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Nexus Auto gives you one model ID, auto, and picks a model for each new user turn. Easy turns can go to a Fireworks open model. Harder turns can go to an Anthropic or OpenAI model on your own account. You pay the selected model’s rate for each turn, not a closed model’s rate on every turn.
Nexus Auto was formerly called FireRouter, and its model ID was firerouter. FireRouter and the firerouter model ID are no longer available. Use auto everywhere, including in model pickers, SDK calls, and gateway configs.

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How Nexus Auto works

For each new user turn, Nexus Auto predicts which available model will handle it well at the lowest cost. That model serves the whole turn, including its tool calls, until the next user turn. Open models run on Fireworks serverless. Closed models run on your own Anthropic or OpenAI account. The response model field names the model that served the turn, such as glm-5p3 or claude-opus-5-5. Nexus Auto works with the Chat Completions, Messages, and Responses APIs. Fireworks updates the models behind auto after evaluating them, so your model ID stays the same.

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Choose a model ID

Use auto for most work. To choose a model yourself, pin one.

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Which models auto can use

Nexus Auto picks the model for each turn as the turn arrives. What it can choose from depends on the credentials that reach the request: Fireworks manages and updates the models behind auto. To see which one served a turn, read the response model field (see Check which model served a turn). Set effort in the harness to ask for a lighter or more thorough attempt. If you connected Amazon Bedrock, Nexus Auto can serve the model families you routed there through your AWS account. To keep Nexus Auto on open models even when you have Anthropic or OpenAI credentials, limit it to Fireworks. Then run your harness command again, replacing claude with your harness:
  • Global setting: Nexus CLI saves it in ~/.nexus/config.json. Each harness picks it up the next time you run its command. Claude Desktop picks it up on its next request.
  • One value today: --providers accepts only fireworks.
  • Saved keys: Anthropic and OpenAI keys you saved with nexus auto stay saved, but Nexus CLI doesn’t send them while the limit is set.
  • Check or undo: run nexus auto to see the current setting, and nexus auto --reset to go back to the default.
  • Account first: your account’s model restrictions always apply first.
  • Cursor IDE: the setting has no effect there.

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Set up

Every setup uses auto and your Fireworks API key. Only the way you provide the closed-model credential changes.

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Nexus CLI

Nexus CLI writes your coding harness settings and restores them with off. To edit the files yourself with nothing to install, use manual setup. Pick your harness and a setup method. The commands and config files update as you choose, including any provider-key headers. A connected Provider Key adds Anthropic and OpenAI models in every harness. Without one, save your own key with nexus auto. Then run your harness command again, replacing opencode with your harness:
Nexus CLI saves the key in ~/.nexus/config.json. It sends the key only from harnesses that have no sign-in of their own for that provider: The Claude Agent SDK matches Claude Code when it reads the user settings that Nexus CLI writes. Claude Desktop’s picker lists auto and Fireworks models only.
  • Where the key goes: Nexus CLI writes it into the harness config as an x-anthropic-api-key or x-openai-api-key header. VS Code gets it on the auto model only. Claude Desktop’s local proxy adds it to auto turns from the next request, so Claude Desktop doesn’t need a restart.
  • No environment keys: Nexus CLI never reads ANTHROPIC_API_KEY or OPENAI_API_KEY from your environment, and it writes nothing to your shell profile. A harness can still read its own variable, as Claude Code does with ANTHROPIC_API_KEY.
  • Check a key: nexus auto shows which keys are saved on this machine and which Provider Keys are connected to your account. A saved key takes precedence over the Provider Key for that provider.
  • Remove a key: nexus auto --unset anthropic-api-key or nexus auto --unset openai-api-key removes the key from Nexus CLI and from every harness that holds a copy. Restart those harnesses afterward. If a harness is open, Nexus CLI skips it and tells you to quit it and run the command again.
  • Only through nexus auto: harness commands such as nexus opencode don’t accept --anthropic-api-key or --openai-api-key. nexus configure still accepts them, but they are deprecated.
  • Upgrading from an earlier release: the upgrade removes the ANTHROPIC_API_KEY and OPENAI_API_KEY exports that earlier versions added to your shell profile. It also clears {env:...} key references from ~/.nexus/config.json. A literal key saved with nexus configure is kept and sent as shown in the table.
Nexus CLI doesn’t send saved keys from Cursor IDE or the Copilot App, so they need Provider Keys. In the other harnesses, you can add the headers yourself with manual setup. For a full walkthrough, see Claude Code. For each harness’s files and restore behavior, see Coding Harnesses.

