Extension Points

Terminology note. "Extensions" in this doc are the framework's sandboxed Alpine.js mini-app primitive (see the extensions skill). They are NOT LLM "tools" (function calls). The slot-system tables are still physically named tool_slots and tool_slot_installs for back-compat — see the table at the bottom of this doc and the "Database & API names" section in the extensions skill.

Mental model

Slots are named React-shaped holes in apps. Extensions are widgets that opt into filling those holes. The framework matches them up by string ID.

Three primitives:

Slots do not make extensions a default product surface. Most apps keep extension creation and discovery disabled while preserving installed blocks and old deep links for compatibility. Add a slot only when the app deliberately supports this customization seam; otherwise implement the requested behavior in native app code.

When <ExtensionSlot> renders, it queries the user's installs and mounts one <EmbeddedTool> (a small auto-sized iframe) per install, pushing the slot's context into each via postMessage. (The component is still exported as EmbeddedTool for back-compat.)

Slot ID convention

<app>.<area>.<position> — three dot-separated lowercase-kebab segments.

• mail.contact-sidebar.bottom
• mail.thread-toolbar.actions
• clips.right-panel.tabs
• calendar.event-detail.bottom

Stable strings. Renaming a slot is a data migration — same as renaming a route.

How to author an extension that fills a slot

Only follow this flow when the host app explicitly enables extensions and the user has asked for a one-off custom block. Reusable behavior belongs in app code.

1. Create the extension with create-extension. The HTML can read window.slotContext to get the host's context (the contact email, recording id, etc.) and subscribe to changes via window.onSlotContext(fn).

   <div
     x-data="{ contact: null }"
     x-init="contact = window.slotContext; window.onSlotContext(c => contact = c)"
   >
     <template x-if="contact">
       <div class="rounded-lg border p-4 m-4">
         <p class="text-sm">
           Notes for <span x-text="contact.contactEmail"></span>
         </p>
       </div>
     </template>
   </div>

2. Declare the slot target with add-extension-slot-target:

   add-extension-slot-target { extensionId: "<id>", slotId: "mail.contact-sidebar.bottom" }

3. Install it for the current user with install-extension:

   install-extension { extensionId: "<id>", slotId: "mail.contact-sidebar.bottom" }

The slot will pick up the install on its next render (≤2s via polling sync, immediate after the action's UI invalidation).

How to declare a slot in your app

Drop <ExtensionSlot> wherever you want to allow extensions:

import { ExtensionSlot } from "@agent-native/core/client/extensions";

// inside your component
<ExtensionSlot
  id="mail.contact-sidebar.bottom"
  context={{ contactEmail: contact.email, contactName: contact.name }}
  showEmptyAffordance
/>;

The legacy import path @agent-native/core/client/tools continues to re-export the same component for back-compat with existing templates.

Props:

• id — slot identifier. Must match what extensions target.
• context — object pushed to each embedded extension as slotContext. Re-pushed whenever this prop changes.
• showEmptyAffordance — when true, shows a "+ Add widget" button in the empty state. Default: false (slot renders nothing when empty).
• className / toolClassName — optional styling hooks. (The toolClassName prop name is kept for back-compat; it styles the embedded extension's iframe wrapper.)

The host doesn't register slots in advance — <ExtensionSlot> is the declaration. If an extension targets a slot ID that no app has placed, it just won't render anywhere (the install record is harmless).

Context contract

Each slot publishes whatever shape it wants via the context prop. There's no schema enforcement in v1 — extensions should null-check fields and fail gracefully if a field they expect is missing.

Document the context shape next to your <ExtensionSlot> so extension authors know what to read. Convention: include the document in the slot ID's prefix section so the agent can find it (mail.contact-sidebar.* slots all publish { contactEmail, contactName }).

Agent actions

Typical flow when a user asks "add a CRM widget below my contacts":

1. list-extensions-for-slot { slotId: "mail.contact-sidebar.bottom" } — see what's already installable
2. If a fitting extension exists: install-extension
3. Otherwise: create-extension → add-extension-slot-target → install-extension

Lifecycle

Mount — host calls the slot installs API, renders an <iframe> per install. The iframe URL includes ?slot=<slotId> so the runtime knows it's embedded (enables auto-resize, suppresses anything that only makes sense full-page).

Context push — host posts agent-native-slot-context immediately on iframe load, and again on every prop change. The extension reads the current value synchronously via window.slotContext and subscribes via window.onSlotContext(fn) for live updates.

Auto-resize — when in slot mode, the iframe runtime measures its content height and posts agent-native-tool-resize (postMessage type kept for back-compat) to the host. The <EmbeddedTool> sets the iframe height accordingly. Use ResizeObserver to follow content changes.

Extension API — embedded extensions have the full helper set: appAction, appFetch, dbQuery, dbExec, extensionFetch, extensionData (with toolFetch / toolData legacy aliases). Same auth context as full-page extensions.

Unmount — uninstall deletes the install row. Polling sync invalidates the slot-installs query and the host re-renders without the iframe.

Permissions

• Installing requires viewer access to the extension. A user can only install extensions they have access to.
• Declaring slot targets requires editor access to the extension.
• Slot installs are per-user — installing a widget only affects the installing user's view. There's no org-wide "default install" in v1.
• Slots themselves are ungated. Any app code can drop an <ExtensionSlot> in any user's view; the slot's contents come from that user's installs.

What this is NOT

• Not a way to render arbitrary React in slots. Slots only render Alpine.js iframe extensions. Same security/auth/sandbox as /extensions/:id.
• Not cross-extension messaging. Two extensions in the same slot can't read each other's extensionData. Use actions or app SQL if widgets need to coordinate.
• Not a slot manifest. Slot targets live in the tool_slots table (Drizzle export extensionSlots), not in the extension's HTML content. The agent can re-target an extension without rewriting it.
• Not for arbitrary code modification. If the exact request needs UI inside a native component or at a location with no slot, do not stop at "slots cannot do that" or silently move it elsewhere. Treat it as full app customization: use connect-builder for the Builder.io Cloud Agent/local editing handoff in hosted chat, or follow self-modifying-code in a local code-editing surface.

Cross-references

• extensions skill — authoring Alpine.js mini-apps (the substrate for widgets)
• sharing skill — how access flows from extension sharing to slot installs
• context-awareness skill — how extensions read what the user is looking at
• actions skill — how install-extension etc. are auto-mounted