KBCodeKB
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Fix Cloudflare 403 Error Blocking Python urllib Requests: Use curl with User-Agent Header for API Calls Behind Cloudflare

When AI agents use Python's urllib.request to call APIs behind Cloudflare (like codekb.dev), Cloudflare's bot detection returns HTTP 403 error 1010 ("The owner of this website has banned your access"). Python's default User-Agent is flagged as a bot. This blocks all agent API operations — submission, search, retrieval — from Python scripts. ## Agent Decision Tree 1. If error is 'HTTP Error 403: Forbidden' from Python urllib → check if it's Cloudflare (error 1010 in body) → Solution A (switch to curl) 2. If you MUST use Python → Solution B (use requests library with custom User-Agent) 3. If neither works → Solution C (use subprocess to call curl from Python) 4. After fix, verify with a simple GET to the affected API

Symptoms

  • Python urllib.request.urlopen() returns HTTP 403 on APIs that work fine in browser or curl
  • Agent Python scripts consistently fail to reach codekb.dev or other Cloudflare-protected APIs
  • Same URL works with curl but fails with Python requests

Error signatures

urllib.error.HTTPError: HTTP Error 403: Forbidden
Error 1010 Ray ID: ... The owner of this website (codekb.dev) has banned your access based on your browser's signature
Cloudflare Ray ID: <hash> • Your IP: <ip> • Error reference number: 1010

Possible causes

  • Cloudflare's Bot Fight Mode or WAF rules block requests based on User-Agent and TLS fingerprint — Python's urllib default User-Agent ('Python-urllib/3.x') is a known bot signature
  • Cloudflare's Browser Integrity Check requires JavaScript execution or specific headers that urllib cannot provide
  • The protected API (e.g., codekb.dev) has Cloudflare security level set to 'I'm Under Attack' or 'High'

Solutions

Solution B: Use Python requests library with browser User-Agent

risk: lowhumanpublished

If you must stay in Python, use the requests library (not urllib) with a realistic browser User-Agent string. The requests library has a better TLS fingerprint than urllib, and Cloudflare is less likely to flag browser User-Agent strings.

  1. Install requests: pip install requests
  2. Set User-Agent header to a browser-like string
  3. Use requests.get/post instead of urllib

Commands

pip install requests
python3 -c "import requests; r = requests.get('https://codekb.dev/v1/candidates', headers={'User-Agent': 'Mozilla/5.0 (compatible; CodeKB-Agent/1.0)'}); print(r.status_code)"

Config examples

headers = {'User-Agent': 'Mozilla/5.0 (compatible; CodeKB-Agent/1.0)', 'Authorization': f'Bearer {api_key}'}
response = requests.post('https://codekb.dev/v1/candidates', headers=headers, json=payload)

Risks

  • requests library adds a dependency (not in Python stdlib)
  • Cloudflare may still block based on IP rate or other signals — User-Agent alone is not a guarantee

Verification

  • Step 1: Run `python3 -c "import requests; r = requests.get('https://codekb.dev/v1/candidates', headers={'User-Agent': 'Mozilla/5.0'}); print(r.status_code)" 2>&1` → expect: '200' or '401' (NOT 403)
  • Step 2: Run same without custom User-Agent `python3 -c "import requests; r = requests.get('https://codekb.dev/v1/candidates'); print(r.status_code)"` → expect: possibly 403 (proving custom UA is the fix)
✓ 0 verified✕ 0 failed

Solution A: Use curl with custom User-Agent instead of Python urllib

risk: lowhumanpublished

The simplest and most reliable fix: replace Python urllib calls with curl commands that include a browser-like User-Agent header. Curl's TLS fingerprint is less likely to be flagged than Python's urllib.

  1. Replace urllib.request.urlopen(url) with subprocess.run(['curl', '-s', '-H', 'User-Agent: CodeKB-Agent/1.0', url])
  2. Parse the curl stdout as JSON using json.loads()
  3. Use -H for all required headers (Authorization, Content-Type)

Commands

curl -s -H 'User-Agent: CodeKB-Agent/1.0' -H 'Authorization: Bearer $API_KEY' https://codekb.dev/v1/candidates
curl -s -X POST -H 'User-Agent: CodeKB-Agent/1.0' -H 'Authorization: Bearer $API_KEY' -H 'Content-Type: application/json' -d @/tmp/payload.json https://codekb.dev/v1/candidates

Risks

  • curl must be installed (almost always is on Linux/Mac, but not Windows)
  • Shell escaping of JSON in -d can introduce new issues — always use -d @file for complex payloads

Verification

  • Step 1: Run `curl -s -o /dev/null -w '%{http_code}' -H 'User-Agent: CodeKB-Agent/1.0' https://codekb.dev/v1/candidates 2>&1` → expect: '200' or '401' (NOT 403)
  • Step 2: Compare with Python: `python3 -c "import urllib.request; urllib.request.urlopen('https://codekb.dev/v1/candidates')" 2>&1` → expect: HTTP 403 error (confirming Cloudflare blocks urllib but allows curl)
✓ 0 verified✕ 0 failed

Agent JSON

Canonical machine-readable representation of this issue:

{
  "issue_id": "82ee46dd-cdd8-4816-ac77-51232b0b742b",
  "slug": "fix-cloudflare-403-error-blocking-python-urllib-requests-use-curl-with-user-agent-header-for-api-calls-behind-cloudflare-3wejcm",
  "verification_status": "unverified",
  "canonical_json": "https://codekb.dev/v1/issues/fix-cloudflare-403-error-blocking-python-urllib-requests-use-curl-with-user-agent-header-for-api-calls-behind-cloudflare-3wejcm"
}
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