designing-hyperlinked-apis

The goal is an API where:

1. An agent starts at the root, follows links, reads relation docs, and completes tasks (exploration)
2. The recorded session can be exported as a deterministic spec that runs without inference (distillation)

<quick_start> A minimal HAL API needs three things:

1. A root resource with links to available actions
2. CURIEs that map link relation prefixes to documentation URLs
3. Relation documentation served as markdown at those URLs

{
  "_links": {
    "curies": [{ "name": "myapi", "href": "https://api.example.com/rels/{rel}", "templated": true }],
    "self": { "href": "/" },
    "myapi:v1:items": { "href": "/items", "title": "All items" },
    "myapi:v1:create-item": { "href": "/items", "title": "Create an item" }
  },
  "title": "My API",
  "description": "A HAL+JSON API"
}

When an agent encounters myapi:v1:items, the CURIE expands it to https://api.example.com/rels/v1:items — a URL serving markdown documentation describing the method, input schema, and response format. </quick_start>

<essential_principles> The API is a graph. Resources are nodes. Links are edges. The root resource is the entry point. Every resource an agent can reach must be reachable by following links from the root. Never require an agent to construct a URL — it should always come from a link.

Relation docs are prompt files. The markdown served at each CURIE-expanded URL is not just documentation — it’s a prompt. An agent reads it to learn the HTTP method, input schema, and response shape. Write relation docs as if you’re explaining the operation to an agent that has never seen your API before. Include a JSON Schema for the input and an example request.

Progressive disclosure drives design. Don’t expose everything at the root. The root shows top-level actions. Each response reveals deeper links. An agent navigating from root → collection → item → history discovers capabilities incrementally, loading only what’s relevant to its current task.

Version relations, not the API. Instead of /v1/pages and /v2/pages, use versioned relation names: myapi:v1:pages. This versions individual transitions rather than the entire API surface. When a relation’s contract changes, introduce myapi:v2:pages alongside the existing version. The CURIE expansion handles the rest — v1:pages and v2:pages resolve to different documentation URLs.

Deprecate through links. When retiring a relation, add deprecated: true and deprecation: "Use myapi:v2:pages instead" to the link object. Agents that understand HAL deprecation will warn and adapt. Eventually remove the deprecated link entirely. </essential_principles>

<entry_point> The root resource is the only URL an agent needs to know. Design it to surface every top-level capability:

{
  "_links": {
    "curies": [{ "name": "wiki", "href": "https://api.example.com/rels/{rel}", "templated": true }],
    "self": { "href": "/" },
    "wiki:v1:pages": { "href": "/pages", "title": "All wiki pages" },
    "wiki:v1:create-page": { "href": "/pages", "title": "Create a new page" },
    "wiki:v1:categories": { "href": "/categories", "title": "Browse categories" },
    "wiki:v1:search": { "href": "/search{?q}", "templated": true, "title": "Search pages" }
  },
  "title": "Agent Wiki",
  "description": "A HAL+JSON wiki API for AI agent exploration"
}

Key points:

• Every link has a title — this helps agents understand what the link does before reading the full relation docs
• Templated links (like search) declare "templated": true so agents know to substitute variables
• The self link is always present
• CURIEs are declared once at the root; agents store them for the session </entry_point>

"curies": [{ "name": "wiki", "href": "https://api.example.com/rels/{rel}", "templated": true }]

The {rel} template variable is replaced with the relation reference. For wiki:v1:pages, the prefix is wiki and the reference is v1:pages, producing https://api.example.com/rels/v1:pages.

Choose a short, memorable prefix. One CURIE per API is typical. The prefix appears on every link, so brevity matters.

<versioned_relations> Version the relation name, not the URL path:

The version is in the relation name, not the resource URL. This means:

• The same resource can be linked via different relation versions
• Each version has its own documentation URL (via CURIE expansion)
• Clients following v1:pages get the v1 contract; clients following v2:pages get the v2 contract
• You can deprecate v1:pages independently while keeping the resource at /pages

Serve relation docs only for the versioned key. If an agent requests docs for the unversioned form (e.g. /rels/pages), return 410 Gone with a message pointing to the versioned equivalent. </versioned_relations>

<relation_docs> Each relation’s documentation is a markdown file served at the CURIE-expanded URL. Structure it consistently:

# wiki:v1:create-page

Creates a new page in the wiki.

