Create, update, and list a Vercel project's environment variables via REST API, including sensitive vars, target environments, and team scoping

Documented steps

1. Create a bearer token: POST /v3/user/tokens with body {"name": "<token-name>", "expiresAt": <ms-optional>}; the response returns bearerToken exactly once — save it immediately.
2. For every subsequent call, send header 'Authorization: Bearer $VERCEL_TOKEN'. If the token/project belongs to a Team, add query param teamId=<team_id> or slug=<team_slug> to every request (omit for personal-scope projects).
3. List existing env vars: GET /v10/projects/{idOrName}/env?teamId=... — optionally add decrypt=true to reveal plaintext/encrypted values (does NOT work for type=sensitive, which is never readable), gitBranch=<branch> to filter preview-branch vars, or customEnvironmentId/customEnvironmentSlug for custom environments.
4. Create one or more vars: POST /v10/projects/{idOrName}/env?teamId=...&upsert=true with JSON body (single object or array) requiring key, value, type (system|encrypted|plain|sensitive), and either target (array of production|preview|development) or customEnvironmentIds. Add gitBranch (requires target=preview), comment (<=500 chars).
5. For a secret API key, set type="sensitive" and target only production and/or preview — sensitive vars are rejected for the development target.
6. Update an existing var's value/targets: PATCH /v9/projects/{idOrName}/env/{id}?teamId=... (note: this is v9, not v10 like create/list) with the changed fields; the key cannot be changed for sensitive vars.
7. Delete a var: DELETE /v9/projects/{idOrName}/env/{id}?teamId=... (single) or use the batch delete endpoint for multiple ids.
8. Handle 403 'already exists' by re-issuing the create call with upsert=true instead, and handle 429 by backing off — env var creation/update/delete/list all have distinct per-minute rate limits (creation 120/min, updates 120/min, deletion 60/min, retrieval 500/min, all scope=owner).
9. Official docs verified: https://vercel.com/docs/rest-api | https://vercel.com/docs/rest-api/authentication/create-an-auth-token | https://vercel.com/docs/environment-variables/sensitive-environment-variables | https://vercel.com/docs/limits | Reference: https://vercel.com/docs/rest-api/reference/endpoints/projects/create-one-or-more-environment-variables