---
title: Meta-Tools
sidebarTitle: Meta-Tools
description: search_skill / load_skill / execute_action — the three tools the MCP client sees, and the prompts to give the LLM.
icon: wrench
keywords: ['frontmcp', 'mcp', 'model context protocol', 'plugin', 'plugins', 'extension', 'skilled openapi', 'openapi', 'skills', 'meta', 'tools', 'typescript']
---

The plugin exposes exactly three tools through standard `tools/list`. Every per-operation OpenAPI tool is hidden behind these — `execute_action` is the **only** path to actually invoke an upstream operation, and it's the sole authorization checkpoint.

## `search_skill`

Search the registry by free-form query.

```json title="input"
{
  "query": "create an invoice",
  "limit": 20,
  "tags": ["billing"]
}
```

```json title="output"
{
  "skills": [
    {
      "skillId": "invoices",
      "name": "Invoices",
      "description": "Issue, query, and refund invoices.",
      "score": 0.42,
      "bundleVersion": "1.0.0"
    }
  ]
}
```

The score comes from the SDK's `SkillRegistry.search()` (TF-IDF in v1.2.0). `bundleVersion` is included when the skill was registered from a bundle — polling clients can use it to detect bundle swaps without depending on `notifications/skills/list_changed`.

## `load_skill`

Load the full markdown instructions and the executable actions for one skill.

```json title="input"
{ "skillId": "invoices" }
```

```json title="output"
{
  "skill": {
    "id": "invoices",
    "name": "Invoices",
    "description": "Issue, query, and refund invoices.",
    "instructions": "# Invoices skill\n\nThree operations: createInvoice, getInvoice, refundInvoice.",
    "bundleVersion": "1.0.0",
    "actions": [
      {
        "actionId": "createInvoice",
        "summary": "Create a new invoice",
        "inputJsonSchema": { "type": "object", "properties": { "customerId": {}, "amount": {} }, "required": ["customerId","amount"] },
        "outputJsonSchema": { "type": "object" }
      }
    ]
  },
  "isComplete": true
}
```

The LLM should call `load_skill` once per skill it intends to use, then keep the action schemas in context. Schemas inside `actions[]` are the same schemas the OpenAPI spec declared; the executor validates input against them on every `execute_action`.

Throws an MCP-protocol error when the `skillId` is unknown.

## `execute_action`

Invoke one action of a previously-loaded skill.

```json title="input"
{
  "skillId": "invoices",
  "actionId": "createInvoice",
  "input": { "customerId": "cus_42", "amount": 1234 }
}
```

```json title="output (success)"
{
  "ok": true,
  "status": 201,
  "contentType": "application/json",
  "data": { "id": "inv_1", "status": "open" }
}
```

```json title="output (failure)"
{
  "ok": false,
  "status": 0,
  "error": "authority denied: missing required role 'admin'"
}
```

### Pipeline

1. **Resolve** `(skillId, actionId)` from the plugin-private `HiddenOpRegistry`. Unknown action returns `ok: false` with `error: "unknown action ..."`.
2. **Pin** the operation descriptor at call entry — a hot bundle-swap mid-call doesn't change the descriptor the in-flight call uses.
3. **Authorize**: evaluate the action's `requiredAuthorities` (if any) against the caller's `authInfo` via `@frontmcp/auth`'s `AuthoritiesEngine`. Denial returns `ok: false` with `error: "authority denied: ..."`.
4. **Build** the outbound HTTP request through `@frontmcp/adapters/openapi`'s `buildRequest` — path interpolation, header injection defenses, body serialization, security context resolution all flow through the adapter.
5. **SSRF gate**: scheme allowlist (`https:` by default), hostname allowlist (built from the active bundle's `services[].baseUrl`), post-DNS IPv4/IPv6 blocklist (RFC 1918, link-local incl. cloud metadata, loopback, ULA), cloud-metadata hostname blocklist.
6. **Fetch** with timeout (`op.timeoutMs ?? defaultTimeoutMs`) and response-size cap (`op.maxResponseBytes ?? defaultMaxResponseBytes`).
7. **Parse** the response through `parseResponse` and return the structured envelope.

`execute_action` **never throws** — every failure (auth, schema, network, SSRF) returns `ok: false` with a structured `error` string. This keeps the meta-tool surface predictable for the LLM.

## Prompts to give the LLM

The exact prompt depends on your agent harness, but these phrasings work well in practice (cribbed from the tool descriptions the plugin ships):

- For `search_skill`: *"Use this tool first to discover what skills exist for the user's request."*
- For `load_skill`: *"Call this tool once per skill you intend to use. The instructions field is markdown — read it carefully before invoking any action."*
- For `execute_action`: *"This is the only way to invoke an upstream operation. The pipeline auto-validates input, applies authority checks, and returns a structured envelope. Failures are returned as `ok: false` with a reason — they don't throw."*

## Skills-only mode

If your FrontMCP server is configured with `skills_only` mode (set via `?mode=skills_only` on the MCP transport URL), the meta-tools stay visible — they're the only way to use skills, and the plugin treats them as exempt from the skills-only filter. See [Skills](/frontmcp/features/skills) for the skills-only contract.

## What about hidden tools?

By default (`exposeOperationsAsInternalTools: true`), per-operation tools are registered in `scope.tools` with **internal** visibility. This means they:

- Are still callable via the SDK / direct client and can participate in DI, hooks, and observability
- **Do NOT appear** in the public MCP `tools/list` response
- Are **NOT directly callable** from MCP clients via `tools/call`
- Always live in the plugin's `HiddenOpRegistry` so the meta-tools (`load_skill`, `execute_action`) can find and run them with the ABAC checkpoint

If you set `exposeOperationsAsInternalTools: false`, the per-operation tools live **only** in `HiddenOpRegistry` (not in `scope.tools`) and the only way to invoke them is `execute_action`. Use this stricter mode when you need to guarantee that nothing — not even other plugins or in-process tooling — can bypass the ABAC checkpoint.
