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API Tool Returns HTML Error Page Instead of JSON — JSONDecodeError on Error Response

Symptom

  • json.JSONDecodeError: Expecting value: line 1 column 1 (char 0)
  • json.decoder.JSONDecodeError: Extra data: line 2 column 1
  • Response starts with <!DOCTYPE html> or <html>
  • Works during normal operation, fails during maintenance or high load
  • WAF (Web Application Firewall) returns HTML block page instead of JSON
  • CDN returns HTML error page on backend failure

Root Cause

HTTP servers can return HTML for any status code — including 500, 503, and even 200 (maintenance pages). If your code unconditionally calls response.json() without checking Content-Type, an HTML response causes a JSON parse failure. The actual error (maintenance, auth block, WAF) is hidden behind the JSON parse exception.

Fix

Option 1: Check Content-Type before parsing

import httpx

async def safe_api_call(url: str, **kwargs) -> dict:
    async with httpx.AsyncClient() as client:
        response = await client.get(url, **kwargs)

    content_type = response.headers.get("content-type", "")

    if "application/json" not in content_type:
        # Not JSON — inspect what we actually got
        body_preview = response.text[:500]
        raise ValueError(
            f"Expected JSON but got {content_type}\n"
            f"Status: {response.status_code}\n"
            f"Body preview: {body_preview}"
        )

    response.raise_for_status()
    return response.json()

Option 2: Try JSON parse, fall back to informative error

import json, httpx

def parse_response(response: httpx.Response) -> dict:
    """Parse response with helpful error on non-JSON content"""
    raw = response.text

    # Try JSON first
    try:
        return json.loads(raw)
    except json.JSONDecodeError:
        pass

    # Detect HTML error pages
    if raw.strip().startswith("<!") or raw.strip().startswith("<html"):
        if response.status_code == 503:
            raise ServiceUnavailableError("API is in maintenance mode (returned HTML 503 page)")
        elif response.status_code == 403:
            raise AuthError("Request blocked (returned HTML 403 — likely WAF or IP block)")
        elif response.status_code == 500:
            raise APIError(f"API internal error (returned HTML 500 page). Body: {raw[:200]}")
        else:
            raise APIError(
                f"API returned HTML instead of JSON (status {response.status_code}). "
                f"This usually means maintenance, WAF block, or misconfigured endpoint. "
                f"Body preview: {raw[:300]}"
            )

    # Non-JSON, non-HTML
    raise APIError(f"Unexpected response format (status {response.status_code}): {raw[:200]}")

Option 3: Robust request wrapper with full diagnosis

import httpx, json

class RobustAPIClient:
    def __init__(self, base_url: str, headers: dict = None):
        self.base_url = base_url
        self.default_headers = {"Accept": "application/json", **(headers or {})}

    async def get(self, path: str, **kwargs) -> dict:
        return await self._request("GET", path, **kwargs)

    async def post(self, path: str, **kwargs) -> dict:
        return await self._request("POST", path, **kwargs)

    async def _request(self, method: str, path: str, **kwargs) -> dict:
        url = f"{self.base_url}{path}"
        headers = {**self.default_headers, **kwargs.pop("headers", {})}

        async with httpx.AsyncClient(timeout=30.0) as client:
            response = await client.request(method, url, headers=headers, **kwargs)

        # Log non-200 for debugging
        if response.status_code >= 400:
            print(f"API error: {method} {url} → {response.status_code}")

        # Check content type
        content_type = response.headers.get("content-type", "")
        if "application/json" not in content_type and "text/json" not in content_type:
            body = response.text[:1000]
            raise APIResponseError(
                f"Non-JSON response from {url}: "
                f"status={response.status_code}, "
                f"content-type={content_type!r}, "
                f"body={body!r}"
            )

        try:
            data = response.json()
        except json.JSONDecodeError as e:
            raise APIResponseError(
                f"Invalid JSON from {url}: {e}\n"
                f"Raw response: {response.text[:500]}"
            ) from e

        response.raise_for_status()  # Raise for 4xx/5xx after JSON parse
        return data

Option 4: Handle HTML responses gracefully in agent

System prompt:
"When calling API tools:
1. Always check if the response is valid JSON before processing
2. If response starts with '<' or '<!DOCTYPE', it's an HTML error page — report the HTTP status code
3. Common causes of HTML responses:
   - 503: API maintenance, retry in 5 minutes
   - 403: IP blocked or authentication issue
   - 500: API internal error, retry once
   - 429: Rate limited, wait and retry
4. Never crash on a non-JSON API response — report what you received"

Option 5: Validate API endpoint is returning JSON during setup

async def validate_api_endpoint(url: str, api_key: str = None):
    """Health check that validates JSON response format"""
    headers = {"Authorization": f"Bearer {api_key}"} if api_key else {}

    async with httpx.AsyncClient() as client:
        try:
            response = await client.get(url, headers=headers, timeout=10.0)
        except httpx.ConnectError:
            raise RuntimeError(f"Cannot connect to {url}")

    content_type = response.headers.get("content-type", "")
    if "json" not in content_type:
        raise RuntimeError(
            f"API endpoint {url} is not returning JSON. "
            f"Got Content-Type: {content_type}. "
            f"Response: {response.text[:300]}"
        )

    print(f"API endpoint {url} validated: returns JSON (status {response.status_code})")

# Run at startup
await validate_api_endpoint("https://api.example.com/health")

Common HTML-Instead-of-JSON Scenarios

Scenario Status Body Fix
Maintenance page 200 HTML page Wait, retry later
WAF block 403 HTML block page Check IP, headers
Load balancer error 502/504 HTML error Retry with backoff
SSL termination error 200 HTML redirect Check HTTPS config
Wrong endpoint URL 404 HTML 404 page Fix URL
CDN caching HTML error 200 Cached HTML Add Cache-Control headers

Expected Token Savings

Debugging JSONDecodeError on HTML response: ~4,000 tokens Content-Type check prevents confusion: immediate clear error message

Environment

  • Any agent calling external APIs, especially during traffic spikes or incidents
  • Source: direct experience; extremely common when APIs go behind CDNs or WAFs

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