fix(history): raise the transcript head-read window to fit restart bookkeeping

Blank firstPrompt rows weren't all oversized messages -- traced one
directly: a session restarted many times (mux deaths, redeploys)
accumulates a batch of small bookkeeping lines (mode/permission-mode/
last-prompt/queue-operation, one batch per restart) ahead of the real
first message. With enough restarts these alone crossed the old 16KB
head-read window, so extraction found nothing even though the actual
first message was tiny (measured case: ~17.5KB of bookkeeping pushed a
189-byte real message just past the boundary).

Raise the head buffer from 16KB to 128KB (matching the existing
precedent at the codex-history head-read a few hundred lines up) and
fix three now-stale `> 16384`/`> 65536` fallback thresholds to
reference headBuf.length instead of hardcoded numbers, so the tail-read
fallbacks stay correctly scoped to "beyond what head already covered."

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

src/web/routes/session-routes.ts

@@ -2760,7 +2760,7 @@ export function registerSessionRoutes(
 
       let foundContent = head ? hasConversation(head) : false;
       let tail: string | null = null;
-      if (!foundContent && fileStat.size > 16384) {
+      if (!foundContent && fileStat.size > headBuf.length) {
         const tailBuf = Buffer.alloc(32768);
         tail = await readFileTail(filePath, tailBuf, fileStat.size);
         if (tail) foundContent = hasConversation(tail);
@@ -2768,7 +2768,7 @@ export function registerSessionRoutes(
       if (!foundContent) continue;
 
       if (head) firstPrompt = extractFirstUserPrompt(head);
-      if (!firstPrompt && fileStat.size > 65536) {
+      if (!firstPrompt && fileStat.size > headBuf.length) {
         if (!tail) {
           const tailBuf = Buffer.alloc(32768);
           tail = await readFileTail(filePath, tailBuf, fileStat.size);
@@ -2778,9 +2778,9 @@ export function registerSessionRoutes(
 
       // COD-145: last (most recent) user prompt lives near the END of the file, so
       // prefer the tail. For large files where no tail was read yet, read one
-      // (mirrors the firstPrompt > 65536 block). Small files fit in `head`, which
-      // then contains the whole transcript — scan it for the last match instead.
-      if (!tail && fileStat.size > 65536) {
+      // (mirrors the firstPrompt > headBuf.length block). Small files fit in `head`,
+      // which then contains the whole transcript — scan it for the last match instead.
+      if (!tail && fileStat.size > headBuf.length) {
         const tailBuf = Buffer.alloc(32768);
         tail = await readFileTail(filePath, tailBuf, fileStat.size);
       }
@@ -2819,7 +2819,14 @@ export function registerSessionRoutes(
   app.get('/api/history/sessions', async (req) => {
     const query = req.query as { projectKey?: string; offset?: string; limit?: string };
     const projectsDir = join(process.env.HOME || '/tmp', '.claude', 'projects');
-    const headBuf = Buffer.alloc(16384);
+    // 128KB (was 16KB): a session restarted many times over the course of a long
+    // conversation accumulates small bookkeeping lines (mode/permission-mode/
+    // last-prompt/queue-operation, one batch per restart) ahead of the real first
+    // message. 16KB was enough margin for a handful of restarts but not dozens —
+    // a genuinely tiny first message still came up blank because the metadata
+    // alone crossed the window. 128KB matches the existing precedent elsewhere in
+    // this file (line ~1431) rather than inventing a new size.
+    const headBuf = Buffer.alloc(131072);
     // Multi-user: this scans the host-wide ~/.claude/projects tree, so a non-admin
     // must only see history whose decoded workingDir is inside their own case space.
     // Do NOT trust the caller-supplied projectKey — confine on the decoded path.
@@ -2926,7 +2933,8 @@ export function registerSessionRoutes(
     const history: HistoryInput[] = [];
     try {
       const projectsDir = join(process.env.HOME || '/tmp', '.claude', 'projects');
-      const headBuf = Buffer.alloc(16384);
+      // 128KB (was 16KB) — see the sibling allocation above for why.
+      const headBuf = Buffer.alloc(131072);
       const projectDirs = await fs.readdir(projectsDir);
       for (const projDir of projectDirs) {
         const projPath = join(projectsDir, projDir);

test/routes/session-routes.test.ts

@@ -1395,6 +1395,40 @@ describe('session-routes', () => {
       expect(ids).toContain(noEntrypointId);
       expect(ids).not.toContain(sdkId);
     });
+
+    it('finds the real first prompt past a large run of pre-message bookkeeping lines', async () => {
+      // A session restarted many times over a long conversation accumulates a batch
+      // of small bookkeeping lines (mode/permission-mode/last-prompt/queue-operation)
+      // per restart, ahead of the real first message. With enough restarts these can
+      // push the genuine first prompt past a 16KB head-read window even though the
+      // message itself is tiny — the row showed up blank despite having real content.
+      const home = process.env.HOME as string;
+      const projPath = join(home, '.claude', 'projects', 'proj-bookkeeping-test');
+      await mkdir(projPath, { recursive: true });
+
+      const sessionId = '66666666-6666-6666-6666-666666666666';
+      const bookkeepingLine = JSON.stringify({ type: 'mode', mode: 'normal', sessionId }) + '\n';
+      // > 16KB (the old head-read size) but well under 128KB (the new one).
+      const prefix = bookkeepingLine.repeat(Math.ceil(20000 / bookkeepingLine.length));
+      const realLine =
+        JSON.stringify({
+          type: 'user',
+          entrypoint: 'cli',
+          message: { role: 'user', content: 'the real first message' },
+        }) + '\n';
+      expect(prefix.length).toBeGreaterThan(16384);
+
+      await writeFile(join(projPath, `${sessionId}.jsonl`), prefix + realLine);
+
+      const res = await harness.app.inject({
+        method: 'GET',
+        url: '/api/history/sessions?projectKey=proj-bookkeeping-test',
+      });
+      expect(res.statusCode).toBe(200);
+      const row = JSON.parse(res.body).data.sessions.find((s: { sessionId: string }) => s.sessionId === sessionId);
+      expect(row).toBeDefined();
+      expect(row.firstPrompt).toBe('the real first message');
+    });
   });
 
   // ========== POST /api/sessions (with resumeSessionId) ==========