You're sending 20 or 30 business emails a day — client pitches, follow-ups, awkward negotiations, internal updates. Each one takes longer than it should because the first draft is either too stiff or too chatty, and the model doesn't distinguish between them. LLMs can close that gap, but the wrong pick creates new problems: an AI-sounding cadence, length that drifts a third longer than you wanted, or a tone that shifts between paragraphs.
This post ranks four API-level models on the three things that actually matter for business email: tone control, length discipline, and instruction following. Plus the per-email cost math, so you know when the subscription beats going API-direct.
The models and what each costs
| Model | Provider | Input / 1M | Output / 1M | Tier |
|---|---|---|---|---|
| Claude Fable 5 | Anthropic | $10 | $50 | Frontier |
| GPT-5.5 | OpenAI | $5 | $30 | Frontier |
| Gemini 3.5 Flash | $0.75 | $4.50 | Budget | |
| Claude Haiku 4.5 | Anthropic | $1 | $5 | Budget |
Pricing verified August 2026 at Anthropic pricing, OpenAI API pricing, and Google AI Studio pricing.
What makes an email LLM actually work
Three criteria separate models that hold up on business email from ones that technically complete the task:
Tone holds across contexts. A model that writes formal English for a cold pitch and then the same formal English for a warm follow-up has failed at what makes email hard. Good models adjust to a relationship, a company voice, and a recipient's status, and keep that register consistent for the entire email.
Length matches the intent. A 5-sentence update that comes back as 12 sentences took longer to edit than to write yourself. Frontier models infer appropriate length from context. Budget models need the instruction stated explicitly.
Instruction compliance is tight. "Keep the ask in the second paragraph, not the last" should produce that result on the first try. This is one of the clearest gaps between tiers, and the one that shows up most in day-to-day email use.
The ranking
Claude Fable 5: best for tone-sensitive emails
Fable 5 is the model when the email is the product. Sales emails where the first three lines determine whether you get a reply. Client communications where a single phrasing choice shifts the whole relationship. Negotiations where warmth and directness have to coexist in the same paragraph. Fable 5 infers appropriate register from context, holds it through the draft, and produces length that fits the ask, without needing explicit instructions for any of that.
For multi-email sequences where the voice needs to stay consistent across a 4-email thread, no other tier-1 model holds up as well.
A typical business email runs around 400 tokens input and 250 output. At Fable 5's rates, that's roughly $0.017 per draft. At 50 emails per day, about $25 per month in API spend.
GPT-5.5: best for direct, short-form emails
GPT-5.5 is sharper for emails that need to land in 10 seconds. Subject lines average better. The opening line is more likely to be the actual ask rather than a preamble. Internal updates, scheduling, status emails: GPT-5.5 outputs tend to need less editing than Fable 5 on these.
Where it slips: relationship-heavy emails where warmth has to come through without sounding like a CRM template. Fable 5 handles those by a visible margin. In a mixed day of 50 emails, using GPT-5.5 for internal and Fable 5 for client-facing is a practical split.
At 50 emails per day, GPT-5.5 runs about $13 per month.
Gemini 3.5 Flash: budget pick with real quality
At $0.75/M input, Flash is the answer when volume is your constraint. Our multilingual support benchmarks showed Flash tying GPT-5.5 on French and German classification tasks at a fraction of the cost, and the quality-per-dollar advantage carries over to English business email. Output is consistently structured and length instruction following is solid for shorter emails.
Where Flash falls behind: tone variance on nuanced drafts, and anything requiring an unusual register. Budget models need more explicit system prompt guidance to hit a specific key. Build a good system prompt once and Flash handles 80% of routine correspondence reliably.
At 50 emails per day, Flash runs about $1.35 per month.
Claude Haiku 4.5: cheapest reliable option
Haiku 4.5 handles short-form email reliably: scheduling, quick replies, internal status updates. It is not the pick for anything where tone matters or where a native reader would notice the email sounds templated. For high-volume, low-stakes emails where you're filling in variable fields (names, dates, amounts), Haiku 4.5 is the cheapest option that follows field instructions without errors.
For context on where this sits relative to free options, our free LLM guide covers the quality ceiling the free tiers hit. Haiku 4.5 sits a meaningful step above on instruction compliance and output consistency.
At 50 emails per day, Haiku 4.5 runs about $2.70 per month.
Subscription vs API
The break-even math for solopreneurs using one of these models as their email drafting tool:
| Provider | Model | API cost per email* | Subscription | Break-even |
|---|---|---|---|---|
| Anthropic | Claude Fable 5 | ~$0.017 | Pro $20/mo, Max $100-200/mo | Pro covers ~33 emails/day; above that, API costs less |
| OpenAI | GPT-5.5 | ~$0.009 | Plus $20/mo, Pro $200/mo | Plus covers ~67 emails/day; above that, API costs less |
| Gemini 3.5 Flash | ~$0.0009 | Gemini Advanced $19.99/mo | API beats subscription above a few hundred emails/day | |
| Anthropic | Claude Haiku 4.5 | ~$0.0018 | included in Claude Pro | Pro covers 330+ emails/day before API costs more |
*Estimated at 400 tokens input + 250 tokens output per email, direct API with no caching.
If you're using an LLM subscription for email as part of a broader workflow (coding, research, drafting), the subscription is usually simpler and cheaper at typical solo volumes. The API makes more sense when you're building something that calls programmatically: a Cursor workflow, an Alfred action, a Python script. Above 33 Fable 5 emails per day, the API costs less than Claude Pro. Above 67 GPT-5.5 emails per day, it costs less than ChatGPT Plus.
Which to use
Client-facing emails and anything where tone sets the outcome: Claude Fable 5. At $0.017 per draft, the tone control is worth it.
Internal updates, scheduling, and direct short-form correspondence: GPT-5.5. More direct, about half the cost, less re-rolling on short emails.
High-volume routine correspondence where budget is the constraint: Gemini 3.5 Flash. One good system prompt and it handles the bulk reliably.
Templated email with variable fields: Claude Haiku 4.5. Cheapest option that follows field instructions without hallucinating the details you gave it.
You can route your own email templates against all four models through LLMTest — a single endpoint, per-call cost tracking, no provider context-switching. If you want ranked results on your actual prompts before committing to a model, start a free run.