# How ReClip Handles Unsupported URLs: A Deep Dive into Error Handling

> Learn how ReClip handles unsupported URLs. ReClip uses yt-dlp for validation, returning HTTP 400 errors and preventing invalid job creation for a robust user experience.

- Repository: [Avery Gan/reclip](https://github.com/averygan/reclip)
- Tags: deep-dive
- Published: 2026-09-03

---

**ReClip delegates all URL validation to yt-dlp, propagates any "unsupported URL" errors back to the frontend as HTTP 400 JSON responses, and prevents invalid download jobs from being created.**

ReClip is a lightweight Flask-based video downloader that relies entirely on yt-dlp for URL parsing and media extraction. When users submit URLs from unsupported sites or malformed links, the application follows a clean, predictable error-handling flow. This article explains exactly how ReClip handles unsupported URLs by examining the source code in `averygan/reclip`.

## The Error Handling Flow for Unsupported URLs

ReClip does not maintain its own URL whitelist or validation logic. Instead, it lets yt-dlp determine whether a URL is valid. The flow spans three layers: the Flask backend, the yt-dlp subprocess, and the JavaScript frontend.

### Step 1: URL Submission to Flask Backend

When a user submits a URL, the frontend sends a POST request to either `/api/info` (to fetch metadata) or `/api/download` (to start a download). Both endpoints accept the same JSON payload:

```json
{ "url": "https://unsupported-site.example/video" }

```

The backend immediately checks for empty strings, then constructs a yt-dlp command.

### Step 2: yt-dlp Execution and Failure Detection

For the info endpoint in [`app.py`](https://github.com/averygan/reclip/blob/main/app.py), the backend runs:

```python
@app.route("/api/info", methods=["POST"])
def get_info():
    data = request.json
    url = data.get("url", "").strip()
    if not url:
        return jsonify({"error": "No URL provided"}), 400

    cmd = ["yt-dlp", "--no-playlist", "-j", url]
    result = subprocess.run(cmd, capture_output=True, text=True, timeout=60)

    # Unsupported URL → yt-dlp returns non-zero exit code

    if result.returncode != 0:
        return jsonify({"error": result.stderr.strip().split("\n")[-1]}), 400

```

If yt-dlp cannot recognize the URL, it exits with a non-zero status and writes an error to **stderr**. Common messages include "URL is not recognized" or "Unsupported URL".

### Step 3: JSON Error Response to Frontend

The Flask handler extracts the **last line** of stderr to keep the error message concise. The response follows this exact structure:

```json
{ "error": "URL is not recognized" }

```

The HTTP status code is **400 Bad Request**, indicating a client-side input error rather than a server failure.

### Step 4: Frontend Error Display

The client-side JavaScript in [`templates/index.html`](https://github.com/averygan/reclip/blob/main/templates/index.html) handles the response:

```javascript
fetch("/api/info", {method: "POST", body: JSON.stringify({url})})
  .then(r => r.json())
  .then(data => {
    if (data.error) {
      showAlert(data.error);   // Renders red alert below input
    } else {
      renderVideoInfo(data);
    }
  });

```

The UI displays the raw yt-dlp message without modification. No download job is created, and the internal `jobs` dictionary remains empty.

## How the Download Endpoint Handles Unsupported URLs

The `/api/download` endpoint follows the same pattern but with additional job tracking. From [`app.py`](https://github.com/averygan/reclip/blob/main/app.py):

```python
@app.route("/api/download", methods=["POST"])
def start_download():
    data = request.json
    url = data.get("url", "").strip()
    if not url:
        return jsonify({"error": "No URL provided"}), 400

    # Job creation and thread spawning...

    # The worker thread runs `run_download`, which sets:

    #   job["status"] = "error"

    #   job["error"] = yt-dlp error message

    # if the download command fails.

```

Key difference: the download endpoint spawns a background thread. If yt-dlp fails immediately with an unsupported URL error, the thread marks the job status as **"error"** and stores the yt-dlp message. The user sees the same error feedback via polling or WebSocket updates.

## Design Philosophy: Why ReClip Doesn't Pre-validate URLs

ReClip intentionally avoids URL filtering for three reasons:

- **yt-dlp coverage** — yt-dlp supports thousands of sites; maintaining a parallel list would be error-prone
- **Clear error messages** — yt-dlp provides specific, actionable failure reasons
- **Simplicity** — The codebase stays lean by delegating all extraction logic

As noted in [`README.md`](https://github.com/averygan/reclip/blob/main/README.md), ReClip promises to handle "Anything yt-dlp supports" — and nothing more.

## Summary

- ReClip **does not validate URLs itself**; all validation is delegated to yt-dlp
- Unsupported URLs trigger a **non-zero exit code** from yt-dlp with stderr error text
- Both `/api/info` and `/api/download` return **HTTP 400** with JSON `{"error": "..."}`
- The **last line of stderr** is extracted to keep error messages concise
- The frontend displays errors directly without creating download jobs
- The internal `jobs` dictionary remains untouched for invalid URLs

## Frequently Asked Questions

### What happens if I submit a completely malformed URL to ReClip?

ReClip passes the URL directly to yt-dlp. For malformed URLs, yt-dlp typically returns "URL is not recognized" or similar. The backend captures this from stderr, returns HTTP 400 with the message in JSON, and the frontend displays it as a red alert. The application does not crash or hang.

### Does ReClip support checking URL validity before starting a download?

Yes. The `/api/info` endpoint exists for this purpose. It runs yt-dlp with `--no-playlist -j` to extract metadata without downloading. If this succeeds, the URL is valid. If it fails, you get the same error response format before any download begins.

### Can I customize the error messages for unsupported URLs?

Not without modifying the source. ReClip returns yt-dlp's raw stderr output (specifically the last line). To customize messages, you would need to edit the error handling in [`app.py`](https://github.com/averygan/reclip/blob/main/app.py) around lines capturing `result.stderr.strip().split("\n")[-1]`.

### Why does ReClip return HTTP 400 instead of 422 or 500?

HTTP 400 Bad Request is appropriate because the error stems from client input (an unsupported URL), not server misconfiguration or unprocessable entity semantics. The 500 status is reserved for actual server failures like yt-dlp not being installed or subprocess timeouts.