BLS CAPTCHA Errors and Troubleshooting

BLS CAPTCHA solving has unique challenges because it uses a custom implementation. Here are the most common errors and their solutions.

API submission errors

ERROR_BAD_PARAMETERS

Cause: Missing required parameters — either instructions or images.

Fix:

# WRONG — missing instructions
response = requests.post("https://ocr.captchaai.com/in.php", data={
    "key": API_KEY, "method": "bls",
    "image_base64_1": img1, "json": 1
})

# CORRECT — include instructions
response = requests.post("https://ocr.captchaai.com/in.php", data={
    "key": API_KEY, "method": "bls",
    "instructions": "Select all images with a car",
    "image_base64_1": img1, "json": 1
})

ERROR_WRONG_FILE_EXTENSION

Cause: Image data is not valid base64 or is an unsupported format.

Fix:

• Ensure images are base64-encoded PNG or JPEG
• Remove the data:image/...;base64, prefix
• Verify the base64 string is not truncated

import base64

# Strip the data URI prefix
src = img_element.get_attribute("src")
if src.startswith("data:image"):
    b64 = src.split(",")[1]
else:
    # Download and encode
    img_data = requests.get(src).content
    b64 = base64.b64encode(img_data).decode()

ERROR_CAPTCHA_UNSOLVABLE

Cause: The images are too low quality, blurry, or the instruction is ambiguous.

Fix:

• Capture images at full resolution
• Ensure the instruction text is extracted correctly
• Retry — some challenges are inherently more difficult

Image extraction errors

Images load dynamically

Problem: Images are not in the DOM when the page first loads.

Fix: Wait for the captcha to fully render:

from selenium.webdriver.support.ui import WebDriverWait
from selenium.webdriver.support import expected_conditions as EC

# Wait for captcha images to load
WebDriverWait(driver, 10).until(
    EC.presence_of_all_elements_located((By.CSS_SELECTOR, ".captcha-image img"))
)

Images are canvases, not img elements

Problem: Some BLS implementations render images on <canvas> elements.

Fix: Extract canvas data as base64:

canvas_elements = driver.find_elements(By.CSS_SELECTOR, ".captcha-canvas")
for i, canvas in enumerate(canvas_elements, 1):
    b64 = driver.execute_script(
        "return arguments[0].toDataURL('image/png').split(',')[1];",
        canvas
    )
    payload[f"image_base64_{i}"] = b64

Images behind anti-hotlinking

Problem: Image URLs return 403 when fetched outside the browser.

Fix: Extract images within the browser context:

# Get image data from within the browser
b64 = driver.execute_script("""
    var img = arguments[0];
    var canvas = document.createElement('canvas');
    canvas.width = img.naturalWidth;
    canvas.height = img.naturalHeight;
    canvas.getContext('2d').drawImage(img, 0, 0);
    return canvas.toDataURL('image/png').split(',')[1];
""", img_element)

Solution application errors

Wrong images selected

Cause: Image ordering mismatch between extraction and display.

Fix: Maintain consistent ordering:

# Ensure images are indexed in display order
captcha_imgs = driver.find_elements(By.CSS_SELECTOR, ".captcha-image img")
# The order of find_elements matches DOM order = display order
for i, img in enumerate(captcha_imgs, 1):
    payload[f"image_base64_{i}"] = extract_base64(img)

Solution indices don't match

Cause: CaptchaAI returns 1-based indices, but your code uses 0-based.

Fix:

solution = result["request"]  # e.g., "1,3,5"
indices = [int(i) for i in solution.split(",")]

# Convert to 0-based for array access
for idx in indices:
    captcha_imgs[idx - 1].click()  # 1-based → 0-based

Form submission fails after correct selection

Cause: Additional form fields or tokens are missing.

Fix: Check for hidden fields that must be submitted alongside the captcha:

# Look for hidden captcha tokens
hidden_fields = driver.find_elements(By.CSS_SELECTOR, "input[type='hidden']")
for field in hidden_fields:
    name = field.get_attribute("name")
    value = field.get_attribute("value")
    print(f"Hidden field: {name}={value}")

Timeout errors

Captcha expires before solve completes

Problem: BLS CAPTCHA has a short validity window.

Fix:

• Extract images and submit to CaptchaAI immediately
• Do not extract images and then wait before submitting
• If the solve takes >60 seconds, the captcha may have expired — refresh and retry

Polling takes too long

Fix: Ensure you are polling correctly:

# Standard polling pattern
for _ in range(30):  # 30 attempts × 5 seconds = 150 seconds max
    time.sleep(5)
    result = requests.get("https://ocr.captchaai.com/res.php", params={
        "key": API_KEY, "action": "get", "id": task_id, "json": 1
    }).json()

    if result.get("status") == 1:
        return result["request"]
    if result.get("request") == "ERROR_CAPTCHA_UNSOLVABLE":
        # Don't keep polling — start over
        raise Exception("Unsolvable")

Debugging checklist

FAQ

How many images should I send to CaptchaAI?

Send all images displayed in the CAPTCHA, typically 3–9. Use image_base64_1 through image_base64_9.

What if the instruction is in a non-English language?

Send the instruction exactly as displayed. CaptchaAI handles multilingual instructions.

Can I preload images to speed up solving?

No. BLS generates unique images per session. You must extract them fresh for each captcha instance.

What if BLS changes their CAPTCHA format?

If the format changes, image extraction code may need updating. The CaptchaAI API parameters (method=bls, instructions, images) will remain the same.

Related guides

• How to Solve BLS CAPTCHA Step by Step
• How BLS CAPTCHA Works
• CaptchaAI Error Codes Reference