Mocking API dependencies with jsonbin.io and ASP.NET Core

When building against external APIs during development, you often hit rate limits, flaky responses, or simply don’t want real side effects while testing. I’ve started reaching for jsonbin.io as a dead-simple JSON mock store.

Why jsonbin?

• Free tier is generous for dev/test usage
• Each “bin” is just a JSON document accessible via REST
• You can validate bins against a JSON schema
• No infra to maintain

Workflow

1. Generate example data with ChatGPT

Give it your JSON schema and ask for a realistic example document. Paste the result as your bin content.

2. Create a bin

curl -v 
  -H "Content-Type: application/json" 
  -H "X-Master-Key: <YOUR_API_KEY>" 
  --request POST 
  --data '{"sample": "Hello World"}' 
  https://api.jsonbin.io/v3/b

The response gives you a bin ID — save it. Your mock endpoint is now https://api.jsonbin.io/v3/b/{BIN_ID}.

3. Wire up the HTTP client in ASP.NET Core

Register a typed HttpClient that points at jsonbin and sets the required headers:

services.AddHttpClient<IMyHttpClient, MyHttpClient>(options =>
{
    options.BaseAddress = new Uri("https://api.jsonbin.io/");
    options.DefaultRequestHeaders.Add("X-Bin-Meta", new[] { "false" });
});

The X-Bin-Meta: false header strips jsonbin’s metadata wrapper so you get your raw JSON back.

4. Swap base addresses per environment

In appsettings.Development.json, point the base address at jsonbin. In appsettings.Production.json, point it at the real API. Your IMyHttpClient code doesn’t change — just the config.

Keep your X-Master-Key in user secrets or environment variables, not in source code. dotnet user-secrets set "JsonBin:ApiKey" "your-key" is the right move.