xAI's latest fast reasoning model with 2M token context window. Combines speed with strong reasoning capabilities, supporting text and image inputs with function calling and structured outputs.
Grok 4.1 Fast Reasoning is a legacy xAI fast-reasoning route that TheRouter still lists as xai/grok-4-1-fast-reasoning. The model row preserves a 2-million-token context window, text+image input, function calling, structured outputs, and a reasoning parameter at the TheRouter API layer. xAI's own May 15, 2026 migration notice is the key fact for this page: grok-4-1-fast-reasoning was retired upstream and redirected to grok-4.3, not maintained as a separate public xAI model card.
Use this page as a compatibility reference, not as a recommendation to start new deployments on the old slug. Existing applications that already reference xai/grok-4-1-fast-reasoning can keep a stable TheRouter model id while they validate behavior, pricing, and latency. For new xAI reasoning work, compare xai/grok-4.3 for the current upstream generation or xai/grok-4.20-reasoning when the 2M context window is the decisive requirement.
The operational trade-off is ambiguity. TheRouter's catalog describes a cheap, fast, reasoning-capable 2M-context route, while xAI's public docs say the named upstream slug was retired and folded into Grok 4.3. That makes source-backed disclosure more important than benchmark claims. This page therefore avoids unsourced score tables for the 4.1 fast variant and focuses on what can be verified: route shape, retirement status, supported chat capabilities, and migration choices.
xai/grok-4-1-fast-reasoning while teams test successor behaviorxai/grok-4.3 and xai/grok-4.20-reasoning before changing production routingxai/grok-4.3 instead of the retired 4.1 fast slugxAI retired the upstream grok-4-1-fast-reasoning slug on May 15, 2026 and redirects it to grok-4.3; the vendor does not present it as a current standalone model with a separate model card.
TheRouter still exposes the listed xai/grok-4-1-fast-reasoning model id through its OpenAI-compatible chat route with TheRouter catalog pricing and a 2M context configuration, so customers should treat it as a compatibility route and validate before migrating.
| Type | Rate |
|---|---|
| Input | $0.324 / 1M tokens |
| Output | $0.810 / 1M tokens |
| TheRouter model ID | xai/grok-4-1-fast-reasoning | verified |
| Upstream slug status | Retired by xAI on May 15, 2026 and redirected to grok-4.3docs.x.ai ↗ | verified |
| Context window | 2,000,000 tokens on TheRouter | verified |
| Input modalities | Text, image | verified |
| Output modalities | Text | verified |
| TheRouter pricing | $0.30 input / $0.75 output per 1M tokens on TheRouter | verified |
| Reasoning parameter | Supported on TheRouter model rowdocs.x.ai ↗ | verified |
| Structured outputs | Supported through response_format on TheRouter | verified |
| Training cutoff | Not publicly disclosed for this retired slug | unknown |
| License | Proprietary | verified |
| Benchmark | Distribution | Score | Source |
|---|---|---|---|
Exact retired slug benchmarks xAI documents the slug retirement and redirection but does not publish standalone benchmark scores for Grok 4.1 Fast Reasoning. | — | Not publicly disclosed | — |
Successor benchmark guidance The upstream redirect target is grok-4.3; benchmark comparisons should use the curated xai/grok-4.3 page rather than inventing scores for this retired alias. | — | Not publicly disclosed | — |
TheRouter route validation The public vendor docs do not verify TheRouter's active catalog row; use TheRouter catalog behavior for route existence and xAI docs for upstream retirement status. | — | Not publicly disclosed | — |
Use the global api.therouter.ai endpoint shown below for new integrations; the legacy China accelerated endpoint is retired.
curl https://api.therouter.ai/v1/chat/completions -H "Content-Type: application/json" -H "Authorization: Bearer $THE_ROUTER_API_KEY" -d '{
"model": "xai/grok-4-1-fast-reasoning",
"messages": [
{"role": "user", "content": "Summarize the key points from this input."}
]
}'Use the OpenAI-compatible chat completions endpoint with the TheRouter model id. Keep this snippet for compatibility tests; prefer successor models for new builds.
curl https://api.therouter.ai/v1/chat/completions \
-H "Authorization: Bearer $THEROUTER_API_KEY" \
-H "Content-Type: application/json" \
-d '{
"model": "xai/grok-4-1-fast-reasoning",
"messages": [
{"role": "system", "content": "Reason carefully and flag uncertainty."},
{"role": "user", "content": "Summarize the migration risks in this plan."}
],
"reasoning": {"effort": "medium"}
}'The May migration guide lists grok-4-1-fast-reasoning among retired API model names. For TheRouter users, that makes the page a compatibility and migration reference rather than a fresh-model launch page.
Not as a TheRouter model id. xAI says the original upstream slug was retired and redirects to grok-4.3; TheRouter still lists xai/grok-4-1-fast-reasoning as its own compatibility route, so validate behavior before assuming exact equivalence.
Usually no. Use xai/grok-4.3 for the current xAI generation, or xai/grok-4.20-reasoning when pinned 2M-context reasoning is required. Keep xai/grok-4-1-fast-reasoning mainly for existing route compatibility and migration tests.
Because xAI's public documentation identifies this slug as retired and does not publish a standalone benchmark table for it. Publishing current Grok 4.3 scores under the retired alias would be misleading, so this page records the evidence gap instead.
| source | URL | retrieved | |
|---|---|---|---|
| TheRouter model ID | — | — | verified |
| Upstream slug status | docs.x.ai ↗ | 2026-08-08 | verified |
| Context window | — | — | verified |
| Input modalities | — | — | verified |
| Output modalities | — | — | verified |
| TheRouter pricing | — | — | verified |
| Reasoning parameter | docs.x.ai ↗ | 2026-08-08 | verified |
| Structured outputs | — | — | verified |
| Training cutoff | — | — | unknown |
| License | — | — | verified |
| Exact retired slug benchmarks | docs.x.ai ↗ | 2026-08-08 | unknown |
| Successor benchmark guidance | docs.x.ai ↗ | 2026-08-08 | unknown |
| TheRouter route validation | docs.x.ai ↗ | 2026-08-08 | unknown |
| xAI retires Grok 4.1 fast slugs and redirects them to Grok 4.3 | docs.x.ai ↗ | 2026-08-08 | verified |
| Is xai/grok-4-1-fast-reasoning the same as Grok 4.3? | docs.x.ai ↗ | 2026-08-08 | to verify |
| Should I use this model for a new production integration? | docs.x.ai ↗ | 2026-08-08 | to verify |
| Why does the page not list exact benchmark scores? | docs.x.ai ↗ | 2026-08-08 | to verify |