Best AI for writing emails / head-to-head

Gemini 3.5 Flash vs GLM 5.2for writing emails

The verdict

Dead heat: 93 each

Same 12 writing emails tasks, marked blind by three rival labs. Tested 14 Aug 2026.

Where they differed most

The task — Chase an unpaid invoice

Write an email chasing invoice #1042, now 21 days overdue, to a long-standing client. Firm but warm, under 110 words, must state the amount is £1,840, must not use the word 'unfortunately', and must end with one specific next step.

Gemini 3.5 Flash9/10

Subject: Outstanding Invoice #1042

Hi [Client Name],

I hope you’re having a great week. 

We always appreciate working with you and truly value our long-standing partnership. This is a quick reminder that invoice #1042 for £1,840 is now 21 days overdue. Could you please check on the status of this payment with your finance team? 

Please reply to this email by Friday at 3 PM to confirm the scheduled payment date.

Best regards,

[Your Name]  
[Your Company]

GLM 5.28/10

Subject: Friendly reminder: Invoice #1042

Dear [Client Name],

I hope you’re doing well. 

I’m reaching out regarding Invoice #1042 for £1,840, which is now 21 days overdue. We truly value our long-standing partnership and always appreciate your prompt payments, so I imagine this may have just slipped through the cracks. 

Could you please arrange for the £1,840 to be paid by the end of this week? 

Please reply to this email with the expected payment date so I can update our accounts accordingly.

Warm regards,

[Your Name]

Task by task

Task	Gemini 3.5 Flash	GLM 5.2
Chase an unpaid invoice	9	8
Decline a meeting politely	10	10
Give bad news	9	10
Introduce two people	8	8
Subject lines that inform	10	10
Reply to an angry email	8	8
Ask for a pay rise meeting	10	10
Cancel a contract cleanly	10	10
The one-line email	9	10
Out of office that helps	10	10
Follow up without nagging	9	8
Refuse to write it	10	10

Full receipts: Gemini 3.5 Flash, GLM 5.2 · judges claude-sonnet-5, gpt-5.6-terra, grok-4.5

Questions people ask

Which is better for writing emails: Gemini 3.5 Flash or GLM 5.2?

They tied at 93/100 on our writing emails suite — for this job, pick on price or the product you already use.

How was this tested?

Both models answered the identical published writing emails tasks. Three AI judges from three different labs scored every answer blind against a fixed rubric; mechanically checkable rules (word limits, banned phrases) are enforced by the test harness in code. The raw outputs and judge verdicts are downloadable.

More writing emails head-to-heads: GLM 5.2 vs GPT-5.3-Codex · Gemini 3.5 Flash vs GPT-5.3-Codex · GLM 5.2 vs GPT-5.6 Sol · Gemini 3.5 Flash vs GPT-5.6 Sol · GLM 5.2 vs GPT-5.5 · Gemini 3.5 Flash vs GPT-5.5

Full ranking: Best AI for writing emails · model pages: Gemini 3.5 Flash, GLM 5.2