I Tested 3 AI Editors on the Same Business Email — Here's What Each One Changed

I’m Bryan Collins. I’ve been editing business writing for over seven years — running writing courses, editing client work as a ghostwriter, and rewriting my own emails to a 10,900-person list. I’ve tested Claude, ChatGPT and Grammarly for polishing drafts before they go out. Today I ran the same business email through all three. The differences were sharper than I expected.

TL;DR

Claude produced the most natural-sounding rewrite. ChatGPT kept more of the original structure but missed two tone issues. Grammarly caught a grammar error the other two ignored. For a non-native English professional, the strongest combination is Grammarly Pro plus one general-purpose AI editor — Grammarly for the line-level fixes, the AI for tone and confidence.

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The Email I Tested

I built a realistic project-update email from a fictional product manager I’ll call Marta. Her first language is Spanish. She’s writing to her manager, Liam, asking for feedback before a Friday stakeholder meeting. The draft is the kind I edit several times a week — clear thinking, hedged language, one passive construction, one missing article, and a sentence that runs too long.

Here is the original:

Subject: Quick update on the onboarding redesign — need your input

Hi Liam,

I would like to perhaps suggest that we move the onboarding redesign forward by one sprint. The customer interviews have been completed by the design team last week and the feedback was very consistent across the five users that we spoke with about new account setup. I think we have enough signal to start building, however I wanted to check with you first before I share update with the wider product team on Friday.

Could you let me know if you are comfortable with this direction, or if you would prefer that we wait for the second round of interviews?

Thanks, Marta

Four issues to watch for:

1. Over-hedging: “I would like to perhaps suggest” is two hedges stacked on top of each other.
2. Passive voice: “The customer interviews have been completed by the design team last week.”
3. Missing article: “before I share update” should be “before I share the update”.
4. A long, comma-heavy sentence that buries the actual recommendation.

Test 1: What Grammarly Changed

I pasted the email into the Grammarly web editor with Pro enabled.

What Grammarly caught:

• Article error. Flagged “before I share update” and suggested “before I share an update”. The other two tools missed this entirely.
• Passive voice. Highlighted “have been completed by the design team” and suggested “the design team completed”.
• Wordiness. Flagged “I would like to perhaps suggest” and recommended “I suggest” or “I recommend”.
• Tone signal. The tone indicator read “uncertain” and recommended sounding more confident.

What Grammarly did not change: the long second sentence, and the over-hedged opening got a swap rather than a rewrite.

Pricing reality (as of May 2026). Grammarly’s free tier covers basic grammar, spelling and punctuation. Tone detection, full-sentence rewrites and the clarity suggestions I used here require Grammarly Pro, which is around $12 per month billed annually. Check the Grammarly pricing page before you commit — vendor pricing moves.

Test 2: What ChatGPT Changed

I used ChatGPT (GPT-4-class model) with this prompt:

Rewrite this business email to sound more confident and professional. Keep it close to the same length. Do not change the meaning.

The rewrite:

Subject: Onboarding redesign — input needed before Friday

Hi Liam,

I’d like to suggest we move the onboarding redesign forward by one sprint. The design team completed customer interviews last week, and feedback was consistent across all five users we spoke with about new account setup. I think we have enough signal to start building, but I wanted to check with you first before I share the update with the wider product team on Friday.

Could you let me know if you’re comfortable with this direction, or if you’d prefer we wait for the second round of interviews?

Thanks, Marta

What ChatGPT changed:

• Dropped “perhaps” from the hedged opening. Kept “I’d like to suggest”, which is still a hedge but a softer one.
• Flipped the passive sentence to active voice.
• Quietly fixed the missing article (“share the update”).
• Tightened the subject line.

What ChatGPT preserved that I’d have cut: “I’d like to suggest” is still a hedge. A confident manager-to-manager email opens with the recommendation. “Let’s move the onboarding redesign forward by one sprint” lands harder. “I think we have enough signal” is another confidence leak that survived.

ChatGPT does what you ask. If you don’t ask it to remove every hedge, it won’t. Write the prompt sharper.

Test 3: What Claude Changed

I used Claude (Sonnet-class model) with the same prompt:

Rewrite this business email to sound more confident and professional. Keep it close to the same length. Do not change the meaning.

The rewrite:

Subject: Onboarding redesign — recommending we move up one sprint

Hi Liam,

I’d like to move the onboarding redesign forward by one sprint. The design team finished customer interviews last week, and feedback was strikingly consistent across all five users on new account setup. We have enough signal to start building.

