idempotency_conflict

HTTP 409. The idempotency key was used for a different request, or that request is still running.

What it means

Flow keeps each Idempotency-Key for 24 hours with the request it came with. A repeat of the same request returns the first answer; anything else with that key is refused.

Why it happens

channel_code names the case:

`channel_code`	Why	What to do
`body_mismatch`	The key was already used for a different request: another method, path or body (often a constant key instead of a fresh UUID).	Make a new key for the new request. Retrying with this key never works.
`in_progress`	A repeat arrived while the first request with this key was still running.	Wait `retry_after` seconds, then repeat the identical request with the same key and body to get its answer.
`secret_not_kept`	The first request went through and its answer carried a secret that is shown only once (`POST /v1/webhook_endpoints`, `POST /v1/webhook_endpoints/{id}/rotate_secret`). Flow does not keep that answer.	Find the endpoint with `GET /v1/webhook_endpoints`; if you lost its secret, rotate it again with a new key.

How to fix it

Make a new key per logical request (a UUID or ULID) and reuse it only to retry that exact request. Switch on channel_code: only in_progress is worth retrying with the same key.

const key = crypto.randomUUID(); // one per message you mean to send
await fetch("https://api.flow.engineer/v1/conversations/" + conv + "/messages", {
  method: "POST",
  headers: { Authorization: `Bearer ${process.env.FLOW_MESSAGING_KEY}`, "Content-Type": "application/json", "Idempotency-Key": key },
  body: JSON.stringify({ content: { type: "text", text: "Hi" } }),
});
// On 409: error.channel_code === "in_progress" -> wait error.retry_after seconds and
// repeat with the same key; "body_mismatch" -> a bug: this key was used for another request.

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.