ERROR_NO_SLOT_AVAILABLE: Queue Management Solutions

ERROR_NO_SLOT_AVAILABLE means all CaptchaAI workers for your requested CAPTCHA type are busy. This is a transient error — wait and retry.

Why This Happens

Basic Retry Logic

import time
import requests


def submit_with_slot_retry(api_key, method, params, max_retries=5):
    """Submit task with automatic retry on NO_SLOT_AVAILABLE."""
    delay = 3

    for attempt in range(max_retries):
        resp = requests.post("https://ocr.captchaai.com/in.php", data={
            "key": api_key,
            "method": method,
            "json": 1,
            **params,
        }, timeout=30)
        result = resp.json()

        if result.get("status") == 1:
            return result["request"]

        error = result.get("request", "")

        if error == "ERROR_NO_SLOT_AVAILABLE":
            print(f"No slot available. Retry in {delay}s (attempt {attempt + 1})")
            time.sleep(delay)
            delay = min(delay * 1.5, 15)  # Cap at 15 seconds
            continue

        # Non-retriable error
        raise RuntimeError(f"Submit error: {error}")

    raise RuntimeError("Max retries exceeded — no slots available")

Throttled Batch Submission

Avoid triggering NO_SLOT by pacing your submissions:

import time
import requests


def submit_batch_throttled(api_key, tasks, rate_per_second=2):
    """Submit tasks at a controlled rate."""
    interval = 1.0 / rate_per_second
    task_ids = []

    for task in tasks:
        while True:
            resp = requests.post("https://ocr.captchaai.com/in.php", data={
                "key": api_key,
                "method": task["method"],
                "json": 1,
                **task["params"],
            }, timeout=30)
            result = resp.json()

            if result.get("status") == 1:
                task_ids.append(result["request"])
                break

            if result.get("request") == "ERROR_NO_SLOT_AVAILABLE":
                time.sleep(5)
                continue

            raise RuntimeError(result.get("request"))

        time.sleep(interval)

    return task_ids

Pre-Check Capacity

Check if slots are likely available before submitting large batches:

def check_capacity(api_key):
    """Quick check: submit a small test to see if slots are open."""
    resp = requests.post("https://ocr.captchaai.com/in.php", data={
        "key": api_key,
        "method": "base64",
        "body": "/9j/4AAQSkZJRg==",  # Tiny test image
        "json": 1,
    }, timeout=10)
    result = resp.json()

    if result.get("request") == "ERROR_NO_SLOT_AVAILABLE":
        return False
    return True

Scheduling Around Peak Hours

CaptchaAI capacity varies by time:

import datetime


def should_batch_now():
    """Check if current time is good for batch processing."""
    hour = datetime.datetime.utcnow().hour
    if 0 <= hour < 6:
        return True  # Off-peak
    if 18 <= hour < 24:
        return True  # Moderate
    return False  # Peak hours — use slower rate

Troubleshooting

FAQ

How long should I wait between retries?

Start with 3 seconds and increase by 50% each retry, capping at 15 seconds. Most slot issues resolve within 5-10 seconds.

Is NO_SLOT billed?

No. Failed submissions are not charged. Only successful solves are billed.

Should I switch providers on NO_SLOT?

No. Brief slot unavailability is normal for all providers. Implement retry logic instead — it's simpler and more reliable.

Related Guides

• Rate Limiting and 429 Responses
• Error Handling Decision Tree

Handle capacity gracefully — get CaptchaAI.