Twitter Add List Member API | Curate an X List

Add List Member adds one account to a Twitter/X List you own. It runs as your registered X session rather than a pooled account, because a List belongs to a specific account. The 200 body returns the List's member_count as proof the write landed, and a not-applied write returns the same field layout with a 422 and a machine-readable reason. One $0.0008 write, billed only when the write actually applies.

curl -X POST "https://example.com/list/add_member" \  -H "Content-Type: application/json" \  -d '{    "list_id": "1234567890123456789",    "user_id": "745273"  }'

{  "ok": true,  "action": "add_member",  "list_id": "1234567890123456789",  "user_id": "745273",  "member_count": 42,  "list": {    "id": "1234567890123456789",    "name": "Founders",    "member_count": 42  }}

{  "error": "bad_request",  "message": "Missing or malformed parameter. Fix the request before retrying."}

{  "error": "unauthorized",  "message": "The API key is missing, malformed, or revoked. Check the Authorization header."}

{  "error": "insufficient_credits",  "message": "Your balance is exhausted. Top up your credits to continue."}

{  "error": "forbidden",  "message": "The acting account is not authorized for this write action, or has no logged-in session."}

{  "error": "not_found",  "message": "The resource does not exist, for example a deleted tweet or a private account."}

{  "error": "rate_limited",  "message": "Too many requests. Back off and retry with exponential backoff."}

{  "error": "server_error",  "message": "Something failed on our side. Retry with backoff; if it persists, contact support."}

Pricing

One $0.0008 write, and you are not charged when the write does not apply.

When to use

Use it to build or curate a List programmatically, for example adding every speaker at a conference to a List as they are announced.

Related endpoints

• Remove List Member: Remove one account from a Twitter/X List you own.
• Create List: Create a new Twitter/X List owned by your session.
• List Members: Page through the members of a public Twitter/X List.

FAQ

Does this need my own X session?

Yes. A List belongs to an account, so this runs as your registered session, not a pooled one. Register with POST /customer/session first, or send auth_token and ct0 inline per call.

How do I know the write landed?

The 200 body carries member_count read back from the List object. If X returned no list object at all, member_count is null, which is itself the not-applied signal.

What happens if the account is already a member?

The write does not apply, so you get the same field layout with a 422 and a reason explaining why, and you are not billed.