---
"@context": https://schema.org
"@type": TechArticle
"@id": https://www.twilio.com/docs/api/errors/80903#article
headline: "80903: Unknown Participant"
url: https://www.twilio.com/docs/api/errors/80903
inLanguage: en
dateModified: 2026-06-08T12:04:59.000Z
author:
  "@type": Organization
  name: Twilio Developer Education Team
publisher:
  "@type": Organization
  name: Twilio
---

# 80903: Unknown Participant

Log Level: WARNING

## Description

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`.

### Possible causes

* 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.

### Possible solutions

* 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.

#### Additional resources

* [Proxy API overview](/docs/proxy/api)
* [Participant resource](/docs/proxy/api/participant)
* [Out-of-Session Callback Response Guide](/docs/proxy/out-session-callback-response-guide)