GPT-5.6 Luna vs Grok 4.20

GPT-5.6 Luna leads Grok 4.20 on coding, 58.0 to 33.3 on the Vibe Coding Index, and GPT-5.6 Luna costs 2.1x less per output token ($1.20 vs $2.50 per million). Updated October 22, 2026; 39 benchmarks have a result for both.

Replace GPT-5.6 Luna Replace Grok 4.20

GPT-5.6 LunaOpenAI

Coding: 58.0
Overall: 61.6
Output $/M: $1.20

Grok 4.20xAI

Coding: 33.3
Overall: 41.0
Output $/M: $2.50

vs boards won of 39 shared, 1 tied

GPT-5.6 Luna or Grok 4.20: which to pick

Pick GPT-5.6 Luna for coding: it scores 58.0 on the Vibe Coding Index against 33.3 for Grok 4.20.

Pick Grok 4.20 for a larger context window (2M against 1.05M tokens) and work measured by Vending-Bench 2 or ARC-AGI, where it leads.

Model AGPT-5.6 LunaOpenAI58.0/1006 independent benchmarks$0.04 per solved task Model BGrok 4.20xAI33.3/1002 independent benchmarks$0.72 per solved task

398016

Shared results: 39
GPT-5.6 Luna only: 80
Grok 4.20 only: 16
Like-for-like categories: 7 of 9

Only benchmarks that tested both models count toward a pick; a result only one model has is shown but not compared.

Which one for your work

Each card is decided by one number both models carry. Where only one has it, the card says so and makes no call.

Coding
Vibe Coding Index, six public coding benchmarks
GPT-5.6 Luna
GPT-5.6 Luna scores 58.0 against 33.3.
6 and 2 benchmarks behind the scores
Intelligence
AA Intelligence Index
GPT-5.6 Luna
GPT-5.6 Luna leads, 37.3 against 25.7.
Same board, both measured
Agentic work
AA Agentic Index
No call
Only GPT-5.6 Luna has a AA Agentic Index result, 42.1.
Directional only
Reasoning
GPQA Diamond
GPT-5.6 Luna
GPT-5.6 Luna leads, 91.7% against 88.6%.
Same board, both measured
Math
AIME
No call
Only Grok 4.20 has a AIME result, 96.5%.
Directional only
Knowledge
MMLU Pro
Grok 4.20
Grok 4.20 leads, 86.3% against 86.0%.
Same board, both measured
Web design
Design Arena: websites
GPT-5.6 Luna
GPT-5.6 Luna leads, 1255 against 1229.
Same board, both measured
Long documents
Listed context window
Grok 4.20
Grok 4.20 takes 2M tokens against 1.05M.
Listed by OpenRouter
Everyday cost
1K input + 500 output tokens
GPT-5.6 Luna
GPT-5.6 Luna $0.0008, Grok 4.20 $0.0025 per turn.
List prices
Cost per solved task
One SWE-bench Verified task, solved
GPT-5.6 Luna
GPT-5.6 Luna $0.04, Grok 4.20 $0.72.
Published attempt cost and pass rate

At a glance

Coding index: GPT-5.6 Luna 58.0 from 6 benchmarks, Grok 4.20 33.3 from 2 benchmarks.
Widest category: Coding, 36.3 points apart on average over 5 shared benchmarks.
Widest single gap: Vibe Code Bench (Vals AI): GPT-5.6 Luna 77.1% against Grok 4.20 4.1%, 73.0 points apart on a 0 to 100 scale.
Output price: GPT-5.6 Luna $1.20 against Grok 4.20 $2.50 per million output tokens, 2.1x apart.
Context window: GPT-5.6 Luna takes 1.05M tokens, Grok 4.20 2M.

Benchmark by benchmark

First the score: boards each model leads, per category, where both have a result. Then every board, a category to a fold; bars are each result on a 0 to 100 scale, so compare the two in a row, not one row with another.

