diff --git a/packages/jsonrpc/src/error.ts b/packages/jsonrpc/src/error.ts
new file mode 100644
index 0000000..98812a6
--- /dev/null
+++ b/packages/jsonrpc/src/error.ts
@@ -0,0 +1,56 @@
+export interface IError {
+  code: number
+  message: string
+}
+
+export interface IErrorWithData<T> extends IError {
+  data: T
+}
+
+export interface IErrorResponse<T> {
+  jsonrpc: '2.0'
+  id: string | number | null
+  result: null
+  error: IErrorWithData<T>
+}
+
+export interface IJSONRPCError<T> extends Error {
+  code: number
+  statusCode: number
+  response: IErrorResponse<T>
+}
+
+export function isJSONRPCError(err: any): err is IJSONRPCError<unknown> {
+  return err.name === 'IJSONRPCError' &&
+    typeof err.message === 'string' &&
+    err.hasOwnProperty('code') &&
+    err.hasOwnProperty('response')
+}
+
+export function createError<T = null>(
+  error: IError,
+  info: {
+    id: number | string | null
+    data: T
+    statusCode: number
+  },
+): IJSONRPCError<T> {
+
+  const err = new Error(error.message) as IJSONRPCError<T>
+
+  err.name = 'IJSONRPCError'
+  err.code = error.code
+  err.statusCode = info.statusCode
+  err.response = {
+    jsonrpc: '2.0',
+    id: info.id,
+    error: {
+      message: error.message,
+      code: error.code,
+      data: info.data,
+    },
+    result: null,
+  }
+
+  return err
+}
diff --git a/packages/jsonrpc/src/express.test.ts b/packages/jsonrpc/src/express.test.ts
index a3753e5..df3e0b5 100644
--- a/packages/jsonrpc/src/express.test.ts
+++ b/packages/jsonrpc/src/express.test.ts
@@ -55,14 +55,15 @@ describe('jsonrpc', () => {
   function createApp() {
     const app = express()
     app.use(bodyParser.json())
-    app.use('/myService', jsonrpc(new Service(5), [
+    app.use('/myService', jsonrpc(req => ({userId: 1000}))
+      .addService(new Service(5), [
       'add',
       'delay',
       'syncError',
       'asyncError',
       'httpError',
       'addWithContext',
-    ], req => ({userId: 1000})))
+    ]))
     return app
   }
 
@@ -97,6 +98,7 @@ describe('jsonrpc', () => {
         result: null,
         error: {
           code: -32000,
+          data: null,
           message: 'Server error',
         },
       })
@@ -106,19 +108,19 @@ describe('jsonrpc', () => {
       expect(err.message).toBe('Server error')
       expect(err.code).toBe(-32000)
     })
-    it('returns an error when message is not in json format', async () => {
+    it('returns an error when message is not readable', async () => {
       const result = await request(createApp())
       .post('/myService')
       .send('a=1')
       .expect(400)
-      expect(result.body.error.message).toEqual('Parse error')
+      expect(result.body.error.message).toEqual('Invalid request')
     })
     it('returns an error when message is not valid', async () => {
       const result = await request(createApp())
       .post('/myService')
       .send({})
       .expect(400)
-      expect(result.body.error.message).toEqual('Invalid Request')
+      expect(result.body.error.message).toEqual('Invalid request')
     })
     it('converts http errors into jsonrpc errors', async () => {
       const err = await getError(client.httpError(403, 'Unauthorized'))
diff --git a/packages/jsonrpc/src/express.ts b/packages/jsonrpc/src/express.ts
index 5126ac6..6cfb244 100644
--- a/packages/jsonrpc/src/express.ts
+++ b/packages/jsonrpc/src/express.ts
@@ -1,182 +1,68 @@
 import {FunctionPropertyNames} from './types'
 import {NextFunction, Request, Response, Router} from 'express'
-
-export const ERROR_PARSE = {
-  code: -32700, message: 'Parse error', data: null}
-export const ERROR_INVALID_REQUEST = {
-  code: -32600,
-  message: 'Invalid Request',
-  data: null}
-export const ERROR_METHOD_NOT_FOUND = {
-  code: -32601,
-  message: 'Method not found',
-  data: null,
-}
-export const ERROR_INVALID_PARAMS = {
-  code: -32602,
-  message: 'Invalid params',
-  data: null,
-}
-export const ERROR_INTERNAL_ERROR = {
-  code: -32603,
-  message: 'Internal error',
-  data: null,
-}
-export const ERROR_SERVER = {
-  code: -32000,
-  message: 'Server error',
-  data: null,
-}
-
-export interface ISuccessResponse<T> {
-  jsonrpc: '2.0'
-  id: number
-  result: T
-  error: null
-}
-
-export interface IJSONRPCError<T> {
-  code: number
-  message: string
-  data: T
-}
-
-export interface IErrorResponse<T> {
-  jsonrpc: '2.0'
-  id: number | null
-  result: null
-  error: IJSONRPCError<T>
-}
-
-export type IRPCResponse<T> = ISuccessResponse<T> | IErrorResponse<T>
-
-export function createSuccessResponse<T>(id: number, result: T)
-  : ISuccessResponse<T> {
-  return {
-    jsonrpc: '2.0',
-    id,
-    result,
-    error: null,
-  }
-}
-
-export function createErrorResponse<T>(
-  id: number | null, error: IJSONRPCError<T>)
-  : IErrorResponse<T> {
-  return {
-    jsonrpc: '2.0',
-    id,
-    result: null,
-    error,
-  }
-}
-
-export function pick<T, K extends FunctionPropertyNames<T>>(t: T, keys: K[])
-  : Pick<T, K> {
-  return keys.reduce((obj, key) => {
-    // tslint:disable-next-line
-    const fn = t[key] as unknown as Function
-    obj[key] = fn.bind(t)
-    return obj
-  }, {} as Pick<T, K>)
-}
-
-function isPromise(value: any): value is Promise<unknown> {
-  return value !== null && typeof value === 'object' &&
-    'then' in value && 'catch' in value &&
-    typeof value.then === 'function' && typeof value.catch === 'function'
-}
+import {createRpcService, ERROR_SERVER} from './jsonrpc'
+import {createError, isJSONRPCError, IJSONRPCError} from './error'
 
