new_contact_limit - Flow

HTTP 429. The sender has used its budget for starting conversations.

What it means

Each sender may start a limited number of new conversations per hour and per day; new lines start low and warm up over days. Replies into existing conversations are not limited this way.

Why it happens

Many POST /v0/messages starts in a short time.
A new sender that is still warming up.

How to fix it

Wait retry_after seconds before starting more conversations. Spread starts out, and prefer replying to people who wrote first.

if (err.type === "new_contact_limit") await sleep(err.retry_after * 1000);

Every error also carries hint, one sentence specific to your request, and doc_url, this page. Coding agents connected to Flow’s MCP server (https://api.flow.engineer/mcp) can call explain_error with the type to read this page.