TokenMix Research Lab · 2026-04-24

"Chat completion API provider returned error" Fix Guide

The error chat completion API provider returned error appears in Cursor, Cline, Continue.dev, and OpenRouter-integrated apps when an upstream LLM provider fails mid-request. It's a generic wrapper error — the actual problem varies. Root causes include: provider rate limit (429), model not available at provider, authentication failure on provider's side, provider outage, malformed request that passes gateway validation but fails at provider, and content filtering blocks. This guide walks through diagnosing and fixing each sub-case. TokenMix.ai includes automatic fallback routing that converts most of these errors into successful retries on alternate providers.

Table of Contents

• Confirmed vs Speculation
• Step 1: Get the Actual Provider Error
• Common Sub-Errors
• Fix by App: Cursor, Cline, OpenRouter
• Implementing Robust Retry Logic
• FAQ

Confirmed vs Speculation

Snapshot note (2026-04-24): Error-wrapping behavior differs across IDE versions (Cursor 3.x logs behave differently from 2.x), Cline release cycles, and OpenRouter's fallback config. Sub-error taxonomy below reflects the most common patterns seen as of April 2026 — specific error-code text varies slightly between provider versions. Always check raw upstream error before applying the "right fix".

Step 1: Get the Actual Provider Error

The wrapper error hides the real problem. To debug:

Cursor: Open ~/.cursor/logs/[latest].log → search for "provider returned" or "completion_error"

Cline: Output panel → Cline → look for full stack trace with upstream_error

OpenRouter: Response body JSON has error.metadata.raw field with original provider error

TokenMix.ai: Response headers include X-Provider-Error-Code and X-Provider-Error-Message showing the underlying cause

Common Sub-Errors

Fix by App: Cursor, Cline, OpenRouter

In Cursor (BYOK mode):

1. Check error log for specific provider error
2. Try different model ID in settings
3. If using OpenRouter: check OpenRouter dashboard for provider health
4. Switch to direct provider (Anthropic direct vs via OpenRouter)

In Cline:

1. Open Output panel → Cline output
2. Look for full error stack
3. Common fix: switch model provider entirely (Anthropic → OpenAI or vice versa)
4. For persistent issues: check Cline GitHub issues

In OpenRouter:

1. OpenRouter dashboard → Request history → expand failed request
2. Shows upstream provider raw error
3. OpenRouter has automatic fallback — ensure it's enabled in model settings
4. Some models configurable to "always fallback on error"

Via TokenMix.ai:

• Multi-provider fallback is default behavior — most errors are transparently handled
• Check response headers for provider attribution
• Contact support with trace ID from failed response

Implementing Robust Retry Logic

from openai import OpenAI, APIError
import time

def robust_call(prompt, models=None):
    """Try models in order, fallback on error"""
    models = models or [
        "anthropic/claude-sonnet-4-6",
        "openai/gpt-5-4",
        "z-ai/glm-5.1"  # budget fallback
    ]
    
    client = OpenAI(base_url="https://api.tokenmix.ai/v1", api_key="key")
    
    for model in models:
        for attempt in range(3):
            try:
                return client.chat.completions.create(
                    model=model,
                    messages=[{"role": "user", "content": prompt}]
                )
            except APIError as e:
                if "rate_limit" in str(e).lower():
                    time.sleep(2 ** attempt)
                    continue
                # Other errors: break to next model
                break
    
    raise RuntimeError("All providers failed")

Key principles: different error types = different strategies. Rate limit = retry same model. Other errors = fall over to different model.

FAQ

Why is the error so unhelpful?

Tools that wrap multiple providers (gateways, IDEs) sometimes catch upstream errors and wrap them generically to provide unified UX. Trade-off: uniformity at cost of debuggability. Always check logs for underlying details.

Does this error mean the provider is down?

Not necessarily. Could be your auth issue, rate limit, malformed request, or content filter — none are "provider down." Check provider status page (status.openai.com, status.anthropic.com) to confirm.

Can I prevent this error?

Not entirely — transient errors happen. Design for resilience: implement exponential backoff, multi-provider fallback, circuit breaker pattern (stop calling failing provider temporarily). TokenMix.ai does this automatically.

What if I get this error with my own Anthropic/OpenAI direct call?

Then the gateway isn't the wrapper — your app / SDK is. Check SDK version is current. Check error response body for specific provider error. Direct calls give you raw error; no gateway mystery.

Do tools like Cursor auto-retry?

Some yes, some no. Cursor 3.x has auto-retry for transient errors. Cline has limited retry. OpenRouter depends on fallback config. For production code, implement your own retry layer — don't rely on tool defaults.

Is this the same as "upstream error"?

Yes, different naming. "Upstream error" is more technical; "chat completion API provider returned error" is user-facing. Both mean the same thing.

How common is this error in production?

With single-provider setup: <1% of requests typical. With good multi-provider fallback (via TokenMix.ai): approaches 0% visible errors since fallbacks succeed.

Sources

• Gemini API 429 Fix — TokenMix
• Claude Rate Exceeded — TokenMix
• Cursor Unauthorized Fix — TokenMix
• OpenAI Error Codes — TokenMix
• AI API Rate Limits — TokenMix

By TokenMix Research Lab · Updated 2026-04-24