astro-webmcp

🤖 Make your Astro site AI-agent ready in one line of code.

Astro integration that automatically exposes your site's content via WebMCP — allowing AI agents to discover, search, and navigate your content directly in the browser.

What is WebMCP?

WebMCP is a proposed web standard by Chrome that lets websites declare structured "tools" for AI agents. Instead of an agent visually interpreting each page element, the site explicitly declares what can be done — search articles, navigate to sections, get page metadata.

• Spec: https://webmachinelearning.github.io/webmcp/
• Chrome docs: https://developer.chrome.com/docs/ai/webmcp
• GitHub: https://github.com/webmachinelearning/webmcp

Installation

npm install astro-webmcp

Basic usage

// astro.config.mjs
import { defineConfig } from 'astro/config';
import webmcp from 'astro-webmcp';

export default defineConfig({
  integrations: [webmcp()],
});

That's it. All your site content is now exposed via WebMCP automatically.

Configuration options

webmcp({
  collections: ['blog', 'docs'], // filter which collections to expose (default: all)
  security: {
    exposedTo: [],          // origins allowed cross-origin access (default: none)
    maxOutputLength: 1500,  // max chars per tool output (default: 1500)
    sanitizeOutputs: true,  // strip prompt injection patterns (default: true)
  },
})

Registered tools

Tool schemas

search_content

{
  "name": "search_content",
  "inputSchema": {
    "type": "object",
    "properties": {
      "query": { "type": "string", "description": "Search term" },
      "collection": { "type": "string", "description": "Filter by collection (optional)" },
      "limit": { "type": "number", "description": "Max results (default: 5)" }
    },
    "required": ["query"]
  }
}

list_sections

{
  "name": "list_sections",
  "inputSchema": { "type": "object", "properties": {} }
}

go_to

{
  "name": "go_to",
  "inputSchema": {
    "type": "object",
    "properties": {
      "slug": { "type": "string", "description": "Page slug or path" }
    },
    "required": ["slug"]
  }
}

get_page_info

{
  "name": "get_page_info",
  "inputSchema": { "type": "object", "properties": {} }
}

Architecture

┌─────────────────────────────────────────────────────────┐
│                      BUILD TIME                          │
│                                                         │
│  Astro pages ────→ Hook astro:build:done                │
│                         │                               │
│                         ▼                               │
│                   /_webmcp/manifest.json                 │
│                   (titles, slugs, descriptions)          │
└─────────────────────────────────────────────────────────┘
                          │
                          ▼
┌─────────────────────────────────────────────────────────┐
│                    RUNTIME (Browser)                     │
│                                                         │
│  Injected script (injectScript)                         │
│       │                                                 │
│       ├─ fetch('/_webmcp/manifest.json')                │
│       │                                                 │
│       └─ navigator.modelContext.registerTool()          │
│            ├── search_content                           │
│            ├── list_sections                            │
│            ├── go_to                                    │
│            └── get_page_info                            │
└─────────────────────────────────────────────────────────┘
                          │
                          ▼
┌─────────────────────────────────────────────────────────┐
│                      AI AGENT                           │
│                                                         │
│  Chrome 149+ discovers tools via WebMCP protocol        │
│  Agent can search, list, and navigate site content      │
└─────────────────────────────────────────────────────────┘

Components

Integration (src/index.ts) — implements AstroIntegration with three hooks:

• astro:config:setup — injects the client-side script into every page via injectScript
• astro:server:setup — serves a dynamic manifest during development
• astro:build:done — generates /_webmcp/manifest.json by extracting titles and descriptions from built HTML

Client script (src/client.ts) — runs in the browser on every page:

1. Feature detection: document.modelContext ?? navigator.modelContext
2. Fetches the manifest from /_webmcp/manifest.json
3. Registers 4 tools with JSON Schema input definitions

Manifest (/_webmcp/manifest.json) — static JSON generated at build time:

{
  "generatedAt": "2026-06-07T00:45:30.260Z",
  "site": "https://example.com",
  "collections": [
    { "name": "blog", "count": 42 },
    { "name": "docs", "count": 15 }
  ],
  "entries": [
    {
      "slug": "blog/my-article",
      "url": "/blog/my-article/",
      "title": "My Article",
      "description": "Article summary",
      "collection": "blog"
    }
  ]
}

Design decisions

Testing

1. Enable WebMCP in Chrome

Navigate to chrome://flags/#enable-webmcp-testing → Enabled → Relaunch.

