# Direct API Wrapper

> Expose an existing API as MCP tools by mapping each operation one-to-one — fastest path from 'we have an API' to 'agents can call it'.

- **Category**: Agentic AI
- **Subcategory**: Tool Use & Environment
- **Canonical URL**: https://designpattern.fyi/patterns/direct_api_wrapper/

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## Description
**Intent**: Make any well-documented API agent-callable with minimal wrapper logic.

**Context**: You already run a stable HTTP API with an OpenAPI spec or typed SDK. You want agents to call it without re-implementing business logic or writing tool definitions by hand.

**Solution**: Map each API operation to one MCP tool, deriving name, input schema, and output shape directly from the API contract. A generator reads the OpenAPI doc and emits the server — tools track the API automatically. FastMCP's `from_openapi`/`from_fastapi`, `fastapi-mcp`, and Speakeasy/Stainless generators all follow this shape. Keep the wrapper free of new business logic so regeneration stays cheap.


## Use Cases
- An existing API is stable and described by an OpenAPI doc or typed SDK.
- The goal is the fastest route to making the API agent-callable.
- The API surface is small enough that one tool per operation doesn't overwhelm the model.
- The team wants tools to track the API automatically, not be hand-maintained.





## Trade-offs


### Advantages

- Fastest path from existing API to agent-callable tool surface.

- Low maintenance — regenerate from the contract when the API changes.

- Works for any OpenAPI-described API regardless of implementation language.

- No new semantics to design or document.




### Considerations & Drawbacks

- Tool sprawl — a large API becomes a long list of low-level tools that overwhelms model selection.

- Programmer-oriented operation names and error shapes can mislead tool selection.

- No orchestration or error smoothing — multi-step tasks still require the model to chain calls.

- Inherits the API's chattiness; token cost scales with round-trips.







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**Reference**: [Original Source](https://www.agentpatternscatalog.org/patterns/direct-api-wrapper/)