Copilot Extensibility: Building Custom Plugins for Microsoft 365 Copilot

Microsoft 365 Copilot’s plugin architecture enables deep integration with enterprise systems. After Build 2025’s enhancements, building custom Copilot extensions is more powerful than ever. Here’s how to create plugins that extend Copilot’s capabilities.

Plugin Architecture Overview

Copilot plugins use the OpenAPI specification to define capabilities. The runtime handles authentication, parameter mapping, and response formatting automatically:

// plugin-manifest.json
{
  "schema_version": "v1.1",
  "name": "Contoso CRM Plugin",
  "description": "Access customer data and manage relationships",
  "auth": {
    "type": "OAuthPluginVault",
    "reference_id": "contoso-crm-oauth"
  },
  "capabilities": {
    "conversation_starters": [
      "Show my top opportunities",
      "Find customer contacts at Fabrikam"
    ]
  },
  "functions": [
    {
      "name": "getCustomerDetails",
      "description": "Retrieve detailed customer information by name or ID",
      "parameters": {
        "type": "object",
        "properties": {
          "customer_id": {"type": "string"},
          "include_history": {"type": "boolean", "default": false}
        }
      }
    }
  ]
}

Implementing the Backend

Your API endpoint receives structured requests from Copilot and returns formatted responses:

import { CopilotPluginRequest, CopilotResponse } from '@microsoft/copilot-sdk';

export async function getCustomerDetails(
  req: CopilotPluginRequest
): Promise<CopilotResponse> {
  const { customer_id, include_history } = req.parameters;

  const customer = await crmService.getCustomer(customer_id);
  const history = include_history
    ? await crmService.getInteractionHistory(customer_id)
    : null;

  return {
    type: "AdaptiveCard",
    body: {
      customer_name: customer.name,
      account_value: customer.annualRevenue,
      primary_contact: customer.primaryContact.email,
      recent_interactions: history?.slice(0, 5),
      actions: [
        { type: "OpenUrl", title: "View in CRM", url: customer.crmUrl }
      ]
    }
  };
}

Testing and Deployment

Use the Copilot Developer Portal to test plugins in a sandboxed environment before enterprise deployment. The new debugging tools show exactly how Copilot interprets your API responses and generates user-facing output.