# http4s

`mcp-http4s` · JVM · Scala.js · Scala Native

Streamable HTTP + SSE transport via http4s Ember. Two builder entry points:

- `McpHttp.basic[IO]` — plain request/response MCP over HTTP
- `McpHttp.streaming[IO]` — adds session management, resource subscriptions, server-initiated logging, cancellation, and per-session state via `.stateful` / `.authenticated` chains

```scala
libraryDependencies += "net.andimiller.mcp" %%% "mcp-http4s" % "0.10.0"
```

A minimal HTTP server built on `McpHttp.basic` looks like:

```scala
import cats.effect.{IO, IOApp}
import com.comcast.ip4s.*
import net.andimiller.mcp.http4s.McpHttp

object MyHttpServer extends IOApp.Simple:
  def run: IO[Unit] =
    McpHttp.basic[IO]
      .name("my-server").version("1.0.0")
      .port(port"8080")
      .serve
      .useForever
```

`.serve` returns a `Resource[IO, http4s.server.Server]`; tools, resources,
and prompts are added with `.withTool` / `.withResource` / `.withPrompt`
before the call.

See [Server construction → HTTP server](../getting-started/server-construction.md#http-server-basic)
for the server side, or [Clients → Client construction](../clients/client-construction.md#http-client)
for connecting to a streamable HTTP server as an `McpClient`.

## Embedded Explorer UI

The `http4s` module bundles the [Explorer](explorer.md) — a Scala.js + Tyrian
browser UI for exploring tools, resources, templates, and prompts on any HTTP
MCP server. Enable it on your server with `.withExplorer(...)`:

```scala
import cats.effect.IO
import net.andimiller.mcp.http4s.McpHttp

val builder =
  McpHttp.streaming[IO]
    .withExplorer(redirectToRoot = true) // serves at /explorer, optionally redirects / there
```

The Explorer is served at `/explorer/index.html` and defaults the connection
URL to the current origin + `/mcp`.

The Explorer assets are pre-built into the http4s module's classpath
resources. To rebuild after changes to the Explorer source:

```bash
sbt buildExplorer
```

This compiles the Scala.js app and runs Parcel to bundle the JS and CSS into
`modules/explorer/dist/`, which is then copied into the http4s classpath
resources.