Boards won, where both have a result
Category	GPT-5.6 Luna	Grok 4.20	Ties
Coding	5	0
Intelligence	2	0
Agentic work	4	1
Reasoning	3	3	1 tie
Knowledge	5	1
Web design	5	3
General	5	1
All shared boards	29	9	1 tie

CodingLike-for-likeSix of these boards feed the Vibe Coding Index50

Coding benchmarks for GPT-5.6 Luna and Grok 4.20
Benchmark	GPT-5.6 Luna	Grok 4.20	Leader
LiveBench coding	73.6%	Not measured	Only GPT-5.6 Luna measured
LMArena WebDev	1518	Not measured	Only GPT-5.6 Luna measured
AA Coding Index	71.4	Not measured	Only GPT-5.6 Luna measured
LiveCodeBench (Vals AI)	Not measured	84.3%	Only Grok 4.20 measured
SWE-bench Verified (Vals AI)	93.0%	72.2%	GPT-5.6 Luna
Terminal-Bench 4 (Vals AI)	11.6%	Not measured	Only GPT-5.6 Luna measured
Vibe Code Bench (Vals AI)	77.1%	4.1%	GPT-5.6 Luna
Code migration (Vals AI)	44.5%	0.3%	GPT-5.6 Luna
ProgramBench (Vals AI)	0.0%	Not measured	Only GPT-5.6 Luna measured
IOI (Vals AI)	61.8%	Not measured	Only GPT-5.6 Luna measured
Terminal-Bench 2.1 (Vals AI)	79.0%	44.2%	GPT-5.6 Luna
Vibe Code Bench 1-100 (Vals AI)	22.6%	Not measured	Only GPT-5.6 Luna measured
SkillsBench (Vals AI)	60.5%	Not measured	Only GPT-5.6 Luna measured
CyberBench (Vals AI)	73.6%	Not measured	Only GPT-5.6 Luna measured
SciCode	53.6%	Not measured	Only GPT-5.6 Luna measured
Terminal-Bench Hard (Artificial Analysis run)	Not measured	37.9%	Only Grok 4.20 measured
Terminal-Bench	Not measured	57.3%	Only Grok 4.20 measured
FrontierCode	39.8%	Not measured	Only GPT-5.6 Luna measured
WeirdML	60.9%	52.3%	GPT-5.6 Luna
ALE-Bench	1667.4	Not measured	Only GPT-5.6 Luna measured
LMArena Text: coding	1466	Not measured	Only GPT-5.6 Luna measured
LMArena Text: software and IT	1463	Not measured	Only GPT-5.6 Luna measured
LMArena WebDev: React	1508	Not measured	Only GPT-5.6 Luna measured
LMArena WebDev: HTML	1520	Not measured	Only GPT-5.6 Luna measured
LMArena WebDev: from an image	1517	Not measured	Only GPT-5.6 Luna measured
SciCode	6.2%	Not measured	Only GPT-5.6 Luna measured
Neither model has a result on the other 13 boards here

IntelligenceLike-for-likePublic boards, shown as published, kept out of the index20

Intelligence benchmarks for GPT-5.6 Luna and Grok 4.20
Benchmark	GPT-5.6 Luna	Grok 4.20	Leader
AA Intelligence Index	37.3	25.7	GPT-5.6 Luna
Epoch Capabilities Index	156.5	152.1	GPT-5.6 Luna

Agentic workLike-for-likePublic boards, shown as published, kept out of the index41

Agentic work benchmarks for GPT-5.6 Luna and Grok 4.20
Benchmark	GPT-5.6 Luna	Grok 4.20	Leader
AA Agentic Index	42.1	Not measured	Only GPT-5.6 Luna measured
Design Arena: agents building web apps	Not measured	1144	Only Grok 4.20 measured
Design Arena: agents building full-stack apps	Not measured	1054	Only Grok 4.20 measured
Design Arena: agents building mobile apps	Not measured	1090	Only Grok 4.20 measured
Design Arena: agents building Android apps	Not measured	1045	Only Grok 4.20 measured
Design Arena: agents building Godot games	Not measured	1089	Only Grok 4.20 measured
Design Arena: HTML slides	Not measured	1180	Only Grok 4.20 measured
LiveBench research	51.0%	Not measured	Only GPT-5.6 Luna measured
LiveBench browser	68.0%	Not measured	Only GPT-5.6 Luna measured
LiveBench long agentic	30.4%	Not measured	Only GPT-5.6 Luna measured
SREBench (Vals AI)	1.5%	Not measured	Only GPT-5.6 Luna measured
Finance Agent (Vals AI)	55.0%	28.5%	GPT-5.6 Luna
Tax Agent Bench (Vals AI)	24.8%	7.0%	GPT-5.6 Luna
Legal Research Bench (Vals AI)	36.5%	13.9%	GPT-5.6 Luna
Excel Modeling Benchmark (Vals AI)	67.1%	11.7%	GPT-5.6 Luna
BioMysteryBench (Vals AI)	61.5%	Not measured	Only GPT-5.6 Luna measured
Terminal-Bench-Science (Vals AI)	0.0%	Not measured	Only GPT-5.6 Luna measured
Vals Science Index	25.3%	Not measured	Only GPT-5.6 Luna measured
τ²-Bench Telecom	Not measured	93.0%	Only Grok 4.20 measured
APEX-Agents	43.0%	Not measured	Only GPT-5.6 Luna measured
BALROG	45.6%	Not measured	Only GPT-5.6 Luna measured
Vending-Bench 2	$4,095	$4,663	Grok 4.20
Neither model has a result on the other 19 boards here

