Which model is best for autonomous coding agents in June 2026?

Claude Fable 5 (released June 9, 2026 by Anthropic). It scores 80.3% on SWE-Bench Pro, an 11-point lead over Claude Opus 4.8 (69.2%) and more than 20 points ahead of GPT-5.5 (58.6%) and Gemini 3.1 Pro (54.2%). On the harder FrontierCode Diamond set the gap is even larger: 29.3% versus 13.4% for GPT-5.5. For long-horizon agent tasks where the model has to run for hours, Fable 5 is the default.

Which model is best for long context (1M+ tokens)?

Both Claude Fable 5 (1M default, 128K output) and GPT-5.5 (1M context) are strong. GPT-5.5 scores 74.0% on OpenAI's MRCR v2 at 512K–1M tokens, a genuine fix vs GPT-5.4's 36.6% at the same range. Claude Fable 5 leads on GraphWalks at the same range. Gemini 3.5 Pro extends to 2M tokens but has not been independently benchmarked at the full range yet. For 1M-token retrieval: GPT-5.5. For 1M-token reasoning: Claude Fable 5.

Which is cheapest per task?

Depends on the task profile. Gemini 3.5 Flash and Claude Haiku 4.5 dominate cheap tasks; this comparison is about the frontier tier. Among frontier models, GPT-5.5 is roughly $5/$15 per million in/out, Claude Fable 5 is $15/$75, Gemini 3.5 Pro is $5/$30 (pre-release pricing). For pure cost per token, GPT-5.5 wins. For cost per successfully completed agent task on SWE-Bench Pro, Claude Fable 5 wins because the success rate is high enough to overcome the price premium.

Claude Mythos 5 is the restricted-availability sibling of Fable 5. Same underlying model with certain safety classifiers lifted, offered to Project Glasswing approved customers — cyber defenders, critical infrastructure operators. Most users will not have access. If you need maximum offensive-security capability and you qualify, Mythos 5. Otherwise, Fable 5 is what you get.

Quick Answer

Claude Fable 5 vs GPT-5.5 vs Gemini 3.5 Pro: SWE-Bench Pro (June 2026)

Published: June 12, 2026

Claude Fable 5 vs GPT-5.5 vs Gemini 3.5 Pro: SWE-Bench Pro

Three frontier models in active use as of June 12, 2026. Claude Fable 5 shipped June 9. GPT-5.5 has been in the API since April 24. Gemini 3.5 Pro is rolling out in June after the Google I/O 2026 announcement. Here is the honest benchmark + use case breakdown.

Last verified: June 12, 2026

TL;DR

Model	SWE-Bench Pro	MRCR v2 (512K–1M)	Context	Price (in/out per 1M)	Release
Claude Fable 5	80.3% ✅	strong (GraphWalks lead)	1M / 128K out	$15 / $75	June 9, 2026
GPT-5.5	58.6%	74.0% ✅	1M	$5 / $15	April 24, 2026
Gemini 3.5 Pro	~54.2% (3.1 baseline)	TBD at 2M	2M	$5 / $30 (est.)	June 2026 GA

Where each one wins

Claude Fable 5 — autonomous agents

Anthropic released Claude Fable 5 on June 9, 2026 along with Claude Mythos 5 (restricted). Fable 5 is the new “Mythos-class” tier above Opus.

SWE-Bench Pro: 80.3% — 11 points above Opus 4.8, 22 points above GPT-5.5.
FrontierCode Diamond: 29.3% — more than double GPT-5.5’s 13.4%.
Long-horizon tasks — designed for multi-hour autonomous agent runs.
1M token context default, up to 128K output per request.
Routing — if safety classifiers refuse a request, response can route to weaker Claude Opus 4.8.
Availability — Claude API, Claude Platform on AWS, Amazon Bedrock, Vertex AI, Microsoft Foundry.

If you are picking a model for Claude Code, an autonomous code review agent, or any long-running SWE workflow, Fable 5 is the default. See Claude Fable 5 vs Opus 4.8: should you upgrade.

GPT-5.5 — long-context retrieval and price

OpenAI released GPT-5.5 in the API on April 24, 2026 and updated ChatGPT’s default model with the GPT-5.5 Instant variant.

MRCR v2 at 512K–1M: 74.0% — genuinely fixes the long-context regression of GPT-5.4 (which scored 36.6% at the same range).
SWE-Bench Pro: 58.6% — behind Fable 5 but improved over GPT-5.4.
Pricing: ~$5 in / $15 out per million tokens — the cheapest of the three frontier models.
GPT-5.5 Instant — smarter, more accurate default ChatGPT experience with reduced hallucinations.

Best fit: long document analysis, RAG over million-token contexts, cost-sensitive frontier workloads. GPT-5.6 leaks suggest a June 2026 release with 1.5M context and UltraFast Codex mode, so watch for that.

Gemini 3.5 Pro — context size and Deep Think

Announced at Google I/O May 2026, GA in June 2026. Sundar Pichai’s commitment is for full availability in June.

2M token context window — largest of the three.
Deep Think reasoning mode — multi-step problem solving with explicit reasoning chains.
Multimodal frontier — text, images, audio, video in one model.
Pricing rollout — $20 Pro tier and $250 Ultra tier consumer plans first, then broader API.

Best fit: massive context retrieval, enterprise multimodal workloads, Vertex AI customers. The Deep Think mode positions it as the alternative to extended thinking in Claude. See Gemini 3.5 Pro vs Claude Fable 5 vs GPT-5.5 long context coding.

Decision matrix

Use case	Pick
Autonomous coding agent	Claude Fable 5
Long-context retrieval (RAG over 1M tokens)	GPT-5.5
Multimodal frontier (video + text + audio)	Gemini 3.5 Pro
Cost-sensitive frontier tier	GPT-5.5
Enterprise on Vertex AI	Gemini 3.5 Pro
Code review and refactor on real repos	Claude Fable 5
Massive single-shot context (>1M tokens)	Gemini 3.5 Pro (2M)
Default for Claude Code / Cursor / Windsurf agent mode	Claude Fable 5

Pricing per successful task

Pure $/token favors GPT-5.5. But for agentic coding workloads, what matters is cost per successful task:

Claude Fable 5 at 80.3% success on SWE-Bench Pro, $15 input → if a task uses 100K tokens, ~$1.50 input cost × 1.25 retry rate = ~$1.87 effective.
GPT-5.5 at 58.6% success, $5 input → ~$0.50 input cost × 1.71 retry rate = ~$0.86 effective.

GPT-5.5 still wins on raw price for code that retries cleanly. Fable 5 wins when retries are expensive (long-running agents, human-in-the-loop reviews).

Bottom line

Claude Fable 5 for agentic coding. GPT-5.5 for long-context retrieval and price. Gemini 3.5 Pro for multimodal and 2M-token enterprise. Three different jobs. Use all three through Cursor 4, Windsurf, or Claude Code routers.

Sources: Anthropic news (June 9, 2026), OpenAI release notes, Google I/O 2026, DataCamp, BenchLM, EdenAI, Digital Applied independent benchmarks (June 2026).