feat(core-trpc): scaffold tRPC client with React Query providers

- createTRPCContext + useTRPC hook for typed client access
- NextTrpcProvider with SSR-safe absolute URL resolution
- TanstackTrpcProvider for TanStack Start apps
- /api/trpc catch-all route handler in web-next
- Wire NextTrpcProvider into app providers
- Add @repo/core-trpc to transpilePackages

packages/core-trpc/eslint.config.js

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+import baseConfig from "@repo/core-eslint/base";
+
+export default baseConfig;

packages/core-trpc/package.json

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+{
+  "name": "@repo/core-trpc",
+  "private": true,
+  "version": "0.0.0",
+  "type": "module",
+  "exports": {
+    ".": "./src/index.ts",
+    "./next": "./src/providers/next-provider.tsx",
+    "./tanstack": "./src/providers/tanstack-provider.tsx"
+  },
+  "scripts": {
+    "build": "tsc --noEmit",
+    "lint": "eslint .",
+    "typecheck": "tsc --noEmit",
+    "test": "vitest run --passWithNoTests"
+  },
+  "dependencies": {
+    "@repo/core-api": "workspace:*",
+    "@tanstack/react-query": "^5.66.0",
+    "@trpc/client": "^11.17.0",
+    "@trpc/react-query": "^11.17.0",
+    "@trpc/server": "^11.17.0",
+    "@trpc/tanstack-react-query": "^11.17.0",
+    "react": "^19.0.0",
+    "superjson": "^2.2.1"
+  },
+  "devDependencies": {
+    "@repo/core-eslint": "workspace:*",
+    "@repo/core-testing": "workspace:*",
+    "@repo/core-typescript": "workspace:*",
+    "@testing-library/jest-dom": "^6.5.0",
+    "@testing-library/react": "^16.0.0",
+    "@types/react": "^19.0.0",
+    "jsdom": "^25.0.0",
+    "vitest": "^3.0.0"
+  }
+}

packages/core-trpc/src/client.test.ts

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+import { describe, it, expect } from "vitest";
+import { useTRPC, TRPCProvider } from "./client";
+
+describe("core-trpc client exports", () => {
+  it("exports useTRPC hook", () => {
+    expect(useTRPC).toBeTypeOf("function");
+  });
+
+  it("exports TRPCProvider component", () => {
+    expect(TRPCProvider).toBeTypeOf("function");
+  });
+});

packages/core-trpc/src/client.ts

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+"use client";
+
+import { createTRPCContext } from "@trpc/tanstack-react-query";
+import type { AppRouter } from "@repo/core-api";
+
+export const { TRPCProvider, useTRPC } = createTRPCContext<AppRouter>();

packages/core-trpc/src/index.ts

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+export { useTRPC, TRPCProvider } from "./client";
+export { getQueryClient } from "./query-client";
+export type { AppRouter } from "@repo/core-api";

packages/core-trpc/src/providers/next-provider.tsx

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+"use client";
+
+import { useState } from "react";
+import { QueryClientProvider } from "@tanstack/react-query";
+import { createTRPCClient, httpBatchLink } from "@trpc/client";
+import superjson from "superjson";
+import type { AppRouter } from "@repo/core-api";
+import { TRPCProvider } from "../client";
+import { getQueryClient } from "../query-client";
+
+function getBaseUrl() {
+  if (typeof window !== "undefined") return "";
+  return `http://localhost:${process.env.PORT ?? 3000}`;
+}
+
+export function NextTrpcProvider({
+  children,
+  trpcUrl = "/api/trpc",
+}: {
+  children: React.ReactNode;
+  trpcUrl?: string;
+}) {
+  const [queryClient] = useState(() => getQueryClient());
+  const [trpcClient] = useState(() =>
+    createTRPCClient<AppRouter>({
+      links: [
+        httpBatchLink({
+          url: `${getBaseUrl()}${trpcUrl}`,
+          transformer: superjson,
+        }),
+      ],
+    }),
+  );
+
+  return (
+    <TRPCProvider trpcClient={trpcClient} queryClient={queryClient}>
+      <QueryClientProvider client={queryClient}>{children}</QueryClientProvider>
+    </TRPCProvider>
+  );
+}

packages/core-trpc/src/providers/tanstack-provider.tsx

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+"use client";
+
+import { useState } from "react";
+import { QueryClientProvider } from "@tanstack/react-query";
+import { createTRPCClient, httpBatchLink } from "@trpc/client";
+import superjson from "superjson";
+import type { AppRouter } from "@repo/core-api";
+import { TRPCProvider } from "../client";
+import { getQueryClient } from "../query-client";
+
+export function TanstackTrpcProvider({
+  children,
+  trpcUrl,
+}: {
+  children: React.ReactNode;
+  trpcUrl: string;
+}) {
+  const [queryClient] = useState(() => getQueryClient());
+  const [trpcClient] = useState(() =>
+    createTRPCClient<AppRouter>({
+      links: [httpBatchLink({ url: trpcUrl, transformer: superjson })],
+    }),
+  );
+
+  return (
+    <TRPCProvider trpcClient={trpcClient} queryClient={queryClient}>
+      <QueryClientProvider client={queryClient}>{children}</QueryClientProvider>
+    </TRPCProvider>
+  );
+}

packages/core-trpc/src/query-client.ts

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+import { QueryClient } from "@tanstack/react-query";
+
+let clientQueryClient: QueryClient | undefined;
+
+const defaultOptions = {
+  queries: {
+    staleTime: 30 * 1000,
+    refetchOnWindowFocus: false,
+  },
+};
+
+export function getQueryClient(): QueryClient {
+  if (typeof window === "undefined") {
+    // Server: always create a new instance per request
+    return new QueryClient({ defaultOptions });
+  }
+  // Browser: singleton
+  if (!clientQueryClient) {
+    clientQueryClient = new QueryClient({ defaultOptions });
+  }
+  return clientQueryClient;
+}

packages/core-trpc/tsconfig.json

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+{
+  "extends": "@repo/core-typescript/base.json",
+  "compilerOptions": {
+    "outDir": "dist",
+    "rootDir": ".",
+    "lib": ["ES2022", "DOM"],
+    "jsx": "preserve",
+    "declaration": false,
+    "declarationMap": false,
+    "types": ["vitest/globals", "@testing-library/jest-dom"],
+    "paths": {
+      "@/*": ["./src/*"]
+    }
+  },
+  "include": ["src/**/*"],
+  "exclude": ["node_modules", "dist"]
+}

packages/core-trpc/turbo.json

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+{
+  "extends": ["//"],
+  "tags": ["core-composition"]
+}

packages/core-trpc/vitest.config.ts

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+import path from "node:path";
+import { mergeConfig } from "vitest/config";
+import { jsdomVitestConfig } from "@repo/core-typescript/vitest.base.jsdom";
+
+export default mergeConfig(jsdomVitestConfig, {
+  resolve: { alias: { "@": path.resolve(__dirname, "./src") } },
+});