HTTP Request and Response Parameter Mapping (Scala)

Overview

When an agent is exposed over HTTP, Golem maps parts of each HTTP request to constructor and method parameters. This skill covers how path segments, query parameters, headers, and request bodies are mapped, which types are supported for each, and how return types map to HTTP responses.

Path Variables

Path variables {varName} in mount or endpoint paths map to parameters by name:

// Mount path variables → constructor parameters (from class Id)
@agentDefinition(mount = "/api/tasks/{name}")
trait TaskAgent extends BaseAgent {
  class Id(val name: String)
 
  // Endpoint path variables → method parameters
  @endpoint(method = "GET", path = "/items/{itemId}")
  def getItem(itemId: String): Future[Item]
}

Remaining (catch-all) path variables capture everything after a prefix:

@endpoint(method = "GET", path = "/files/{*filePath}")
def getFile(filePath: String): Future[String]
// GET .../files/docs/readme.md → filePath = "docs/readme.md"

Catch-all variables can only appear as the last path segment and are not allowed in mount paths.

Query Parameters

Specified in the endpoint path using ?key={var} syntax:

@endpoint(method = "GET", path = "/search?q={query}&limit={n}")
def search(query: String, n: Int): Future[String]
// GET .../search?q=hello&limit=10

Header Variables

Map HTTP headers to parameters using the @header annotation on individual parameters:

import golem.runtime.annotations.header
 
@endpoint(method = "POST", path = "/report")
def submitReport(
  @header("X-Tenant") tenantId: String,
  data: String
): Future[String]

Supported Types for Path, Query, and Header Variables

Only these types can be used for parameters bound to path/query/header variables (the value is parsed from the URL/header string):

For query parameters and headers only (not path variables), two additional wrapper types are supported:

All other types (case classes, tuples, sealed traits with data, etc.) can only be used as body parameters.

POST Request Body Mapping

User-supplied method parameters not bound to path variables, query parameters, or headers are body parameters. Ordinary structured parameters are populated from the JSON request body:

@endpoint(method = "POST", path = "/items/{id}")
def updateItem(id: String, name: String, count: Int): Future[Item]
// POST .../items/123
// Body: { "name": "Widget", "count": 5 }
// → id from path, name and count from body

Each ordinary structured body parameter becomes a top-level field in the expected JSON body object. All custom types require a zio.blocks.schema.Schema instance.

⚠️ Important: Ordinary structured bodies use a JSON object with body parameter names only as keys, even for one parameter. def decide(decision: String) expects {"decision": "approved"}, not a bare string. A single object parameter named payload also needs wrapping: {"payload": {"name": "Widget", "count": 5}}, not {"name": "Widget", "count": 5}. Do not include path/query/header parameters. Unstructured text and binary parameters instead consume the raw body; see below.

CLI positional invocation is different: pass one argument per user-supplied method parameter in signature order, including parameters bound to path/query/header values over HTTP. Skip host-injected Principal fields without a principal value or placeholder. Use Scala native literals for each value, not one JSON object wrapping the full input. A single object parameter is one native case-class literal without the HTTP payload wrapper. Load golem-invoke-agent-scala for CLI examples and caller-only input rules.

Raw Text and Binary Bodies

Use golem.schema.UnstructuredText[L] for raw text/plain and golem.schema.UnstructuredBinary[M] for raw binary request and response bodies. The type argument is the allow-list: String for no restriction, or a string-literal union such as "en" | "de" / "image/png" | "image/jpeg".

@endpoint(method = "POST", path = "/upload/{bucket}")
def upload(bucket: String, payload: golem.schema.UnstructuredBinary["image/png" | "image/jpeg"]): Future[Long]
 
@endpoint(method = "POST", path = "/notes/{id}")
def addNote(id: String, body: golem.schema.UnstructuredText[String]): Future[Int]
 
@endpoint(method = "GET", path = "/notes/{id}")
def getNote(id: String): Future[golem.schema.UnstructuredText["en" | "de"]]

import golem.schema.{UnstructuredBinary, UnstructuredText}
 
override def upload(bucket: String, payload: UnstructuredBinary["image/png" | "image/jpeg"]): Future[Long] =
  Future.successful(payload match {
    case UnstructuredBinary.Inline(data, _) => data.length.toLong
    case UnstructuredBinary.Url(_)          => -1L
  })
 
override def getNote(id: String): Future[UnstructuredText["en" | "de"]] =
  Future.successful(UnstructuredText.fromInline("hello", "en"))

• Values: UnstructuredText.Inline(text, languageCode) / UnstructuredText.Url(url) and UnstructuredBinary.Inline(data, mimeType) / UnstructuredBinary.Url(url); build them with fromInline(...) (optionally with a language code / MIME type) or fromUrl(url).
• A method with an unstructured body parameter may have only one body parameter, and it cannot be bound to a path/query/header variable.
• Text request bodies must have no Content-Type, text/plain, or text/plain; charset=utf-8 (otherwise 415); an optional single Content-Language is checked against the allow-list.
• Responses: text is sent as text/plain; charset=utf-8 with the language code as Content-Language; binary uses its MIME type (default application/octet-stream); a Url value is a 307 redirect.
• Generated RPC clients do not copy the agent trait file’s imports, so use the fully qualified golem.schema.… names in the agent trait.

Return Type to HTTP Response Mapping

Scala Either[E, T] maps to WIT result<T, E> (Right = Ok, Left = Err).

The value inside Some / Right follows the same rules as a top-level value: an unstructured value nested in Option, Either[E, _], or Either[E, Option[_]] is sent as the raw body, not as JSON. For example, Future[Either[String, Option[golem.schema.UnstructuredBinary[String]]]] gives raw bytes (200), 404 for Right(None), and a JSON error (500) for Left.

Data Type to JSON Mapping

Related Guides

• Load golem-add-http-endpoint-scala for the high-level workflow of defining and mounting HTTP endpoints on an agent