fix(security): escape field names in transcript regex extraction

extractJsonStringFieldPrefix and extractJsonNullableStringFieldPrefix
interpolate the `field` parameter into `new RegExp(...)` without
escaping.  All current callers pass hardcoded strings ("id",
"parentId", "type", "role"), but the function signature accepts
any string.  A future caller passing a field containing regex
metacharacters (e.g. "foo.bar") would match unintended patterns.

Wrap the interpolation with escapeRegExp() from src/shared/regexp.ts
so metacharacters are treated literally.

Signed-off-by: Sebastien Tardif <sebtardif@ncf.ca>

src/gateway/session-utils.fs.test.ts

@@ -1982,6 +1982,39 @@ describe("oversized transcript line guards", () => {
     expectUsageFields(usage, { modelProvider: "test-provider" });
   });
 
+  test("oversized line metadata extraction preserves id and parentId", async () => {
+    const sessionId = "test-oversized-metadata-extract";
+    const transcriptPath = path.join(tmpDir, `${sessionId}.jsonl`);
+    const oversizedContent = "w".repeat(300 * 1024);
+    const lines = [
+      JSON.stringify({ type: "session", version: 3, id: sessionId }),
+      JSON.stringify({
+        type: "message",
+        id: "root-msg",
+        parentId: null,
+        message: { role: "user", content: "root" },
+      }),
+      JSON.stringify({
+        type: "message",
+        id: "oversized-child",
+        parentId: "root-msg",
+        message: { role: "assistant", content: oversizedContent },
+      }),
+    ];
+    fs.writeFileSync(transcriptPath, `${lines.join("\n")}\n`, "utf-8");
+
+    const out = await readRecentSessionMessagesAsync(sessionId, storePath, undefined, {
+      maxMessages: 10,
+    });
+
+    const serialized = JSON.stringify(out);
+    // The oversized line's id and parentId must be extracted correctly
+    // from the prefix via regex-based field extraction.
+    expect(serialized).toContain("oversized-child");
+    expect(serialized).toContain("root-msg");
+    expect(serialized).not.toContain(oversizedContent);
+  });
+
   test("readSessionTitleFieldsFromTranscriptAsync delegates to bounded sync reader", async () => {
     const sessionId = "test-async-title-bounded";
     writeTranscript(

src/gateway/session-utils.fs.ts

@@ -4,6 +4,7 @@ import { deriveSessionTotalTokens, hasNonzeroUsage, normalizeUsage } from "../ag
 import { jsonUtf8Bytes } from "../infra/json-utf8-bytes.js";
 import { hasInterSessionUserProvenance } from "../sessions/input-provenance.js";
 import { extractAssistantVisibleText } from "../shared/chat-message-content.js";
+import { escapeRegExp } from "../shared/regexp.js";
 import { normalizeLowercaseStringOrEmpty } from "../shared/string-coerce.js";
 import { estimateStringChars, estimateTokensFromChars } from "../utils/cjk-chars.js";
 import { stripInlineDirectiveTagsForDisplay } from "../utils/directive-tags.js";
@@ -273,7 +274,7 @@ function isOversizedTranscriptLine(line: string): boolean {
 }
 
 function extractJsonStringFieldPrefix(prefix: string, field: string): string | undefined {
-  const match = new RegExp(`"${field}"\\s*:\\s*"((?:\\\\.|[^"\\\\])*)"`).exec(prefix);
+  const match = new RegExp(`"${escapeRegExp(field)}"\\s*:\\s*"((?:\\\\.|[^"\\\\])*)"`).exec(prefix);
   if (!match) {
     return undefined;
   }
@@ -289,7 +290,7 @@ function extractJsonNullableStringFieldPrefix(
   prefix: string,
   field: string,
 ): string | null | undefined {
-  if (new RegExp(`"${field}"\\s*:\\s*null`).test(prefix)) {
+  if (new RegExp(`"${escapeRegExp(field)}"\\s*:\\s*null`).test(prefix)) {
     return null;
   }
   return extractJsonStringFieldPrefix(prefix, field);