diff --git a/.changeset/merge-dict.md b/.changeset/merge-dict.md
new file mode 100644
index 000000000..73db3bf77
--- /dev/null
+++ b/.changeset/merge-dict.md
@@ -0,0 +1,5 @@
+---
+"braintrust": patch
+---
+
+fix(security): reject `__proto__`, `constructor`, and `prototype` keys in `mergeDicts` / `mergeDictsWithPaths` to prevent prototype pollution from untrusted merge sources
diff --git a/js/util/object_util.test.ts b/js/util/object_util.test.ts
index e22ddc63f..794f601d6 100644
--- a/js/util/object_util.test.ts
+++ b/js/util/object_util.test.ts
@@ -1,4 +1,4 @@
-import { expect, test, describe } from "vitest";
+import { expect, test, describe, afterEach } from "vitest";
 import {
   mapAt,
   forEachMissingKey,
@@ -242,3 +242,52 @@ describe("tags set-union merge", () => {
     expect(a.metadata.tags).toEqual(["c", "d"]);
   });
 });
+
+describe("mergeDicts prototype pollution resistance", () => {
+  const POLLUTION_KEY = "polluted";
+
+  afterEach(() => {
+    Reflect.deleteProperty(Object.prototype, POLLUTION_KEY);
+  });
+
+  test.each([
+    {
+      name: "__proto__ at top level",
+      payload: '{"__proto__":{"polluted":"yes"}}',
+    },
+    {
+      name: "nested __proto__",
+      payload: '{"a":{"__proto__":{"polluted":"yes"}}}',
+    },
+    {
+      name: "constructor.prototype",
+      payload: '{"constructor":{"prototype":{"polluted":"yes"}}}',
+    },
+    {
+      name: "top-level prototype",
+      payload: '{"prototype":{"polluted":"yes"}}',
+    },
+  ])("mergeDicts ignores $name", ({ payload }) => {
+    const target: Record<string, unknown> = {};
+    mergeDicts(target, JSON.parse(payload));
+    expect(Object.prototype).not.toHaveProperty(POLLUTION_KEY);
+    expect(target).not.toHaveProperty(POLLUTION_KEY);
+  });
+
+  test("nested __proto__ leaves the legitimate subtree intact", () => {
+    const target: Record<string, unknown> = { a: { b: 1 } };
+    mergeDicts(target, JSON.parse('{"a":{"__proto__":{"polluted":"yes"}}}'));
+    expect(target).toEqual({ a: { b: 1 } });
+  });
+
+  test("forbidden key alongside safe key only drops the forbidden one", () => {
+    const target: Record<string, unknown> = {};
+    mergeDictsWithPaths({
+      mergeInto: target,
+      mergeFrom: JSON.parse('{"safe":1,"__proto__":{"polluted":"yes"}}'),
+      mergePaths: [],
+    });
+    expect(target).toEqual({ safe: 1 });
+    expect(Object.prototype).not.toHaveProperty(POLLUTION_KEY);
+  });
+});
diff --git a/js/util/object_util.ts b/js/util/object_util.ts
index 7925eac06..607bacbb8 100644
--- a/js/util/object_util.ts
+++ b/js/util/object_util.ts
@@ -3,6 +3,13 @@ import { isArray, isObject, isObjectOrArray } from "./type_util";
 // Fields that automatically use set-union merge semantics (unless in mergePaths).
 const SET_UNION_FIELDS = new Set(["tags"]);
 
+// Block certain keys from being merged to prevent prototype pollution and other unsafe merges.
+const FORBIDDEN_MERGE_KEYS: ReadonlySet<string> = new Set([
+  "__proto__",
+  "constructor",
+  "prototype",
+]);
+
 // Mutably updates `mergeInto` with the contents of `mergeFrom`, merging objects
 // deeply. Does not merge any further than `merge_paths`.
 // For fields in SET_UNION_FIELDS (like "tags"), arrays are merged as sets (union)
@@ -39,6 +46,7 @@ function mergeDictsWithPathsHelper({
   mergePaths: Set<string>;
 }) {
   Object.entries(mergeFrom).forEach(([k, mergeFromV]) => {
+    if (FORBIDDEN_MERGE_KEYS.has(k)) return;
     const fullPath = path.concat([k]);
     const fullPathSerialized = JSON.stringify(fullPath);
     const mergeIntoV = recordFind(mergeInto, k);