diff --git a/sdk/typescript/README.md b/sdk/typescript/README.md
index 9e1f4a51..3ae5bb25 100644
--- a/sdk/typescript/README.md
+++ b/sdk/typescript/README.md
@@ -380,7 +380,9 @@ including cached input and cache writes; fees and surcharges are not included.
 
 Use `--max-cost USD` to stop a scan, including its delegated workers, when its
 running cost exceeds the limit. Partial results are preserved. Requests
-already in progress can finish above the limit.
+already in progress can finish above the limit. Cost tracking accepts Codex
+session events up to 1 MiB; an oversized event stops the scan because its
+running cost can no longer be verified safely.
 
 Run `npx @openai/codex-security scan --help` or `npx @openai/codex-security bulk-scan --help`
 for the complete CLI references.
diff --git a/sdk/typescript/src/cost.ts b/sdk/typescript/src/cost.ts
index 912ccd27..1481e9dc 100644
--- a/sdk/typescript/src/cost.ts
+++ b/sdk/typescript/src/cost.ts
@@ -28,7 +28,9 @@ interface ScanTokenUsage {
 
 interface SessionUsage {
   offset: number;
-  remainder: Buffer;
+  pendingLine: Buffer[];
+  pendingLineBytes: number;
+  unreadable: boolean;
   threadId: string | null;
   parentThreadId: string | null;
   usage: ScanTokenUsage | null;
@@ -56,6 +58,7 @@ const MODEL_PRICING_NANODOLLARS: Readonly<Record<string, ModelPricing>> = {
 
 const COST_POLL_INTERVAL_MS = 100;
 const SESSION_READ_SIZE = 64 * 1_024;
+const MAX_SESSION_EVENT_BYTES = 1 * 1_024 * 1_024;
 
 export class ScanCostTracker {
   readonly #options: ScanCostTrackerOptions;
@@ -115,7 +118,9 @@ export class ScanCostTracker {
       if (session === undefined) {
         session = {
           offset: 0,
-          remainder: Buffer.alloc(0),
+          pendingLine: [],
+          pendingLineBytes: 0,
+          unreadable: false,
           threadId: null,
           parentThreadId: null,
           usage: null,
@@ -187,6 +192,7 @@ async function readSessionUsage(
   path: string,
   session: SessionUsage,
 ): Promise<void> {
+  if (session.unreadable) return;
   let file;
   try {
     file = await open(path, "r");
@@ -205,27 +211,54 @@ async function readSessionUsage(
       );
       if (bytesRead === 0) return;
       session.offset += bytesRead;
-      const contents = Buffer.concat([
-        session.remainder,
-        buffer.subarray(0, bytesRead),
-      ]);
-      let lineStart = 0;
-      while (true) {
-        const lineEnd = contents.indexOf(0x0a, lineStart);
-        if (lineEnd === -1) break;
-        readSessionEvent(
-          contents.subarray(lineStart, lineEnd).toString("utf8"),
-          session,
-        );
-        lineStart = lineEnd + 1;
+      try {
+        readSessionChunk(buffer.subarray(0, bytesRead), session);
+      } catch (error) {
+        session.unreadable = true;
+        session.pendingLine = [];
+        session.pendingLineBytes = 0;
+        throw error;
       }
-      session.remainder = Buffer.from(contents.subarray(lineStart));
     }
   } finally {
     await file.close();
   }
 }
 
+function readSessionChunk(contents: Buffer, session: SessionUsage): void {
+  let lineStart = 0;
+  while (lineStart < contents.length) {
+    const newline = contents.indexOf(0x0a, lineStart);
+    const lineEnd = newline === -1 ? contents.length : newline;
+    const fragment = contents.subarray(lineStart, lineEnd);
+    const lineBytes = session.pendingLineBytes + fragment.length;
+    if (lineBytes > MAX_SESSION_EVENT_BYTES) {
+      throw new Error("Codex session event exceeds the 1 MiB safety limit.");
+    }
+
+    if (newline === -1) {
+      if (fragment.length > 0) {
+        session.pendingLine.push(Buffer.from(fragment));
+        session.pendingLineBytes = lineBytes;
+      }
+      return;
+    }
+
+    if (session.pendingLineBytes === 0) {
+      readSessionEvent(fragment.toString("utf8"), session);
+    } else {
+      if (fragment.length > 0) session.pendingLine.push(Buffer.from(fragment));
+      readSessionEvent(
+        Buffer.concat(session.pendingLine, lineBytes).toString("utf8"),
+        session,
+      );
+      session.pendingLine = [];
+      session.pendingLineBytes = 0;
+    }
+    lineStart = newline + 1;
+  }
+}
+
 function readSessionEvent(line: string, session: SessionUsage): void {
   if (line.length === 0) return;
   let event: unknown;
diff --git a/sdk/typescript/tests-ts/cost.test.ts b/sdk/typescript/tests-ts/cost.test.ts
index 5e7d588b..9647d950 100644
--- a/sdk/typescript/tests-ts/cost.test.ts
+++ b/sdk/typescript/tests-ts/cost.test.ts
@@ -226,6 +226,62 @@ describe("live scan cost tracking", () => {
     });
   });
 
+  test("retains a bounded partial event across incremental reads", async () => {
+    const home = await codexHome();
+    const path = await writeSession(home, "scan-thread", {
+      input_tokens: 100,
+      output_tokens: 10,
+    });
+    const tracker = new ScanCostTracker({
+      codexHome: home,
+      model: "gpt-5.6-terra",
+    });
+    tracker.start("scan-thread");
+    await tracker.refresh();
+
+    const event = JSON.stringify({
+      type: "event_msg",
+      payload: {
+        type: "token_count",
+        info: {
+          total_token_usage: { input_tokens: 250, output_tokens: 20 },
+        },
+      },
+    });
+    const padding = " ".repeat(128 * 1_024);
+    await appendFile(path, `${padding}${event.slice(0, 40)}`);
+    expect((await tracker.refresh()).cost?.inputTokens).toBe(100);
+
+    await appendFile(path, `${event.slice(40)}\n`);
+    expect((await tracker.stop()).cost?.inputTokens).toBe(250);
+  });
+
+  test("rejects and quarantines a session event larger than 1 MiB", async () => {
+    const home = await codexHome();
+    const path = await writeSession(home, "scan-thread", {
+      input_tokens: 100,
+      output_tokens: 10,
+    });
+    await appendFile(path, "x".repeat(1 * 1_024 * 1_024 + 1));
+    const tracker = new ScanCostTracker({
+      codexHome: home,
+      model: "gpt-5.6-terra",
+    });
+    tracker.start("scan-thread");
+
+    await expect(tracker.refresh()).rejects.toThrow(
+      "Codex session event exceeds the 1 MiB safety limit.",
+    );
+    expect((await tracker.refresh()).cost).toEqual({
+      model: "gpt-5.6-terra",
+      inputTokens: 100,
+      cachedInputTokens: 0,
+      cacheWriteInputTokens: 0,
+      outputTokens: 10,
+      estimatedUsd: 0.0004,
+    });
+  });
+
   test("reports a changed running cost only once", async () => {
     const home = await codexHome();
     await writeSession(home, "scan-thread", {