Trigger a CircleCI pipeline via API, choosing the correct endpoint for your VCS integration

Documented steps

1. Identify your project's VCS integration type first - CircleCI documents two different trigger endpoints depending on it.
2. Legacy/standard endpoint (GitHub OAuth, Bitbucket Cloud, CircleCI Server): POST https://circleci.com/api/v2/project/{project-slug}/pipeline, headers Circle-Token + Content-Type: application/json, body {"branch": "<branch-name>"} OR {"tag": "<tag-name>"}, plus optional "parameters": {...} (limits: 100 max entries, 128-char max key, 512-char max value).
3. Newer endpoint (GitHub App and Bitbucket Data Center integrations; GitHub OAuth/Bitbucket Cloud can also use it): POST https://circleci.com/api/v2/project/{provider}/{organization}/{project}/pipeline/run, where provider is one of github, gh, bitbucket, bb, or circleci.
4. Body for the pipeline/run endpoint requires "definition_id" (uuid of the pipeline definition to run), plus "config": {"branch"|"tag": ...}, "checkout": {"branch"|"tag": ...}, and optional "parameters" (same size limits as above).
5. GitLab projects: per the docs, triggering a pipeline via the CircleCI v2 API is not currently supported for GitLab projects - trigger via git push or another supported mechanism instead.
6. CircleCI Server (self-hosted): use the legacy path form https://<your-hostname>/api/v2/project/<VCS>/<org>/<repo>/pipeline where VCS is gh or bb.
7. A successful trigger returns 201 with the new pipeline object (id, number, state); poll GET /pipeline/{pipeline-id} afterward to track it.
8. Reference: https://circleci.com/docs/guides/orchestrate/triggers-overview/