HTTP Request and Response Parameter Mapping (Rust)

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 {var_name} in mount or endpoint paths map to parameters by name:


// Mount path variables → constructor parameters
#[agent_definition(mount = "/api/tasks/{task_name}")]
pub trait TaskAgent {
    fn new(task_name: String) -> Self;
 
    // Endpoint path variables → method parameters
    #[endpoint(get = "/items/{item_id}")]
    fn get_item(&self, item_id: String) -> Item;
}

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


#[endpoint(get = "/files/{*path}")]
fn get_file(&self, path: String) -> FileContent;
// GET .../files/docs/readme.md → path = "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(get = "/search?q={query}&limit={max_results}")]
fn search(&self, query: String, max_results: u64) -> Vec<SearchResult>;
// GET .../search?q=hello&limit=10

Header Variables

Map HTTP headers to parameters using the headers(...) block on #[endpoint]:


#[endpoint(
    get = "/data",
    headers("X-Request-Id" = "request_id", "Authorization" = "token")
)]
fn get_data(&self, request_id: String, token: String) -> Data;

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):

Rust Type	Parsed From
`String`	Used as-is
`char`	Single character
`bool`	Parsed from `"true"` / `"false"`
`u8`, `u16`, `u32`, `u64`	Parsed as unsigned integer
`i8`, `i16`, `i32`, `i64`	Parsed as signed integer
`f32`, `f64`	Parsed as floating-point number
Enum (unit variants only)	Matched against known case names

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

Rust Type	Behavior
`Option<T>` (where `T` is a supported type above)	Optional — absent query param or header produces `None`
`Vec<T>` (where `T` is a supported type above)	Repeated query params or comma-separated header values

All other types (structs, tuples, enums with data, HashMap, 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(post = "/items/{id}")]
fn update_item(&mut self, id: String, name: String, count: u64) -> 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 must derive Schema.

⚠️ 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 fn decide(&mut self, 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.

Binary Request and Response Bodies

Use UnstructuredBinary from the SDK for raw binary payloads:


use golem_rust::agentic::UnstructuredBinary;
use golem_rust::AllowedMimeTypes;
 
// Accepting any binary content type
#[endpoint(post = "/upload/{bucket}")]
fn upload(&self, bucket: String, payload: UnstructuredBinary<String>) -> i64;
 
// Restricting to specific MIME types
#[derive(AllowedMimeTypes, Clone, Debug)]
pub enum ImageTypes {
    #[mime_type("image/gif")]
    ImageGif,
    #[mime_type("image/png")]
    ImagePng,
}
 
#[endpoint(post = "/upload-image/{bucket}")]
fn upload_image(&self, bucket: String, payload: UnstructuredBinary<ImageTypes>) -> i64;
 
// Returning binary data
#[endpoint(get = "/download")]
fn download(&self) -> UnstructuredBinary<String>;

In the implementation:


fn upload(&self, _bucket: String, payload: UnstructuredBinary<String>) -> i64 {
    match payload {
        UnstructuredBinary::Url(_) => -1,
        UnstructuredBinary::Inline { data, .. } => data.len() as i64,
    }
}
 
fn download(&self) -> UnstructuredBinary<String> {
    UnstructuredBinary::Inline {
        data: vec![1, 2, 3, 4],
        mime_type: "application/octet-stream".to_string(),
    }
}

Return Type to HTTP Response Mapping

Return Type	HTTP Status	Response Body
`()` (unit / no return)	204 No Content	empty
`T` (any type)	200 OK	JSON-serialized `T`
`Option<T>`	200 OK if `Some`, 404 Not Found if `None`	JSON `T` or empty
`Result<T, E>`	200 OK if `Ok`, 500 Internal Server Error if `Err`	JSON `T` or JSON `E`
`Result<(), E>`	204 No Content if `Ok`, 500 if `Err`	empty or JSON `E`
`Result<T, ()>`	200 OK if `Ok`, 500 if `Err`	JSON `T` or empty
`UnstructuredBinary<M>`	200 OK	Raw binary with Content-Type

Data Type to JSON Mapping

Rust Type	JSON Representation
`String`	JSON string
`u8`–`u64`, `i8`–`i64`	JSON number (integer)
`f32`, `f64`	JSON number (float)
`bool`	JSON boolean
`Vec<T>`	JSON array
Struct (with `Schema`)	JSON object (camelCase field names)
`Option<T>`	value or `null`
`Result<T, E>`	value (see response mapping above)
Enum (unit variants)	JSON string
Enum (with data)	JSON object with tag
`HashMap<K, V>`	JSON array of `[key, value]` tuples