fix(core): preserve message payloads during validation

Author: carderne

packages/core/src/v3/zodMessageHandler.test.ts

@@ -0,0 +1,41 @@
+import { describe, expect, it, vi } from "vitest";
+import { z as z3 } from "zod/v3";
+import { z as z4 } from "zod/v4";
+import { ZodMessageSender, sendMessageInCatalog } from "./zodMessageHandler.js";
+
+const schemas = [
+  [
+    "Zod 3",
+    z3.object({ value: z3.string().transform(Number), added: z3.string().default("default") }),
+  ],
+  [
+    "Zod 4",
+    z4.object({ value: z4.string().transform(Number), added: z4.string().default("default") }),
+  ],
+] as const;
+
+describe("Zod message sending", () => {
+  it.each(schemas)("ZodMessageSender preserves the original %s payload", async (_name, schema) => {
+    const catalog = { TEST: schema };
+    const payload = { value: "42", extra: "preserved" };
+    const sender = vi.fn(async (_message: unknown) => {});
+    const messageSender = new ZodMessageSender({ schema: catalog, sender });
+
+    await messageSender.send("TEST", payload);
+
+    expect(sender).toHaveBeenCalledWith({ type: "TEST", payload, version: "v1" });
+  });
+
+  it.each(schemas)(
+    "sendMessageInCatalog preserves the original %s payload",
+    async (_name, schema) => {
+      const catalog = { TEST: schema };
+      const payload = { value: "42", extra: "preserved" };
+      const sender = vi.fn(async (_message: unknown) => {});
+
+      await sendMessageInCatalog(catalog, "TEST", payload, sender);
+
+      expect(sender).toHaveBeenCalledWith({ type: "TEST", payload, version: "v1" });
+    }
+  );
+});

packages/core/src/v3/zodMessageHandler.ts

@@ -282,8 +282,13 @@ export class ZodMessageSender<TMessageCatalog extends ZodMessageCatalogSchema> {
     }
 
     try {
-      // send the validated/normalized output so zod transforms, defaults, and coercions are applied
-      await this.#sender({ type, payload: parsedPayload.data, version: "v1" });
+      // Preserve the original payload on the wire for backwards compatibility. Validation is
+      // intentionally side-effect-free here; receivers parse the payload into the schema output.
+      await this.#sender({
+        type,
+        payload: payload as unknown as inferZodSchemaOutput<TMessageCatalog[keyof TMessageCatalog]>,
+        version: "v1",
+      });
     } catch (error) {
       console.error("[ZodMessageSender] Failed to send message", error);
     }
@@ -338,8 +343,13 @@ export async function sendMessageInCatalog<TMessageCatalog extends ZodMessageCat
     throw new ZodSchemaParsedError(parsedPayload.error, payload);
   }
 
-  // send the validated/normalized output so zod transforms, defaults, and coercions are applied
-  await sender({ type, payload: parsedPayload.data, version: "v1" });
+  // Preserve the original payload on the wire for backwards compatibility. Validation is
+  // intentionally side-effect-free here; receivers parse the payload into the schema output.
+  await sender({
+    type,
+    payload: payload as unknown as inferZodSchemaOutput<TMessageCatalog[keyof TMessageCatalog]>,
+    version: "v1",
+  });
 }
 
 export type MessageCatalogToSocketIoEvents<TCatalog extends ZodMessageCatalogSchema> = {