diff --git a/adapters/http.ts b/adapters/http.ts
index 50d485a..e9daf1e 100644
--- a/adapters/http.ts
+++ b/adapters/http.ts
@@ -1,4 +1,5 @@
 import type { RelayAdapter, RelayRequestInput, RelayResponse } from "../libraries/adapter.ts";
+import { RelayError, UnprocessableContentError } from "../libraries/errors.ts";
 
 export class HttpAdapter implements RelayAdapter {
   #id: number = 0;
@@ -6,10 +7,28 @@ export class HttpAdapter implements RelayAdapter {
   constructor(readonly url: string) {}
 
   async send({ method, params }: RelayRequestInput): Promise<RelayResponse> {
-    const res = await fetch(this.url, { method: "POST", headers: { "content-type": "application/json" }, body: JSON.stringify({ method, params, id: this.#id++ }) });
-    if (res.headers.get("content-type")?.includes("application/json") === false) {
-      throw new Error("Unexpected return type");
+    const id = this.#id++;
+    const res = await fetch(this.url, {
+      method: "POST",
+      headers: { "content-type": "application/json" },
+      body: JSON.stringify({ relay: "1.0", method, params, id }),
+    });
+    const contentType = res.headers.get("content-type");
+    if (contentType !== "application/json") {
+      return {
+        relay: "1.0",
+        error: new UnprocessableContentError(`Invalid 'content-type' in header header, expected 'application/json', received '${contentType}'`),
+        id,
+      };
     }
-    return res.json();
+    const json = await res.json();
+    if ("error" in json) {
+      return {
+        relay: "1.0",
+        error: RelayError.fromJSON(json.error),
+        id,
+      };
+    }
+    return json;
   }
 }
diff --git a/deno.json b/deno.json
index 71adc30..a1dd375 100644
--- a/deno.json
+++ b/deno.json
@@ -1,6 +1,6 @@
 {
   "name": "@valkyr/relay",
-  "version": "0.3.0",
+  "version": "0.3.1",
   "exports": {
     ".": "./mod.ts",
     "./http": "./adapters/http.ts"
diff --git a/libraries/adapter.ts b/libraries/adapter.ts
index c810390..f0db0f0 100644
--- a/libraries/adapter.ts
+++ b/libraries/adapter.ts
@@ -1,3 +1,5 @@
+import { RelayError } from "./errors.ts";
+
 export type RelayAdapter = {
   send(input: RelayRequestInput): Promise<RelayResponse>;
 };
@@ -9,12 +11,12 @@ export type RelayRequestInput = {
 
 export type RelayResponse =
   | {
+      relay: "1.0";
       result: unknown;
-      id: string;
+      id: string | number;
     }
   | {
-      error: {
-        message: string;
-      };
-      id: string;
+      relay: "1.0";
+      error: RelayError;
+      id: string | number;
     };
diff --git a/libraries/api.ts b/libraries/api.ts
index e8b87f3..0895f5b 100644
--- a/libraries/api.ts
+++ b/libraries/api.ts
@@ -2,6 +2,7 @@ import z from "zod";
 
 import { BadRequestError, InternalServerError, NotFoundError, RelayError } from "./errors.ts";
 import { Procedure } from "./procedure.ts";
+import { RelayRequest, request } from "./request.ts";
 
 export class RelayApi<TProcedures extends Procedure[]> {
   /**
@@ -21,6 +22,15 @@ export class RelayApi<TProcedures extends Procedure[]> {
     }
   }
 