Before I take this to the wider product team on Friday, I wanted to check that you’re comfortable with the direction. If you’d rather wait for the second round of interviews, tell me now and I’ll hold.

Thanks, Marta

What Claude changed:

• Removed the stacked hedges. The opening is a recommendation, not a request to suggest.
• Split the long sentence into three shorter ones.
• Restructured the second paragraph into a clearer ask: “tell me now and I’ll hold” gives Liam a concrete decision to make.
• Picked a more vivid word (“strikingly consistent”) that, in fairness, is a small editorial liberty.

Where Claude went too far: “strikingly consistent” was not in Marta’s original. If you want a faithful rewrite rather than a redraft in someone else’s voice, that word is a problem. The subject line also shifted to a more formal register — whether that’s right depends on Marta’s relationship with Liam.

Claude is the most willing of the three to rewrite for confidence. That is its strength and its risk.

Side-by-Side

Tool	Length change	Tone	Grammar fixes	Best for
Grammarly Pro	Minimal (suggestions only)	Flags uncertain tone	Article, passive voice, wordiness	Line-level fixes inside Gmail and Slack
ChatGPT	Slightly shorter	Softens hedges, doesn’t remove them	Article, passive voice	Faithful rewrites that respect your structure
Claude	Shorter, restructured	Confident, sometimes too confident	Article, passive voice	Confidence rewrites when you want a stronger draft

Which One Marta Should Use

If you can pay for only one tool, pay for Grammarly Pro. The reason is volume. Marta writes dozens of emails and Slack messages every week, and she is not going to paste each one into ChatGPT or Claude. Grammarly works inside Gmail, Outlook and Slack in real time, catching article errors and passive constructions on every message — not only the high-stakes ones.

Use ChatGPT or Claude as a second pass for the messages that matter — the project update to the manager, the escalation to the executive sponsor, the email to the client. Both have generous free tiers as of May 2026, though limits change. Check current free-tier caps before relying on them daily.

The combination that worked best in my test: Grammarly on every message, plus Claude for one rewrite per week on whatever email has the highest stakes. That covers the grammar floor and the tone ceiling.

For a fuller comparison, see Best AI Writing Tools for ESL Professionals.

What None of Them Caught

Here is the uncomfortable bit. All three tools improved Marta’s email at the sentence level. None of them asked the question I would have asked as her editor.

Is Friday too late?

Marta is asking Liam for input before a meeting on Friday. The email gives him no deadline. Liam runs three other teams and triages email by urgency. If Marta needs his sign-off by Wednesday so she has time to incorporate it, she needs to say so. None of the three AI editors caught that.

This is the gap between language and judgement. Tools can fix the words. They cannot tell you whether you are asking the right person, at the right time, for the right decision. That part is on you — and it is the part that gets you taken seriously at work.

FAQ

Is Grammarly’s free tier enough for a non-native English speaker at work?

For grammar and spelling, yes. For tone detection, sentence rewrites and clarity suggestions — the features that change how confident your email sounds — you need Grammarly Pro. The free tier is a fine way to test whether the tool fits your workflow before you pay.

Can I use ChatGPT or Claude for confidential work email?

Check your employer’s AI policy first. Many companies prohibit pasting client data, financial information or internal strategy into consumer chatbots. If your employer has an approved enterprise version (ChatGPT Enterprise, Claude for Work), use that. If not, use the consumer tools only on emails that contain no confidential information.

Which AI editor sounds least like a robot?

In my test, Claude. ChatGPT preserves structure well but tends to soften rather than restructure. Claude is more willing to rewrite a paragraph from the ground up, which produces more natural prose but occasionally introduces words the original writer would not have used. Always read the rewrite and adjust before sending.

How do I write a prompt that gets a better rewrite?

Be specific about what to change and what to leave alone. A prompt like “rewrite to remove every hedge, keep my voice, do not introduce new words” produces a tighter result than “make it sound more professional”. The more constraints you give the model, the closer the output stays to your intent.

Do these tools work for British and Irish English?

Grammarly lets you set your dialect to British English in settings — do this if you write for UK or Irish workplaces, because American spellings will otherwise be flagged. ChatGPT and Claude will follow whatever dialect you ask for in the prompt. Include “use British English spelling” in your instruction if that matters for your reader.