ReasoningLike-for-likePublic boards, shown as published, kept out of the index33

Reasoning benchmarks for GPT-5.6 Luna and Grok 4.20
Benchmark	GPT-5.6 Luna	Grok 4.20	Leader
GPQA Diamond	91.7%	88.6%	GPT-5.6 Luna
LiveBench reasoning	85.6%	Not measured	Only GPT-5.6 Luna measured
MysteryMechanism (Vals AI)	14.4%	Not measured	Only GPT-5.6 Luna measured
MMMU Pro (Vals AI)	85.0%	83.5%	GPT-5.6 Luna
GPQA Diamond (Artificial Analysis run)	91.1%	91.1%	Tie
Humanity's Last Exam (Artificial Analysis run)	39.5%	34.5%	GPT-5.6 Luna
GPQA Diamond (Epoch AI run)	91.6%	Not measured	Only GPT-5.6 Luna measured
ARC-AGI	88.0%	89.5%	Grok 4.20
ARC-AGI-2	59.5%	65.1%	Grok 4.20
SimpleBench	46.8%	Not measured	Only GPT-5.6 Luna measured
Chess puzzles	43.0%	Not measured	Only GPT-5.6 Luna measured
Mystery game puzzles	21.0%	Not measured	Only GPT-5.6 Luna measured
LMArena Text: hard prompts	1451	Not measured	Only GPT-5.6 Luna measured
LMArena Text: expert	1478	Not measured	Only GPT-5.6 Luna measured
LMArena Text: hard prompts in English	1460	Not measured	Only GPT-5.6 Luna measured
LMArena Vision: diagrams	1273	Not measured	Only GPT-5.6 Luna measured
LMArena Vision: homework	1282	Not measured	Only GPT-5.6 Luna measured
Kagi LLM Benchmark	49.1%	75.0%	Grok 4.20
Neither model has a result on the other 4 boards here

MathDirectional onlyPublic boards, shown as published, kept out of the index7 boards, none shared

Math benchmarks for GPT-5.6 Luna and Grok 4.20
Benchmark	GPT-5.6 Luna	Grok 4.20	Leader
AIME	Not measured	96.5%	Only Grok 4.20 measured
OTIS Mock AIME	98.3%	Not measured	Only GPT-5.6 Luna measured
FrontierMath, tiers 1 to 3	82.1%	Not measured	Only GPT-5.6 Luna measured
FrontierMath, tier 4	61.0%	Not measured	Only GPT-5.6 Luna measured
ProofBench	60.0%	Not measured	Only GPT-5.6 Luna measured
LMArena Text: math	1459	Not measured	Only GPT-5.6 Luna measured
LMArena Text: mathematical work	1470	Not measured	Only GPT-5.6 Luna measured
Neither model has a result on the other 6 boards here

