Cursor API Errors: 401, 404, 429, and Connection Failures (2026)

When you enter a custom API key and Base URL in Cursor, errors usually fall into four layers: credentials, paths, rate limits, and network. Many people see a red warning and immediately switch models, turning a configuration problem into half a day of wasted work. The right order is to read the status code first, reproduce with a minimal request, and only then change the model or prompt.

This article breaks the problem down into 401, 404, 429, connection failures, streaming interruptions, and context overflows, with diagnostic commands, log fields, and an actionable support checklist.

Note: Cursor is a trademark of Anysphere, Inc. Cursor releases frequently, so settings menus and error text may change. This article uses common versions as of October 2026; refer to your installed version and the official documentation for specifics.

1. Diagnostic order: layer first, then change configuration

Status code / symptom	Common meaning	Check first	Retry?
401	Key missing, wrong, expired, or lacking permission	Key, auth field, account status	No
403	Key lacks model permission or team policy blocks it	Model permissions, team settings	No
404	Wrong path or nonexistent model	Base URL, model ID	No
429	Request rate, concurrency, or token quota exceeded	Usage, shared keys, retry policy	Retry after backoff
400/413	Malformed request or context too long	Context, parameters, max_tokens	Retry after changing the request
5xx	Server error or gateway overload	Service status, request ID	Retry after backoff
Connection failure	DNS, TLS, proxy, firewall	Network path	Retry after fixing

Fix one variable at a time: same key, same Base URL, same model ID, change only one item before testing again. If you change three things at once, a success tells you nothing about which change mattered.

2. 401: check the key, then the auth field

A 401 means authentication failed; it has nothing to do with model capability or context length. Troubleshoot in this order:

Was the key copied completely, without leading or trailing spaces, line breaks, or quotes? Copying from a chat window is the most common source of hidden characters.
Was the key entered in the correct provider field? Cursor has separate fields for OpenAI, Anthropic, Google, and Azure; the wrong field produces an immediate 401.
Was the key deleted, disabled, or expired? Check its status in the gateway console, not just in a local file.
Does the account have available balance or quota? Some gateways return 401 or 403 when the balance is insufficient; read the error body.
Does the team prohibit personal third-party keys? Cursor Teams/Enterprise can block BYOK at the admin level, in which case even a correct local configuration will fail.
Does the Base URL point to the environment that issued the key? A test-environment key configured against a production address usually also returns 401.
Are environment variables actually loaded? Print only the first and last four characters of the key; never write the full key into logs.

# Minimal auth test: verify only the address, key, and protocol
export API_KEY="sk-your-key"

curl -sS -o /tmp/cursor_resp.json -w "%{http_code}\n" \
  "https://api.zhongshangqihang.com/v1/models" \
  -H "Authorization: Bearer $API_KEY"

cat /tmp/cursor_resp.json

A 200 means the key and address are basically usable. If you get 401, try the same key in the gateway console's test entry to distinguish a key problem from a Cursor field or network-layer problem.

3. 404: Base URL and model ID each account for half

A 404 does not necessarily mean the model was retired. The most common cause is a malformed path:

The Base URL is missing /v1, so requests hit the site's web routes and return HTML or a 404.
The Base URL includes /v1/chat/completions, and the client appends the path again, producing /v1/chat/completions/chat/completions.
Anthropic's /v1/messages is entered in an OpenAI-compatible field, or the reverse.
The domain is misspelled, or the console address is used as the API address.

Model IDs can also cause 404 or model_not_found:

Case, hyphens, and version numbers must match the pricing page or model list exactly.
The model may not be available to the current account or plan.
The same model may have different IDs under different protocols.
Cursor's Verify button usually tests only key and address connectivity; it does not guarantee that the model ID is correct. Confirm with a real request.

When troubleshooting, first visit /v1/models if the gateway provides it, then copy the model ID from the returned list instead of typing it by hand.

4. 429: rate limiting is not insufficient balance

A 429 means requests exceeded a rate, concurrency, or token quota. Common triggers:

Requests per minute or tokens per minute exceeded the limit.
Multiple people share one key and compete for the same quota.
An Agent reads files and runs commands continuously, sending many requests in a short time.
Retries have no backoff, so failures generate more requests and create a retry storm.
Long-context requests consume more quota, leaving less room for concurrency.

When handling a 429, read the response headers and error body first rather than blindly multiplying retries. If Retry-After is present, wait as instructed; if not, use exponential backoff with random jitter. Do not retry 401 or 404; retries only amplify the problem.

import random
import time

def retry_delay(attempt, base=1.0, cap=30.0):
    delay = min(cap, base * (2 ** attempt))
    return delay + random.uniform(0, delay * 0.3)

for attempt in range(4):
    response = send_request()  # Replace with your request function
    if response.status_code not in (429, 500, 502, 503, 504):
        break
    time.sleep(retry_delay(attempt))

If 429s cluster in fixed time windows, introduce queuing and smooth the send rate. If one key serves multiple services, split keys by project. If an Agent loop is responsible, set a maximum number of rounds and a spending cap.

