Grok 4.3 is the April 2026 Grok with a full 1M tokens context window at $1.25 / $2.50 per Mtok (input / output), and it is the cheapest 1M context reasoning model from a US lab by a wide margin. Inside Atlas, Grok 4.3 is the model for reading: feed it large codebase-index result sets, ask it to reason about architecture, and let it plan. The limit to design around is 30,000 max output tokens, the tightest ceiling of any frontier-adjacent model, so large diffs must be emitted across multiple turns.
Why is Grok 4.3 the cheapest 1M context reasoning model?
Grok 4.3 charges $2.50 per Mtok output on a 1M window, versus $12 for Gemini 3.1 Pro and $30 for GPT-5.6. That gap is why Grok 4.3 is the cheapest 1M context reasoning model from a US lab by a wide margin, at $1.25 / $2.50 per Mtok.
Price per million tokens on a million-token window is the whole argument for Grok 4.3. Gemini 3.1 Pro charges $12 per Mtok output and GPT-5.6 charges $30, so a large analysis session that reads a lot and writes a moderate amount costs several times more on those models than on Grok 4.3. Atlas searches code with hybrid semantic and keyword retrieval fused by reciprocal rank fusion, which produces large result sets, and 1,000,000 token context makes it practical to feed large codebase-index result sets without paging. Grok 4.3 turns that into a cheap operation instead of an expensive one.
What is Grok 4.3 best at inside Atlas?
Grok 4.3 is best inside Atlas for read and analysis work. Grok 4.3 has reasoning and tool calling both enabled, so it is a viable Atlas build-agent model, not just a cheap reader, but its 1,000,000 token context is what makes it the natural fit for architecture questions across a large repository.
Atlas drafts a plan in a read-only plan agent and asks before switching to a build agent, and Grok 4.3 is well matched to that plan agent role. With 1M tokens of context, Atlas can pour an entire codebase-index result set into a single Grok 4.3 turn and get reasoning over the whole thing rather than a paged approximation. Atlas indexes code by AST declarations using tree-sitter, not blind line windows, so what fills that million-token window is real declarations rather than arbitrary chunks. Run `atlas models xai` and look for grok-4.3, then select it from `/models` when the task is understanding rather than editing.
Why does Grok 4.3's 30,000 token output limit matter?
Grok 4.3 caps output at 30,000 max output tokens, the tightest ceiling of any frontier-adjacent model, so large diffs must be emitted across multiple turns. The narrow output budget interacts badly with Atlas's diff-reviewed edit flow on big refactors, where the diff itself is the model's output.
The asymmetry is severe on Grok 4.3: a 1,000,000 token input window paired with a 30,000 token output ceiling. Atlas computes a unified diff for every file edit and surfaces it for approval before writing, so on a big refactor the patch has to fit inside 30,000 tokens or be split. Splitting works, but it means several turns and several approvals for what should be one review. Because output is capped at 30K tokens, prefer Grok 4.3 for read and analysis work and switch to Grok 4.5 for large edits. Atlas lets you switch the active model and provider on the fly, so that handoff is a single keystroke.
Can Grok 4.3 run as an Atlas build agent?
Grok 4.3 can run as an Atlas build agent because reasoning and tool calling are both enabled, so it is a viable build-agent model, not just a cheap reader. The practical limit is the 30,000 max output tokens ceiling, which constrains how much of a change Grok 4.3 can write per turn.
For small and medium edits, Grok 4.3 works as a build agent inside Atlas. Every Atlas tool call is permission-gated against allow, ask, and deny rules before it runs, and Grok 4.3's tool calling holds up in that loop. Where Grok 4.3 breaks down is volume: a change producing more than 30,000 tokens of diff cannot land in one turn. A workable pattern is to keep Grok 4.3 on the plan agent, where its 1M context and $1.25 per Mtok input shine, and hand execution to a model with a bigger output budget when the plan calls for a sweeping patch.
When should you pick a different model than Grok 4.3?
Pick a different model than Grok 4.3 whenever a single turn needs to produce more than 30,000 tokens of output. Grok 4.3's 1M context at $1.25 / $2.50 per Mtok is unmatched for reading, but its 30,000 token output ceiling is the tightest of any frontier-adjacent model.
The switching rule for Grok 4.3 is mechanical rather than a judgment call. Big refactor, multi-file rename, wide API migration: leave Grok 4.3 and pick a model with a larger output budget. Large-repository analysis, architecture questions, bug hunting across a big result set, plan drafting: Grok 4.3 is the cheapest good answer, since $2.50 per Mtok output against Gemini 3.1 Pro's $12 and GPT-5.6's $30 makes long reading sessions inexpensive. Atlas favorites and recents make this a two-model workflow rather than a compromise on a single one.
Setup
- 01Export `XAI_API_KEY` in your shell so Atlas can authenticate against xAI.
- 02Run `atlas models xai` and look for grok-4.3 in the resolved list.
- 03Select Grok 4.3 from `/models` in the Atlas TUI.
- 04Because output is capped at 30K tokens, prefer Grok 4.3 for read and analysis work and switch to Grok 4.5 for large edits.
- 05Keep Grok 4.5 in your Atlas favorites so the handoff for a big refactor is one keystroke.
Frequently asked questions
- How much does Grok 4.3 cost per million tokens?
- Grok 4.3 costs $1.25 / $2.50 per Mtok (input / output). On a 1M window that output rate compares to $12 for Gemini 3.1 Pro and $30 for GPT-5.6.
- What is Grok 4.3's context window?
- Grok 4.3 has a full 1M tokens context window. The 1,000,000 token context makes it practical to feed Atlas's large codebase-index result sets without paging.
- What is Grok 4.3's max output token limit?
- Grok 4.3 caps output at 30,000 max output tokens, the tightest ceiling of any frontier-adjacent model, so large diffs must be emitted across multiple turns.
- How do I switch Atlas to Grok 4.3?
- Export `XAI_API_KEY`, run `atlas models xai` and look for grok-4.3, then select it from `/models`. Atlas lets you switch the active model and provider on the fly.
- Is Grok 4.3 or Grok 4.5 better for Atlas?
- Grok 4.3 is better for reading, with a 1M context at $1.25 per Mtok input. Grok 4.5 is better for editing, because Grok 4.3's 30,000 token output ceiling truncates large diffs.
- Can Grok 4.3 do agentic coding?
- Yes, within limits. Reasoning and tool calling are both enabled on Grok 4.3, so it is a viable Atlas build-agent model, but the narrow 30,000 token output budget interacts badly with Atlas's diff-reviewed edit flow on big refactors.
- What is the cheapest 1M context reasoning model in 2026?
- Grok 4.3, at $1.25 / $2.50 per Mtok, is the cheapest 1M context reasoning model from a US lab by a wide margin.
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