Common OCR CAPTCHA Errors and Fixes

Image/text CAPTCHA solving can fail due to image quality, format issues, or incorrect hint parameters. Here is how to diagnose and fix the most common errors.

Submission errors

ERROR_WRONG_FILE_EXTENSION

Cause: The image is not in a supported format or the base64 is invalid.

Fix:

import base64

# Ensure proper encoding
with open("captcha.png", "rb") as f:
    b64 = base64.b64encode(f.read()).decode()

# Don't include the data URI prefix
# WRONG: "data:image/png;base64,iVBOR..."
# RIGHT: "iVBOR..."

ERROR_ZERO_CAPTCHA_FILESIZE

Cause: The image file is empty or the download failed.

Fix:

import os

# Check file size before submitting
if os.path.getsize("captcha.png") == 0:
    print("Image file is empty — re-download")
    # Re-capture the captcha

ERROR_TOO_BIG_CAPTCHA_FILESIZE

Cause: Image exceeds maximum size (typically 600KB).

Fix:

from PIL import Image
import io

img = Image.open("captcha.png")
# Reduce quality without losing text clarity
buffer = io.BytesIO()
img.save(buffer, format="PNG", optimize=True)

Wrong text returned

Characters consistently misread

Cause: The solver confuses similar characters (0/O, 1/l/I, 5/S).

Fix: Use hint parameters to constrain the character set:

# If captcha is digits only
response = requests.post("https://ocr.captchaai.com/in.php", data={
    "key": API_KEY, "method": "base64", "body": b64,
    "numeric": 1,  # 1 = digits only
    "json": 1
})

# If captcha is letters only
response = requests.post("https://ocr.captchaai.com/in.php", data={
    "key": API_KEY, "method": "base64", "body": b64,
    "numeric": 2,  # 2 = letters only
    "json": 1
})

Wrong case (uppercase vs lowercase)

Cause: The solver defaults to lowercase.

Fix: Set regsense=1 to enable case sensitivity:

response = requests.post("https://ocr.captchaai.com/in.php", data={
    "key": API_KEY, "method": "base64", "body": b64,
    "regsense": 1,  # Case-sensitive
    "json": 1
})

Extra or missing characters

Cause: Noise is interpreted as characters, or characters are merged.

Fix: Set min/max length constraints:

# If you know the CAPTCHA is always 6 characters
response = requests.post("https://ocr.captchaai.com/in.php", data={
    "key": API_KEY, "method": "base64", "body": b64,
    "min_len": 6,
    "max_len": 6,
    "json": 1
})

Math expression not computed

Cause: The solver reads the text "3+7" instead of computing the answer "10".

Fix: Set calc=1:

response = requests.post("https://ocr.captchaai.com/in.php", data={
    "key": API_KEY, "method": "base64", "body": b64,
    "calc": 1,  # Compute the math expression
    "json": 1
})

Image quality issues

CAPTCHA too small

Problem: Very small images (under 50px height) lose character detail.

Fix: Capture at the largest available size. If the page renders the captcha small, check for a higher-resolution source URL:

# Check for higher-res version
img_src = captcha_el.get_attribute("src")
# Some sites use ?size=small — try removing or changing the parameter
high_res_src = img_src.replace("size=small", "size=large")

CAPTCHA is animated

Problem: Some CAPTCHAs use animated GIFs where the text is only visible in certain frames.

Fix: Extract the correct frame:

from PIL import Image

gif = Image.open("captcha.gif")
# Extract each frame and find the one with text
for i in range(gif.n_frames):
    gif.seek(i)
    gif.save(f"frame_{i}.png")

CAPTCHA has transparent background

Problem: PNG with transparent background may not render correctly for OCR.

Fix: Add a white background:

from PIL import Image

img = Image.open("captcha.png").convert("RGBA")
background = Image.new("RGBA", img.size, (255, 255, 255, 255))
background.paste(img, mask=img)
background.convert("RGB").save("captcha_white_bg.png")

Reporting incorrect solutions

If CaptchaAI returns the wrong text, report it:

# Report bad answer
requests.get("https://ocr.captchaai.com/res.php", params={
    "key": API_KEY,
    "action": "reportbad",
    "id": task_id
})

This improves solver accuracy and may refund the solve cost.

Accuracy improvement checklist

FAQ

Why is my CAPTCHA always wrong by one character?

Use min_len and max_len to constrain the answer length. Also verify the image quality — blurry images cause misreads.

Should I use file upload or base64?

Both work equally well. Base64 is convenient for programmatic use; file upload works better with multipart form tools.

How do I solve CAPTCHAs in other languages?

Set the language parameter: 1 for Cyrillic, 2 for Latin. For other scripts, omit the parameter and let the solver detect automatically.

Can I solve audio CAPTCHAs with this method?

No. Audio CAPTCHAs require a different solving approach. Check CaptchaAI documentation for audio CAPTCHA support.

Related guides

• Image CAPTCHA Solving Using API
• How Image CAPTCHA Solving Works (OCR)
• CaptchaAI Error Codes Reference