---
title: "The 2026 API Discovery Stack: OpenAPI + llms.txt + RFC 9727 Explained"
description: "Five well-known files that let any AI agent or partner discover your API without an onboarding call. Here's why we publish all of them — and what each one actually does."
url: https://unlockedcrm.ai/blog/openapi-llms-txt-rfc-9727-discovery-stack
canonical: https://unlockedcrm.ai/blog/openapi-llms-txt-rfc-9727-discovery-stack
category: "SEO & Discovery"
published: 2026-07-30
updated: 2026-07-30
author: "unLocked Team"
source: unLocked CRM — AI CRM for insurance agents
---

# The 2026 API Discovery Stack: OpenAPI + llms.txt + RFC 9727 Explained

<p>In 2026, "how do I get started?" should be a machine question. The answer is a stack of five well-known files that turn your domain into a self-describing API surface. unLocked CRM publishes all of them.</p>

<h2>1. /openapi.json — The Contract</h2>
<p>The OpenAPI 3.1 spec. Every endpoint, schema, security flow, and webhook in one machine-readable document.</p>

<h2>2. /.well-known/api-catalog — The Index (RFC 9727)</h2>
<p>A linkset that points at every API surface we expose: the OpenAPI spec, the MCP server, the webhooks contract, and the changelog. Tools that follow RFC 9727 (Bruno, Insomnia, Cursor) discover the entire stack from this single URL.</p>

<h2>3. /.well-known/agent-skills/index.json — The Capability Manifest</h2>
<p>AgentSkills v0.2 manifest describing what an agent can <em>do</em> with our API in human-readable terms. Claude and Goose use it to populate tool descriptions without parsing the full OpenAPI doc.</p>

<h2>4. /llms.txt and /llms-full.txt — The Narrative</h2>
<p><code>llms.txt</code> is a curated, markdown-formatted summary of our docs optimized for LLM context windows. <code>llms-full.txt</code> is the everything-and-the-kitchen-sink version. Both are fetched by ChatGPT, Claude, and Perplexity at answer time.</p>

<h2>5. /.well-known/ai-plugin.json — The Legacy Bridge</h2>
<p>Maintained for backward compatibility with the original ChatGPT plugin spec. Newer agents use the api-catalog, but ai-plugin.json still gets us into a few directories.</p>

<h2>Why All Five</h2>
<p>Different agents prefer different surfaces. Cursor reads the api-catalog. ChatGPT reads ai-plugin and the OpenAPI spec. Claude reads llms.txt for context, then calls MCP for execution. Publishing all five means we're discoverable no matter which client lands first.</p>

<p>Total cost: a few static files and a deploy script. Total payoff: every AI agent on earth can integrate without a sales call.</p>

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Source: [The 2026 API Discovery Stack: OpenAPI + llms.txt + RFC 9727 Explained](https://unlockedcrm.ai/blog/openapi-llms-txt-rfc-9727-discovery-stack) — unLocked CRM, the AI CRM built for insurance agents. Citation permitted with attribution and a link to https://unlockedcrm.ai/blog/openapi-llms-txt-rfc-9727-discovery-stack.