diff --git a/packages/server/src/builder.test.ts b/packages/server/src/builder.test.ts
index eb9ba0f00..9bbb964ed 100644
--- a/packages/server/src/builder.test.ts
+++ b/packages/server/src/builder.test.ts
@@ -130,7 +130,7 @@ describe('builder', () => {
 
     expect(applied).toBe(decorateMiddlewareSpy.mock.results[0]?.value)
     expect(decorateMiddlewareSpy).toBeCalledTimes(1)
-    expect(decorateMiddlewareSpy).toBeCalledWith(mid)
+    expect(decorateMiddlewareSpy).toBeCalledWith(mid, def.errorMap)
   })
 
   it('.errors', () => {
diff --git a/packages/server/src/builder.ts b/packages/server/src/builder.ts
index c1afe03fe..e094fd816 100644
--- a/packages/server/src/builder.ts
+++ b/packages/server/src/builder.ts
@@ -6,7 +6,7 @@ import type { Context, MergedCurrentContext, MergedInitialContext } from './cont
 import type { ORPCErrorConstructorMap } from './error'
 import type { Lazy } from './lazy'
 import type { AnyMiddleware, MapInputMiddleware, Middleware } from './middleware'
-import type { DecoratedMiddleware } from './middleware-decorated'
+import type { AnyDecoratedMiddleware, DecoratedMiddleware } from './middleware-decorated'
 import type { ProcedureHandler } from './procedure'
 import type { Router } from './router'
 import type { EnhancedRouter, EnhanceRouterOptions } from './router-utils'
@@ -146,7 +146,7 @@ export class Builder<
   middleware<UOutContext extends IntersectPick<TCurrentContext, UOutContext>, TInput, TOutput = any>( // = any here is important to make middleware can be used in any output by default
     middleware: Middleware<TInitialContext, UOutContext, TInput, TOutput, ORPCErrorConstructorMap<TErrorMap>, TMeta>,
   ): DecoratedMiddleware<TInitialContext, UOutContext, TInput, TOutput, any, TMeta> { // any ensures middleware can used in any procedure
-    return decorateMiddleware(middleware)
+    return decorateMiddleware(middleware, this['~orpc'].errorMap)
   }
 
   /**
@@ -190,16 +190,24 @@ export class Builder<
   >
 
   use(
-    middleware: AnyMiddleware,
+    middleware: AnyMiddleware | AnyDecoratedMiddleware,
     mapInput?: MapInputMiddleware<any, any>,
   ): BuilderWithMiddlewares<any, any, any, any, any, any> {
     const mapped = mapInput
       ? decorateMiddleware(middleware).mapInput(mapInput)
       : middleware
 
+    if (!('errorMap' in middleware) || !middleware.errorMap) {
+      return new Builder({
+        ...this['~orpc'],
+        middlewares: addMiddleware(this['~orpc'].middlewares, mapped),
+      }) as any
+    }
+
     return new Builder({
       ...this['~orpc'],
       middlewares: addMiddleware(this['~orpc'].middlewares, mapped),
+      errorMap: mergeErrorMap(this['~orpc'].errorMap, middleware.errorMap),
     }) as any
   }
 
diff --git a/packages/server/src/middleware-decorated.ts b/packages/server/src/middleware-decorated.ts
index 592500ab2..c38cefe62 100644
--- a/packages/server/src/middleware-decorated.ts
+++ b/packages/server/src/middleware-decorated.ts
@@ -1,4 +1,4 @@
-import type { Meta } from '@orpc/contract'
+import type { ErrorMap, Meta } from '@orpc/contract'
 import type { IntersectPick } from '@orpc/shared'
 import type { Context, MergedCurrentContext, MergedInitialContext } from './context'
 import type { ORPCErrorConstructorMap } from './error'
@@ -12,6 +12,11 @@ export interface DecoratedMiddleware<
   TErrorConstructorMap extends ORPCErrorConstructorMap<any>,
   TMeta extends Meta,
 > extends Middleware<TInContext, TOutContext, TInput, TOutput, TErrorConstructorMap, TMeta> {
+  /**
+   * Error map associated with this middleware (if any)
+   * @internal
+   */
+  errorMap?: ErrorMap
   /**
    * Change the expected input type by providing a map function.
    */
@@ -78,6 +83,8 @@ export interface DecoratedMiddleware<
   >
 }
 
