diff --git a/docs/superpowers/refactor-logs/2026-05-06-input-output-unification.md b/docs/superpowers/refactor-logs/2026-05-06-input-output-unification.md
index d864073..c3d7a9f 100644
--- a/docs/superpowers/refactor-logs/2026-05-06-input-output-unification.md
+++ b/docs/superpowers/refactor-logs/2026-05-06-input-output-unification.md
@@ -15,11 +15,14 @@ doc-update items so docs are written once for the post-Plan-9 state.
 
 ## 1. Files added
 
-(populated as work progresses)
+- packages/core-shared/src/trpc/define-error-middleware.ts — middleware factory mapping [ErrorCtor, TRPC_CODE] tuples to TRPCError translation
+- packages/core-shared/src/trpc/define-error-middleware.test.ts — 4 tests covering mapped translation, multiple codes, unmapped passthrough (verifies INTERNAL_SERVER_ERROR + cause preservation), cause preservation
 
 ## 2. Files modified
 
-(populated as work progresses)
+- packages/core-shared/src/trpc/init.ts — `t` instance now exported (was internal const) so feature procedures.ts can do `t.procedure.use(...)`
+- packages/core-shared/package.json — added "./trpc/define-error-middleware" subpath export
+- packages/core-shared/tsconfig.json — set `rootDir: "."` and added `@/*` path alias so test files using `@/` resolve correctly under `tsc --noEmit`
 
 ## 3. Pattern changes (code-level)
 
@@ -34,7 +37,10 @@ doc-update items so docs are written once for the post-Plan-9 state.
 
 ## 4. Error-middleware adoption
 
-(populated when defineErrorMiddleware is wired in core-shared and consumed by features)
+- core-shared infrastructure landed; feature routers will adopt in Tasks 3-7.
+- Discriminator: `instanceof Ctor` (not error name string), so duck-typing is impossible — features pass their own class constructors.
+- Cause preservation: TRPCError carries the original domain error in `.cause` for client structured-error inspection.
+- Note on tRPC v11 behavior: unmapped errors still surface as `TRPCError(code: INTERNAL_SERVER_ERROR)` because tRPC's `createCaller` wraps all procedure errors. Our middleware does not interfere — the original error is preserved as `.cause` for inspection.
 
