OpenAI

GPT-5.5

Released Apr 23, 2026 · knowledge cutoff 2025-12

Status: Superseded
Location: United States
Modality: Multimodal
Context window: 1M
Max output: 128K
Speed: —
Price ($/MTok in / out): $5 / $30
Cost per task (medium effort): $0.41
Open weights: No

Benchmarks5 of 8 reported

GPQA Diamond

93.5%

SWE-bench Verified

88.7%

Humanity's Last Exam

41.4%

LMArena Elo

1481

ARC-AGI-2

85%

MMLU-Pro

—

Terminal-Bench 2.1

—

AIME (latest)

—

Strengths & weaknesses

Strengths

Tenacious on long agentic runs — keeps pushing rather than declaring premature victory
Reliably pulls the one relevant detail out of a very long prompt
Deliberately built for agent workloads rather than for chat

Weaknesses

Chat quality regressed — shorter, flatter answers and more refusals than the 5.1 line
Struggles to recover once a long thread already contains a wrong assumption
Training artefacts surfaced in production, including a tic for inserting creature words that OpenAI later patched out

GPT-5.5 Pro variant ($30/$180) sits #2 on LMArena. Context reported as 1M API / 1.1M by some trackers. Knowledge cutoff Dec 2025. SWE-bench Verified 88.7 lab-reported; LMArena Elo 1481 is the GPT-5.5 (high) listing.

For developers

API model strings

Not researched

Licence

Proprietary — weights not released

Retirement

No retirement announced

Lineage

No recorded predecessor