What is Deep Think in Gemini 3.5 Pro?

Deep Think is Google's explicit reasoning mode in Gemini 3.5 Pro, announced at Google I/O May 2026 and rolling out in June 2026. It lets the model spend additional compute on multi-step problems before answering, producing explicit reasoning traces. It is positioned as Google's response to Claude's extended thinking and OpenAI's o-series-style hidden reasoning. Best fit: hard math, complex code planning, multi-document analysis.

What is extended thinking in Claude Fable 5?

Extended thinking is Anthropic's controlled reasoning mode, inherited from Claude 3.7 Sonnet and refined in Fable 5. You set a thinking token budget; the model produces an explicit thinking block before its final response. In Fable 5 it is part of the Mythos-class capability — designed for long-horizon agentic tasks where the model needs to plan, execute, observe, and replan over hours.

Which gives better answers on hard problems?

Depends on the problem. Claude Fable 5 with extended thinking leads on SWE-Bench Pro (80.3%) and FrontierCode Diamond (29.3% vs 13.4% for GPT-5.5). Gemini 3.5 Pro with Deep Think has not been independently benchmarked at scale yet (June 12, 2026 — GA rolling out). Early Google internal numbers claim parity on AIME and competitive code generation. For now, default to Claude Fable 5 for coding agents and re-evaluate Gemini after 30 days of public benchmarks.

What about context window — 2M vs 1M?

Gemini 3.5 Pro's 2M-token window is the largest of the three frontier models. Claude Fable 5 ships 1M default with 128K output. GPT-5.5 is 1M context. For massive single-shot context (entire monorepos, large legal corpora, multi-document research), Gemini 3.5 Pro is the only option. For 1M-token retrieval quality, GPT-5.5 leads MRCR v2 at 74.0%. The 2M is most useful when you genuinely need it; for most workloads 1M is enough.

Quick Answer

Gemini 3.5 Pro Deep Think vs Claude Fable 5 Extended Thinking (June 2026)

Published: June 12, 2026

Gemini 3.5 Pro Deep Think vs Claude Fable 5 Extended Thinking

Two flagship reasoning modes from two frontier labs. Google’s Deep Think rolled out with Gemini 3.5 Pro in June 2026. Anthropic’s extended thinking is the core of Claude Fable 5, released June 9, 2026. Here is the comparison that matters.

Last verified: June 12, 2026

TL;DR

Capability	Gemini 3.5 Pro Deep Think	Claude Fable 5 Extended Thinking
Release	June 2026 GA (rolling out)	June 9, 2026
Context window	2M tokens	1M tokens / 128K output
Reasoning mode	Deep Think (explicit traces)	Extended thinking (budget-controlled)
SWE-Bench Pro	~54% (3.1 baseline; 3.5 TBD)	80.3% ✅
FrontierCode Diamond	TBD	29.3%
Pricing (est.)	$5 in / $30 out per 1M	$15 in / $75 out per 1M
Best fit	Massive multimodal context	Long-horizon coding agents

How each mode works

Gemini 3.5 Pro Deep Think

Announced at Google I/O May 2026. GA promised for June. The mode:

Engages explicit multi-step reasoning before final answer.
Designed for hard math, complex planning, multi-document synthesis.
Combines with the 2M-token context window — you can dump a whole codebase + docs and Deep Think over the corpus.
Available via Vertex AI for enterprise and the $20 Pro / $250 Ultra consumer tiers.

For the broader Gemini 3.5 Pro fit, see Gemini 3.5 Pro vs Claude Fable 5 vs GPT-5.5 long context coding.

Claude Fable 5 Extended Thinking

Anthropic’s controlled reasoning, refined from earlier Claude generations:

You set a thinking_budget in tokens.
The model emits a thinking block, then the final response.
Designed for long-horizon agent loops — plan, execute, observe, replan over many turns.
Mythos-class tier above Opus 4.8.
Routes to Opus 4.8 fallback if safety classifiers refuse the original request.

For the Fable 5 tier picker, see Claude Fable 5 vs Sonnet 4.7 vs Haiku 4.5.

Where each one wins

Claude Fable 5 — agentic SWE

SWE-Bench Pro 80.3%. FrontierCode Diamond 29.3%. These are the benchmarks that matter for autonomous coding agents that run in Claude Code, Cursor agent mode, Windsurf Cascade. Fable 5 is the leader by a wide margin.

If you build a code-fixing bot, a Claude Code workflow, or a CI agent — Fable 5.

Gemini 3.5 Pro — massive context

2M tokens is genuinely useful when:

You need to reason over an entire legal corpus.
You analyze multiple full books or research papers in one call.
You feed a complete enterprise codebase + dependency docs in one shot.
You combine text + video + image in one multimodal call (Gemini’s native multimodal is strongest).

If you need 2M context, Gemini 3.5 Pro is the only frontier option today.

GPT-5.5 — 1M-token retrieval

Not in this comparison directly, but worth noting: at 512K–1M tokens, GPT-5.5 scores 74.0% on MRCR v2, the best 1M-context retrieval today. See Claude Fable 5 vs GPT-5.5 vs Gemini 3.5 Pro SWE-Bench.

Pricing per call

Estimates (provider pricing as of June 12, 2026; Gemini 3.5 Pro pricing finalizing):

Model	Input ($/1M)	Output ($/1M)	Deep Think / extended thinking cost
Gemini 3.5 Pro	$5 (est.)	$30 (est.)	Bundled in output
Claude Fable 5	$15	$75	Counts against output budget

Claude is structurally more expensive per token. The trade-off is success rate on agentic workloads.

Decision matrix

Use case	Pick
Autonomous code review on GitHub PRs	Claude Fable 5
Multi-hour Claude Code session	Claude Fable 5
Reason over 1.5M-token legal corpus	Gemini 3.5 Pro
Video + transcript + code multimodal task	Gemini 3.5 Pro
Hard math (competition problems)	Test both, default Gemini 3.5 Pro Deep Think
Tool-use chain with replanning	Claude Fable 5
Cost-sensitive frontier reasoning	Gemini 3.5 Pro
Enterprise on Vertex AI	Gemini 3.5 Pro
Enterprise on AWS Bedrock	Claude Fable 5

What to watch in the next 30 days

Public Gemini 3.5 Pro benchmarks. Internal Google numbers are not enough. Look for independent SWE-Bench Pro and AIME runs.
Deep Think cost transparency. Google has not finalized the per-call cost of Deep Think mode. Pricing may differentiate from base Gemini 3.5 Pro.
Claude Fable 5 Mythos behavior. Anthropic released Mythos 5 (restricted) alongside Fable 5. Watch for Project Glasswing case studies.
GPT-5.6 release. If it lands in June with the leaked 1.5M context and UltraFast Codex, the three-way comparison shifts. See GPT-5.6 leaked features.

Bottom line

Claude Fable 5 wins for autonomous coding agents (SWE-Bench Pro 80.3%, FrontierCode Diamond 29.3%). Gemini 3.5 Pro wins for massive context (2M tokens) and multimodal frontier. Use both via Vertex AI / Bedrock / Anthropic API. Re-evaluate after 30 days of public Gemini 3.5 Pro benchmarks.

Sources: Anthropic news (June 9, 2026), Google I/O 2026, Vertex AI docs, DataCamp benchmark coverage, Enterprise DNA.