diff --git a/.changeset/zod-v4-json-schema-fallback.md b/.changeset/zod-v4-json-schema-fallback.md
new file mode 100644
index 0000000000..2ac93a93d4
--- /dev/null
+++ b/.changeset/zod-v4-json-schema-fallback.md
@@ -0,0 +1,5 @@
+---
+"@browserbasehq/stagehand": patch
+---
+
+Use the Zod v4 subpath JSON schema converter when root zod does not expose `toJSONSchema`.
diff --git a/packages/core/lib/v3/zodCompat.ts b/packages/core/lib/v3/zodCompat.ts
index 2ff7ea0575..76aa688aa1 100644
--- a/packages/core/lib/v3/zodCompat.ts
+++ b/packages/core/lib/v3/zodCompat.ts
@@ -1,4 +1,5 @@
 import { z } from "zod";
+import { z as zodV4 } from "zod/v4";
 import type {
   ZodObject as Zod4Object,
   ZodRawShape as Zod4RawShape,
@@ -35,15 +36,18 @@ export function toJsonSchema(schema: StagehandZodSchema): JsonSchemaDocument {
     return zodToJsonSchema(schema);
   }
 
-  // For v4 schemas, use built-in z.toJSONSchema() method
-  const zodWithJsonSchema = z as typeof z & {
-    toJSONSchema?: (schema: Zod4TypeAny) => JsonSchemaDocument;
-  };
+  const converters = [zodV4, z] as Array<
+    typeof z & {
+      toJSONSchema?: (schema: Zod4TypeAny) => JsonSchemaDocument;
+    }
+  >;
 
-  if (zodWithJsonSchema.toJSONSchema) {
-    return zodWithJsonSchema.toJSONSchema(schema as Zod4TypeAny);
+  for (const zodWithJsonSchema of converters) {
+    if (zodWithJsonSchema.toJSONSchema) {
+      return zodWithJsonSchema.toJSONSchema(schema as Zod4TypeAny);
+    }
   }
 
-  // This should never happen with Zod v4.1+
+  // This should never happen with Zod v4.1+ or transitional Zod v3.25+ packages.
   throw new Error("Zod v4 toJSONSchema method not found");
 }
diff --git a/packages/core/tests/unit/zod-compat.test.ts b/packages/core/tests/unit/zod-compat.test.ts
new file mode 100644
index 0000000000..0d00e6ebac
--- /dev/null
+++ b/packages/core/tests/unit/zod-compat.test.ts
@@ -0,0 +1,28 @@
+import { afterEach, describe, expect, it, vi } from "vitest";
+
+describe("zodCompat", () => {
+  afterEach(() => {
+    vi.resetModules();
+    vi.doUnmock("zod");
+    vi.doUnmock("zod/v4");
+    vi.doUnmock("zod-to-json-schema");
+  });
+
+  it("uses zod/v4 JSON schema conversion when root zod lacks toJSONSchema", async () => {
+    const schema = { _zod: { def: { type: "object" } } };
+    const jsonSchema = {
+      type: "object",
+      properties: { ok: { type: "boolean" } },
+    };
+    const toJSONSchema = vi.fn(() => jsonSchema);
+
+    vi.doMock("zod", () => ({ z: {} }));
+    vi.doMock("zod/v4", () => ({ z: { toJSONSchema } }));
+    vi.doMock("zod-to-json-schema", () => ({ default: vi.fn() }));
+
+    const { toJsonSchema } = await import("../../lib/v3/zodCompat.js");
+
+    expect(toJsonSchema(schema as never)).toBe(jsonSchema);
+    expect(toJSONSchema).toHaveBeenCalledWith(schema);
+  });
+});