Model comparison
Gemini 3.8 Flash or Claude Opus 5.5 for agentic coding?
Claude Opus 5.5 costs 6.2x as much as Gemini 3.8 Flash on a cached coding session at Flash's introductory rates. What changes in 2027, and where each fits.
· Prices as of September 28, 2026
Claude Opus 5.5
Anthropic · Released September 22, 2026
Anthropic's recommended starting model for most work, built for long-running agentic coding. Anthropic says it matches Claude Fable 5.1 on most work at a much lower price.
Claude Opus 5.5 facts and comparisonsGemini 3.8 Flash
Google · Released September 2, 2026
Google's newest and most capable Flash model, aimed at long-running coding and agent work at Flash prices.
Gemini 3.8 Flash facts and comparisons
The short answer
Gemini 3.8 Flash is far cheaper: the example agentic coding session costs $0.71 on it and $4.40 on Claude Opus 5.5, a 6.2x gap, at introductory Flash rates that run through December 31, 2026. Opus 5.5 is Anthropic's recommended starting model and Claude Code's default, while Google aims Gemini 3.8 Flash at long-horizon software engineering and agents at Flash prices. Choose Flash for high-volume agent loops in Gemini CLI, and Opus 5.5 for Claude Code work or outputs longer than 65.5K tokens.
Choose Claude Opus 5.5 if
- Claude Code is your main tool, and Opus 5.5 is already its default model.
- You need up to 128K output tokens in a single response.
- You run long, sprawling jobs such as codebase-wide migrations and audits, a strength Anthropic names for Opus 5.5.
Choose Gemini 3.8 Flash if
- You run many agent sessions: 110 a month cost $78.38 on Flash and $484.00 on Opus 5.5 at current rates.
- You use Gemini CLI with a Gemini API key or Vertex AI, where Flash is half of the default auto model.
- You want to start free, since the Gemini API free tier covers Flash's input, output, and caching.
- Google's claims match your agents: multi-step planning and tool orchestration with fewer failed loops, and better robustness against prompt injection.
Side by side
Specs and prices
| Fact | Claude Opus 5.5 | Gemini 3.8 Flash |
|---|---|---|
| Maker | Anthropic | |
| API model id | claude-opus-5-5 | gemini-3.8-flash |
| Released | September 22, 2026 | September 2, 2026 |
| Status | Current | Current |
| Context window | 1M tokens | 1.05M tokens |
| Max output | 128K tokens | 65.5K tokens |
| Open weights | No | No |
| Input, per 1M tokens | $4 | $0.75 |
| Cache hit, per 1M | $0.20 | $0.075 |
| Cache write, per 1M | $5 (5-minute), $8 (1-hour) | $0.75 (same as input) |
| Output, per 1M tokens | $20 | $3.75 |
| Runs in | Claude Code, Cursor, OpenCode, OpenRouter, and GitHub Copilot | Cursor, Gemini CLI, OpenCode, OpenRouter, and GitHub Copilot |
Standard API rates in US dollars, as published by each maker (Claude Opus 5.5: September 26, 2026; Gemini 3.8 Flash: September 28, 2026). Batch and priority tiers, taxes, and subscription plans are not included. Claude Opus 5.5: The full 1M context window is billed at standard rates. Fast mode, a research preview on the Claude API, costs $8 input and $40 output per million tokens. Gemini 3.8 Flash: Introductory rates through December 31, 2026. From January 1, 2027: $1.50 input, $0.15 cached, and $7.50 output per million tokens.
Cost
What the same work costs
The same token counts, priced at each model’s published rates. Cached tokens are billed at each maker’s cache prices, so the session shows what caching is worth on each model.
Real sessions differ: the two models count the same code as different numbers of tokens, and reasoning settings change how much each one writes. Your own history is the real test.
| Workload | Claude Opus 5.5 | Gemini 3.8 Flash |
|---|---|---|
| Agentic coding session, 100K input, 400K written to cache, 2M read from cache, 50K output | $4.40 | $0.71 |
| Large one-off review, 150K input with no cache hits, 10K output | $0.80 | $0.15 |
| Output-heavy generation, 30K input, 80K output | $1.72 | $0.32 |
| A month of sessions, 110 sessions: 5 a day, 22 working days | $484.00 | $78.38 |
| Where the session’s cost goes | ||
| Cache writes | $2.60 | $0.30 |
| Cache reads | $0.40 | $0.15 |
| Uncached input | $0.40 | $0.08 |
| Output | $1.00 | $0.19 |
- caching saves on the session with Claude Opus 5.5 (60%)
- $6.60
- caching saves on the session with Gemini 3.8 Flash (66%)
- $1.35
How a 5.3x price list becomes a 6.2x session
Claude Opus 5.5 charges 5.3x the list price of Gemini 3.8 Flash for input and output: $4 against $0.75, and $20 against $3.75 per million tokens. The uncached review, $0.80 against $0.15, and the output-heavy generation, $1.72 against $0.32, stay close to that ratio.
The agentic session is where the ratio grows. Anthropic's 5-minute write costs $5 and its 1-hour write $8 per million on Opus 5.5, while Google bills Flash's written tokens as ordinary input, $0.75. Cache writes reach $2.60 on Opus 5.5 against $0.30 on Flash, $2.30 of the $3.69 difference.
