Fix plugin RPCs with schema transforms (#8550)

* fix(plugins): parse RPC values once at the receiver

Sending parsed values caused transforms to run again on incompatible schema outputs. Keep the wire values in schema-input form and parse only where consumed.

* fix(plugins): let omitted RPC inputs reach schema defaults

* fix(plugins): accept omitted RPC outputs for client defaults

packages/plugin/src/client/host.test.ts

@@ -24,17 +24,23 @@ describe("plugin RPC contracts", () => {
     expectTypeOf<RpcOutput<typeof contract>>().toEqualTypeOf<{ value: number }>();
   });
 
-  it("validates input before invoking the daemon", async () => {
-    let invocations = 0;
-    const invoke = async () => {
-      invocations += 1;
-      return { value: 2 };
+  it("sends input unchanged for the plugin to parse", async () => {
+    let parses = 0;
+    const transformed = defineRpc({
+      name: "transformed",
+      input: z.string().transform((text) => {
+        parses += 1;
+        return Number(text);
+      }),
+      output: z.number(),
+    });
+    const invoke = async (_method: string, input: unknown) => {
+      expect(input).toBe("2");
+      expect(parses).toBe(0);
+      return transformed.input.parseAsync(input);
     };
-
-    await expect(callPluginRpc(contract, invoke, { value: "wrong" })).rejects.toBeInstanceOf(
-      z.ZodError,
-    );
-    expect(invocations).toBe(0);
+    await expect(callPluginRpc(transformed, invoke, "2")).resolves.toBe(2);
+    expect(parses).toBe(1);
   });
 
   it("validates output before resolving", async () => {

packages/plugin/src/rpc.ts

@@ -39,8 +39,7 @@ export async function callPluginRpc<InputSchema extends ZodType, OutputSchema ex
   invoke: (method: string, input: unknown) => Promise<unknown>,
   input: PluginWireInput<ZodInput<InputSchema>>,
 ): Promise<ZodOutput<OutputSchema>> {
-  const parsedInput = await contract.input.parseAsync(input);
-  const output = await invoke(contract.name, parsedInput);
+  const output = await invoke(contract.name, input);
   return contract.output.parseAsync(output);
 }
 

packages/protocol/src/messages.ts

@@ -1603,7 +1603,7 @@ export const PluginRpcInvokeRequestSchema = z.object({
   requestId: z.string(),
   pluginId: PluginIdSchema,
   method: z.string().min(1),
-  input: z.unknown(),
+  input: z.unknown().optional(),
 });
 
 export const PluginRpcSubscribeRequestSchema = z.object({
@@ -6969,7 +6969,7 @@ export const PluginRpcInvokeResponseSchema = z.object({
   type: z.literal("plugin.rpc.invoke.response"),
   payload: z.object({
     requestId: z.string(),
-    output: z.unknown(),
+    output: z.unknown().optional(),
   }),
 });
 

packages/protocol/tests/validation/ws-outbound.test.ts

@@ -5,7 +5,7 @@ import { fileURLToPath, pathToFileURL } from "node:url";
 import { createJiti } from "jiti";
 import type { z } from "zod";
 import { describe, expect, it } from "vitest";
-import { WSOutboundMessageSchema } from "../../src/messages.js";
+import { PluginRpcInvokeResponseSchema, WSOutboundMessageSchema } from "../../src/messages.js";
 import { validateWSOutboundMessage } from "../../src/validation/ws-outbound.js";
 
 interface GeneratedSchema {
@@ -401,3 +401,8 @@ const SourceSchema = z.object({
     });
   });
 });
+
+it("accepts omitted RPC output so the receiving client can apply schema defaults", () => {
+  const message = { type: "plugin.rpc.invoke.response", payload: { requestId: "default-output" } };
+  expect(PluginRpcInvokeResponseSchema.safeParse(message).success).toBe(true);
+});

packages/server/src/server/plugins/plugin-process.ts

@@ -139,7 +139,7 @@ export function createPluginWorker(options: {
       registerSubscription(contract, handler as PluginSubscriptionHandler);
       return;
     }
-    register(contract, (input, context) => handler(contract.input.parse(input), context as never));
+    register(contract, (input, context) => handler(input, context as never));
   }) as PluginServerContext["handle"];
 
   function register(contract: PluginRpcContract, handler: RpcHandler): void {
@@ -593,7 +593,6 @@ export function createPluginWorker(options: {
         if (!paseo) throw new Error("Plugin Paseo API is unavailable");
         return registered.handler(input, { paseo });
       })
-      .then((output) => registered.contract.output.parseAsync(output))
       .then((output: unknown) => {
         assertPluginWireValue(output, "output");
         return output;

packages/server/src/server/plugins/plugin-subscriptions.e2e.test.ts

@@ -8,6 +8,9 @@ import {
   type WebSocketLike,
 } from "@getpaseo/client/internal/daemon-client";
 import { afterEach, expect, test } from "vitest";
+import { defineRpc } from "@getpaseo/plugin";
+import { callPluginRpc } from "@getpaseo/plugin/client/host";
+import { z } from "zod";
 import { resolveDaemonVersion } from "../daemon-version.js";
 import { createTestPaseoDaemon, type TestPaseoDaemon } from "../test-utils/paseo-daemon.js";
 
