The customer has a broken path. You have already done the obvious restarts. Engineering needs to see a defect, not a feeling. The draft that feels complete is a forward of the ticket. They now have your job for forty minutes before they can start theirs.
This is not a customer escalation handoff. You may still own the customer. You are handing the defect. It is closer to handing off a project in miniature: reasons, dead ends, and what looks wrong on purpose.
Four things above the ticket
The outcome. Present tense. "They cannot export a CSV for March. They can see the month on screen." Not "export is weird."
The repro. Smallest steps, environment, time. If you cannot reproduce it, say that, and say what you tried. "Could not reproduce" with no list is how the bug comes back as cannot reproduce.
What you already tried. Each attempt and what it ruled out. This is the section that saves an engineer from doing your Tuesday again.
The question. A fix estimate, a workaround you can send today, or a confirmation this is expected. One question. Write it so a cold reader on the next rotation can answer without paging you.
Keep the customer's words, mark your reads
Quote the failure and any date they set. "This is the third date we have given our own client" is different from "they are frustrated." The first is something engineering can help you answer with a sentence the customer can forward.
"Sounds like a race in the exporter" is a conclusion. Put it next to the observation, or leave it out. An assistant that sat with you will happily promote a hunch into a root cause. You approve the package, so you keep the label honest.
You do not need their pager channel to start
Send a briefing. Link the ticket. No attachments, no screenshot dump unless the screenshot is the only repro and you have nowhere else to put a link to it — even then, describe what it shows. If they have an account, you need an accepted contact. If they do not, they get email and a page.
A nickname you use for the customer stays on your side. Engineering should see the account name they already have in the logs.
Default: "If I have not heard by 17:00 UTC I will send the workaround in the brief and I will not promise a fix date." Then do that.
If the afternoon is in a session, connect your assistant and have it list what you already tried before you write the question.