> ## Documentation Index
> Fetch the complete documentation index at: https://docs.sideren.io/llms.txt
> Use this file to discover all available pages before exploring further.

# Pay as you go

> Purchased credit in US dollars that never expires: for PAYG mode, where every model is available, and for extra usage past a plan's limits.

## Purchased credit

Purchased credit is in US dollars and never expires. Buy it on the [billing page](https://www.sideren.io/dashboard/billing) in packs of \$5, \$20 and \$50, or any amount from \$2 to \$1,000. \$5 adds \$5 of credit.

A request on credit is charged at the model's list price plus 20%, for its input, cached input and output; list prices are on the plan pages and below. In PAYG mode, cache writes count at the model's cache-write rate ([Prompt caching](https://docs.sideren.io/guides/prompt-caching.md)).

Purchased credit is separate from a plan's monthly usage. It is spent in two cases: in PAYG mode, and as extra usage past a plan's limits.

## PAYG mode

* Turn on PAYG mode for a key on the [keys page](https://www.sideren.io/dashboard/keys), or send the `x-sideren-payment-mode: payg` header with a request. The setup helper's `npx sideren setup --payment-mode payg` stamps that header into the agents it configures.
* In PAYG mode every model is available, whatever your plan: all 143, including the ones [below](#models-only-in-payg-mode) that no plan includes.
* An empty balance returns `402 paid_credit_required`; only a top-up clears it.

## Extra usage

Extra usage keeps a plan going past its limits. Turn it on on the [billing page](https://www.sideren.io/dashboard/billing#extra-usage): past your monthly usage or a 5-hour or weekly limit, requests carry on using purchased credit, with your plan's models, while you have enough. It doesn't cover the PAYG-only models below. See [Usage limits](https://docs.sideren.io/plans/usage-limits.md).

## Models only in PAYG mode

These 16 models are on no plan. They run only in PAYG mode, never use a plan's usage, and extra usage doesn't cover them. Requested without PAYG mode, they return `400 payg_required`.

Prices are list prices in US dollars per 1M tokens. A dash under Cached input means the model has no cached-input discount: cached input costs the input price.

### Anthropic

| Model | Context | Input | Cached input | Output |
| - | - | -: | -: | -: |
| Claude Fable 5.1 `fable-5.1` | 1M | \$10 | \$0.25 | \$50 |
| Claude Opus 5 `opus-5` | 1M | \$5 | \$0.5 | \$25 |
| Claude Sonnet 5 `sonnet-5` | 1M | \$3 | \$0.3 | \$15 |
| Claude Opus 4.8 `opus-4.8` | 1M | \$5 | \$0.5 | \$25 |
| Claude Opus 4.6 `opus-4.6` | 1M | \$5 | \$0.5 | \$25 |
| Sonnet 4.6 `sonnet-4.6` | 200K | \$3 | \$0.3 | \$15 |
| Claude Opus 4.5 `opus-4.5` | 200K | \$5 | \$0.5 | \$25 |

`opus-4.5` retires on Nov 24, 2026: move to `opus-4.6`.

### Meta

| Model | Context | Input | Cached input | Output |
| - | - | -: | -: | -: |
| Muse Wonder 1.0 `muse-wonder-1.0` | 1.05M | \$4 | \$0.2 | \$20 |

### Google

| Model | Context | Input | Cached input | Output |
| - | - | -: | -: | -: |
| Gemini 4 Argon `gemini-4-argon` | 1.05M | \$2 | \$0.1 | \$10 |
| Gemini 3.1 Pro Preview `gemini-3.1-pro-preview` | 200K | \$2 | — | \$12 |
| Gemini 3.8 Flash `gemini-3.8-flash` | 1M | \$0.75 | \$0.075 | \$3.75 |
| Gemini 3.7 Flash `gemini-3.7-flash` | 1.05M | \$0.75 | — | \$3.75 |
| Gemini 3.6 Flash `gemini-3.6-flash` | 1.05M | \$0.75 | — | \$3.75 |
| Gemini 3.5 Flash `gemini-3.5-flash` | 1.05M | \$1.5 | — | \$9 |
| Gemini 3.5 Flash Lite `gemini-3.5-flash-lite` | 1.05M | \$0.3 | — | \$2.5 |
| Gemini 3.1 Flash Lite `gemini-3.1-flash-lite` | 1.05M | \$0.25 | — | \$1.5 |


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