Agree on the record that owns each fact
An integration is easier to maintain when both systems agree on ownership. Let the ATS own the candidate identity, the vacancy and the hiring stage. The interview tool owns the invitation, the recording, the review status and the feedback. Write that division down before you map a single field.
A shared field with two owners will disagree at some point. Decide which system writes the value and which system only reads it. That rule prevents a discussion before it reaches a recruiter.
Start with the candidate and the vacancy
The candidate record needs a stable id, an email address and a name that a recruiter recognises. The vacancy needs an id and a title. If the ATS creates a new candidate for every invitation, link the invitation to the existing candidate instead.
Check what happens when a candidate applies for a second vacancy. The integration should keep the person and create a separate application. Without that split, a stage change on one role can overwrite the other.
Separate invitation status from hiring stage
Recruiters need to know whether an invitation was sent, opened, started or submitted. That is not the same as the hiring stage. Keep the invitation status in the interview tool and map the hiring stage to the ATS. If the ATS has room for only one status field, choose the hiring stage and show the invitation status in the interview tool.
Use the same list of stage names on both sides. A short, agreed list is easier to test than a free-text field that changes with every team.
Share the review result, not the whole review
The ATS needs to show a reviewer that an interview was reviewed and what the outcome was. It does not always need the full transcript or the video file. Send a review status, a short summary and a link to the recording when your process allows it.
Keep the evidence in the system where reviewers work. The ATS can show that the review happened and which decision followed, while the interview tool keeps the recording and the notes.
Map the decision and its reason
A decision field should carry the outcome the team recorded: advance, hold, decline or another value your process uses. Add a separate field for the reason when reviewers need to record it. Do not put the reason in the stage field, because the stage summary becomes unreadable.
Confirm that a decision made in the interview tool does not silently reopen a closed application. The ATS should accept the decision once and keep its own audit trail.
Test the handoffs in order
Use a test candidate and move it through the process. Check the record after each handoff: candidate, application, invitation, review and decision. Note the field, the expected value and the value that arrived. Fix one handoff at a time.
Write the field map in a shared document with the owner, the direction and an example value. Review it when the hiring process changes. A short map that the team understands is easier to maintain than a long map that nobody opens.