+export type AnyDecoratedMiddleware = DecoratedMiddleware<any, any, any, any, any, any>
+
 export function decorateMiddleware<
   TInContext extends Context,
   TOutContext extends Context,
@@ -86,23 +93,39 @@ export function decorateMiddleware<
   TErrorConstructorMap extends ORPCErrorConstructorMap<any>,
   TMeta extends Meta,
 >(
-  middleware: Middleware<TInContext, TOutContext, TInput, TOutput, TErrorConstructorMap, TMeta>,
+  middleware: Middleware<TInContext, TOutContext, TInput, TOutput, TErrorConstructorMap, TMeta>
+    | DecoratedMiddleware<TInContext, TOutContext, TInput, TOutput, TErrorConstructorMap, TMeta>,
+  errorMap?: ErrorMap,
 ): DecoratedMiddleware<TInContext, TOutContext, TInput, TOutput, TErrorConstructorMap, TMeta> {
   const decorated = ((...args) => middleware(...args)) as DecoratedMiddleware<TInContext, TOutContext, TInput, TOutput, TErrorConstructorMap, TMeta>
 
+  // Attach error map if provided
+  if (errorMap) {
+    decorated.errorMap = errorMap
+  }
+  if ('errorMap' in middleware) {
+    decorated.errorMap = middleware.errorMap
+  }
+
   decorated.mapInput = (mapInput) => {
     const mapped = decorateMiddleware(
       (options, input, ...rest) => middleware(options as any, mapInput(input as any), ...rest as [any]),
+      decorated.errorMap, // Preserve error map
     )
 
     return mapped as any
   }
 
-  decorated.concat = (concatMiddleware: AnyMiddleware, mapInput?: MapInputMiddleware<any, any>) => {
+  decorated.concat = (concatMiddleware: AnyMiddleware | AnyDecoratedMiddleware, mapInput?: MapInputMiddleware<any, any>) => {
     const mapped = mapInput
       ? decorateMiddleware(concatMiddleware).mapInput(mapInput)
       : concatMiddleware
 
+    const combinedErrorMap = {
+      ...decorated.errorMap,
+      ...('errorMap' in concatMiddleware ? concatMiddleware.errorMap : undefined),
+    }
+
     const concatted = decorateMiddleware((options, input, output, ...rest) => {
       const merged = middleware({
         ...options,
@@ -114,7 +137,7 @@ export function decorateMiddleware<
       } as any, input as any, output as any, ...rest)
 
       return merged
-    })
+    }, combinedErrorMap)
 
     return concatted as any
   }
diff --git a/packages/server/src/procedure-decorated.ts b/packages/server/src/procedure-decorated.ts
index 5a6fb4c8e..41f0e9302 100644
--- a/packages/server/src/procedure-decorated.ts
+++ b/packages/server/src/procedure-decorated.ts
@@ -12,6 +12,7 @@ import type { IntersectPick, MaybeOptionalOptions } from '@orpc/shared'
 import type { Context, MergedCurrentContext, MergedInitialContext } from './context'
 import type { ORPCErrorConstructorMap } from './error'
 import type { AnyMiddleware, MapInputMiddleware, Middleware } from './middleware'
+import type { AnyDecoratedMiddleware } from './middleware-decorated'
 import type { ProcedureActionableClient } from './procedure-action'
 import type { CreateProcedureClientOptions, ProcedureClient } from './procedure-client'
 import { mergeErrorMap, mergeMeta, mergeRoute } from '@orpc/contract'
@@ -136,14 +137,22 @@ export class DecoratedProcedure<
     TMeta
   >
 
-  use(middleware: AnyMiddleware, mapInput?: MapInputMiddleware<any, any>): DecoratedProcedure<any, any, any, any, any, any> {
+  use(middleware: AnyMiddleware | AnyDecoratedMiddleware, mapInput?: MapInputMiddleware<any, any>): DecoratedProcedure<any, any, any, any, any, any> {
     const mapped = mapInput
       ? decorateMiddleware(middleware).mapInput(mapInput)
       : middleware
 
