## TL;DR
GitHub only lets you request reviews from people who can see the repo. The person you picked has no access, so add them as a collaborator (or to a team with access) and request the review again. There is no way to request a review from a true outsider.

## The error
```text
failed to request review: reviewer is not a collaborator: how to fix
```

## Steps

### 1. Confirm the person lacks access
Check the repo's collaborator list or team membership for the username.
Expected output: the user is absent, which confirms the diagnosis.

### 2. Invite them as a collaborator
In repo Settings, Collaborators, send an invite. For org repos, add them to a team with at least read access instead.
Expected output: the invite is sent.

### 3. Wait for the invite to be accepted
The user must accept before GitHub treats them as a collaborator.
Expected output: the user appears in the collaborator list.

### 4. Request the review again
```bash
gh pr edit 123 --add-reviewer USERNAME
```
Replace 123 with the PR number and USERNAME with their handle.
Expected output: the review request posts without the collaborator error.

## When to use
- The exact "not a collaborator" error
- Requesting review from a new contractor or external contributor
- Org repos where team membership drives access

## When not to use
- Review requests failing silently (different cause)
- Requesting reviews from yourself (blocked by a different rule)
- Team review requests (teams need write access, covered in edge cases)

## Tool compatibility
- GitHub review requests (web UI and API)
- gh CLI (pr edit with the add-reviewer flag)
- GitHub org teams

## Variant phrasings
### cannot request review from outside collaborator
Same fix. They need repo access first.

### add reviewer who is not a collaborator
Invite first, request second. The order matters.

### review request failed user not found
If the username is also wrong, fix the handle. But the collaborator error specifically means access, not identity.

## Why it happens
Review requests grant the reviewer read access to the PR diff. GitHub refuses to hand diff access to someone with no relationship to the repo, so the request API validates collaborator status first.

## Edge cases
- Outside collaborators on org repos: they need explicit repo access, not just org membership.
- Team review requests require the team to have write access. Read-only teams cannot be requested.
- Pending invites do not count. The user must accept first.
- For public feedback without access, share the PR link. Anyone can comment on a public PR without being a requested reviewer.

## Provenance

Resolved from the public thread: https://vectle.com/posts/pst_clitKU3Ip8RRq_2Zxb5hUg