 export type TGetContext<Context> = (req: Request) => Context
 
-/**
- * Adds middleware for handling JSON RPC requests. Expects JSON middleware to
- * already be configured.
- */
-export function jsonrpc<T, F extends FunctionPropertyNames<T>, Context>(
-  service: T,
-  methods: F[],
+export function jsonrpc<Context>(
   getContext: TGetContext<Context>,
+  idempotentMethodRegex = /^(find|fetch|get)/,
 ) {
-  const rpcService = pick(service, methods)
-
-  const router = Router()
-
-  router.post('/', (req, res, next) => {
-    if (req.headers['content-type'] !== 'application/json') {
-      res.status(400)
-      return res.json(createErrorResponse(null, ERROR_PARSE))
-    }
-    const {id, method, params} = req.body
-    const isNotification = id === null || id === undefined
-    if (
-      req.body.jsonrpc !== '2.0' ||
-      typeof method !== 'string' ||
-      !Array.isArray(params)
+  return {
+   /**
+    * Adds middleware for handling JSON RPC requests. Expects JSON middleware to
+    * already be configured.
+    */
+    addService<T, F extends FunctionPropertyNames<T>>(
+      service: T,
+      methods: F[],
     ) {
-      res.status(400)
-      return res.json(createErrorResponse(id, ERROR_INVALID_REQUEST))
-    }
+      const callRpcService = createRpcService(service, methods)
 
-    if (
-      !rpcService.hasOwnProperty(method) ||
-      typeof (rpcService as any)[method] !== 'function'
-    ) {
-      res.status(404)
-      return res.json(createErrorResponse(id, ERROR_METHOD_NOT_FOUND))
-    }
+      const router = Router()
 
-    // if (rpcService[method].arguments.length !== params.length) {
-    //   return res.json(createErrorResponse(null, ERROR_INVALID_PARAMS))
-    // }
+      router.post('/', (req, res, next) => {
+        callRpcService(req.body, getContext(req))
+        .then(response => {
+          if (response === null) {
+            // notification
+            res.status(204).send()
+          } else {
+            res.json(response)
+          }
+        })
+        .catch(err => handleError(req.body.id, err, req, res, next))
+      })
 
