fix(claude): scan large transcript records once

Retain unfinished record chunks and assemble each record once instead of copying and rescanning a growing buffer on every backward read. Cover import of a large record after the last explicit rename.

packages/app/e2e/browser/composer-recall.spec.ts

@@ -3,7 +3,7 @@ import {
   composerLocator,
   expectComposerDraft,
   expectComposerVisible,
-  submitMessage,
+  fillComposerDraft,
   typeIntoFocusedComposer,
 } from "../support/helpers/composer";
 import { openAgentRoute, seedMockAgentWorkspace } from "../support/helpers/mock-agent";
@@ -12,8 +12,14 @@ function userMessages(page: Page) {
   return page.getByTestId("user-message");
 }
 
+async function sendDraftWhenReady(page: Page) {
+  await expect(page.getByRole("button", { name: "Send message", exact: true })).toBeEnabled();
+  await pressInComposer(page, "Enter");
+}
+
 async function sendAndWaitForMessage(page: Page, text: string, expectedCount: number) {
-  await submitMessage(page, text);
+  await fillComposerDraft(page, text);
+  await sendDraftWhenReady(page);
   await expect(userMessages(page)).toHaveCount(expectedCount, { timeout: 30_000 });
   await expectComposerDraft(page, "");
 }
@@ -49,7 +55,7 @@ test.describe("Composer recall", () => {
       await pressInComposer(page, "ArrowUp");
       await expectComposerDraft(page, "add tests for login");
 
-      await pressInComposer(page, "Enter");
+      await sendDraftWhenReady(page);
       await expect(userMessages(page)).toHaveCount(3, { timeout: 30_000 });
       await expect(userMessages(page).last()).toContainText("add tests for login");
     } finally {

packages/server/src/server/agent/providers/claude/agent.import-titles.test.ts

@@ -1,3 +1,4 @@
+import { constants as bufferConstants } from "node:buffer";
 import * as fs from "node:fs/promises";
 import * as os from "node:os";
 import * as path from "node:path";
@@ -236,4 +237,64 @@ describe("importing a renamed Claude session", () => {
 
     await expect(importSession(sessionId)).rejects.toThrow();
   });
+
+  test("reads the rename from a transcript too large to hold as one string", async () => {
+    const sessionId = "oversized-transcript";
+    const projectDir = claudeProjectDirSync(SESSION_CWD, { configDir });
+    await fs.mkdir(projectDir, { recursive: true });
+    const transcript = path.join(projectDir, `${sessionId}.jsonl`);
+    // A hole stands in for the long session this used to fail on: the file only
+    // has to be longer than a string can be, and the rename sits at its end.
+    await fs.writeFile(transcript, "");
+    await fs.truncate(transcript, bufferConstants.MAX_STRING_LENGTH + 1);
+    await fs.appendFile(
+      transcript,
+      `\n${[
+        userRecord({ sessionId, content: "Trace the slow query" }),
+        { type: "custom-title", customTitle: "Slow query hunt", sessionId },
+      ]
+        .map((record) => JSON.stringify(record))
+        .join("\n")}\n`,
+    );
+
+    const imported = await importSession(sessionId);
+
+    expect(imported.config.title).toBe("Slow query hunt");
+  });
+
+  test("reads the rename from behind more records than one read covers", async () => {
+    const sessionId = "late-records-rename";
+    await writeSession({
+      sessionId,
+      records: [
+        userRecord({ sessionId, content: "Review this project" }),
+        { type: "custom-title", customTitle: "Scrollback review", sessionId },
+        // One record longer than a single read, then enough records to leave the
+        // rename several reads back from the end of the file.
+        userRecord({ sessionId, content: "x".repeat(128 * 1024) }),
+        ...Array.from({ length: 64 }, (_, index) =>
+          userRecord({ sessionId, content: `${"y".repeat(4 * 1024)} ${index}` }),
+        ),
+      ],
+    });
+
+    const imported = await importSession(sessionId);
+
+    expect(imported.config.title).toBe("Scrollback review");
+  });
+
+  test("imports a large record after the last rename without repeatedly copying it", async () => {
+    const sessionId = "large-record-after-rename";
+    await writeSession({
+      sessionId,
+      records: [
+        { type: "custom-title", customTitle: "Large attachment review", sessionId },
+        userRecord({ sessionId, content: "x".repeat(96 * 1024 * 1024) }),
+      ],
+    });
+
+    const imported = await importSession(sessionId);
+
+    expect(imported.config.title).toBe("Large attachment review");
+  });
 });

packages/server/src/server/agent/providers/claude/agent.ts

@@ -6853,40 +6853,79 @@ function normalizeClaudeSessionTitle(title: string | null): string | null {
 }
 
