Fix RequestEntityTooLarge: 413 Request Entity Too Large in Flask

This error occurs when an uploaded file exceeds Flask's MAX_CONTENT_LENGTH setting, which defaults to unlimited but is commonly set low. The request is rejected before your handler runs. Fix it by increasing MAX_CONTENT_LENGTH to an appropriate limit, validating file size client-side, and handling the 413 error gracefully with a custom error handler.

Reading the Stack Trace

Here's what each line means:

• File "/app/venv/lib/python3.12/site-packages/flask/app.py", line 869, in full_dispatch_request: Flask checks content length before dispatching to your route handler, rejecting oversized requests early.
• File "/app/venv/lib/python3.12/site-packages/werkzeug/exceptions.py", line 223, in __call__: Werkzeug raises RequestEntityTooLarge as an HTTP exception, which Flask converts to a 413 response.
• werkzeug.exceptions.RequestEntityTooLarge: 413 Request Entity Too Large: The uploaded content exceeds the configured MAX_CONTENT_LENGTH limit.

Common Causes

app.config['MAX_CONTENT_LENGTH'] = 1 * 1024 * 1024  # 1 MB — too small for images

# No @app.errorhandler(413) registered
# Users see raw werkzeug HTML error page

<form method='POST' enctype='multipart/form-data'>
  <input type='file' name='photo'>
  <button type='submit'>Upload</button>
</form>

The Fix

Increase MAX_CONTENT_LENGTH to an appropriate limit for your use case (e.g., 16 MB for image uploads) and add a custom error handler that returns a clear JSON response instead of a raw HTML error page.

app.config['MAX_CONTENT_LENGTH'] = 1 * 1024 * 1024  # 1 MB

app.config['MAX_CONTENT_LENGTH'] = 16 * 1024 * 1024  # 16 MB

@app.errorhandler(413)
def too_large(e):
    return jsonify({'error': 'File is too large. Maximum size is 16 MB.'}), 413

Testing the Fix

import pytest
import io
from app import create_app

@pytest.fixture
def client():
    app = create_app()
    app.config['TESTING'] = True
    app.config['MAX_CONTENT_LENGTH'] = 16 * 1024 * 1024
    return app.test_client()

def test_upload_within_limit_succeeds(client):
    data = {'file': (io.BytesIO(b'x' * 1024), 'small.txt')}
    response = client.post('/upload', data=data, content_type='multipart/form-data')
    assert response.status_code in (200, 201)

def test_upload_exceeding_limit_returns_413(client):
    # Simulate oversized content-length header
    app = client.application
    app.config['MAX_CONTENT_LENGTH'] = 1024  # 1 KB for test
    data = {'file': (io.BytesIO(b'x' * 2048), 'big.txt')}
    response = client.post('/upload', data=data, content_type='multipart/form-data')
    assert response.status_code == 413

Run your tests:

pytest tests/ -v

Pushing Through CI/CD

git checkout -b fix/flask-file-upload-413,git add app/__init__.py app/config.py,git commit -m "fix: increase upload limit and add 413 error handler",git push origin fix/flask-file-upload-413

Your CI config should look something like this:

name: CI
on:
  pull_request:
    branches: [main]
jobs:
  test:
    runs-on: ubuntu-latest
    steps:
      - uses: actions/checkout@v4
      - uses: actions/setup-python@v5
        with:
          python-version: '3.12'
          cache: 'pip'
      - run: pip install -r requirements.txt
      - run: pytest tests/ -v --tb=short
      - run: flake8 app/

The Full Manual Process: 18 Steps

Here's every step you just went through to fix this one bug:

1. Notice the error alert or see it in your monitoring tool
2. Open the error dashboard and read the stack trace
3. Identify the file and line number from the stack trace
4. Open your IDE and navigate to the file
5. Read the surrounding code to understand context
6. Reproduce the error locally
7. Identify the root cause
8. Write the fix
9. Run the test suite locally
10. Fix any failing tests
11. Write new tests covering the edge case
12. Run the full test suite again
13. Create a new git branch
14. Commit and push your changes
15. Open a pull request
16. Wait for code review
17. Merge and deploy to production
18. Monitor production to confirm the error is resolved

Total time: 30-60 minutes. For one bug.

pip install bugstack

import bugstack

bugstack.init(api_key=os.environ["BUGSTACK_API_KEY"])

Deploying the Fix (Manual Path)

1. Run pytest locally to verify uploads within the new limit succeed and oversized uploads return 413.
2. Open a pull request with the config and error handler changes.
3. Wait for CI checks to pass on the PR.
4. Have a teammate review and approve the PR.
5. Merge to main and test file uploads in staging.

Frequently Asked Questions