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LLM gateway

The gateway sends auto and your Fireworks API key to Fireworks, so you don’t need Nexus CLI. Clients behind the gateway call auto with only their gateway key. Choose LiteLLM or Portkey, a model ID, and where the provider keys live. The widget writes the gateway config and a test request. For other gateways and each credential option, see LLM Gateways.

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API and SDK

Point the SDK you already use at Fireworks. Its API key is your Fireworks API key. For an Anthropic or OpenAI model, connect a Provider Key or send the provider’s key as a header (see Provide Anthropic and OpenAI credentials). Anthropic-style clients add /v1/messages to the base URL themselves, so their base URL ends at /inference.
Pick an API and a language. The code adds the provider header that API needs, or leaves it out if you have Provider Keys connected. The Anthropic SDK’s base URL is https://api.fireworks.ai/inference, without /v1. Some models return a thinking block before the text, so the snippet reads the first text block rather than content[0]. For errors and limits, see APIs and SDKs.

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Provide Anthropic and OpenAI credentials

Nexus Auto calls closed models through your own Anthropic, OpenAI, or Amazon Bedrock account. For Anthropic and OpenAI, store the key once as a Provider Key or send it with each request. Bedrock uses Provider Keys only, with each model mapped to a Bedrock model ID and region. A request header takes precedence over the stored Provider Key for that provider.
Provider Keys isn’t available on every account yet. If Provider Keys doesn’t appear in Settings, contact the Fireworks team to turn it on.
Without a credential for a closed provider, Nexus Auto leaves that provider’s models out and serves the turn with the others. A request returns 400 no_credential when you call a closed model ID such as claude-opus-5-5 directly and that credential is missing.

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Check which model served a turn

  1. Send a request with auto:
  2. Check the response model field. It names the model that served the turn, such as glm-5p3 or claude-opus-5-5.
  3. An easy prompt usually gets an open model. A harder prompt, such as debugging a concurrency bug or writing a proof, is more likely to get the closed model. No single request is guaranteed to use either. In a coding harness, set effort to ask for a lighter or more thorough attempt.
  4. To confirm a closed-model credential, call that model ID directly, such as claude-opus-5-5. A 400 no_credential error means the credential is missing.

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Watch orchestration live

In Claude Code, you can watch each turn’s model choice as you work. Connect Claude Code with auto. The status line shows the model that served the latest turn and the estimated session cost. For a per-turn breakdown of model choices, tokens, and estimated cost, run:
Pick a session to open its live meter, or pass --session <id> to open one directly. See Usage and Cost for the columns and pricing.

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Balance quality and savings

Set effort in the harness. Higher effort asks for a more thorough attempt. Lower effort asks for a lighter one. Nexus Auto uses that when it picks a model.

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Pay for the model that serves each turn

Open models bill to your Fireworks account at their serverless rates. Closed models bill to the Anthropic or OpenAI account whose credential served the turn. In the Codex CLI, a ChatGPT sign-in covers most OpenAI model turns; see Who pays for OpenAI model turns. Cached input bills at the serving model’s cached-input rate. Use Usage and Cost to review Fireworks usage and Claude Code session estimates.

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Supported models

auto decides which model serves each turn from the credentials you provide. To send every turn to one model below, pin it. Anthropic serves Claude Fable only to organizations with data retention enabled. Otherwise, Fable requests return a 400 error from Anthropic.

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Pin a specific model

To choose a model yourself, use its ID instead of auto. In a harness, pass it with --model. In a request, set it as the model value:
A pinned model serves every turn. A full model ID such as glm-5p3 stays on that version. Aliases such as glm-latest track the current version in a family and also skip Nexus Auto. See Open Models.
Custom routes such as firerouter/opus are no longer available, and Nexus Auto does not accept auto/... route IDs. The latency-first auto-instant mix is retired too. Pin a model ID as shown above, or use auto. See Deprecated and removed.

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Limitations

  • Nexus Auto is not available through Microsoft Foundry. See Nexus for Microsoft Foundry.
  • Accounts with data residency enabled cannot use Nexus Auto. Use a residency-compatible pinned serverless model instead.

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Troubleshooting

No closed-model credential is reaching Nexus Auto. Save a key with nexus auto --anthropic-api-key or --openai-api-key, then run your harness command again. Or connect the matching Provider Key.Run nexus auto to see what’s set:
  • “saved, not sent while auto is limited to fireworks”: run nexus auto --reset, then run your harness command again.
  • “Sign in to see status”: run nexus login so Nexus CLI can read your account’s Provider Keys.
  • “Not enabled on this account”: your account can’t use Provider Keys yet. Save your own key with nexus auto instead.
Nexus CLI doesn’t send exported ANTHROPIC_API_KEY or OPENAI_API_KEY values. In Codex, OpenAI model turns need a Codex sign-in or an OpenAI Provider Key.