Conversation not found
The message
Conversation not foundWhat it means
ChatGPT asked its servers for a chat and they answered that it doesn't exist (a 404). Most reports come from Temporary Chats, which aren't saved, failing on the second message.
What to do
Copy anything you need out of the screen first, then start a fresh chat. If a saved chat you need says this, wait a few hours before assuming it's gone, and report it to Support with the time and device.
ChatGPT shows this in place of a reply, sometimes in orange text under your message, and the message you just sent doesn't go through:
Conversation not foundSome people get it with the status code in front ("404: Conversation not found") and part of the chat's ID after. A 404 is the standard web code for "that thing doesn't exist", so ChatGPT's servers had no chat under that ID when your app asked. OpenAI's troubleshooting page for ChatGPT errors doesn't list this message as of October 1, 2026, so what follows comes from OpenAI's forum and its Temporary Chat article.
Most often: a Temporary Chat on its second message
The pattern with the most reports goes like this. You open a Temporary Chat, send one message, get an answer, and the next message fails with "Conversation not found". In September 2024 someone on OpenAI's forum spotted what was happening in the address bar. The URL started as chatgpt.com/?temporary-chat=true, then after the first reply it switched to a normal chatgpt.com/c/<id> chat link. Their reading was that the follow-up then went looking for a saved chat that didn't exist. That wave lasted about three days before it stopped on its own.
It has come back since. People posted the same failure in August 2025, January 2026 and June 2026 ("the conversation randomly 404s mid-chat in temporary chats"), and an August 2026 report from the Android app (version 1.2026.209) described a Temporary Chat that stuck in the history list as "New chat" and returned this error even after a restart. Another person found that opening two Temporary Chats at once broke the first with "conversation key not found".
OpenAI Support replied to the main thread on September 16, 2026: "We'll pass this along for review." They asked anyone still hitting it to email support@openai.com with the approximate time, the browser or app, device and OS, screenshots and the steps that seem to trigger it.
Why a lost Temporary Chat can't be reopened
This is how OpenAI says Temporary Chats work. Its Temporary Chat article says: "If you do not save a temporary chat, it will not appear in your history and cannot be reopened from history after you leave it." OpenAI may keep a copy for up to 30 days for safety purposes, but that copy isn't one you can open. So when this error hits a Temporary Chat, copy out anything you want before you refresh. If the chat matters, the same article says you can save it as a regular chat, which puts it in your history.
When a saved chat says it
Rarer, and more worrying when it happens. During the GPT-5 launch in August 2025 one user got "Conversation not found" while a reply was still thinking, and after a reload the chat had vanished from the sidebar. Others in that thread had chats reappear six or seven hours later. In August 2026 someone reported a chat lost in a move to a ChatGPT Business workspace that still returned "Conversation not found" months on, with a Support case open.
For a saved chat, check status.openai.com, give it a few hours, and search your chat history for a phrase from it. If it's still missing, open a Support ticket with the chat's URL if you have it.
One look-alike to keep apart: "Message in conversation not found" was a different error, mostly from custom GPTs in November 2023.
Other lines the same feature prints
Match yours against these if the one at the top of the page is not quite it. They come from the same code and mean related things.
Conversation Not Found404: Conversation not foundconversation key not found