Model comparison

GPT-5.5 vs GPT-5.6 Luna

Two OpenAI tiers compared on the figures that decide which one a workload actually needs.

Catalog record checked September 3, 2026Individual provider fields may changeEvidence confidence: High — see receipts below

OpenAI

GPT-5.5

Frontier

OpenAI

GPT-5.6 Luna

Budget

AI model capability comparison
Specification	GPT-5.5	GPT-5.6 Luna
Provider	OpenAI	OpenAI
Tier	Frontier	Budget
Context window	1.05M	1.05M
Max output	128K	128K
Input / 1M tokens	$5	Winner: $0.20
Output / 1M tokens	$30	Winner: $1.20
Weights	Closed	Closed
Parameters	Not disclosedUnverified	cheapest GPT-5.6 tier
Reasoning levels	none, low, medium, high, xhigh, max	none, low, medium, high
Modalities	text, image	text, image
API model id	gpt-5.5	gpt-5.6-luna
Released	April 23, 2026	July 9, 2026
Artificial Analysis Intelligence Index [xhigh] (2026-08-14)	58	Not verifiedUnverified
Artificial Analysis Intelligence Index [high] (2026-08-14)	Not verifiedUnverified	47
SWE-bench Verified (2026-09-01)	82.6	Winner: 93
Terminal-Bench 2.1 (2026-07-27)	83.4	Winner: 84.7
GPQA Diamond (2026-07-27)	93.5	Not verifiedUnverified
Humanity's Last Exam (2026-04-23)	52.2	Not verifiedUnverified
LiveBench (2026-09-05)	Not verifiedUnverified	73.6

Prices are USD per million tokens at standard rates, excluding batch and caching discounts. Bold indicates the better figure where one is objectively better. Values we could not confirm from the provider are shown as “Not verified” rather than estimated.

Benchmark receipts

2026-09-05: LiveBench official leaderboard (benchmark-owned), max effortContamination-resistant general capability across reasoning, coding, math, data analysis, and language, with monthly question refreshes. Comparability: comparable with caveat — Rolling question set: observations months apart measure different question mixes. Record the measurement date and compare within ~1 month windows.
2026-09-01: vals.ai SWE-bench Verified leaderboard (independent, mini-swe-agent bash-only) (independent; OpenAI did not report SWE-bench Verified for the GPT-5.6 family)Real GitHub issue resolution: does the model's patch pass the hidden tests. Comparability: comparable with caveat — Post-audit vendor claims and pre-audit scores sit on different task trust levels; scaffolding (agent harness, compute budget) also dominates results. Never aggregate across scaffolds.
2026-08-14: Artificial AnalysisComposite index blending reasoning, knowledge, and coding evals into one 0–100 score. Comparability: directly comparable — AA occasionally rebaselines the index scale between snapshots — a score captured on one date is only comparable to same-snapshot scores (check measuredAt).
2026-07-27: Kimi K3 model card (vendor-reported, xhigh); announcement charts show 93.6 — minor conflict preservedAgentic terminal work: multi-step tasks executed in a sandboxed shell environment. Comparability: comparable with caveat — Scores across Terminal-Bench major versions (v1 vs v2) are NOT comparable; harness configuration (container, time limits) also shifts results.
2026-07-09: OpenAI GPT-5.6 announcement (vendor; chart transcribed by Vellum)Agentic terminal work: multi-step tasks executed in a sandboxed shell environment. Comparability: comparable with caveat — Scores across Terminal-Bench major versions (v1 vs v2) are NOT comparable; harness configuration (container, time limits) also shifts results.
2026-04-23: OpenAI — Introducing GPT-5.5 (vendor, with tools; 41.4 no tools)Frontier-knowledge ceiling: extremely hard multi-domain questions written to be near-impossible without deep expertise. Comparability: comparable with caveat — Subset/tool configurations (text-only vs with-tools) differ between vendors and materially change scores; must match configuration to compare.

Pricing tiers: GPT-5.5: Standard $5/$30 per MTok; >272K input billed 2x in / 1.5x out. Cached input $0.50/MTok; batch $2.50/$15. · GPT-5.6 Luna: Launch list was $1/$6; cut to $0.20/$1.20 on 2026-07-30. Cached input $0.02/MTok; batch $0.10/$0.60.

FrontierRecord checked September 3, 2026

GPT-5.5

GPT-5.5 is OpenAI's previous flagship — strong on Terminal-Bench 2.0 (82.7%) and 1M-token context.

Best for

Reasoning
Agentic coding
Long-context work

Watch out

Superseded by GPT-5.6 Sol on quality; still a capable, widely integrated model.