## Method

POST

## Input

```json
{
  "type": "object",
  "properties": {
    "title": { "type": "string", "description": "Page title, must be unique" },
    "body": { "type": "string", "description": "Page content (markdown)" },
    "tags": { "type": "array", "items": { "type": "string" }, "description": "Optional tags" }
  },
  "required": ["title", "body"]
}

Example

POST /pages
Content-Type: application/json

{
  "title": "Agent Communication",
  "body": "# Overview\n\nAgents communicate via...",
  "tags": ["protocols"]
}

Response

Returns the created page as a HAL resource. Status: 201 Created.

Key points:
- The Input section is a **JSON Schema** — agents use this to validate their request before sending
- Always include Method, Input (with schema), Example, and Response sections
- Serve with `Content-Type: text/markdown`
- Keep docs concise — an agent needs to understand the operation, not read a tutorial
</relation_docs>

<embedded_resources>
Use `_embedded` for collections and nested resources. The key should be a CURIE-expanded relation:

```json
{
  "_links": {
    "curies": [{ "name": "wiki", "href": "https://api.example.com/rels/{rel}", "templated": true }],
    "self": { "href": "/pages" },
    "wiki:v1:create-page": { "href": "/pages", "title": "Create a new page" }
  },
  "_embedded": {
    "wiki:v1:page": [
      {
        "_links": {
          "self": { "href": "/pages/1" },
          "wiki:v1:history": { "href": "/pages/1/history" }
        },
        "id": "1",
        "title": "Hello World",
        "tags": ["test"]
      }
    ]
  },
  "totalCount": 1
}

Embedded items should include their own _links so agents can navigate to individual resources. Keep embedded representations minimal (summaries) — the full resource is at the self link. </embedded_resources>

<link_design> Each response should include links that tell the agent what it can do next:

• Navigation links: self, next, prev — where am I, where can I go
• Action links: wiki:v1:edit-page, wiki:v1:delete-page — what can I do here
• Context links: wiki:v1:pages, wiki:v1:history — related resources

Name relations by what they do, not how they work:

• wiki:v1:create-page not wiki:v1:post-to-pages
• wiki:v1:edit-page not wiki:v1:put-page
• wiki:v1:history not wiki:v1:get-versions

Use title on links to give agents a quick description without needing to fetch the full relation docs. </link_design>

{
  "wiki:v1:pages": { "href": "/pages", "title": "All pages" },
  "wiki:pages": { "href": "/pages", "title": "All pages", "deprecated": true, "deprecation": "Use wiki:v1:pages instead" }
}

HAL clients that understand deprecation will warn the agent. The deprecation string tells them what to use instead.

Lifecycle:

1. Introduce versioned relation (wiki:v1:pages)
2. Mark unversioned as deprecated (keep both for a transition period)
3. Remove deprecated relation entirely (clients following old links get no match)

For relation docs: serve full docs only for the versioned key. Return 410 Gone for deprecated unversioned lookups, pointing to the versioned equivalent.

<designing_for_distillation> An API designed for distillation makes it easy for agents to record a workflow and export it as a deterministic spec. This means:

• Stable relation names — the spec references relations by name, so names must be stable across requests
• Self-contained relation docs — the JSON Schema in the docs becomes the contract in the exported spec
• Consistent response shapes — each relation should always return the same structure so the spec is predictable
• Idempotent navigation — following the same link from the same state should produce the same result

The exported spec looks like:

[
  { "action": "start", "url": "/" },
  { "action": "follow", "relation": "wiki:v1:pages", "method": "GET" },
  { "action": "follow", "relation": "wiki:v1:create-page", "method": "POST",
    "input": {
      "bodySchema": { "type": "object", "properties": { "title": { "type": "string" }, "body": { "type": "string" } }, "required": ["title", "body"] },
      "body": { "title": "Example", "body": "Content" }
    }
  }
]

The bodySchema comes directly from your relation docs. If your docs include a clear JSON Schema, the agent preserves it in the export, and a runner can use it to validate inputs without any inference. </designing_for_distillation>

<success_criteria> A well-designed HAL API:

• Is fully navigable from the root through link-following alone
• Has CURIEs that expand every relation to a documentation URL
• Serves relation docs with Method, Input (JSON Schema), Example, and Response sections
• Uses versioned relation names (myapi:v1:*) rather than versioned URL paths
• Includes title on links for quick agent comprehension
• Returns embedded resource summaries with their own self links in collections
• Can be explored by an agent with no prior knowledge and distilled into a deterministic spec </success_criteria>