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):

Scala Type	Parsed From
String	Used as-is
Boolean	Parsed from "true" / "false"
Int	Parsed as 32-bit signed integer
Long	Parsed as 64-bit signed integer
Float	Parsed as 32-bit float
Double	Parsed as 64-bit float
Enum / sealed trait (unit cases only)	Matched against known case names

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

Scala Type	Behavior
Option[T] (where T is a supported type above)	Optional — absent query param or header produces None
List[T] / Array[T] (where T is a supported type above)	Repeated query params or comma-separated header values

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

POST Request Body Mapping

For POST/PUT/DELETE endpoints, method parameters not bound to path variables, query parameters, or headers 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 unmapped parameter becomes a top-level field in the expected JSON body object. All custom types require a zio.blocks.schema.Schema instance.

⚠️ Important: The request body is always a JSON object with parameter names as keys — even when there is only a single body parameter. For example, an endpoint def decide(decision: String) expects {"decision": "approved"}, never a bare string like "approved". Sending a non-object JSON value or plain text will fail with REQUEST_JSON_BODY_PARSING_FAILED.

Return Type to HTTP Response Mapping

Return Type	HTTP Status	Response Body
Future[Unit]	204 No Content	empty
Future[T]	200 OK	JSON-serialized T
Future[Option[T]]	200 OK if Some, 404 Not Found if None	JSON T or empty

Data Type to JSON Mapping

Scala Type	JSON Representation
String	JSON string
Int, Long	JSON number (integer)
Float, Double	JSON number (float)
Boolean	JSON boolean
Array[T], List[T]	JSON array
case class (with Schema)	JSON object (camelCase field names)
Option[T]	value or null
sealed trait / enum	JSON variant
Tuple	JSON array

Related Skills

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