Let a voice agent use other apps' MCP servers during a call

An agent can call tools on another app’s MCP server while it is on the phone: look up an order in your backend, check a calendar, add a note to a CRM contact. There is no integration to build per app. Any app with a remote MCP server (Streamable HTTP or SSE) can be connected. Three steps:

Connect the server with POST /v1/mcp-servers. Its tools are read straight away.
Choose which tools agents may use with enabled_tools.
Attach the server to an agent with mcp_server_ids.

The console does the same under Integrations.

Connect a server

curl https://api.voice.wixzel.com/v1/mcp-servers \
  -H "Authorization: Bearer $WIXZEL_API_KEY" \
  -H "Content-Type: application/json" \
  -d '{
    "name": "Orders",
    "url": "https://mcp.example.com/mcp",
    "auth": {
      "type": "headers",
      "headers": [{ "name": "Authorization", "value": "Bearer sk_live_..." }]
    }
  }'

const server = await wixzel.mcpServers.create({
  name: 'Orders',
  url: 'https://mcp.example.com/mcp',
  auth: { type: 'headers', headers: [{ name: 'Authorization', value: 'Bearer sk_live_...' }] },
});
console.log(server.status, server.tools.map((t) => t.name));

name is what the agent is told the app is called, so use the name a caller would say. The answer lists every tool the server offers and a status:

status	meaning
`ready`	Tools were read. Agents can use it.
`needs_auth`	An OAuth server that has not been signed in to, or whose sign-in expired.
`error`	It could not be reached. `last_error` says why.
`pending`	Not tried yet.

Agents only use servers that are ready. POST /v1/mcp-servers/{id}/refresh reads the tools again and updates the status.

Authentication

none for a public server.
headers for an API key or any fixed header. Values are encrypted at rest and never returned; responses list only the header names. On update, send a header without value to keep the stored one.
oauth for a server that signs in. Most hosted MCP servers do.

Create the server with "auth": { "type": "oauth" }, then start the sign-in:

curl -X POST https://api.voice.wixzel.com/v1/mcp-servers/$ID/oauth/start \
  -H "Authorization: Bearer $WIXZEL_API_KEY"

{
  "object": "mcp_server_oauth",
  "authorization_url": "https://auth.example.com/authorize?client_id=...&state=...",
  "redirect_uri": "https://api.voice.wixzel.com/mcp-oauth/callback",
  "expires_at": "2026-10-02T10:10:00.000Z"
}

Open authorization_url in a browser and sign in. When the provider sends the browser back, the server is connected and its tools are read. The link works once and expires after ten minutes. Wixzel Voice registers itself with the server’s authorization server automatically when the server allows it. If it does not (GitHub’s, for one), register an OAuth app with the provider using the redirect_uri above, and send its id and secret as auth.oauth.client_id and auth.oauth.client_secret. Tokens are refreshed automatically during calls. If a refresh fails, the server moves to needs_auth and agents stop using it until it is connected again.

Choose the tools

Only tools in enabled_tools are ever offered to an agent. Until you set it, the first discovery enables only the tools the server marks read-only (readOnlyHint). Anything that can change data has to be switched on on purpose.

curl -X PATCH https://api.voice.wixzel.com/v1/mcp-servers/$ID \
  -H "Authorization: Bearer $WIXZEL_API_KEY" \
  -H "Content-Type: application/json" \
  -d '{ "enabled_tools": ["lookup_order", "create_note"] }'

The list is replaced as a whole. A tool that disappears from the server is removed from it at the next refresh.

Attach it to an agent

curl -X PATCH https://api.voice.wixzel.com/v1/agents/$AGENT_ID \
  -H "Authorization: Bearer $WIXZEL_API_KEY" \
  -H "Content-Type: application/json" \
  -d '{ "mcp_server_ids": ["'$ID'"] }'

An agent can use up to five servers and 40 tools in total. Send [] to detach all of them. Deleting a server detaches it from every agent.

What happens on a call

The tools are declared to the model when the call starts, from the stored list, so connecting a server never delays the greeting. The connections open in the background.
Each tool is named <app>__<tool>, for example orders__lookup_order, and its description starts with the app’s name.
The agent says a few words before it uses a tool, so the caller is not left in silence. On the classic and Sarvam engines, if the model goes straight to the tool, the agent says “One moment, let me check that.” for it.
Before using a tool the server marks as changing data, the agent asks the caller to confirm.
A tool call times out after 10 seconds. Failures become a sentence the agent can say (“I couldn’t reach Orders right now”); they never end the call.
Results are cut to 4,000 characters before the model sees them.
At most 20 tool calls per call, and three rounds of tool calls per caller turn before the agent has to answer in words.

Every tool call is recorded on the call. GET /v1/calls/{id} returns them in tool_calls, with the arguments, whether it worked, and how long it took. All four engines support tools: classic, sarvam, gemini-live and deepgram-agent.

Cost

Tool calls are free. The extra language-model tokens a tool result adds to the conversation are billed as usual, on the agent’s LLM line in /v1/usage/events.

Security

Allowlist. Only enabled tools are declared, and a call to any other name is refused even if the model guesses it.
Untrusted results. The agent is told that tool results are data, not instructions.
Network. Server URLs must be public https (or http) on port 443 or 80. Private, loopback and link-local addresses are refused when the server is saved and again on every request, including each redirect, and the connection goes to the address that was checked. A redirect to another host does not carry your headers.
Secrets. Header values, OAuth tokens and client secrets are encrypted at rest and never returned by the API.
Scopes. Managing servers needs mcp_servers:read and mcp_servers:write.

Getting started

Guides

Carriers

Concepts

API reference

Let a voice agent use other apps' MCP servers during a call

Connect a server

Authentication

Choose the tools

Attach it to an agent

What happens on a call

Cost

Security

​Connect a server

​Authentication

​Servers that sign in with OAuth

​Choose the tools

​Attach it to an agent

​What happens on a call

​Cost

​Security

Connect a server

Authentication

Servers that sign in with OAuth

Choose the tools

Attach it to an agent

What happens on a call

Cost

Security