+  /**
+   * Takes a request candidate and parses its json body.
+   *
+   * @param candidate - Request candidate to parse.
+   */
+  async parse(candidate: Request): Promise<RelayRequest> {
+    return request.parseAsync(await candidate.json());
+  }
+
   /**
    * Handle a incoming fetch request.
    *
@@ -28,7 +38,7 @@ export class RelayApi<TProcedures extends Procedure[]> {
    * @param params - Parameters provided with the method request.
    * @param id     - Request id used for response identification.
    */
-  async call(method: string, params: unknown, id: string): Promise<Response> {
+  async call({ method, params, id }: RelayRequest): Promise<Response> {
     const procedure = this.#index.get(method);
     if (procedure === undefined) {
       return toResponse(new NotFoundError(`Method '' does not exist`), id);
@@ -112,10 +122,11 @@ export class RelayApi<TProcedures extends Procedure[]> {
  * @param result - Result to send back as a Response.
  * @param id     - Request id which can be used to identify the response.
  */
-function toResponse(result: object | RelayError | Response | void, id: string): Response {
+export function toResponse(result: object | RelayError | Response | void, id: string | number): Response {
   if (result === undefined) {
     return new Response(
       JSON.stringify({
+        relay: "1.0",
         result: null,
         id,
       }),
@@ -133,6 +144,7 @@ function toResponse(result: object | RelayError | Response | void, id: string):
   if (result instanceof RelayError) {
     return new Response(
       JSON.stringify({
+        relay: "1.0",
         error: result,
         id,
       }),
@@ -146,6 +158,7 @@ function toResponse(result: object | RelayError | Response | void, id: string):
   }
   return new Response(
     JSON.stringify({
+      relay: "1.0",
       result,
       id,
     }),
diff --git a/libraries/errors.ts b/libraries/errors.ts
index 2003d62..c51b145 100644
--- a/libraries/errors.ts
+++ b/libraries/errors.ts
@@ -1,17 +1,50 @@
-export abstract class RelayError<D = unknown> extends Error {
+export abstract class RelayError<TData = unknown> extends Error {
   constructor(
     message: string,
     readonly status: number,
-    readonly data?: D,
+    readonly data?: TData,
   ) {
     super(message);
   }
 
-  toJSON(): {
-    status: number;
-    message: string;
-    data: any;
-  } {
+  /**
+   * Converts a server delivered JSON error to its native instance.
+   *
+   * @param value - Error JSON.
+   */
+  static fromJSON(value: RelayErrorJSON): RelayErrorType {
+    switch (value.status) {
+      case 400:
+        return new BadRequestError(value.message, value.data);
+      case 401:
+        return new UnauthorizedError(value.message, value.data);
+      case 403:
+        return new ForbiddenError(value.message, value.data);
+      case 404:
+        return new NotFoundError(value.message, value.data);
+      case 405:
+        return new MethodNotAllowedError(value.message, value.data);
+      case 406:
+        return new NotAcceptableError(value.message, value.data);
+      case 409:
+        return new ConflictError(value.message, value.data);
+      case 410:
+        return new GoneError(value.message, value.data);
+      case 415:
+        return new UnsupportedMediaTypeError(value.message, value.data);
+      case 422:
+        return new UnprocessableContentError(value.message, value.data);
+      case 503:
+        return new ServiceUnavailableError(value.message, value.data);
+      default:
+        return new InternalServerError(value.message, value.data);
+    }
+  }
+
+  /**
+   * Convert error instance to a JSON object.
+   */
+  toJSON(): RelayErrorJSON {
     return {
       status: this.status,
       message: this.message,
@@ -20,7 +53,7 @@ export abstract class RelayError<D = unknown> extends Error {
   }
 }
 
-export class BadRequestError<D = unknown> extends RelayError<D> {
+export class BadRequestError<TData = unknown> extends RelayError<TData> {
   /**
    * Instantiate a new BadRequestError.
    *
@@ -30,12 +63,12 @@ export class BadRequestError<D = unknown> extends RelayError<D> {
    *
    * @param data - Optional data to send with the error.
    */
-  constructor(message = "Bad Request", data?: D) {
+  constructor(message = "Bad Request", data?: TData) {
     super(message, 400, data);
   }
 }
 
-export class UnauthorizedError<D = unknown> extends RelayError<D> {
+export class UnauthorizedError<TData = unknown> extends RelayError<TData> {
   /**
    * Instantiate a new UnauthorizedError.
    *
@@ -56,12 +89,12 @@ export class UnauthorizedError<D = unknown> extends RelayError<D> {
    * @param message - Optional message to send with the error. Default: "Unauthorized".
    * @param data    - Optional data to send with the error.
    */
-  constructor(message = "Unauthorized", data?: D) {
+  constructor(message = "Unauthorized", data?: TData) {
     super(message, 401, data);
   }
 }
 