 ## 5. Public-API surface
 
diff --git a/packages/core-shared/package.json b/packages/core-shared/package.json
index bb63b07..241719f 100644
--- a/packages/core-shared/package.json
+++ b/packages/core-shared/package.json
@@ -7,7 +7,8 @@
     ".": "./src/index.ts",
     "./payload": "./src/payload/index.ts",
     "./trpc/init": "./src/trpc/init.ts",
-    "./trpc/context": "./src/trpc/context.ts"
+    "./trpc/context": "./src/trpc/context.ts",
+    "./trpc/define-error-middleware": "./src/trpc/define-error-middleware.ts"
   },
   "scripts": {
     "build": "tsc --noEmit",
diff --git a/packages/core-shared/src/trpc/define-error-middleware.test.ts b/packages/core-shared/src/trpc/define-error-middleware.test.ts
new file mode 100644
index 0000000..d3a370e
--- /dev/null
+++ b/packages/core-shared/src/trpc/define-error-middleware.test.ts
@@ -0,0 +1,90 @@
+import { describe, expect, it } from "vitest";
+import { TRPCError } from "@trpc/server";
+import { t } from "@/trpc/init";
+import { defineErrorMiddleware } from "@/trpc/define-error-middleware";
+
+class FooNotFoundError extends Error {
+  constructor(message: string) {
+    super(message);
+    this.name = "FooNotFoundError";
+  }
+}
+
+class FooBadRequestError extends Error {
+  constructor(message: string) {
+    super(message);
+    this.name = "FooBadRequestError";
+  }
+}
+
+const errorRouter = t.router({
+  notFound: t.procedure
+    .use(
+      defineErrorMiddleware([
+        [FooNotFoundError, "NOT_FOUND"],
+        [FooBadRequestError, "BAD_REQUEST"],
+      ]),
+    )
+    .query(() => {
+      throw new FooNotFoundError("nope");
+    }),
+  badRequest: t.procedure
+    .use(
+      defineErrorMiddleware([
+        [FooNotFoundError, "NOT_FOUND"],
+        [FooBadRequestError, "BAD_REQUEST"],
+      ]),
+    )
+    .query(() => {
+      throw new FooBadRequestError("oops");
+    }),
+  unmapped: t.procedure
+    .use(defineErrorMiddleware([[FooNotFoundError, "NOT_FOUND"]]))
+    .query(() => {
+      throw new Error("plain");
+    }),
+});
+
+describe("defineErrorMiddleware", () => {
+  it("translates a mapped error to TRPCError with the configured code", async () => {
+    const caller = errorRouter.createCaller({});
+    await expect(caller.notFound()).rejects.toMatchObject({
+      code: "NOT_FOUND",
+    });
+  });
+
+  it("translates a different mapped error to a different code", async () => {
+    const caller = errorRouter.createCaller({});
+    await expect(caller.badRequest()).rejects.toMatchObject({
+      code: "BAD_REQUEST",
+    });
+  });
+
+  it("does not translate unmapped errors but preserves the original via cause", async () => {
+    const caller = errorRouter.createCaller({});
+    try {
+      await caller.unmapped();
+      throw new Error("expected throw");
+    } catch (e) {
+      expect(e).toBeInstanceOf(TRPCError);
+      const trpcErr = e as TRPCError;
+      expect(trpcErr.code).toBe("INTERNAL_SERVER_ERROR");
+      expect(trpcErr.cause).toBeInstanceOf(Error);
+      expect(trpcErr.cause).not.toBeInstanceOf(TRPCError);
+      expect((trpcErr.cause as Error).message).toBe("plain");
+    }
+  });
+
+  it("preserves the original error as the cause", async () => {
+    const caller = errorRouter.createCaller({});
+    try {
+      await caller.notFound();
+      throw new Error("expected throw");
+    } catch (e) {
+      expect(e).toBeInstanceOf(TRPCError);
+      const trpcErr = e as TRPCError;
+      expect(trpcErr.cause).toBeInstanceOf(FooNotFoundError);
+      expect((trpcErr.cause as Error).message).toBe("nope");
+    }
+  });
+});
diff --git a/packages/core-shared/src/trpc/define-error-middleware.ts b/packages/core-shared/src/trpc/define-error-middleware.ts
new file mode 100644
index 0000000..a9ef641
--- /dev/null
+++ b/packages/core-shared/src/trpc/define-error-middleware.ts
@@ -0,0 +1,36 @@
+import { TRPCError } from "@trpc/server";
+import type { TRPC_ERROR_CODE_KEY } from "@trpc/server/rpc";
+import { t } from "./init";
+
+type ErrorCtor = new (...args: never[]) => Error;
+
+/**
+ * Build a tRPC middleware that translates domain errors to TRPCError.
+ *
+ * Each tuple pairs a constructor with a TRPC error code. The middleware
+ * runs the procedure body inside a try/catch; on `instanceof Ctor`,
+ * it throws a TRPCError with the configured code and the original error
+ * preserved as `.cause`. Unmapped errors propagate untouched (tRPC's
+ * default INTERNAL_SERVER_ERROR handling applies).
+ *
+ * Owned by features: each feature passes its own constructors in.
+ * core-shared never enumerates feature-specific error classes.
+ */
+export function defineErrorMiddleware(
+  map: ReadonlyArray<readonly [ErrorCtor, TRPC_ERROR_CODE_KEY]>,
+) {
+  return t.middleware(async ({ next }) => {
+    const result = await next();
+    if (!result.ok) {
+      const cause = result.error.cause;
+      if (cause instanceof Error) {
+        for (const [Ctor, code] of map) {
+          if (cause instanceof Ctor) {
+            throw new TRPCError({ code, message: cause.message, cause });
+          }
+        }
+      }
+    }
+    return result;
+  });
+}
diff --git a/packages/core-shared/src/trpc/init.ts b/packages/core-shared/src/trpc/init.ts
index 1a4a034..ee8ef78 100644
--- a/packages/core-shared/src/trpc/init.ts
+++ b/packages/core-shared/src/trpc/init.ts
@@ -1,7 +1,7 @@
 import { initTRPC } from "@trpc/server";
 import superjson from "superjson";
 
-const t = initTRPC.create({
+export const t = initTRPC.create({
   transformer: superjson,
 });
 
diff --git a/packages/core-shared/tsconfig.json b/packages/core-shared/tsconfig.json
index 3bdbcfc..652e804 100644
--- a/packages/core-shared/tsconfig.json
+++ b/packages/core-shared/tsconfig.json
@@ -2,7 +2,10 @@
   "extends": "@repo/core-typescript/base.json",
   "compilerOptions": {
     "outDir": "dist",
-    "rootDir": "src"
+    "rootDir": ".",
+    "paths": {
+      "@/*": ["./src/*"]
+    }
   },
   "include": ["src/**/*"],
   "exclude": ["node_modules", "dist"]