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Three commands answer most of it before you read any further:

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Authentication

The key is missing, malformed, or in the wrong header for the dialect you’re calling. Anthropic-style endpoints take x-api-key; OpenAI-style endpoints take Authorization: Bearer. /v1/messages accepts either. Keys start with sdn_. See Authentication.
You are still logged in to Claude Code’s own account, which takes precedence over the environment variables. Run /logout first, then re-run npx sideren. Check ANTHROPIC_BASE_URL is the bare origin — https://api.sideren.io, with no /v1; the Anthropic SDK appends the path itself.
Sideren keys intentionally start with sdn_; they are not Anthropic sk-ant- credentials. Configure Claude Code with npx sideren setup --tool claude-code --key sdn_your_key, or set ANTHROPIC_BASE_URL=https://api.sideren.io and put the key in ANTHROPIC_AUTH_TOKEN from PowerShell. Do not paste it into the consumer Claude desktop app’s Anthropic key/sign-in field; that app does not configure a custom gateway. See Claude Code.
Codex Desktop does not inherit User environment variables. Re-run npx sideren setup --tool codex --key sdn_your_key so the key is stored as experimental_bearer_token in ~/.codex/config.toml. See Codex CLI.

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Models

The id did not resolve. Check it against GET /v1/models. Unknown ids fail closed rather than silently becoming something else, and the compatibility handles for foreign SDK names are a finite list — an invented id is a 404 by design. See Model routing.
It is a preview id. The Models page marks these in its Availability column: the name is accepted, but no dedicated placement is routed for it yet, so a request for it is served by a comparable available model. Pin a Live id for anything whose exact behavior you depend on.
That model does not accept image input. It is a short list — check capabilities.vision on the catalog entry. See Vision.
The request declared a hard requirement (a forced tool_choice, a strict response_format, a reasoning effort level) that no placement of that model can honour exactly. Switch to a model whose supported_parameters includes it, or drop the requirement.

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Output

finish_reason: "length" — you hit max_tokens. On a reasoning model, hidden reasoning is charged against the same ceiling and can consume all of it, leaving nothing for the answer. Raise the budget; 4,000+ is a safe floor on reasoning models. See Reasoning.
Reasoning is hidden unless the request opts in. If you are seeing it, something in your stack is sending thinking, reasoning_effort or reasoning — often an SDK default. Remove it.
A stream that cannot finish ends with a protocol-shaped error event, never a silent truncation — so check for one before assuming the model ran out of things to say. Your agent may already have acted on tool calls from that turn, so decide in your client whether to send it again. See Streaming.
The catalog’s delivery field says which ids are pinned and which are compatibility handles: optimized ids are served by whichever comparable model the gateway picks today, exact ids keep their own placement. The response always echoes the id you sent. Pin a delivery: "exact" id for anything whose exact behavior you depend on. See Model routing.

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Limits and cost

A rate limit — your plan’s request window or concurrency. Honor retry-after exactly and pace off the x-ratelimit-* headers instead of discovering the limit by hitting it. Parallel agent fleets usually hit concurrency first. See Plans & limits.
Past 100% of your plan’s monthly credits, new requests stop: they return 402 allowance_exhausted. The message says how much of the month you have used, when your credits renew and which plan has more, and the error object carries the same facts as fields (limit, used_percent, resets_at, upgrade_url). In-flight requests finish: nothing is cut off mid-stream, and your subscription is never billed extra for it. A 402 carries no retry-after, because waiting a few seconds doesn’t clear it.Two ways to keep going:
  • Use purchased credit in PAYG mode. Add credit on the billing page, then turn on PAYG mode for your key or send the x-sideren-payment-mode: payg header. The setup helper’s npx sideren setup --payment-mode payg stamps it into the agents it configures.
  • Upgrade. The higher plan’s credits apply right away. Downgrades take effect at renewal.
Free works the same way when its 200 credits are used: requests return 402 credit_used_up. Free credits don’t renew, so only purchased credit or a subscription clears it.
The model isn’t in your plan. The message names the plan that includes it and, on Free, the models you can use. Upgrade, pick a model your plan includes (Plans & limits), or send the request in PAYG mode on purchased credit. Retrying won’t help.
Free’s 200 credits are used. There is no renewal date to wait for, so there is no retry-after. Subscribe, or add credit and turn on PAYG mode for your key. Don’t retry it in a loop. (Before 2026-10-17 this code was credits_exhausted.)
The request ran in PAYG mode (the key’s PAYG setting or the x-sideren-payment-mode: payg header) and there is no purchased credit left. Only a top-up clears it; or turn PAYG mode off to use your plan’s credits.
When capacity is tight, a request waits for a slot rather than being refused. It shows up as latency, not an error — so give client timeouts headroom. Paid plans are admitted first; Free requests may wait longer under heavy load. See Reliability.
Each request uses credits for the model that served it, so larger models and long outputs use credits faster. The usage page shows what every request used, and x-sideren-monthly-used-percent on every response shows how much of the month is gone. Two levers: move the hot path to a lighter model, and keep the prompt prefix stable so repeated context is served from the cache (Prompt caching).

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Getting help

support@sideren.io

The fastest route for anything not answered above.
Include these and the first reply can usually be the answer rather than a question:
  • What you sent — endpoint, model id, and the request body with secrets removed.
  • What came back — the status code and the full error.message.
  • When — a rough timestamp with timezone, so the trace can be found.
  • The sideren-model response header, if you have it — it names what actually served the request.
Never send your API key. If you think one has leaked, revoke it in the dashboard first and tell us second. Found a security issue? Report it to security@sideren.io rather than the support address.