80903: Unknown Participant

This warning appears when Proxy cannot identify the participant for the current action. Proxy Sessions route communications between Participants, and both inbound handling and MessageInteraction requests depend on a valid participant match by ParticipantSid or by the participant's identifier.

• The ParticipantSid in your request does not belong to an existing Participant in the target Session. Proxy message creation uses /Participants/{ParticipantSid}/MessageInteractions, and Participant records are scoped to a specific Session.
• The caller or sender does not match any Participant identifier in the Session. In Proxy, the identifier is the participant's phone number or channel identifier, and inbound calls or messages that are not from a Participant in an open Session are handled through the out-of-session flow.
• The Participant was deleted from the Session. Deleted Participants have a dateDeleted, and Proxy does not let you add a removed participant back to the same Session.

• Fetch or list the Session's Participants and confirm that you are using the correct KP... Participant SID for the request. Create the missing Participant before you send a message or start routing traffic through the Session.
• Verify that the caller or sender is using the same phone number or channel identifier you stored in the Participant identifier field. If the contact value has changed, create a new Participant with the correct identifier.
• If you removed a Participant, create a new Session and add fresh Participants instead of trying to reuse the deleted one. Proxy does not allow a removed participant to be added back to the same Session.
• If calls or messages can arrive before a Session exists or after it closes, configure outOfSessionCallbackUrl so you can return TwiML or auto-create session JSON for that traffic.

• Proxy API overview
• Participant resource
• Out-of-Session Callback Response Guide