 const CLAUDE_CUSTOM_TITLE_TYPE = "custom-title";
+const CLAUDE_CUSTOM_TITLE_NEEDLE = Buffer.from(`"${CLAUDE_CUSTOM_TITLE_TYPE}"`);
+const CLAUDE_TITLE_SCAN_CHUNK_BYTES = 64 * 1024;
+
+function readClaudeCustomTitleFromRecord(record: Buffer): string | null {
+  // The needle also matches a prompt that quotes it, so a record still has to
+  // parse as a `custom-title` entry carrying a usable name. Records without the
+  // needle are never decoded, so a long one costs a byte search and nothing more.
+  if (!record.includes(CLAUDE_CUSTOM_TITLE_NEEDLE)) {
+    return null;
+  }
+  let parsed: unknown;
+  try {
+    parsed = JSON.parse(record.toString("utf8").trim());
+  } catch {
+    return null;
+  }
+  const entry = toObjectRecord(parsed);
+  if (!entry || entry.isSidechain || entry.type !== CLAUDE_CUSTOM_TITLE_TYPE) {
+    return null;
+  }
+  return normalizeClaudeSessionTitle(
+    typeof entry.customTitle === "string" ? entry.customTitle : null,
+  );
+}
 
 /**
  * The `custom-title` record is re-emitted every time the session is saved, so
- * the current name is the last usable one in the file. Searching backwards finds it without
- * walking the transcript, which matters because sessions reach tens of
- * megabytes.
+ * the current name is the last usable one in the file. Reading backwards a chunk
+ * at a time finds it without walking the transcript, which matters because
+ * sessions reach tens of megabytes, and never holds the file as one string, so a
+ * transcript past V8's maximum string length still imports.
  */
-function readLastClaudeCustomTitle(content: string): string | null {
-  const needle = `"${CLAUDE_CUSTOM_TITLE_TYPE}"`;
-  let at = content.lastIndexOf(needle);
-  while (at >= 0) {
-    const lineStart = content.lastIndexOf("\n", at) + 1;
-    const lineEnd = content.indexOf("\n", at);
-    const line = lineEnd === -1 ? content.slice(lineStart) : content.slice(lineStart, lineEnd);
-    let parsed: unknown;
-    try {
-      parsed = JSON.parse(line.trim());
-    } catch {
-      parsed = undefined;
-    }
-    const entry = toObjectRecord(parsed);
-    if (entry && !entry.isSidechain && entry.type === CLAUDE_CUSTOM_TITLE_TYPE) {
-      const title = normalizeClaudeSessionTitle(
-        typeof entry.customTitle === "string" ? entry.customTitle : null,
+async function readLastClaudeCustomTitle(filePath: string): Promise<string | null> {
+  const handle = await fsPromises.open(filePath, "r");
+  try {
+    const stats = await handle.stat();
+    if (!stats.isFile()) {
+      // Opening a directory succeeds on Windows even though reading it does not,
+      // so reject the way reading the transcript as a file would.
+      throw Object.assign(
+        new Error(`EISDIR: illegal operation on a directory, read '${filePath}'`),
+        { code: "EISDIR", path: filePath },
       );
-      if (title) {
-        return title;
-      }
     }
-    // The needle also matches a prompt that quotes it, and a record can carry no
-    // usable name, so keep looking backwards for one that does.
-    at = at <= 0 ? -1 : content.lastIndexOf(needle, at - 1);
+    let end = stats.size;
+    // Retain unfinished record pieces in reverse file order. Assemble each
+    // complete record once, so a large tool/image record costs linear work.
+    let pieces: Buffer[] = [];
+    while (end > 0) {
+      const length = Math.min(CLAUDE_TITLE_SCAN_CHUNK_BYTES, end);
+      const chunk = Buffer.alloc(length);
+      await handle.read(chunk, 0, length, end - length);
+      let recordEnd = chunk.length;
+      for (
+        let at = chunk.lastIndexOf(10);
+        at >= 0;
+        at = at === 0 ? -1 : chunk.lastIndexOf(10, at - 1)
+      ) {
+        pieces.push(chunk.subarray(at + 1, recordEnd));
+        const record = Buffer.concat(pieces.toReversed());
+        const title = readClaudeCustomTitleFromRecord(record);
+        if (title) return title;
+        pieces = [];
+        recordEnd = at;
+      }
+      if (recordEnd > 0) pieces.push(chunk.subarray(0, recordEnd));
+      end -= length;
+    }
+    // The first record in the file has no newline in front of it.
+    return readClaudeCustomTitleFromRecord(Buffer.concat(pieces.toReversed()));
+  } finally {
+    await handle.close();
   }
-  return null;
 }
 
 function isErrnoException(error: unknown): error is NodeJS.ErrnoException {
@@ -6898,9 +6937,8 @@ async function readClaudeRenamedSessionTitle(input: {
   sessionId: string;
   configDir: string;
 }): Promise<string | null> {
-  let content: string;
   try {
-    content = await fsPromises.readFile(await claudeTranscriptPath(input), "utf8");
+    return await readLastClaudeCustomTitle(await claudeTranscriptPath(input));
   } catch (error) {
     // A session with no transcript on disk has no rename to carry over. Every
     // other read failure is a real fault and stays explicit.
@@ -6909,7 +6947,6 @@ async function readClaudeRenamedSessionTitle(input: {
     }
     throw error;
   }
-  return readLastClaudeCustomTitle(content);
 }
 
 function normalizeImportablePromptPreview(text: string): string | null {