@@ -39,6 +42,13 @@ const repeated = defineSubscription({
   input: z.object({ times: z.string().transform(Number) }),
   event: z.string().transform((text) => text.length),
 });
+const transformed = defineRpc({
+  name: "transformed",
+  input: z.object({ when: z.string().transform(async (text) => new Date(text)) }),
+  output: z.object({ n: z.string().transform(async (text) => Number(text)) }),
+});
+const defaulted = defineRpc({ name: "defaulted", input: z.string().default("x"), output: z.string() });
+const defaultedOutput = defineRpc({ name: "defaulted-output", input: z.object({}), output: z.string().default("x") });
 const watchers = defineRpc({
   name: "watchers",
   input: z.object({}),
@@ -81,6 +91,9 @@ export default function contribute(server: PluginServerContext) {
   server.handle(repeated, async ({ times }, { push }) => {
     await push(typeof times === "number" ? "x".repeat(times) : "input arrived untransformed");
   });
+  server.handle(transformed, ({ when }) => ({ n: String(when.getUTCFullYear()) }));
+  server.handle(defaulted, (input) => input);
+  server.handle(defaultedOutput, () => undefined);
   server.handle(watchers, () => ({ starts, active }));
   server.handle(thumbnail, ({ size }) => Buffer.alloc(size, 7));
   server.handle(nestedOutput, () => ({ data: new Uint8Array(1) }));
@@ -352,3 +365,64 @@ export default function contribute(server: PluginServerContext) {
     await harness.close();
   }
 }, 60_000);
+
+test("plain RPC parses async input and output transforms once at the receiver", async () => {
+  const transformed = defineRpc({
+    name: "transformed",
+    input: z.object({ when: z.string().transform(async (text) => new Date(text)) }),
+    output: z.object({ n: z.string().transform(async (text) => Number(text)) }),
+  });
+
+  const harness = await startHarness();
+  try {
+    await expect(
+      callPluginRpc(
+        transformed,
+        (method, input) => harness.client.invokePluginRpc(PLUGIN_ID, method, input),
+        { when: "2026-01-01T00:00:00.000Z" },
+      ),
+    ).resolves.toEqual({ n: 2026 });
+  } finally {
+    await harness.close();
+  }
+});
+
+test("plain RPC applies a top-level input default in the plugin process", async () => {
+  const contract = defineRpc({
+    name: "defaulted",
+    input: z.string().default("x"),
+    output: z.string(),
+  });
+  const harness = await startHarness();
+  try {
+    await expect(
+      callPluginRpc(
+        contract,
+        (method, input) => harness.client.invokePluginRpc(PLUGIN_ID, method, input),
+        undefined,
+      ),
+    ).resolves.toBe("x");
+  } finally {
+    await harness.close();
+  }
+});
+
+test("plain RPC applies a top-level output default on the client", async () => {
+  const contract = defineRpc({
+    name: "defaulted-output",
+    input: z.object({}),
+    output: z.string().default("x"),
+  });
+  const harness = await startHarness();
+  try {
+    await expect(
+      callPluginRpc(
+        contract,
+        (method, input) => harness.client.invokePluginRpc(PLUGIN_ID, method, input),
+        {},
+      ),
+    ).resolves.toBe("x");
+  } finally {
+    await harness.close();
+  }
+}, 30_000);

packages/server/src/server/plugins/runtime.posix.test.ts

@@ -1853,7 +1853,7 @@ export function inspect() { void Surface; return {}; }`,
     await runtime.stopAll();
   });
 
-  it("rejects a handler result that does not match its RPC output schema", async () => {
+  it("forwards a handler result for the client to validate against its RPC output schema", async () => {
     const directory = await createPlugin(
       "invalid-output",
       `import { z } from "zod";
@@ -1872,7 +1872,9 @@ export default function contribute(plugin: any) {
 
     await runtime.startPlugin("invalid-output", directory);
 
-    await expect(runtime.invoke("invalid-output", "broken", {})).rejects.toThrow();
+    await expect(runtime.invoke("invalid-output", "broken", {})).resolves.toEqual({
+      value: "wrong",
+    });
     await runtime.stopAll();
   });
 

public-docs/plugins/reference.md

@@ -2222,7 +2222,7 @@ export default function contribute(server: PluginServerContext) {
 }
 ```
 
-Inputs and outputs are validated on both sides. RPC names start with a lowercase letter and contain lowercase letters, numbers, dots, hyphens, or underscores. `useRpc()` returns a typed async function. Use TanStack Query for request state, caching, and mutations.
+Inputs are parsed once in the plugin process; outputs are parsed once on the client. Send schema inputs from the client and return output-schema inputs from the handler; schema transforms run at the receiving side. RPC names start with a lowercase letter and contain lowercase letters, numbers, dots, hyphens, or underscores. `useRpc()` returns a typed async function. Use TanStack Query for request state, caching, and mutations.
 
 Backend handlers receive the same `PaseoApi` as `{ paseo }`. Their connection belongs to the subprocess and closes when the plugin stops. It does not subscribe to timelines or catalog events until plugin code subscribes. Follow the [SDK event contract](/docs/sdk/events) for cleanup and timeline replacements. Backend code can use Node APIs and dependencies installed in the plugin directory.