KnowledgeLike-for-likePublic boards, shown as published, kept out of the index51

Knowledge benchmarks for GPT-5.6 Luna and Grok 4.20
Benchmark	GPT-5.6 Luna	Grok 4.20	Leader
MMLU Pro	86.0%	86.3%	Grok 4.20
LegalBench (Vals AI)	84.0%	77.7%	GPT-5.6 Luna
CaseLaw (Vals AI)	Not measured	54.4%	Only Grok 4.20 measured
CorpFin (Vals AI)	64.2%	63.7%	GPT-5.6 Luna
TaxEval (Vals AI)	76.2%	74.1%	GPT-5.6 Luna
MortgageTax (Vals AI)	67.3%	45.4%	GPT-5.6 Luna
MedQA (Vals AI)	Not measured	94.5%	Only Grok 4.20 measured
MedCode (Vals AI)	42.4%	32.2%	GPT-5.6 Luna
SimpleQA Verified	41.0%	Not measured	Only GPT-5.6 Luna measured
LMArena Text: science	1433	Not measured	Only GPT-5.6 Luna measured
LMArena Text: medicine and healthcare	1399	Not measured	Only GPT-5.6 Luna measured
LMArena Text: legal and government	1446	Not measured	Only GPT-5.6 Luna measured
LMArena Text: business and finance	1439	Not measured	Only GPT-5.6 Luna measured
FACTS Benchmark Suite	35.0%	Not measured	Only GPT-5.6 Luna measured
Neither model has a result on the other 4 boards here

Web designLike-for-likePublic boards, shown as published, kept out of the index53

Web design benchmarks for GPT-5.6 Luna and Grok 4.20
Benchmark	GPT-5.6 Luna	Grok 4.20	Leader
Design Arena: websites	1255	1229	GPT-5.6 Luna
Design Arena: UI components	1248	1200	GPT-5.6 Luna
Design Arena: game dev	1194	1193	GPT-5.6 Luna
Design Arena: data viz	1211	1217	Grok 4.20
Design Arena: 3D	1121	1193	Grok 4.20
Design Arena: all code categories	1231	1221	GPT-5.6 Luna
Design Arena: SVG	1194	1165	GPT-5.6 Luna
Design Arena: ASCII art	1142	1175	Grok 4.20
LiveBench design	76.0%	Not measured	Only GPT-5.6 Luna measured

GeneralLike-for-likePublic boards, shown as published, kept out of the index51

General benchmarks for GPT-5.6 Luna and Grok 4.20
Benchmark	GPT-5.6 Luna	Grok 4.20	Leader
LiveBench data analysis	78.0%	Not measured	Only GPT-5.6 Luna measured
Public Benefits Bench (Vals AI)	61.2%	Not measured	Only GPT-5.6 Luna measured
SAGE (Vals AI)	44.2%	38.2%	GPT-5.6 Luna
MedScribe (Vals AI)	84.4%	63.4%	GPT-5.6 Luna
Vals Multimodal Index	69.1%	39.1%	GPT-5.6 Luna
IFBench	Not measured	81.2%	Only Grok 4.20 measured
Long context reasoning (AA-LCR)	83.7%	69.0%	GPT-5.6 Luna
DTBench	89.1%	90.1%	Grok 4.20
LMCA	48.5%	38.7%	GPT-5.6 Luna
CL-bench	Not measured	22.2%	Only Grok 4.20 measured
Surface Evolver Bench	61.9%	Not measured	Only GPT-5.6 Luna measured
Furniture assembly	42.5%	Not measured	Only GPT-5.6 Luna measured
CL-bench Life	Not measured	11.9%	Only Grok 4.20 measured
LMArena Text	1431	Not measured	Only GPT-5.6 Luna measured
LMArena Text: instruction following	1437	Not measured	Only GPT-5.6 Luna measured
LMArena Text: creative writing	1396	Not measured	Only GPT-5.6 Luna measured
LMArena Vision	1257	Not measured	Only GPT-5.6 Luna measured
LMArena Text: English	1441	Not measured	Only GPT-5.6 Luna measured
LMArena Text: non-English	1417	Not measured	Only GPT-5.6 Luna measured
LMArena Text: Chinese	1470	Not measured	Only GPT-5.6 Luna measured
LMArena Text: German	1455	Not measured	Only GPT-5.6 Luna measured
LMArena Text: Japanese	1415	Not measured	Only GPT-5.6 Luna measured
LMArena Text: Korean	1415	Not measured	Only GPT-5.6 Luna measured
LMArena Text: French	1458	Not measured	Only GPT-5.6 Luna measured
LMArena Text: Spanish	1446	Not measured	Only GPT-5.6 Luna measured
LMArena Text: Russian	1427	Not measured	Only GPT-5.6 Luna measured
LMArena Text: Polish	1405	Not measured	Only GPT-5.6 Luna measured
LMArena Text: multi-turn	1434	Not measured	Only GPT-5.6 Luna measured
LMArena Text: longer queries	1436	Not measured	Only GPT-5.6 Luna measured
LMArena Text: writing and language	1419	Not measured	Only GPT-5.6 Luna measured
LMArena Text: entertainment and media	1402	Not measured	Only GPT-5.6 Luna measured
LMArena Vision: English	1263	Not measured	Only GPT-5.6 Luna measured
LMArena Vision: Chinese	1294	Not measured	Only GPT-5.6 Luna measured
LMArena Vision: reading text	1264	Not measured	Only GPT-5.6 Luna measured
LMArena Vision: humor	1190	Not measured	Only GPT-5.6 Luna measured
LMArena Vision: creative writing	1257	Not measured	Only GPT-5.6 Luna measured
LMArena Document	1457	Not measured	Only GPT-5.6 Luna measured
Neither model has a result on the other 9 boards here

