diff --git a/packages/adapter-utils/src/usage-checkpoint.test.ts b/packages/adapter-utils/src/usage-checkpoint.test.ts
index 2b622ef60f0..a158f587c15 100644
--- a/packages/adapter-utils/src/usage-checkpoint.test.ts
+++ b/packages/adapter-utils/src/usage-checkpoint.test.ts
@@ -29,6 +29,29 @@ describe("usage checkpoint stream", () => {
     expect(parser.mock.calls[0][0]).not.toContain("private content");
   });
 
+  it("bounds large content strings before buffering and preserves usage on either side", async () => {
+    const saved = vi.fn(); const parser = vi.fn(createParser()); const output = vi.fn();
+    const log = createUsageCheckpointLog(output, saved, parser);
+    const line = JSON.stringify({ type: "result", usage: { inputTokens: 2, costUsd: 0.25 }, messages: [{ content: '"\\\n'.repeat(2 * 1024 * 1024) }], model: "after-large-content" });
+    for (let offset = 0; offset < line.length; offset += 4093) await log("stdout", line.slice(offset, offset + 4093));
+    await log.flush();
+    expect(saved).toHaveBeenLastCalledWith(expect.objectContaining({ costUsd: 0.25, complete: true }));
+    expect(parser.mock.calls[0][0]).toContain("after-large-content");
+    expect(parser.mock.calls[0][0].length).toBeLessThan(1000);
+    expect(output.mock.calls.map(call => call[1]).join("")).toBe(line);
+  });
+
+  it("preserves Unicode and escaped quotes across the string limit and chunk boundaries", async () => {
+    for (const suffix of ['\\u1234', '\\"', '\\\\']) {
+      const saved = vi.fn(); const parser = vi.fn(createParser());
+      const log = createUsageCheckpointLog(vi.fn(), saved, parser);
+      const line = '{"type":"result","text":"' + 'a'.repeat(249) + suffix + 'discarded","usage":{"costUsd":0.25}}\n';
+      for (const character of line) await log("stdout", character);
+      await log.flush();
+      expect(saved).toHaveBeenCalledWith(expect.objectContaining({ costUsd: 0.25 }));
+    }
+  });
+
   it("surfaces a persistence failure outside the local process's best-effort log path", async () => {
     const failure = new Error("Receipt storage unavailable");
     const output = vi.fn(); const saved = vi.fn().mockRejectedValue(failure);
diff --git a/packages/adapter-utils/src/usage-checkpoint.ts b/packages/adapter-utils/src/usage-checkpoint.ts
index 1e0c44bd86e..123ae66615e 100644
--- a/packages/adapter-utils/src/usage-checkpoint.ts
+++ b/packages/adapter-utils/src/usage-checkpoint.ts
@@ -24,6 +24,41 @@ function project(value: unknown, depth = 0, field = ""): unknown {
       : project(entry, depth + 1, key)]));
 }
 
+/** Bound strings before buffering whole JSON lines. Provider records can carry
+ * many MiB of conversation text alongside a small usage object. Preserve JSON
+ * escapes across chunks so discarded text cannot corrupt the accounting fields. */
+function createStringCompactor() {
+  let inString = false, length = 0, escapeRemaining = 0;
+  let escapeCode = false, keepEscape = false;
+  return (chunk: string) => {
+    let output = "";
+    for (const character of chunk) {
+      if (character === "\n") {
+        inString = false; escapeRemaining = 0; escapeCode = false;
+        output += character;
+      } else if (!inString) {
+        output += character;
+        if (character === '"') { inString = true; length = 0; }
+      } else if (escapeRemaining > 0) {
+        if (keepEscape) output += character;
+        escapeRemaining = escapeCode && character === "u" ? 4 : escapeRemaining - 1;
+        escapeCode = false;
+        length++;
+      } else if (character === '"') {
+        output += character; inString = false;
+      } else if (character === "\\") {
+        keepEscape = length < 250;
+        if (keepEscape) output += character;
+        escapeRemaining = 1; escapeCode = true; length++;
+      } else {
+        if (length < 250) output += character;
+        length++;
+      }
+    }
+    return output;
+  };
+}
+
 /** The consumer incrementally parses only new records (never the full history).
  * A fresh instance belongs to one CLI attempt. Call flush() after its process
  * exits, before retrying or final result handling. Checkpoint failures are
@@ -34,6 +69,7 @@ export function createUsageCheckpointLog(
   consume: (records: string) => AdapterUsageCheckpoint | null,
 ) {
   const attemptId = randomUUID();
+  const compactStrings = createStringCompactor();
   let remainder = "", accounting = "", previous = "";
   let snapshot: AdapterUsageCheckpoint | null = null;
   let failure: unknown;
@@ -63,7 +99,7 @@ export function createUsageCheckpointLog(
   const log = async (stream: "stdout" | "stderr", chunk: string) => {
     if (onUsage && !failed && stream === "stdout") {
       try {
-        remainder += chunk;
+        remainder += compactStrings(chunk);
         const lines = remainder.split("\n"); remainder = lines.pop() ?? "";
         if (remainder.length > 8 * 1024 * 1024) throw new Error("Accounting protocol line exceeds 8 MiB");
         for (const line of lines) retain(line);
diff --git a/server/src/__tests__/adapter-cost-failure-paths.test.ts b/server/src/__tests__/adapter-cost-failure-paths.test.ts
index bde2346f785..b831a1e0476 100644
--- a/server/src/__tests__/adapter-cost-failure-paths.test.ts
+++ b/server/src/__tests__/adapter-cost-failure-paths.test.ts
@@ -98,10 +98,12 @@ describe("CLI adapter accounting on timeout", () => {
       }
       const end = fixture.name === "pi" && !timedOut ? '\n{"type":"agent_end","messages":[]}' : "";
       const first = JSON.stringify(early) + "\n";
-      const noise = JSON.stringify({ type: "text", part: { text: "x".repeat(5 * 1024 * 1024) } }) + "\n";
+      const noise = JSON.stringify({ type: "text", part: { text: "x".repeat(9 * 1024 * 1024) } }) + "\n";
       const last = JSON.stringify(fixture.event) + end;
       processResult.mockImplementation(async (_run, _target, _command, _args, options) => {
-        for (const chunk of [first, noise, last]) await options.onLog("stdout", chunk);
+        for (const record of [first, noise, last]) {
+          for (let offset = 0; offset < record.length; offset += 65521) await options.onLog("stdout", record.slice(offset, offset + 65521));
+        }
         return { exitCode: timedOut ? null : 0, signal: timedOut ? "SIGTERM" : null, timedOut, stdout: (first + noise + last).slice(-4 * 1024 * 1024), stderr: "", pid: 123, startedAt: new Date().toISOString() };
       });
       const onUsage = checkpoint ? vi.fn() : undefined;