2. Install the test extension

Model Context Tool Inspector

3. Verify tools in DevTools Console

// Chrome 149
const tools = await navigator.modelContext.getTools();
console.table(tools.map(t => ({ name: t.name, description: t.description })));

// Chrome 150+
const tools = await document.modelContext.getTools();
console.table(tools.map(t => ({ name: t.name, description: t.description })));

4. Execute a tool manually

const tools = await navigator.modelContext.getTools();
const search = tools.find(t => t.name === 'search_content');
const result = await navigator.modelContext.executeTool(search, '{"query": "astro"}');
console.log(JSON.parse(result));

Combining with Declarative API

This plugin uses the Imperative API for search and navigation. For existing forms on your site, you can add the Declarative API manually — both coexist:

<form toolname="send_message"
      tooldescription="Send a contact message."
      toolautosubmit
      action="/api/contact">
  <label for="email">Email</label>
  <input type="email" name="email" required>
  <label for="message">Message</label>
  <textarea name="message" required></textarea>
  <button type="submit">Send</button>
</form>

The agent will see both the integration tools + declarative form tools.

Browser support

The integration is a progressive enhancement. On unsupported browsers, the injected script detects the absence of navigator.modelContext / document.modelContext and exits immediately. No errors thrown, no extra bytes parsed.

Enabling WebMCP

WebMCP requires explicit opt-in. There are two paths depending on your use case:

Local development: Chrome flag

For testing on your own machine:

1. Open chrome://flags/#enable-webmcp-testing
2. Set to Enabled
3. Restart Chrome

Tools will appear for any localhost site that registers them. No token needed.

Production: Origin Trial

For deployed sites where real visitors should have WebMCP active (without requiring them to flip a flag), Chrome offers an Origin Trial.

Step 1 — Register your origin

Go to the WebMCP Origin Trial registration page and register the domain(s) where your Astro site is deployed (e.g. https://mysite.com). You'll receive a trial token — a long Base64 string.

Step 2 — Store the token

Add the token to your .env file:

# .env
WEBMCP_ORIGIN_TRIAL_TOKEN=AjfC0e...your-long-token-here...Qw==

Never hardcode the token in source. It's tied to a specific origin and has an expiration date — keeping it in .env makes rotation easy.

Step 3 — Inject the meta tag

In your base layout (typically src/layouts/Layout.astro or src/layouts/Base.astro), add the origin trial meta tag inside <head>:

---
// src/layouts/Layout.astro
---
<html lang="en">
  <head>
    <meta charset="utf-8" />
    <meta name="viewport" content="width=device-width, initial-scale=1" />

    {/* WebMCP Origin Trial — enables navigator.modelContext for visitors */}
    {import.meta.env.WEBMCP_ORIGIN_TRIAL_TOKEN && (
      <meta
        http-equiv="origin-trial"
        content={import.meta.env.WEBMCP_ORIGIN_TRIAL_TOKEN}
      />
    )}

    <title>My Site</title>
  </head>
  <body>
    <slot />
  </body>
</html>

The conditional (&&) means the tag only renders when the token exists — development builds without the env var won't emit an empty meta tag.

Step 4 — Verify

After deploying, open DevTools → Application → Frames → top → Origin Trials. You should see "WebMCP" listed as active. Alternatively, check the Console:

// Should return the registered tools if everything works
const tools = await navigator.modelContext.getTools();
console.log(tools.length, 'WebMCP tools active');

Origin trial caveats

• Tokens are origin-bound — https://mysite.com and https://staging.mysite.com need separate tokens.
• Tokens expire (typically 6-12 weeks). Chrome will email you before expiration. Renew and update your .env.
• The trial is available starting Chrome 149. Users on older Chrome versions simply won't have modelContext — the integration handles this gracefully.
• Native support (no flag or token) is targeted for H2 2026.

Security

This plugin implements security measures aligned with the Chrome Agent Security Guidelines and WebMCP Tool Security recommendations.

