> ## Documentation Index
> Fetch the complete documentation index at: https://docs.domesystems.ai/llms.txt
> Use this file to discover all available pages before exploring further.

# Tools

> Why Dome attaches MCP servers as tools agents call without holding upstream credentials

export const tool = "A tool is an MCP server (or related backend) you attach so agents can call it without holding upstream secrets. Dome authorizes each call, injects the credential, and audits the result.";

<p>
  {tool}
</p>

## High-level overview

When you add a tool, Dome stores its endpoint, protocol, credentials, and egress headers. After it connects, every operation the server exposes appears in that tool's **discovered catalog**. You can block or deprecate one operation without removing the whole tool.

For an agent to use a tool:

1. Add the tool for the upstream MCP server.
2. Configure upstream authentication when the server requires it.
3. Attach the tool to a [Gateway](/concepts/gateways).
4. Allow that Gateway on the agent.
5. Confirm discovery with the agent's key.

Adding a tool and calling it looks like this:

1. An operator adds a Streamable HTTP tool for Notion, sets shared API-key auth, attaches it to `prod-tools`, and grants an agent access.
2. The agent lists tools at `/gateways/<id>/mcp`.
3. Each `tools/call` runs admission, Rules, request Guards, egress with injected credentials, response Guards, and audit.

The agent never sees the Notion credential.

A tool is a callable MCP connection. Gateway membership makes it reachable. Rules still decide each call.

### Upstream authentication

Agent API keys authenticate inbound requests to Dome. They never pass through as upstream credentials. Dome stores upstream secrets and injects them only on the outbound call.

Two settings control which credential the gateway sends: the **authentication method** (how the upstream verifies the request) and the **credential type** (whose credential is sent: shared or per-user). The pairings are fixed. Catalog of methods, types, and defaults is on the [Tools](/reference/resources/tools#upstream-authentication) reference.

With per-user credentials, each request must carry a verified act-as identity. When the end user has no credential yet, Dome returns `401` with a short-lived `provision_url`. Refer to [Errors and denials](/develop#errors-and-denials).

### Egress headers

Egress headers are values the gateway adds on the outbound request. A header can be a literal, a stored secret, or the verified act-as identity. Only Streamable HTTP tools can carry them. An act-as header requires OIDC or HMAC verification and is incompatible with OAuth auth on the same tool. Details are on the [Tools](/reference/resources/tools#egress-headers) reference.

### Discovered tools

After the gateway connects, every operation the server exposes appears in the tool's catalog. Status (`active`, `deprecated`, `blocked`) lets you change one operation without removing the tool. Blocking denies the operation even when a rule would permit it. Status behavior is on the [Tools](/reference/resources/tools#discovered-tools) reference.

### Protocols

Dome supports Streamable HTTP for remote MCP servers and stdio for local processes. OAuth and egress headers require Streamable HTTP. Protocol notes are on the [Tools](/reference/resources/tools#protocols) reference and the [Tools gateway](/concepts/gateways/tools-gateway#egress-protocols) concept.

## Next steps

With the tool connection model in mind, continue with:

* [Resources](/concepts/resources) concept for how tools sit among resource types
* [Tools gateway](/concepts/gateways/tools-gateway) concept for the MCP ingress under a Gateway
* [Gateways](/concepts/gateways) concept for membership and reachability
* [Guards](/concepts/controls/guards) concept for JSON Filters on tool arguments and results
* [Tools](/reference/resources/tools) reference for auth, headers, catalog status, and protocols
* [Tools](/connect/resources/tools) to add tools, manage catalogs, and attach Gateways