-export class ForbiddenError<D = unknown> extends RelayError<D> {
+export class ForbiddenError<TData = unknown> extends RelayError<TData> {
   /**
    * Instantiate a new ForbiddenError.
    *
@@ -77,12 +110,12 @@ export class ForbiddenError<D = unknown> extends RelayError<D> {
    * @param message - Optional message to send with the error. Default: "Forbidden".
    * @param data    - Optional data to send with the error.
    */
-  constructor(message = "Forbidden", data?: D) {
+  constructor(message = "Forbidden", data?: TData) {
     super(message, 403, data);
   }
 }
 
-export class NotFoundError<D = unknown> extends RelayError<D> {
+export class NotFoundError<TData = unknown> extends RelayError<TData> {
   /**
    * Instantiate a new NotFoundError.
    *
@@ -99,12 +132,12 @@ export class NotFoundError<D = unknown> extends RelayError<D> {
    * @param message - Optional message to send with the error. Default: "Not Found".
    * @param data    - Optional data to send with the error.
    */
-  constructor(message = "Not Found", data?: D) {
+  constructor(message = "Not Found", data?: TData) {
     super(message, 404, data);
   }
 }
 
-export class MethodNotAllowedError<D = unknown> extends RelayError<D> {
+export class MethodNotAllowedError<TData = unknown> extends RelayError<TData> {
   /**
    * Instantiate a new MethodNotAllowedError.
    *
@@ -116,12 +149,12 @@ export class MethodNotAllowedError<D = unknown> extends RelayError<D> {
    * @param message - Optional message to send with the error. Default: "Method Not Allowed".
    * @param data    - Optional data to send with the error.
    */
-  constructor(message = "Method Not Allowed", data?: D) {
+  constructor(message = "Method Not Allowed", data?: TData) {
     super(message, 405, data);
   }
 }
 
-export class NotAcceptableError<D = unknown> extends RelayError<D> {
+export class NotAcceptableError<TData = unknown> extends RelayError<TData> {
   /**
    * Instantiate a new NotAcceptableError.
    *
@@ -133,12 +166,12 @@ export class NotAcceptableError<D = unknown> extends RelayError<D> {
    * @param message - Optional message to send with the error. Default: "Not Acceptable".
    * @param data    - Optional data to send with the error.
    */
-  constructor(message = "Not Acceptable", data?: D) {
+  constructor(message = "Not Acceptable", data?: TData) {
     super(message, 406, data);
   }
 }
 
-export class ConflictError<D = unknown> extends RelayError<D> {
+export class ConflictError<TData = unknown> extends RelayError<TData> {
   /**
    * Instantiate a new ConflictError.
    *
@@ -154,12 +187,12 @@ export class ConflictError<D = unknown> extends RelayError<D> {
    * @param message - Optional message to send with the error. Default: "Conflict".
    * @param data    - Optional data to send with the error.
    */
-  constructor(message = "Conflict", data?: D) {
+  constructor(message = "Conflict", data?: TData) {
     super(message, 409, data);
   }
 }
 
-export class GoneError<D = unknown> extends RelayError<D> {
+export class GoneError<TData = unknown> extends RelayError<TData> {
   /**
    * Instantiate a new GoneError.
    *
@@ -177,12 +210,12 @@ export class GoneError<D = unknown> extends RelayError<D> {
    * @param message - Optional message to send with the error. Default: "Gone".
    * @param data    - Optional data to send with the error.
    */
-  constructor(message = "Gone", data?: D) {
+  constructor(message = "Gone", data?: TData) {
     super(message, 410, data);
   }
 }
 
