Document PUT /conversations/{conversation_id}/conversation_parts/{id}

[salmanbshah]

Why?

PUT /conversations/{conversation_id}/conversation_parts/{id} was implemented in ConversationPartsController but completely absent from all specs. Customers building integrations that need to update conversation part state (e.g. marking parts as seen, updating send state for external replies) had no documented API surface to work with.

How?

Added PUT /conversations/{conversation_id}/conversation_parts/{id} to the Unstable spec under the Conversations tag. The endpoint supports two operations via the same path: updating the send state of an external reply (send_state + info_message) and marking a part as seen by an admin (seen + admin_id). Includes a new update_conversation_part_request schema with 200/401/403/404 responses.

Decisions

• Unstable only: This endpoint uses experimental HMAC auth fields not yet stabilised across API versions.
• Single endpoint, two operations: The controller uses a single update action for both send state and seen state updates — documented as one endpoint with a flexible request body rather than two separate paths, matching the backend design.

Implementation Plan

API Documentation Gap Plan

Confirmed Gaps (this PR covers gap #3)

#	Controller	Missing Endpoints	Spec Target
1	api/v3/data_connectors_controller.rb	GET /data_connectors/{id}	Unstable only
2	api/v3/fin_voice_controller.rb	GET /fin_voice/conversation/{conversation_id}	Unstable + v2.15
3	api/v3/conversation_parts_controller.rb	PUT /conversations/{conversation_id}/conversation_parts/{id}	Unstable only
4	api/v3/messages_controller.rb	GET /messages/whatsapp/status	Unstable only

Skipped (feature-flagged, not yet GA)

• fin/tasks_controller.rb — ai_answers_billing_feature
• fin/api_controller.rb — fin-agent-api
• procedures_controller.rb — ProcedureRuns::Constants::FEATURE_FLAG

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