-    // TODO handle synchronous errors
-    let retValue
-    try {
-      retValue = (rpcService as any)[method](...params)
-      if (typeof retValue === 'function') {
-        retValue = retValue(getContext(req))
-      }
-    } catch (err) {
-      return handleError(err, req, res, next)
-    }
-
-    if (!isPromise(retValue)) {
-      if (isNotification) {
-        return res.status(204).send()
-      }
-      return res.json(createSuccessResponse(id, retValue))
-    }
-
-    retValue
-    .then(result => {
-      return res.json(createSuccessResponse(id, result))
-    })
-    .catch(err => handleError(err, req, res, next))
-  })
-
-  return router
+      return router
+    },
+  }
 }
 
 function handleError(
+  id: number | string | null,
   err: any,
   req: Request,
   res: Response,
   next: NextFunction,
 ) {
-  const statusCode: number = 'statusCode' in err &&
-    typeof err.statusCode === 'number'
-    ? err.statusCode
-    : 500
+  // TODO log error
+  // TODO make this nicer
 
-  const message = statusCode >= 400 && statusCode < 500
-    ? err.message
-    : ERROR_SERVER.message
+  const error: IJSONRPCError<unknown> = isJSONRPCError(err)
+    ? err
+    : createError({
+      code: ERROR_SERVER.code,
+      message: err.statusCode >= 400 && err.statusCode < 500
+        ? err.message
+        : ERROR_SERVER.message,
+    }, {
+      id,
+      data: null,
+      statusCode: err.statusCode || 500,
+    })
 
-  res.status(statusCode)
-  res.json(createErrorResponse(req.body.id, {
-    code: ERROR_SERVER.code,
-    message,
-    data: err.errors,
-  }))
+  res.status(error.statusCode)
+  res.json(error.response)
 }
diff --git a/packages/jsonrpc/src/index.ts b/packages/jsonrpc/src/index.ts
index 603f87f..e98ef36 100644
--- a/packages/jsonrpc/src/index.ts
+++ b/packages/jsonrpc/src/index.ts
@@ -1,4 +1,6 @@
+export * from './error'
 export * from './express'
+export * from './jsonrpc'
 export * from './local'
 export * from './redux'
 export * from './remote'
diff --git a/packages/jsonrpc/src/isPromise.ts b/packages/jsonrpc/src/isPromise.ts
new file mode 100644
index 0000000..1bfd79a
--- /dev/null
+++ b/packages/jsonrpc/src/isPromise.ts
@@ -0,0 +1,5 @@
+export function isPromise(value: any): value is Promise<unknown> {
+  return value !== null && typeof value === 'object' &&
+    'then' in value && 'catch' in value &&
+    typeof value.then === 'function' && typeof value.catch === 'function'
+}
diff --git a/packages/jsonrpc/src/jsonrpc.ts b/packages/jsonrpc/src/jsonrpc.ts
new file mode 100644
index 0000000..0db5a4a
--- /dev/null
+++ b/packages/jsonrpc/src/jsonrpc.ts
@@ -0,0 +1,139 @@
+export type TId = number | string
+import {ArgumentTypes, FunctionPropertyNames, RetType} from './types'
+import {isPromise} from './isPromise'
+import {createError, IErrorWithData} from './error'
+
+export const ERROR_PARSE = {
+  code: -32700,
+  message: 'Parse error',
+}
+
+export const ERROR_INVALID_REQUEST = {
+  code: -32600,
+  message: 'Invalid request',
+}
+
+export const ERROR_METHOD_NOT_FOUND = {
+  code: -32601,
+  message: 'Method not found',
+}
+
+export const ERROR_INVALID_PARAMS = {
+  code: -32602,
+  message: 'Invalid params',
+}
+
+export const ERROR_INTERNAL_ERROR = {
+  code: -32603,
+  message: 'Internal error',
+}
+
+export const ERROR_SERVER = {
+  code: -32000,
+  message: 'Server error',
+}
+
+export function pick<T, K extends FunctionPropertyNames<T>>(t: T, keys: K[])
+  : Pick<T, K> {
+  return keys.reduce((obj, key) => {
+    // tslint:disable-next-line
+    const fn = t[key] as unknown as Function
+    obj[key] = fn.bind(t)
+    return obj
+  }, {} as Pick<T, K>)
+}
+
+export interface IRequest<M extends string | symbol | number = any, A = any[]> {
+  jsonrpc: '2.0'
+  id: TId | null
+  method: M
+  params: A
+}
+
+export interface ISuccessResponse<T> {
+  jsonrpc: '2.0'
+  id: TId
+  result: T
+  error: null
+}
+
+export interface IErrorResponse<T> {
+  jsonrpc: '2.0'
+  id: TId | null
+  result: null
+  error: IErrorWithData<T>
+}
+
+export type IResponse<T = any> = ISuccessResponse<T> | IErrorResponse<T>
+
+export function createSuccessResponse<T>(id: number | string, result: T)
+  : ISuccessResponse<T> {
+  return {
+    jsonrpc: '2.0',
+    id,
+    result,
+    error: null,
+  }
+}
+
+export function createErrorResponse<T>(
+  id: number | string | null, error: IErrorWithData<T>)
+  : IErrorResponse<T> {
+  return {
+    jsonrpc: '2.0',
+    id,
+    result: null,
+    error,
+  }
+}
+
+export type TGetContext<Context> = (req: Request) => Context
+
+export const createRpcService =
+  <T, M extends FunctionPropertyNames<T>>(
+    service: T,
+    methods: M[],
+  ) => async <Context>(req: IRequest<M, ArgumentTypes<T[M]>>, context: Context)
+  : Promise<ISuccessResponse<RetType<T[M]>> | null> => {
+    const {id, method, params} = req
+
+    if (
+      req.jsonrpc !== '2.0' ||
+      typeof method !== 'string' ||
+      !Array.isArray(params)
+    ) {
+      throw createError(ERROR_INVALID_REQUEST, {
+        id,
+        data: null,
+        statusCode: 400,
+      })
+    }
+
+    const isNotification = req.id === null || req.id === undefined
+
+    const rpcService = pick(service, methods)
+
+    if (
+      !rpcService.hasOwnProperty(method) ||
+      typeof rpcService[method] !== 'function'
+    ) {
+      throw createError(ERROR_METHOD_NOT_FOUND, {
+        id,
+        data: null,
+        statusCode: 404,
+      })
+    }
+
+    let retValue = (rpcService[method] as any)(...params)
+
+    if (typeof retValue === 'function') {
+      retValue = retValue(context)
+    }
+
+    if (!isPromise(retValue) && isNotification) {
+      return null
+    }
+
+    retValue = await retValue
+    return createSuccessResponse(req.id as any, retValue)
+  }
diff --git a/packages/jsonrpc/src/redux.test.ts b/packages/jsonrpc/src/redux.test.ts
index acf7124..fa6c761 100644
--- a/packages/jsonrpc/src/redux.test.ts
+++ b/packages/jsonrpc/src/redux.test.ts
@@ -54,9 +54,10 @@ describe('createActions', () => {
 