-export class UnsupportedMediaTypeError<D = unknown> extends RelayError<D> {
+export class UnsupportedMediaTypeError<TData = unknown> extends RelayError<TData> {
   /**
    * Instantiate a new UnsupportedMediaTypeError.
    *
@@ -194,12 +227,12 @@ export class UnsupportedMediaTypeError<D = unknown> extends RelayError<D> {
    * @param message - Optional message to send with the error. Default: "Unsupported Media Type".
    * @param data    - Optional data to send with the error.
    */
-  constructor(message = "Unsupported Media Type", data?: D) {
+  constructor(message = "Unsupported Media Type", data?: TData) {
     super(message, 415, data);
   }
 }
 
-export class UnprocessableContentError<D = unknown> extends RelayError<D> {
+export class UnprocessableContentError<TData = unknown> extends RelayError<TData> {
   /**
    * Instantiate a new UnprocessableContentError.
    *
@@ -216,12 +249,12 @@ export class UnprocessableContentError<D = unknown> extends RelayError<D> {
    * @param message - Optional message to send with the error. Default: "Unprocessable Content".
    * @param data    - Optional data to send with the error.
    */
-  constructor(message = "Unprocessable Content", data?: D) {
+  constructor(message = "Unprocessable Content", data?: TData) {
     super(message, 422, data);
   }
 }
 
-export class InternalServerError<D = unknown> extends RelayError<D> {
+export class InternalServerError<TData = unknown> extends RelayError<TData> {
   /**
    * Instantiate a new InternalServerError.
    *
@@ -239,12 +272,12 @@ export class InternalServerError<D = unknown> extends RelayError<D> {
    * @param message - Optional message to send with the error. Default: "Internal Server Error".
    * @param data    - Optional data to send with the error.
    */
-  constructor(message = "Internal Server Error", data?: D) {
+  constructor(message = "Internal Server Error", data?: TData) {
     super(message, 500, data);
   }
 }
 
-export class ServiceUnavailableError<D = unknown> extends RelayError<D> {
+export class ServiceUnavailableError<TData = unknown> extends RelayError<TData> {
   /**
    * Instantiate a new ServiceUnavailableError.
    *
@@ -259,7 +292,27 @@ export class ServiceUnavailableError<D = unknown> extends RelayError<D> {
    * @param message - Optional message to send with the error. Default: "Service Unavailable".
    * @param data    - Optional data to send with the error.
    */
-  constructor(message = "Service Unavailable", data?: D) {
+  constructor(message = "Service Unavailable", data?: TData) {
     super(message, 503, data);
   }
 }
+
+export type RelayErrorJSON = {
+  status: number;
+  message: string;
+  data: any;
+};
+
+export type RelayErrorType =
+  | BadRequestError
+  | UnauthorizedError
+  | ForbiddenError
+  | NotFoundError
+  | MethodNotAllowedError
+  | NotAcceptableError
+  | ConflictError
+  | GoneError
+  | UnsupportedMediaTypeError
+  | UnprocessableContentError
+  | ServiceUnavailableError
+  | InternalServerError;
diff --git a/libraries/request.ts b/libraries/request.ts
new file mode 100644
index 0000000..d9ff296
--- /dev/null
+++ b/libraries/request.ts
@@ -0,0 +1,10 @@
+import z from "zod";
+
+export const request = z.object({
+  relay: z.literal("1.0"),
+  method: z.string(),
+  params: z.unknown(),
+  id: z.string().or(z.number()),
+});
+
+export type RelayRequest = z.infer<typeof request>;
diff --git a/mod.ts b/mod.ts
index f3e91f4..e0bb905 100644
--- a/mod.ts
+++ b/mod.ts
@@ -4,3 +4,4 @@ export * from "./libraries/api.ts";
 export * from "./libraries/errors.ts";
 export * from "./libraries/procedure.ts";
 export * from "./libraries/relay.ts";
+export * from "./libraries/request.ts";
diff --git a/tests/procedure.test.ts b/tests/procedure.test.ts
index bf9fd8f..af0dfdb 100644
--- a/tests/procedure.test.ts
+++ b/tests/procedure.test.ts
@@ -19,8 +19,7 @@ describe("Procedure", () => {
         },
       },
       async (request) => {
-        const { method, params, id } = await request.json();
-        return api.call(method, params, id);
+        return api.call(await api.parse(request));
       },
     );
     client = relay.client({