+    if (!('errorMap' in middleware) || !middleware.errorMap) {
+      return new DecoratedProcedure({
+        ...this['~orpc'],
+        middlewares: addMiddleware(this['~orpc'].middlewares, mapped),
+      })
+    }
+
     return new DecoratedProcedure({
       ...this['~orpc'],
       middlewares: addMiddleware(this['~orpc'].middlewares, mapped),
+      errorMap: mergeErrorMap(this['~orpc'].errorMap, middleware.errorMap),
     })
   }
 
diff --git a/packages/server/tests/error-middleware-pattern.test.ts b/packages/server/tests/error-middleware-pattern.test.ts
new file mode 100644
index 000000000..d3bc956e2
--- /dev/null
+++ b/packages/server/tests/error-middleware-pattern.test.ts
@@ -0,0 +1,80 @@
+import { createSafeClient } from '@orpc/client'
+import { createRouterClient, os } from '../src'
+
+describe('error middleware patterns', () => {
+  it('should have defined=true when using original supported pattern (same base for middleware and procedure)', async () => {
+    // ✅ CORRECT: Define errors on base, use same base for both middleware and procedure
+    const base = os.errors({
+      UNAUTHORIZED: {},
+    })
+
+    const middleware = base.middleware(async ({ next, context, errors }) => {
+      throw errors.UNAUTHORIZED()
+    })
+
+    const router = base.use(middleware).handler(async () => {})
+
+    const client = createSafeClient(createRouterClient(router))
+    const [error,, isDefined] = await client()
+
+    // Should have defined=true because error was thrown from defined error map
+    expect((error as any).defined).toBe(true)
+    expect(isDefined).toBe(true)
+    expect((error as any).code).toBe('UNAUTHORIZED')
+  })
+
+  it('should have defined=true with automatic error map merging (new behavior)', async () => {
+    // ✅ NEW BEHAVIOR: Middleware error maps are automatically merged
+    const middleware = os.errors({
+      UNAUTHORIZED: {},
+    }).middleware(async ({ next, context, errors }) => {
+      throw errors.UNAUTHORIZED()
+    })
+
+    // Using base os (no errors) but middleware error map should be automatically merged
+    const router = os.use(middleware).handler(async () => {})
+
+    const client = createSafeClient(createRouterClient(router))
+    const [error,, isDefined] = await client()
+
+    // Should now have defined=true because middleware error map gets merged automatically
+    expect((error as any).defined).toBe(true)
+    expect(isDefined).toBe(true)
+    expect((error as any).code).toBe('UNAUTHORIZED')
+    // Verify the error map was merged
+    expect(router['~orpc'].errorMap).toHaveProperty('UNAUTHORIZED')
+  })
+
+  it('should merge errors from different sources correctly', async () => {
+    const middleware = os.errors({
+      UNAUTHORIZED: {},
+    }).middleware(async ({ next, context, errors }) => {
+      // Don't throw, just continue to test error map merging
+      return next({ context })
+    })
+    const router = os
+      .use(middleware)
+      .errors({ NOT_FOUND: {} })
+      .handler(async ({ errors }) => {
+        // Should have access to both UNAUTHORIZED and NOT_FOUND
+        expect('UNAUTHORIZED' in errors).toBe(true)
+        expect('NOT_FOUND' in errors).toBe(true)
+
+        // @ts-expect-error TODO: Currently, errors defined in middleware is not inferred into the procedure
+        const unauthorizedError = errors.UNAUTHORIZED()
+        const notFoundError = errors.NOT_FOUND()
+
+        expect(unauthorizedError.defined).toBe(true)
+        expect(notFoundError.defined).toBe(true)
+
+        throw notFoundError
+      })
+
+    const client = createSafeClient(createRouterClient(router))
+    const [error,, isDefined] = await client()
+
+    expect((error as any).defined).toBe(true)
+    expect(isDefined).toBe(true)
+    expect((error as any).code).toBe('NOT_FOUND')
+  })
+})