Security configuration

webmcp({
  security: {
    // Origins allowed to access tools cross-origin (default: none — same-origin only)
    exposedTo: ['https://trusted-partner.com'],

    // Max characters per tool output (default: 1500, per Chrome recommendation)
    maxOutputLength: 1500,

    // Sanitize outputs against indirect prompt injection (default: true)
    sanitizeOutputs: true,
  },
})

What is exposed

The plugin only exposes information that is already publicly accessible on your site:

The manifest (/_webmcp/manifest.json) contains no more information than your sitemap.xml already provides to every crawler.

What is NOT exposed

• ❌ No server-side data, APIs, or endpoints
• ❌ No authentication tokens or credentials
• ❌ No admin routes or private pages
• ❌ No user data (the plugin is fully static/client-side)
• ❌ No environment variables or build secrets

Tool annotations

All built-in tools include security annotations to help agents make safe decisions:

Defenses against prompt injection

The plugin implements multiple layers of defense per Chrome's defense-in-depth strategy:

1. Deterministic guardrails

• Output character limits — All tool outputs are truncated to maxOutputLength (default 1.5K chars). This prevents context window overflow and limits the surface area for sophisticated prompt injection attacks.
• Cross-origin isolation — Tools are only accessible same-origin by default. Use exposedTo to explicitly allowlist trusted origins.
• Result cap — search_content enforces a maximum of 20 results regardless of the limit parameter.

2. Output sanitization

When sanitizeOutputs: true (default), the plugin strips common prompt injection patterns from tool outputs:

• "ignore previous instructions" patterns
• Role-play injection ("you are now...")
• Fake system/assistant/user message delimiters
• XML-style instruction tags (<system>, <instruction>, etc.)

This acts as a lightweight classifier for indirect prompt injection embedded in page content (e.g., malicious text in blog comments that gets indexed in titles/descriptions).

3. Annotations as signals

The readOnlyHint and untrustedContentHint annotations signal to the agent:

• Which tools are safe to call without user confirmation (read-only tools)
• Which outputs need heightened scrutiny (untrusted content that could contain injected instructions)

Character budgets

Following Chrome's recommendations:

Design principles

• No arbitrary code execution — tools only read static data or navigate
• No innerHTML — all output is JSON.stringify, no XSS vector
• No external requests — the manifest is fetched from same-origin only
• Progressive enhancement — on unsupported browsers, the script exits immediately with zero side effects
• Origin-isolated — WebMCP requires origin isolation; cross-origin iframes cannot access tools unless explicitly allowed
• Annotations-first — all tools declare their security posture via readOnlyHint and untrustedContentHint
• Defense-in-depth — multiple independent layers (limits, sanitization, annotations, origin control)

For agent developers consuming this plugin

If you are building an agent that consumes tools registered by astro-webmcp, follow these additional recommendations from Chrome Agent Security:

1. Respect untrustedContentHint — Apply spotlighting (delimiting or Base64-encoding) to outputs from tools marked with this annotation.
2. Set token limits — Implement agent-level token limits on all inbound tool responses.
3. Verify intent alignment — Use a critic/validator to ensure tool calls align with the user's original request.
4. Confirm state-changing actions — Tools without readOnlyHint: true (like go_to) should trigger user confirmation.
5. Restrict origins — Only interact with origins relevant to the user's task.

Troubleshooting

Tools don't appear

1. Check that chrome://flags/#enable-webmcp-testing is enabled
2. Verify in the Network tab that /_webmcp/manifest.json returns 200
3. In Console, check navigator.modelContext (Chrome 149) or document.modelContext (150+)

Empty manifest

• Confirm the build completed without errors
• The manifest is generated from built HTML pages — ensure astro build succeeds

Search returns no results

• Search is case-insensitive across title, description, and tags fields
• If your pages have no <meta name="description">, search only matches on title

Requirements

• Astro 6+
• Chrome 149+ with WebMCP enabled — see Browser support for setup options
• Origin-isolated document (Astro default)

Status

🚧 Early development — WebMCP is an evolving standard (developer trial in Chrome 149+).

License

MIT

Author

Contact the author: https://ft.ia.br