Voice

How Customer.io UI sounds — sentence case, direct verbs, no marketing tone.

The product is a tool, not a brochure. Copy is the part of the design system that drifts most often when AI agents or engineers ship UI without a designer in the loop. These rules give every surface the same voice without requiring per-screen review.

Apply everywhere

• Sentence case throughout — titles, labels, headers, columns, button labels.
• Imperative for buttons. "Save segment", not "Click here to save".
• Single action verb per button label. Prefer one verb over a phrase.
• Add an ellipsis when the action requires an additional step ("Export…").
• No exclamation marks anywhere in product UI.
• Say "people", not "users".
• Numbers as plain numbers (12,403, not 12.4K) in body copy. Abbreviate only in badges above 1k (1.2k, 42.7k, 1.4M).
• Tabular numerals on all numbers (font-variant-numeric: tabular-nums).
• No "please", "sorry", "oops", "easy", "simply", or "just".
• Match tone to severity. Direct and factual for errors and warnings. Conversational for features and info.

Voice violations to never ship

• Exclamation marks anywhere in product UI.
• "Click here" or any pronoun-led button label.
• Marketing-tone copy: "Awesome", "Amazing", "Just", "Simply".
• "Sorry" or "Oops" in error messages. Apologize by fixing the underlying issue.
• "Users" — say "people".
• Title case mixed with sentence case in the same surface.

Buttons

• Use a specific verb that matches the action. "Delete campaign", not "OK" or "Submit".
• One verb. "Save and continue" is rare and reserved for save-with-redirect; for everything else, pick one.
• For destructive actions, the button label names the destruction. "Delete", "Archive", "Revoke" — never "Confirm".

Empty states

• Lead with the noun. "No segments yet", not "There's nothing here".
• One-line "what this is" plus a single primary CTA with a verb.
• Never "Nothing to show" or blank.

Error messages

• What went wrong + why (if knowable) + the action that moves forward.
• Never raw API errors or stack traces.
• Primary action is a recovery path. "Re-upload corrected file", "Reconnect account" — never "Dismiss" or "OK".
• For partial results, lead with what succeeded: "8,556 of 12,403 rows imported". Then the gap. Then the action. "Failure" framing overstates the damage when most of the operation worked.

Confirmations

• Action-oriented title. "Delete campaign", never "Are you sure?".
• Body names the consequence — what breaks, what data is lost.
• Footer verb matches the title's verb.

Tone by severity

table

Severity	Tone	Example
Critical error	Direct, factual	Connection lost. Reconnect to resume sending.
Caution	Direct, brief	Sending limit at 90% of monthly quota.
Information	Neutral	Recipients added to segment.
Feature	Conversational, calm	Multivariate testing is now available for campaigns.
Success	Quiet	Segment saved.