Making Outgoing HTTP Requests (Scala)

Overview

Golem Scala agents are compiled to JavaScript via Scala.js and run in a QuickJS-based WASM runtime. The recommended way to make HTTP requests is using the standard fetch API via Scala.js interop, or using ZIO HTTP which internally uses the same WASI HTTP layer.

Option 1: Using `fetch` via Scala.js (Recommended for Simple Requests)

Since Golem Scala apps compile to JavaScript, the global fetch function is available:


import scala.scalajs.js
import scala.scalajs.js.Thenable.Implicits._
import scala.concurrent.Future
import scala.concurrent.ExecutionContext.Implicits.global
 
def fetchData(url: String): Future[String] = {
  val options = js.Dynamic.literal(
    method = "GET",
    headers = js.Dynamic.literal(
      "Accept" -> "application/json"
    )
  )
 
  for {
    response <- js.Dynamic.global.fetch(url, options)
                  .asInstanceOf[js.Promise[js.Dynamic]].toFuture
    text     <- response.text().asInstanceOf[js.Promise[String]].toFuture
  } yield text
}

POST with JSON Body


def postData(url: String, payload: String): Future[String] = {
  val options = js.Dynamic.literal(
    method = "POST",
    headers = js.Dynamic.literal(
      "Content-Type" -> "application/json",
      "Accept" -> "application/json"
    ),
    body = payload
  )
 
  for {
    response <- js.Dynamic.global.fetch(url, options)
                  .asInstanceOf[js.Promise[js.Dynamic]].toFuture
    text     <- response.text().asInstanceOf[js.Promise[String]].toFuture
  } yield text
}

Option 2: Using ZIO HTTP (Recommended for ZIO-Based Agents)

If you already use ZIO in your agent, zio-http provides a typed Scala HTTP client:


import zio._
import zio.http._
import scala.concurrent.Future
 
def fetchFromUrl(url: String): Future[String] = {
  val effect =
    (for {
      response <- ZIO.serviceWithZIO[Client] { client =>
                    client.url(URL.decode(url).toOption.get).batched.get("/")
                  }
      body <- response.body.asString
    } yield body).provide(ZClient.default)
 
  Unsafe.unsafe { implicit u =>
    Runtime.default.unsafe.runToFuture(effect)
  }
}

ZIO HTTP POST Example


import zio._
import zio.http._
import scala.concurrent.Future
 
def postJson(url: String, jsonBody: String): Future[String] = {
  val effect =
    (for {
      response <- ZIO.serviceWithZIO[Client] { client =>
                    client
                      .url(URL.decode(url).toOption.get)
                      .addHeader(Header.ContentType(MediaType.application.json))
                      .batched
                      .post("/")(Body.fromString(jsonBody))
                  }
      body <- response.body.asString
    } yield body).provide(ZClient.default)
 
  Unsafe.unsafe { implicit u =>
    Runtime.default.unsafe.runToFuture(effect)
  }
}

Note: ZIO HTTP requires zio-http as a Scala.js-compatible dependency in your build.sbt.

Complete Example in an Agent


import golem.runtime.annotations.{agentDefinition, agentImplementation, endpoint}
import golem.BaseAgent
import scala.scalajs.js
import scala.scalajs.js.Thenable.Implicits._
import scala.concurrent.Future
import scala.concurrent.ExecutionContext.Implicits.global
 
@agentDefinition(mount = "/weather/{value}")
trait WeatherAgent extends BaseAgent {
  class Id(val value: String)
 
  @endpoint(method = "GET", path = "/current")
  def getCurrent(): Future[String]
}
 
@agentImplementation()
final class WeatherAgentImpl(private val city: String) extends WeatherAgent {
 
  override def getCurrent(): Future[String] = {
    val url = s"https://api.weather.example.com/current?city=$city"
 
    val options = js.Dynamic.literal(
      method = "GET",
      headers = js.Dynamic.literal("Accept" -> "application/json")
    )
 
    for {
      response <- js.Dynamic.global.fetch(url, options)
                    .asInstanceOf[js.Promise[js.Dynamic]].toFuture
      text     <- response.text().asInstanceOf[js.Promise[String]].toFuture
    } yield text
  }
}

Calling Golem Agent HTTP Endpoints

When making HTTP requests to other Golem agent endpoints (or your own), the request body must match the Golem HTTP body mapping convention: non-binary body parameters are always deserialized from a JSON object where each top-level field corresponds to a method parameter name. This is true even when the endpoint has a single body parameter.

For example, given this endpoint definition:


@endpoint(method = "POST", path = "/record")
def record(body: String): Future[Unit]

The correct HTTP request must send a JSON object with a body field — not a raw text string:


// ✅ CORRECT — JSON object with field name matching the parameter
val options = js.Dynamic.literal(
  method = "POST",
  headers = js.Dynamic.literal("Content-Type" -> "application/json"),
  body = """{"body": "a"}"""
)
js.Dynamic.global.fetch("http://my-app.localhost:9006/recorder/main/record", options)
  .asInstanceOf[js.Promise[js.Dynamic]].toFuture
 
// ❌ WRONG — raw text body does NOT match Golem's JSON body mapping
val options = js.Dynamic.literal(
  method = "POST",
  headers = js.Dynamic.literal("Content-Type" -> "text/plain"),
  body = "a"
)

Rule of thumb: If the target endpoint is a Golem agent, always send application/json with parameter names as JSON keys. See the golem-http-params-scala skill for the full body mapping rules.

Key Constraints

Golem Scala apps are compiled to JavaScript via Scala.js — the fetch API is available as a global function
For simple requests, use js.Dynamic.global.fetch directly
For ZIO-based agents, use zio-http which provides a typed Scala API
Third-party JVM HTTP clients (Apache HttpClient, OkHttp, sttp with non-JS backends) will NOT work — they depend on JVM networking APIs
Libraries must be Scala.js-compatible (use %%% in build.sbt)
All HTTP requests go through the WASI HTTP layer under the hood