API Debugging Assistant — Diagnose HTTP & REST Errors

This API debugging assistant helps you systematically diagnose failing HTTP and REST API calls. It walks you outward-in from the status code — 4xx means your request was wrong, 5xx means the server failed — through headers, authentication, and request body so you can isolate the real cause fast. It explains common culprits like expired JWT tokens, missing Content-Type headers, CORS blocks, and rate limiting (429). Everything runs in your browser, so you can paste requests and responses without sending sensitive payloads to a third party.

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Format
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Analyze
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Document
1

Paste & Validate JSON

Fix syntax errors and beautify raw API strings.

Formatted JSON will appear here...
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Transform to CSV

Flatten hierarchical data for easy analysis in Excel or Sheets.

💡 Pro Tip: We've automatically identified the data arrays in your payload and flattened nested objects using dot notation.

Why flatten JSON?

Hierarchical JSON is hard to analyze in spreadsheets. Flattening converts `user.address.city` into a single column, making it sortable and filterable in Excel or Google Sheets.

Security Note

This transformation happens entirely in your browser memory. Your API keys, tokens, or sensitive user data are never transmitted to our servers.
CSV output...
3

Generate Schema / JSON-LD

Create standardized documentation for your API response.

Schema will appear here...

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✨ Pro Tips for API Debugging

  • Webhooks: Paste raw webhook payloads here to quickly see the data structure and field names.
  • Null Handling: Our CSV converter automatically handles missing keys in large arrays to keep your columns aligned.
  • Privacy: We never log your data. All formatting and conversion happens locally in your browser's V8 engine.

Frequently asked questions

Can I test POST requests?

Yes, you can configure the method, headers, and body for any HTTP request.

Does it support CORS?

Since it runs in the browser, it is subject to CORS policies. You may need a proxy or extension for some requests.

Is it like Postman?

It is a lightweight, browser-based alternative for quick API testing without installing software.

Are my API keys safe?

Your requests are made directly from your browser. We do not proxy or log your keys or sensitive data.

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A systematic way to debug any API call

API bugs feel mysterious but almost always resolve to one of a few layers. Work outside-in. First read the status code: 2xx means the server accepted it (your bug is in handling the response), 4xx means you sent something wrong (auth, payload, URL), 5xx means the server failed (often still triggered by your malformed input). The code alone narrows the search dramatically.

Then check, in order: the URL and method (a GET where POST was expected returns 404/405), the headers (missing Content-Type: application/json or Authorization is the most common 400/401), and the request body (malformed JSON, a trailing comma, wrong field names or types). Reproduce the call in curl or an HTTP client to isolate it from your application code.

Common culprits: an expired or wrongly-formatted token, CORS blocking a browser request (a server-side fix, not client), sending form-encoded data where JSON was expected, timezone or encoding mismatches, and rate limiting (429). Log the full request and response — headers included — and compare a working call against the failing one line by line. The difference is your bug.

Related guides

API Payload Optimization →
Reviewed by the ToolsmithPro editorial team · Last updated June 2026. Every calculation and conversion runs entirely in your browser — your inputs are never uploaded, stored or shared. Formulas and methodology are documented on our about page; spot an error? tell us and we'll fix it.

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JSON formatter → JWT decoder → URL encoder → Base64 encoder →