GitHub Actions OIDC to AWS failing — could not assume role

The problem: GitHub Actions workflow was failing with Could not assume role with ARN arn:aws:iam::123456789:role/my-role. Error: Not authorized to perform sts:AssumeRoleWithWebIdentity.

Everything looked correct — OIDC provider was registered, IAM role existed. But it kept failing.

The fix:

# The trust policy had the wrong subject format
# WRONG:
"StringEquals": {
  "token.actions.githubusercontent.com:sub": "repo:my-org/my-repo"
}
 
# CORRECT (must include ref):
"StringEquals": {
  "token.actions.githubusercontent.com:sub": "repo:my-org/my-repo:ref:refs/heads/main"
}
 
# Or use StringLike to allow any branch/tag:
"StringLike": {
  "token.actions.githubusercontent.com:sub": "repo:my-org/my-repo:*"
}

# workflow fix — permissions block is mandatory:
permissions:
  id-token: write   # THIS IS REQUIRED — missing this = silent failure
  contents: read

Why it happens: Two issues can cause this independently. First: the IAM trust policy sub condition must exactly match the OIDC token subject, which includes the branch ref. Second: the workflow permissions.id-token: write must be explicitly set — without it, GitHub doesn't issue the OIDC token at all, but the error message still says "assume role failed."