Image generationNot comparablePublic boards, shown as published, kept out of the index0 boards, none shared

Image generation benchmarks for GPT-5.6 Luna and Grok 4.20
Benchmark	GPT-5.6 Luna	Grok 4.20	Leader
Neither model has a result on 5 boards here

What each workload costs

Three fixed token mixes at each model's listed price, so per-token rates become a bill.

Chat turn

1K input + 500 output tokens

GPT-5.6 Luna$0.0008Fits in one request

Grok 4.20$0.0025Fits in one request

GPT-5.6 Luna costs less

Repository review

50K input + 3K output tokens

GPT-5.6 Luna$0.014Fits in one request

Grok 4.20$0.070Fits in one request

GPT-5.6 Luna costs less

Agent loop

200K input + 10K output tokens

GPT-5.6 Luna$0.052Fits in one request

Grok 4.20$0.275Fits in one request

GPT-5.6 Luna costs less

Listed OpenRouter rates, without cache or batch discounts. A workload that does not fit a model's context window would need splitting.

Specification differences

The leading value in a row is marked; a row with a missing value names no leader. Our two rows are marked in red.

Specifications of GPT-5.6 Luna and Grok 4.20
Specification	OpenAI GPT-5.6 Luna	SpaceXAI Grok 4.20
Maker	OpenAI	xAI
Weights	No public weights listed	No public weights listed
Released	July 9, 2026	March 31, 2026
Context window	1.05M	2M
Input $/Mtok	$0.20	$1.25
Output $/Mtok	$1.20	$2.50
Coding index	58.0	33.3
Independent measurements behind it	6	2
Cost per solved task	$0.04	$0.72
Source	OpenRouter listing	OpenRouter listing

Questions

Is GPT-5.6 Luna better than Grok 4.20 for coding?

Pick GPT-5.6 Luna for coding: it scores 58.0 on the Vibe Coding Index against 33.3 for Grok 4.20. Pick Grok 4.20 for a larger context window (2M against 1.05M tokens) and work measured by Vending-Bench 2 or ARC-AGI, where it leads.

Which is cheaper, GPT-5.6 Luna or Grok 4.20?

GPT-5.6 Luna costs $0.20 per million input tokens and $1.20 per million output tokens; Grok 4.20 costs $1.25 and $2.50. Per solved SWE-bench Verified task, GPT-5.6 Luna costs $0.04 and Grok 4.20 $0.72.

Which has the bigger context window, GPT-5.6 Luna or Grok 4.20?

Grok 4.20, with 2M tokens against 1.05M.

How many benchmarks is the GPT-5.6 Luna vs Grok 4.20 comparison based on?

39 benchmarks have a result for both models as of October 22, 2026. GPT-5.6 Luna's coding index rests on 6 independent benchmarks and Grok 4.20's on 2. Where only one model has a result, the page says so instead of naming a winner.

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GPT-5.6 Luna or Grok 4.20: which to pick

Which one for your work

Coding

Intelligence

Agentic work

Reasoning

Math

Knowledge

Web design

Long documents

Everyday cost

Cost per solved task

At a glance

Benchmark by benchmark

What each workload costs

Chat turn

Repository review

Agent loop

Specification differences

Questions

Compare next