Cursor Composer 2.5 is fast and cheap enough to let an agent write whole API clients and route handlers for you. The catch shows up the moment the code touches a real service: the model writes a clean-looking request to /v2/orders when your service actually exposes /orders and expects a different payload. The code compiles. It just doesn’t work, and you find out three files later.
This guide shows the workflow that fixes that: point Composer 2.5 at your real API specification through MCP, let it generate code against the actual contract, then verify the result in Apidog before it reaches a teammate. If you’re new to the model, the Cursor Composer 2.5 guide covers what it is and how to access it.
Why agentic models guess API shapes
Composer 2.5 is built for long, multi-step agent tasks. Ask it to “add a client for our billing service and wire it into the checkout flow” and it will plan, edit several files, and run tests until they pass. That’s the upgrade over Composer 2, and it’s genuinely useful.
The weakness is structural, not a bug. When the model doesn’t have your API contract in context, it fills the gap with the most statistically likely shape: common field names, REST conventions, the version prefix it saw most in training. The output looks right. It passes a lint. It fails against your server because your server is not the average of every API on the internet.
Three symptoms of this:
- Endpoints that almost match (
/api/users/{id}vs your/users/{userId}). - Invented or wrong fields in request bodies.
- Auth handled the generic way instead of your service’s actual scheme.
You can prompt your way around some of it, but pasting your whole OpenAPI file into chat is brittle and burns context. The durable fix is to give the model structured access to the spec.
The fix: ground Composer 2.5 in your real API spec via MCP
Model Context Protocol (MCP) is the open standard for feeding tools and data to AI models. Cursor supports MCP servers, and the Apidog MCP server exposes your Apidog API specification to the model as a structured source it can query while it codes.
The difference in practice: instead of guessing, Composer 2.5 reads your actual endpoints, schemas, parameters, and response shapes, then writes code against them. This is the same idea behind vibe coding with the Apidog MCP server, applied to a model that’s now strong enough to do the whole task.
Step 1: prepare your API spec in Apidog
Your API contract needs to live somewhere the model can read. Design or import your API in Apidog so the schema, endpoints, and examples are current. If you’re starting from existing docs, Apidog imports OpenAPI and Postman collections directly. The spec is the source of truth the model will follow, so keeping it accurate is the whole game.
Step 2: connect the Apidog MCP server to Cursor
Cursor reads MCP servers from a config file in your project (commonly .cursor/mcp.json). The Apidog MCP server runs through npx and points at your project. A typical configuration looks like this:
{
"mcpServers": {
"apidog-api-spec": {
"command": "npx",
"args": ["-y", "apidog-mcp-server@latest", "--project=<your-project-id>"],
"env": { "APIDOG_ACCESS_TOKEN": "<your-access-token>" }
}
}
}
Use the exact command, project ID, and token from the Apidog MCP setup walkthrough, since those values are specific to your account and the server version. Restart Cursor after saving so it picks up the new server.
Step 3: confirm Composer 2.5 can see the spec
Open an agent session, select composer-2.5 in the model picker, and ask a read-only question first:
“Using the apidog-api-spec MCP server, list the endpoints under the orders resource and the required fields for creating an order.”
If it returns your real endpoints and fields, the connection works. If it answers from generic knowledge, the server isn’t wired in; recheck the config and restart.
Step 4: let it build against the contract
Now give it the real task and name the spec explicitly:
“Using the apidog-api-spec server as the source of truth, write a typed TypeScript client for the orders API, including the create-order and get-order calls. Match the request and response schemas exactly. Add error handling for the 422 validation response the spec defines.”
Because Composer 2.5 sustains long tasks well, it can do this across multiple files and keep the contract consistent. Naming the MCP source in the prompt keeps it anchored instead of drifting back to assumptions.
Verify before you trust: the Apidog test loop
Grounding the model cuts hallucinations sharply. It doesn’t make verification optional. A spec can be slightly behind the running service, and a model can still misread an edge case.
Close the loop:
- Send the generated calls as real requests. Take the endpoints Composer 2.5 wrote and run them in Apidog against a real or mock environment. Check status codes, response bodies, and auth behave as the code assumes.
- Turn working calls into tests. Save the validated requests as automated test scenarios so the next regression is caught by CI, not by a user.
- Mock what isn’t built yet. If the model wrote a client for an endpoint the backend hasn’t shipped, Apidog’s mock server returns realistic responses so frontend work continues. This pairs well with the patterns in AI agents and API testing.
The principle: the model writes the first draft against the contract, and you confirm the draft behaves against a real server. Speed from the agent only compounds if you’re not paying it back in debugging later.
A realistic end-to-end example
Say you’re adding a refunds feature to a payments service.
- The refunds endpoints and schema already exist in your Apidog project from the design phase.
- Apidog MCP is connected to Cursor; Composer 2.5 is selected.
- You prompt: “Using apidog-api-spec, build the refund client and a React hook that calls it. Follow the schema exactly, including the idempotency-key header the spec requires.”
- Composer 2.5 reads the real contract, writes the client, hook, and types, and runs the project’s tests.
- You open Apidog, fire a real create-refund request, confirm the idempotency behavior and the 409 on duplicate, then save both as test scenarios.
What you avoided: a client that forgot the idempotency header, shipped, and double-refunded a customer in staging. That’s the class of bug grounding plus verification removes.
Frequently asked questions
Does Composer 2.5 support MCP? Yes. It has access to Cursor’s full agent tool set, including MCP servers. Select it in the model picker and configure the server in your project. The Composer 2.5 guide covers model selection.
Do I need Apidog to use MCP with Composer 2.5? You need a structured spec source. The Apidog MCP server is the path covered here because it also gives you testing and mocking in the same place. Other options exist in the best MCP servers for Cursor roundup.
Will grounding the model in a spec stop all hallucinations? It removes the biggest category, wrong endpoints and schemas, because the model reads the real contract instead of guessing. It does not replace testing; specs drift from running services, so you still verify.
Is this worth it for a small project? If the model touches any real API, yes. The setup is a one-time config file. The payoff is every generated call matching your contract instead of a plausible guess.
The bottom line
Composer 2.5 is fast and cheap enough to let an agent own real API work. That only pays off if the model codes against your actual contract instead of an averaged guess. Connect your spec through the Apidog MCP server so Composer 2.5 reads the truth, then Download Apidog to send live requests, confirm the responses, and lock the working calls into automated tests and mocks. Grounded generation plus verification is the workflow that turns agent speed into shipped features.