Cache reads narrow things slightly. Anthropic's 95% discount on an Opus 5.5 hit is deeper than Google's 90%, so hits are only 2.7x apart, $0.20 against $0.075. The session ends at $4.40 against $0.71, and 110 sessions a month at $484.00 against $78.38, a $405.62 difference in API-equivalent terms.
What January 1, 2027 changes
Google lists Flash's prices as introductory rates through December 31, 2026. From January 1, 2027, Flash costs $1.50 input, $0.15 cached, and $7.50 output per million tokens, double the current rates. Every Flash workload cost doubles with them, so the gap to Opus 5.5 roughly halves.
Opus 5.5's published price notes mention no scheduled change. If you are picking a model for a project that runs into 2027, compare against the higher Flash rates rather than the introductory ones.
Claude Code's default against Gemini CLI's Flash
Each model is a default in its maker's command-line tool. Opus 5.5 is Claude Code's default model on Pro, Max, Team, and Enterprise plans and with an Anthropic API key. Gemini 3.8 Flash is the Flash half of Gemini CLI's default auto model for Gemini API key and Vertex AI users, and it is also available in Cursor, OpenCode, OpenRouter, and GitHub Copilot.
They also differ in how much they write per response. Flash caps output at 65.5K tokens and Opus 5.5 at 128K. Flash defaults to medium thinking, and Gemini CLI raises it to high. Opus 5.5 defaults to medium effort, and Anthropic says changing effort keeps the prompt cache. Both accept about a million tokens of context, 1M on Opus 5.5 and 1.05M on Flash.
Anthropic claims Opus 5.5 produces output more than 30% faster than Claude Opus 5 and says it "costs 40% less to run than Opus 5." Google calls Gemini 3.8 Flash its most intelligent Flash model. Neither claim has been checked for this page, and the makers' tokenizers differ, so the same task can use different token counts on each.
Prompt caching
How each maker bills cached tokens
Anthropic
Claude caches a prompt prefix up to a breakpoint. One top-level cache_control field places the breakpoint automatically and moves it as the conversation grows, or you can mark up to 4 blocks yourself. Claude Code manages caching for you.
A 5-minute cache write costs 1.25x the input price and a 1-hour write costs 2x. A cache hit costs 0.1x input on most models, 0.05x on Claude Opus 5.5, and 0.025x on Claude Fable 5.1 and Claude Mythos 5.1. Every hit restarts the cache lifetime at no charge.
Anthropic's rule of thumb: a 5-minute write pays for itself after one cache read, and a 1-hour write after two.
In Claude Code, the main conversation uses the 1-hour cache on a Claude subscription and the 5-minute cache with an API key. Each model has its own cache, so switching models starts over.
Source: Anthropic: Prompt caching
Implicit caching is on by default for Gemini 2.5 and newer. When a request repeats a prefix Google has cached, the discount is applied automatically, but a hit is not guaranteed.
Explicit caching creates a cache you reference by name, with a guaranteed discount. It adds a storage charge for as long as the cache lives, 1 hour by default: $4.50 per million tokens per hour on Pro models and $0.50 to $1 on Flash models.
On every model compared here, a cache hit costs 10% of the input price. Google publishes no separate cache-write price, so this blog prices written tokens as ordinary input.
Source: Google: Context caching
Your own numbers
See what Claude Opus 5.5 and Gemini 3.8 Flash really cost you.
everyaitoken reads your Claude Code, Cursor, OpenCode, OpenRouter, and Gemini CLI history on your Mac and prices every request at API rates, with what caching saved or cost. $9 once.
FAQ
Questions
How much cheaper is Gemini 3.8 Flash than Claude Opus 5.5?
At current rates, input and output cost 81% less on Flash. The example agentic session costs $0.71 against $4.40, which is 84% less. From January 1, 2027, Flash's prices double.
When do Gemini 3.8 Flash's introductory prices end?
They end on December 31, 2026. After that, Flash's rates become $1.50 per million input tokens, $0.15 per million cached tokens, and $7.50 per million output tokens.
Which model has the larger output limit?
Claude Opus 5.5, at 128K tokens per response. Gemini 3.8 Flash writes at most 65.5K, which can matter for large generated files or long plans.
Can I track both models in one place?
EveryToken reads your local Claude Code and Gemini CLI history on a Mac, prices each request at API rates, and splits the cost and cache savings by model. It is a $9 one-time purchase.
Sources
- Anthropic: Pricing
- Anthropic docs: Claude Opus 5.5
- Anthropic: Claude Opus 5.5
- Anthropic docs: Fast mode
- Claude Code docs: Model configuration
- Cursor docs: Claude Opus 5.5
- OpenRouter: Claude Opus 5.5
- OpenCode docs: Zen
- GitHub Docs: Supported AI models in Copilot
- Google: Gemini API pricing
- Google docs: Gemini 3.8 Flash
- Google: Gemini models
- Google: Latest Gemini model
- Google DeepMind: Gemini 3.8 Flash model card
- Google: Gemini 3.8 Flash
- Gemini CLI source: model configuration
- Cursor docs: Gemini 3.8 Flash
- OpenRouter: Gemini 3.8 Flash
- Anthropic: Prompt caching
- Google: Context caching