5. Connection failures: SSL, proxy, and DNS

Connection failures usually return no HTTP status code. They appear as timeouts, ECONNRESET, certificate errors, or "cannot connect to the server." Troubleshoot in this order:

Use curl -Iv to see where DNS resolution, the TCP connection, or the TLS handshake stops.
Check whether the certificate is expired, self-signed, or replaced by the corporate network. When an enterprise proxy performs TLS inspection, the company root certificate must be installed in the system trust store.
Check whether proxy environment variables apply only to the terminal while Cursor itself bypasses the proxy. Review Cursor's network/proxy settings and compare direct versus proxy connections if needed.
Check whether a VPN, firewall, or security product blocks long-lived connections or streaming responses.
Check that the system clock is accurate. A large time offset causes certificate validation to fail.
Check whether HTTP/2 or connection reuse is being disrupted by an intermediary device. Test first with a non-streaming, short request.

Do not use "ping works" to judge API availability. Ping only proves the host is reachable; it does not prove that TLS, certificates, and paths are correct.

6. Streaming interruptions and context overflows

Streaming output breaks in three common situations: a proxy buffers SSE, an idle connection times out, or the model is rate-limited during generation. Symptoms include a reply stopping halfway, stream ended unexpectedly, or Cursor spinning indefinitely.

Troubleshooting methods:

Send the same request with "stream": false first. If non-streaming works, the problem is in the streaming path or proxy.
Test directly with curl -N to rule out the Cursor client.
Lower max_tokens and the context length, and see whether interruptions persist.
Check whether the proxy enables response buffering; if necessary, route the API domain through a direct connection.
Set timeouts and maximum rounds for long Agent tasks to avoid hanging connections.

Context overflow usually returns 400 or 413, with error text such as context_length_exceeded or maximum context length. Cursor sends open files, retrieved snippets, and conversation history together, so the actual context is often longer than you expect. Fixes: use an ignore file to exclude build artifacts and logs, close irrelevant tabs, summarize long documents first, and start a new session for lengthy conversations.

7. What logs to keep for support

Record at least these fields: timestamp, status code, error code, model ID, Base URL hostname, request ID, latency, and retry count. The request ID is usually in the response headers; the header may be named request-id, x-request-id, or x-oneapi-request-id, depending on the gateway.

A usable log line looks like this:

2026-10-19T10:12:33+08:00 status=429 code=rate_limit_exceeded model=your-model-id host=api.zhongshangqihang.com request_id=... latency_ms=1840 retry=2

Do not record full keys, full prompts, or user code. Keep only the first and last four characters of a key, and mask sensitive content. When contacting support, one log line with a request ID is more useful than ten screenshots.

8. Support escalation checklist

Record the original error text, status code, time of occurrence, and request ID.
Reproduce the same request with curl to confirm whether the problem occurs only inside Cursor.
Check the Base URL: OpenAI-compatible endpoints usually go through /v1; Claude Code uses the root address.
Check the model ID: copy it from the model list and confirm case and version number.
Test with a new key to rule out key invalidation, quota, or team policy issues.
Start with a non-streaming, short-context request, then gradually add streaming and long context back.
Switch networks or proxies to rule out DNS, TLS, and enterprise proxy factors.
If all of the above pass, submit the logs and reproduction steps to gateway or Cursor support.

9. FAQ

The key works elsewhere, but Cursor returns 401. Why?

First check whether it was entered in the wrong provider field, then whether the Base URL matches the environment that issued the key. On Team plans, also confirm that the administrator has not disabled personal third-party keys.

Does a 404 always mean the model was retired?

Not necessarily. In most cases the Base URL has an extra or missing /v1, or the model ID is misspelled. Visit the model list first, then copy the ID and test again.

Is a 429 caused by insufficient balance?

Not necessarily. A 429 indicates a rate or quota limit; insufficient balance may return 401, 402, or 403 depending on the gateway. Use the error body and billing page as the reference.

Can I use Claude models directly in Cursor?

You can use an Anthropic key, or call Claude through a compatible gateway using the OpenAI protocol. Common versions of the Anthropic field do not offer a Base URL override, so when connecting a gateway, choose OpenAI Compatible and enter the exact model ID.

Why do only streaming requests fail?

Suspect proxy buffering, idle timeouts, and connection reuse first. Compare with a non-streaming request, then test directly with curl -N; this usually identifies which segment of the path is at fault.

Next steps

First create a test key at the ZSQH AI Gateway, run through authentication, model, and streaming with the curl commands in this article, and then enter the key in Cursor. Model availability and prices follow the site's model pricing. When troubleshooting, fix one variable first, then replace items one by one.