diff --git a/src/plugins/provider-auth-choice-helpers.test.ts b/src/plugins/provider-auth-choice-helpers.test.ts
index e87eb937914..9ac05235baa 100644
--- a/src/plugins/provider-auth-choice-helpers.test.ts
+++ b/src/plugins/provider-auth-choice-helpers.test.ts
@@ -42,6 +42,14 @@ describe("applyProviderAuthConfigPatch", () => {
     expect(next.agents?.defaults?.model).toEqual(base.agents.defaults.model);
   });
 
+  it("drops prototype-pollution keys from the merge", () => {
+    const patch = JSON.parse('{"__proto__":{"polluted":true},"agents":{"defaults":{}}}');
+    const next = applyProviderAuthConfigPatch(base, patch);
+    expect(next.agents?.defaults?.models).toEqual(base.agents.defaults.models);
+    expect(({} as Record<string, unknown>).polluted).toBeUndefined();
+    expect(Object.getPrototypeOf(next).polluted).toBeUndefined();
+  });
+
   it("keeps normal recursive merges for unrelated provider auth patch fields", () => {
     const base = {
       agents: {
diff --git a/src/plugins/provider-auth-choice-helpers.ts b/src/plugins/provider-auth-choice-helpers.ts
index b7dd3bfc063..0f5f53d0c48 100644
--- a/src/plugins/provider-auth-choice-helpers.ts
+++ b/src/plugins/provider-auth-choice-helpers.ts
@@ -46,6 +46,10 @@ function isPlainRecord(value: unknown): value is Record<string, unknown> {
   return Boolean(value && typeof value === "object" && !Array.isArray(value));
 }
 
+// Guard the recursive merge against prototype-pollution payloads if a
+// patch ever arrives from a JSON-parsed source that preserves these keys.
+const BLOCKED_MERGE_KEYS = new Set(["__proto__", "prototype", "constructor"]);
+
 export function mergeConfigPatch<T>(base: T, patch: unknown): T {
   if (!isPlainRecord(base) || !isPlainRecord(patch)) {
     return patch as T;
@@ -53,6 +57,9 @@ export function mergeConfigPatch<T>(base: T, patch: unknown): T {
 
   const next: Record<string, unknown> = { ...base };
   for (const [key, value] of Object.entries(patch)) {
+    if (BLOCKED_MERGE_KEYS.has(key)) {
+      continue;
+    }
     const existing = next[key];
     if (isPlainRecord(existing) && isPlainRecord(value)) {
       next[key] = mergeConfigPatch(existing, value);