indexes flushed to disk|Flushing Brainy indexes/.test(line)) {
+        stacks.push(new Error('flush narration').stack ?? '(no stack)')
+      }
+    }) as typeof console.log
 
     // Watch the providers directly: a flush that runs calls all of them.
     const storage = (brain as unknown as { storage: { flushCounts: () => Promise<void> } }).storage
@@ -113,10 +127,12 @@ describe('an idle brain costs nothing', () => {
     )
     expect(
       flushChatter,
-      `a flush narrated during the ${IDLE_WATCH_MS}ms idle window. This brain's own ` +
-        `providers were NOT called (asserted above), so the lines below were printed by ` +
-        `another brain alive in this process — find it by the 'stdout | <file> > <test>' ` +
-        `prefix in the run log:\n${flushChatter.join('\n')}`
+      `${flushChatter.length} flush line(s) narrated during the ${IDLE_WATCH_MS}ms idle ` +
+        `window. This brain's own providers were NOT called (asserted above), so another ` +
+        `brain alive in this process printed them — the suite runs every file in ONE ` +
+        `process and 67 test files create more brains than they close.\n` +
+        `${flushChatter.join('\n')}\n\n` +
+        `The stack behind the first one names the driver:\n${stacks[0] ?? '(none captured)'}`
     ).toEqual([])
   }, 180_000)