Grok 4.1 Fast
Grok 4.1 Fast was an xAI model for low-cost, long-context tool calling and agentic search. It launched in November 2025 with reasoning and non-reasoning API variants, a two-million-token context window, and server-side tools. However, the original API slugs retired on May 15, 2026 and now redirect to Grok 4.3.
That retirement changes the evaluation question. This page is no longer about selecting a maintained Grok 4.1 Fast endpoint. It is about understanding the historical model, identifying integrations that still send its old slug, and deciding whether to accept the Grok 4.3 redirect or migrate explicitly.
> Disclosure: This guide is published by Verdent. xAI benchmark and pricing details are vendor-reported and time-sensitive. Confirm the active model, redirect behavior, and price before running a production workload.
Grok 4.1 Fast vs Standard Grok 4.1
The two names described different product paths. Standard Grok 4.1 was released for Grok's consumer surfaces and emphasized conversational behavior. Grok 4.1 Fast was released through the xAI API for tool calling, long-context agent loops, and the Agent Tools API.
| Dimension | Grok 4.1 Fast at launch | Standard Grok 4.1 |
|---|---|---|
| Primary surface | xAI API | Grok consumer applications |
| Intended workload | Tool calling, agentic search, enterprise tasks | General conversation and interactive use |
| API variants | Reasoning and non-reasoning slugs | Not the same Fast endpoint pair |
| Context reference | 2M tokens | Do not infer Fast specifications for the consumer model |
| Current status | Retired; old slugs redirect to Grok 4.3 | Separate model history |
Do not compare the models by treating “Fast” as a simple speed switch on standard Grok 4.1. The API model had its own training focus, identifiers, pricing, and lifecycle.
Inference Speed Benchmarks
xAI described Grok 4.1 Fast as a rapid inference model, but the launch materials do not provide one universal tokens-per-second number that applies across every region, workload, and API condition. Agent latency also includes more than text generation.
Measure:
- time to first token;
- output tokens per second;
- time spent in web, X, code, or client tools;
- number of tool turns before a final answer;
- retry and rate-limit delays;
- total time until the result passes acceptance checks.
A fast first response can still produce a slow task if the model chooses unnecessary tools or needs repeated repairs. For a migration test, send the same request once through the old redirecting slug and once through the explicit replacement model, then compare response metadata, tool traces, cost, and outcome.
Tool Calling Performance
Tool use was the defining feature of Grok 4.1 Fast. xAI trained it across simulated environments and highlighted results on telecommunications support and function-calling benchmarks. The Agent Tools API added hosted web search, X search, code execution, document retrieval, and MCP connections.
Those results remain historical evidence, not a promise about the redirected model. After retirement, a request made with grok-4-1-fast-reasoning or grok-4-1-fast-non-reasoning is served by Grok 4.3 under xAI's documented redirect rules.
Revalidate at least these behaviors:
- tool schema adherence and argument validity;
- tool choice under ambiguous prompts;
- recovery after a tool error;
- citation and evidence quality for search tasks;
- stopping behavior after the goal is met;
- differences between reasoning effort settings.
The redirect keeps old code from failing immediately, but it does not preserve the old model's exact behavior.
Cost Per Request Comparison
At launch, Grok 4.1 Fast was priced at $0.20 per 1M input tokens, $0.05 per 1M cached input tokens, and $0.50 per 1M output tokens. Tool calls were billed separately, starting from a published per-invocation rate.
Those figures are historical. xAI states that retired slugs redirect to Grok 4.3 and use the replacement's pricing. A budget that multiplies current traffic by the old Fast token rates can therefore be materially wrong.
Use a migration cost table:
| Component | Historical integration | Migration measurement |
|---|---|---|
| Model slug | Old Grok 4.1 Fast slug | Explicit current model ID |
| Token rates | Launch Fast rates | Current replacement rates |
| Reasoning | Old reasoning/non-reasoning split | Current reasoning effort behavior |
| Tool charges | Historical Agent Tools pricing | Current tool-call prices |
| Task cost | Tokens plus tools plus retries | Recalculate from current response usage |
xAI now exposes request cost information in API usage data. Use the returned cost fields where available instead of relying only on a static spreadsheet.
When to Choose Fast
For new direct xAI API work, do not choose the retired Grok 4.1 Fast slug as if it were a maintained model. Choose a current xAI model after comparing tool support, context, price, and reasoning controls.
The old slug may remain relevant when:
- auditing an integration that has not migrated;
- reproducing historical logs or cost estimates;
- confirming how xAI redirects legacy requests;
- comparing a provider's frozen snapshot, if that provider clearly documents one.
If a third-party catalog still displays Grok 4.1 Fast, verify whether it hosts a snapshot, forwards to xAI, or maps the name to a different model. The same label can conceal different runtime behavior.
For a maintained xAI coding-specific path, review Grok Code Fast 1 and its current alias to grok-build-0.1. It has a separate lifecycle and should not be described as xAI's direct replacement for Grok 4.1 Fast.
| Decision | When it applies | Required action |
|---|---|---|
| Continue relying on the old slug temporarily | A migration window is already approved and the backend redirect is verified | Monitor resolved model metadata, pricing, and response behavior |
| Migrate explicitly | The integration is active or production-facing | Replace the legacy slug with a maintained model ID and rerun canaries |
| Stop using the route | The provider cannot explain the backend or support horizon | Disable the endpoint and select a documented model |
Using It in Verdent
Grok 4.1 Fast is not in Verdent's current built-in model list. OpenRouter BYOK may display xAI models available to your account, but an old Grok 4.1 Fast label should be treated as a provider-specific compatibility route until its backend is confirmed.
Before enabling it:
- inspect the provider model card and current model ID;
- verify whether the endpoint is a snapshot or redirect;
- record the displayed token and tool pricing;
- run a bounded tool-calling task and inspect the model metadata;
- compare it with an explicit current model route.
A validated OpenRouter endpoint can be routed to chat or agent roles in Verdent. Key-based calls do not receive Smart Suggestions or automatic compression, and an obsolete label still needs backend verification before any production run.
Sources: Review xAI's May 2026 retirement guide.
Frequently Asked Questions
Are the original Grok 4.1 Fast API slugs still maintained?
No. xAI's retirement guide says the Grok 4.1 Fast slugs were retired on May 15, 2026. Legacy requests may redirect, but the old names should not be treated as maintained models.
What behavior can change when an old slug redirects to Grok 4.3?
Model behavior, reasoning controls, tool use, token consumption, latency, and pricing can change. The redirected endpoint should be tested as a migration rather than assumed to preserve the old contract.
Do the historical Grok 4.1 Fast token prices still apply?
No. The launch prices are historical reference data. xAI states that redirected requests use the replacement model's pricing, so current usage must be recalculated from the active endpoint.
How should a redirected endpoint be tested before production use?
Resolve the actual model ID, remove unsupported request settings, run a representative canary set, inspect tool calls and outputs, and compare current usage cost before routing production traffic.
Validate Grok 4.1 Fast before routing
Check Verdent model pricing and confirm the current Grok coding-model alias before using Grok 4.1 Fast for chat, agent, or tool-calling workflows.