diff --git a/src/features/mcp/codexcli-mcp.ts b/src/features/mcp/codexcli-mcp.ts
index e034e9c9c..65b4328d0 100644
--- a/src/features/mcp/codexcli-mcp.ts
+++ b/src/features/mcp/codexcli-mcp.ts
@@ -6,7 +6,7 @@ import { ValidationResult } from "../../types/ai-file.js";
 import { McpServers } from "../../types/mcp.js";
 import { readFileContentOrNull, readOrInitializeFileContent } from "../../utils/file.js";
 import { warnWithFallback } from "../../utils/logger.js";
-import { isRecord } from "../../utils/type-guards.js";
+import { isPlainObject, isRecord } from "../../utils/type-guards.js";
 import { RulesyncMcp } from "./rulesync-mcp.js";
 import {
   ToolMcp,
@@ -21,12 +21,6 @@ const MAX_REMOVE_EMPTY_ENTRIES_DEPTH = 32;
 
 const PROTOTYPE_POLLUTION_KEYS = new Set(["__proto__", "constructor", "prototype"]);
 
-function isPlainObject(value: unknown): value is Record<string, unknown> {
-  if (!isRecord(value)) return false;
-  const proto = Object.getPrototypeOf(value);
-  return proto === null || proto === Object.prototype;
-}
-
 function convertFromCodexFormat(codexMcp: Record<string, unknown>): McpServers {
   const result: McpServers = {};
 
diff --git a/src/utils/frontmatter.ts b/src/utils/frontmatter.ts
index e8ed0949a..2b8441e96 100644
--- a/src/utils/frontmatter.ts
+++ b/src/utils/frontmatter.ts
@@ -2,12 +2,7 @@ import matter from "gray-matter";
 import { dump, load } from "js-yaml";
 
 import { formatError } from "./error.js";
-
-function isPlainObject(value: unknown): value is Record<string, unknown> {
-  if (value === null || typeof value !== "object") return false;
-  const prototype = Object.getPrototypeOf(value);
-  return prototype === Object.prototype || prototype === null;
-}
+import { isPlainObject } from "./type-guards.js";
 
 function deepRemoveNullishValue(value: unknown): unknown {
   if (value === null || value === undefined) {
diff --git a/src/utils/type-guards.ts b/src/utils/type-guards.ts
index b87d8a42a..ab8ea94b7 100644
--- a/src/utils/type-guards.ts
+++ b/src/utils/type-guards.ts
@@ -5,3 +5,20 @@
 export function isRecord(value: unknown): value is Record<string, unknown> {
   return typeof value === "object" && value !== null && !Array.isArray(value);
 }
+
+/**
+ * Stricter sibling of {@link isRecord}: narrows to *plain* objects whose
+ * prototype is either ``Object.prototype`` or ``null`` — i.e. object
+ * literals, ``Object.create(null)`` bags, and the output of ``JSON.parse``.
+ *
+ * Rejects class instances even though they pass ``isRecord``. This is the
+ * check needed for prototype-pollution hardening: anything walking
+ * arbitrary user-supplied keys (frontmatter parsing, MCP config
+ * conversion, etc.) should reject inputs whose prototype could carry
+ * malicious accessor descriptors.
+ */
+export function isPlainObject(value: unknown): value is Record<string, unknown> {
+  if (!isRecord(value)) return false;
+  const proto = Object.getPrototypeOf(value);
+  return proto === null || proto === Object.prototype;
+}