   const app = express()
   app.use(bodyParser.json())
-  app.use('/service', jsonrpc(new Service(), keys<IService>(), () => ({
-    userId: 1000,
-  })))
+  app.use('/service', jsonrpc(() => ({userId: 1000})).addService(
+    new Service(),
+    keys<IService>(),
+  ))
 
   let baseUrl: string
   let server: Server
diff --git a/packages/jsonrpc/src/remote.test.ts b/packages/jsonrpc/src/remote.test.ts
index d262254..7e2a56c 100644
--- a/packages/jsonrpc/src/remote.test.ts
+++ b/packages/jsonrpc/src/remote.test.ts
@@ -28,7 +28,7 @@ describe('remote', () => {
   function createApp() {
     const a = express()
     a.use(bodyParser.json())
-    a.use('/myService', jsonrpc(service, IServiceKeys, () => ({})))
+    a.use('/myService', jsonrpc(() => ({})).addService(service, IServiceKeys))
     return a
   }
 
diff --git a/packages/jsonrpc/src/types.ts b/packages/jsonrpc/src/types.ts
index 95dd5df..539949e 100644
--- a/packages/jsonrpc/src/types.ts
+++ b/packages/jsonrpc/src/types.ts
@@ -1,8 +1,8 @@
 import {IPendingAction, IResolvedAction, IRejectedAction} from '@rondo/client'
 
-type ArgumentTypes<T> =
+export type ArgumentTypes<T> =
   T extends (...args: infer U) => infer R ? U : never
-type RetType<T> = T extends (...args: any[]) => infer R ? R : never
+export type RetType<T> = T extends (...args: any[]) => infer R ? R : never
 type UnwrapHOC<T> = T extends (...args: any[]) => infer R ? R : T
 type UnwrapPromise<T> = T extends Promise<infer V> ? V : T
 type RetProm<T> = T extends Promise<any> ? T : Promise<T>