Teams error 404 ConversationNotFound: ConversationNotFound - conversation missing or deleted

The conversation ID in your request path does not correspond to any live conversation. The status table documents ConversationNotFound as "Conversation wasn't found as it doesn't exist or is deleted," with retry No and the action "Check if conversation ID sent is an expected value. Remove the ID if it was cached." The sibling code ActivityNotFoundInConversation covers the case where the conversation is fine but the message inside it is gone.

For Teams bots this is overwhelmingly a stale conversation reference problem, and proactive messaging is where it bites. The recommended pattern is to create a conversation once and "store the resulting conversationId for future proactive messages" - but stored references age: chats are deleted, teams are archived or rebuilt, users leave the tenant, threads move. A reference written at install time and replayed a year later can point at nothing. The other production classic is environment bleed: conversation IDs from a test tenant replayed against production, or vice versa, since IDs are meaningless outside the tenant and cloud they were minted in.

Malformed IDs land here too. Teams conversation IDs are long, contain @thread markers and message-ID suffixes (;messageid=), and survive neither truncation in a database column nor careless URL encoding. If the send path built the ID by string concatenation, suspect the plumbing before the platform.

{
  "error": {
    "code": "ConversationNotFound",
    "message": "Conversation not found."
  }
}

Store complete conversation references (ID, serviceUrl, tenant) written only by conversation events, and treat them as expirable cache, not permanent truth. Keep environments' data strictly separated so IDs cannot cross tenants. Validate column widths for conversation IDs - they are longer than people guess. Bots that also handle installationUpdate removals (see BotNotInConversationRoster) rarely accumulate dead references in the first place.