How to Debug a Failed HTTP Request From a HAR File
The fastest way to debug a failed HTTP request is to export the browser network log as HAR, find the bad row, and copy it as cURL. FindUtils HAR Viewer parses that JSON in your browser so you can read method, URL, status, size, and time without uploading the file.
This post walks that path end to end. It also states when you should stop: localhost, private hosts, and secret headers do not belong in a public builder.
Why Start From a HAR File
A screenshot of DevTools loses headers and timing. A HAR file keeps the session as JSON. Anyone with the file can open the same table.
Start from HAR when:
- The failure only happens in the browser. The log shows redirects, CORS preflights, and the real URL.
- You need a command, not a story. One row becomes cURL.
- You must keep the capture local. Client-side parse does not send the file to FindUtils.
Treat the export as sensitive. HAR files often include cookies and authorization headers.
A Four-Step Debug Path
Step 1: Capture only the failing flow
Open the network panel. Clear it. Reproduce the bug. Save HAR. A smaller file is easier to read and stays under the viewer’s 20 MB text limit.
Step 2: Inspect the table
Open the FindUtils HAR Viewer. Drop the file. Find the 4xx or 5xx row, or the slow one. The viewer does not render HAR as HTML.
Step 3: Copy cURL, then strip secrets
Copy the row. Replace live tokens with placeholders before you paste the command into a ticket. If you need many commands, use HAR to cURL.
Step 4: Replay only public hosts
If the URL is a public http or https host, rebuild it in the HTTP Request Builder. Private, local, and metadata addresses are rejected. Timeout is 15 seconds. Body limit is 64 KB. For a local API, run cURL on your machine instead.
Convert a clean command into code with cURL to Code when the fix belongs in a script.
HAR Workflow vs Guessing in the UI
| Approach | What you see | Secret risk | Replay |
|---|---|---|---|
| Screenshot of DevTools | One moment, incomplete | Low, but URLs leak | None |
| HAR in FindUtils viewer | Method, URL, status, size, time | File stays in the browser | Copy cURL only |
| Public HTTP Request Builder | Live status, headers, body | Only if you paste secrets | Public hosts only |
| Desktop client or local cURL | Full control | Your machine | Localhost allowed |
Best for: Use the viewer to understand the capture. Use the builder for a short public replay. Use local cURL when the host is private.
Common Mistakes
Mistake 1: Sharing the raw HAR
Strip cookies and tokens first. Share the row index and a redacted command.
Mistake 2: Replaying localhost through the builder
The builder rejects loopback and private ranges on purpose.
Mistake 3: Converting the whole session when you need one row
The viewer copies one entry. That is usually enough for a bug report.
Tools Used in This Guide
- HAR Viewer — Parse HAR JSON and copy one cURL
- HAR to cURL — Convert many HAR entries
- HTTP Request Builder — Send a public HTTP request
- cURL to Code — Turn cURL into code
FAQ
Q1: Is this HAR workflow free? A: Yes. FindUtils HAR Viewer, HAR to cURL, HTTP Request Builder, and cURL to Code are free, with no signup.
Q2: Does FindUtils upload my HAR? A: No. Parse runs in the browser. The page may still load analytics or ads.
Q3: Can I replay any captured URL?
A: The HTTP Request Builder allows public http and https hosts only. Localhost and private ranges are rejected.
Q4: What is the HAR size limit on the viewer page? A: 20 MB of text.
Q5: Does the viewer execute JavaScript from the HAR? A: No. It does not render HAR as HTML.
Q6: Should I put authorization headers in a public builder? A: Prefer placeholders. If you must replay auth, do it on your own machine.
Next Steps
- Follow the HAR viewer guide
- Follow the HTTP request builder guide
- Convert collections with the Postman to cURL guide
- Browse the complete guide to online developer tools