BudgetRecord checked September 3, 2026

GPT-5.6 Luna

GPT-5.6 Luna is OpenAI's cheapest tier — roughly 1/25 of Opus 5-class input cost, suitable where quality requirements are modest.

Best for

Classification
High-volume chat
Latency-sensitive pipelines

Watch out

Smallest GPT-5.6 tier; verify quality holds before routing frontier work to it.

When the cheaper one wins

GPT-5.6 Luna is cheaper on output at $1.20 per million tokens against $30 for GPT-5.5 — about 25×. Use the cheaper tier for classification, extraction, summarisation, and any task where the expensive model’s extra score does not change the accepted output. The expensive one only pays if your hardest task actually fails on the cheap tier. These are standard-tier API rates, excluding batch and cache discounts.

Run the model picker

Evidence confidence: High

How strong and complete the evidence behind this comparison is — not a prediction of which model is better.

Pricing verified on both sides — Input and output rates are verified for both models.
4/5 core specs verified on both sides — Not published for at least one side: parameter count.
3 shared named benchmarks with differing scores — Measured on: Artificial Analysis Intelligence Index, SWE-bench Verified, Terminal-Bench 2.1.
Verified within the last 90 days — Newest catalog check was 8 days ago.
Both models carry source citations — Each side has at least two catalog sources on record.

Source receipts

Each catalog figure was checked against the provider or an independent second source on the date shown.

GPT-5.5: OpenAI — Introducing GPT-5.5 (accessed 2026-08-29)
GPT-5.5: OpenAI API pricing (gpt-5.5 $5/$30) (accessed 2026-08-29)
GPT-5.6 Luna: OpenAI — GPT-5.6 (accessed 2026-08-29)
GPT-5.6 Luna: OpenAI API pricing (gpt-5.6-luna $0.20/$1.20) (accessed 2026-08-29)

Related comparisons

Diving deeper on one model? GPT-5.5 · GPT-5.6 Luna

Common questions

GPT-5.5 vs GPT-5.6 Luna

Answered from the verified figures on this page rather than general guidance.

Is GPT-5.5 or GPT-5.6 Luna cheaper for input?

GPT-5.6 Luna is cheaper at $0.20 per million input tokens, against $5 for GPT-5.5 — roughly 25× the price. Output tokens usually dominate a real bill, so weigh the output rate more heavily than the input rate. These are base rates; GPT-5.5 has tiered pricing: Standard $5/$30 per MTok; >272K input billed 2x in / 1.5x out. Cached input $0.50/MTok; batch $2.50/$15. GPT-5.6 Luna has tiered pricing: Launch list was $1/$6; cut to $0.20/$1.20 on 2026-07-30. Cached input $0.02/MTok; batch $0.10/$0.60.

Is GPT-5.5 or GPT-5.6 Luna cheaper for output?

GPT-5.6 Luna is cheaper at $1.20 per million output tokens, against $30 for GPT-5.5 — roughly 25× the price. Output tokens usually dominate a real bill, so weigh the output rate more heavily than the input rate. These are base rates; GPT-5.5 has tiered pricing: Standard $5/$30 per MTok; >272K input billed 2x in / 1.5x out. Cached input $0.50/MTok; batch $2.50/$15. GPT-5.6 Luna has tiered pricing: Launch list was $1/$6; cut to $0.20/$1.20 on 2026-07-30. Cached input $0.02/MTok; batch $0.10/$0.60.

Which has the larger context window, GPT-5.5 or GPT-5.6 Luna?

Both accept about 1.05M tokens of context, so document length will not decide between them.

Do GPT-5.5 and GPT-5.6 Luna support the same reasoning levels?

GPT-5.5 exposes none, low, medium, high, xhigh, max, while GPT-5.6 Luna exposes none, low, medium, high.

Should I use GPT-5.5 or GPT-5.6 Luna?

GPT-5.5 is the frontier tier and GPT-5.6 Luna the budget tier. The useful question is whether your hardest task actually fails on the cheaper one — most production volume such as classification, extraction and summarisation does not.

Next step

Choosing between them

The verified figures that separate this pair, computed from the catalog rather than restated boilerplate.

Input price: GPT-5.6 Luna costs $0.20 per 1M tokens versus $5 for GPT-5.5 — a 25x difference at the headline tier.
Measured capability: GPT-5.5 leads Artificial Analysis Intelligence Index 58 to 47 (measured 2026-08-14).
Positioning: GPT-5.5 sits in the frontier tier, GPT-5.6 Luna in the budget tier — most production volume (classification, extraction, summarisation) does not need the pricier tier.

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