diff --git a/src/agents/tools/sessions-history-tool.ts b/src/agents/tools/sessions-history-tool.ts
index 90261c7ac26..18d9576f0b2 100644
--- a/src/agents/tools/sessions-history-tool.ts
+++ b/src/agents/tools/sessions-history-tool.ts
@@ -2,6 +2,7 @@ import { Type } from "@sinclair/typebox";
 import { loadConfig } from "../../config/config.js";
 import { callGateway } from "../../gateway/call.js";
 import { capArrayByJsonBytes } from "../../gateway/session-utils.fs.js";
+import { jsonUtf8Bytes } from "../../infra/json-utf8-bytes.js";
 import { redactSensitiveText } from "../../logging/redact.js";
 import { truncateUtf16Safe } from "../../utils.js";
 import type { AnyAgentTool } from "./common.js";
@@ -140,14 +141,6 @@ function sanitizeHistoryMessage(message: unknown): {
   return { message: entry, truncated, redacted };
 }
 
-function jsonUtf8Bytes(value: unknown): number {
-  try {
-    return Buffer.byteLength(JSON.stringify(value), "utf8");
-  } catch {
-    return Buffer.byteLength(String(value), "utf8");
-  }
-}
-
 function enforceSessionsHistoryHardCap(params: {
   items: unknown[];
   bytes: number;
diff --git a/src/gateway/server-methods/chat.ts b/src/gateway/server-methods/chat.ts
index e5202392a36..d1c585df18f 100644
--- a/src/gateway/server-methods/chat.ts
+++ b/src/gateway/server-methods/chat.ts
@@ -9,6 +9,7 @@ import { createReplyDispatcher } from "../../auto-reply/reply/reply-dispatcher.j
 import type { MsgContext } from "../../auto-reply/templating.js";
 import { createReplyPrefixOptions } from "../../channels/reply-prefix.js";
 import { resolveSessionFilePath } from "../../config/sessions.js";
+import { jsonUtf8Bytes } from "../../infra/json-utf8-bytes.js";
 import { resolveSendPolicy } from "../../sessions/send-policy.js";
 import {
   stripInlineDirectiveTagsForDisplay,
@@ -198,14 +199,6 @@ function sanitizeChatHistoryMessages(messages: unknown[]): unknown[] {
   return changed ? next : messages;
 }
 
-function jsonUtf8Bytes(value: unknown): number {
-  try {
-    return Buffer.byteLength(JSON.stringify(value), "utf8");
-  } catch {
-    return Buffer.byteLength(String(value), "utf8");
-  }
-}
-
 function buildOversizedHistoryPlaceholder(message?: unknown): Record<string, unknown> {
   const role =
     message &&
diff --git a/src/gateway/session-utils.fs.ts b/src/gateway/session-utils.fs.ts
index 53be7392d10..3712c8c8272 100644
--- a/src/gateway/session-utils.fs.ts
+++ b/src/gateway/session-utils.fs.ts
@@ -10,6 +10,7 @@ import {
   resolveSessionTranscriptPathInDir,
 } from "../config/sessions.js";
 import { resolveRequiredHomeDir } from "../infra/home-dir.js";
+import { jsonUtf8Bytes } from "../infra/json-utf8-bytes.js";
 import { hasInterSessionUserProvenance } from "../sessions/input-provenance.js";
 import { stripInlineDirectiveTagsForDisplay } from "../utils/directive-tags.js";
 import { extractToolCallNames, hasToolCall } from "../utils/transcript-tools.js";
@@ -265,14 +266,6 @@ export async function cleanupArchivedSessionTranscripts(opts: {
   return { removed, scanned };
 }
 
-function jsonUtf8Bytes(value: unknown): number {
-  try {
-    return Buffer.byteLength(JSON.stringify(value), "utf8");
-  } catch {
-    return Buffer.byteLength(String(value), "utf8");
-  }
-}
-
 export function capArrayByJsonBytes<T>(
   items: T[],
   maxBytes: number,
diff --git a/src/infra/json-utf8-bytes.test.ts b/src/infra/json-utf8-bytes.test.ts
new file mode 100644
index 00000000000..3418359ae5f
--- /dev/null
+++ b/src/infra/json-utf8-bytes.test.ts
@@ -0,0 +1,16 @@
+import { describe, expect, it } from "vitest";
+import { jsonUtf8Bytes } from "./json-utf8-bytes.js";
+
+describe("jsonUtf8Bytes", () => {
+  it("returns utf8 byte length for serializable values", () => {
+    expect(jsonUtf8Bytes({ a: "x", b: [1, 2, 3] })).toBe(
+      Buffer.byteLength(JSON.stringify({ a: "x", b: [1, 2, 3] }), "utf8"),
+    );
+  });
+
+  it("falls back to string conversion when JSON serialization throws", () => {
+    const circular: { self?: unknown } = {};
+    circular.self = circular;
+    expect(jsonUtf8Bytes(circular)).toBe(Buffer.byteLength("[object Object]", "utf8"));
+  });
+});
diff --git a/src/infra/json-utf8-bytes.ts b/src/infra/json-utf8-bytes.ts
new file mode 100644
index 00000000000..ec677cffb32
--- /dev/null
+++ b/src/infra/json-utf8-bytes.ts
@@ -0,0 +1,7 @@
+export function jsonUtf8Bytes(value: unknown): number {
+  try {
+    return Buffer.byteLength(JSON.stringify(value), "utf8");
+  } catch {
+    return Buffer.byteLength(String(value), "utf8");
+  }
+}