[WRONG BRANCH] fix(usage): bound incremental append reads

src/usage/log.ts

@@ -841,6 +841,10 @@ async function readUsageEntriesIncrementally(
     // A shrink means truncation or replacement-in-place; the retained rows may no
     // longer correspond to file contents, so refuse to extend them.
     if (size < retained.coveredThroughBytes) return null;
+    // Extending retained state is only an optimization; never let a large burst turn
+    // the bounded management read into an unbounded read of everything appended since
+    // the previous poll. A full read below will load only the requested tail window.
+    if (size - retained.coveredThroughBytes > maxReadBytes) return null;
     // Verify the retained REGION is unchanged before anything is reused. Identity keeps
     // dev/ino/birthtime, and an append and an in-place rewrite both move mtime/ctime
     // forward, so only the bytes themselves settle it.

tests/api-usage.test.ts

@@ -578,6 +578,35 @@ describe("GET /api/usage", () => {
     }
   });
 
+  test("an append burst larger than the byte window falls back to a bounded full read", async () => {
+    const now = Date.now();
+    const maxReadBytes = 512;
+    const row = (id: string): string => `${JSON.stringify({
+      requestId: id,
+      timestamp: now,
+      provider: "openai",
+      model: "gpt-5.5",
+      status: 200,
+      durationMs: 1,
+      usageStatus: "reported",
+      usage: { inputTokens: 1, outputTokens: 1 },
+      totalTokens: 2,
+    })}\n`;
+    const path = join(testDir, "usage.jsonl");
+    writeFileSync(path, row("seed"));
+
+    await usageLogModule.readUsageSnapshotForManagement(maxReadBytes);
+    const parsedBeforeBurst = usageReadCacheStatsForTests().parsedLines;
+    appendFileSync(path, Array.from({ length: 100 }, (_, index) => row(`burst-${index}`)).join(""));
+
+    const snapshot = await usageLogModule.readUsageSnapshotForManagement(maxReadBytes);
+    const stats = usageReadCacheStatsForTests();
+    expect(stats.fullReads).toBe(2);
+    expect(stats.tailReads).toBe(0);
+    expect(stats.parsedLines - parsedBeforeBurst).toBe(snapshot.entries.length);
+    expect(snapshot.entries.length).toBeLessThan(100);
+  });
+
   test("appends to an over-window ledger stay incremental and bounded", async () => {
     const now = Date.now();
     writeFixture(now);