We are looking for a highly experienced HubSpot Marketplace App Developer to lead the end-to-end development, certification, and publishing of an official HubSpot App. This is a full-scale Marketplace app build, not a basic integration. Only apply if you have proven experience shipping public HubSpot Marketplace apps — especially involving Custom Channels API and Workflow Extensions. Project Overview The goal is to design, develop, certify, and publish a public HubSpot Marketplace app that integrates a third-party messaging platform with HubSpot. The app must support two integration levels: Case 1: General Integration (Starter & Pro Users) CRM Card (right sidebar) integration on Contact, Deal, Company, and Ticket records Embedded iFrame chat interface per associated contact Conversation-based display logic (phone number matching) Real-time activity logging (inbound/outbound messages to timeline) Two-way contact sync (name, phone, opt-in status) Duplicate handling with one primary contact per phone number Custom Workflow Action: Send message template Dynamic channel selection (real-time) Dynamic template selection (real-time) Placeholder handling Native HubSpot reporting support: Number of conversations Templates used Opt-ins collected Unread conversations (per channel) Message volume (inbound, outbound, AI agent) Suggest the creation of new conversation if none exists Case 2: Enterprise Integration (Service/Sales Hub Enterprise Only) Full Custom Channels API integration Native Inbox messaging support Agents reply from HubSpot Inbox (messages sent via external API) 24-hour session handling logic: Within 24h: free text allowed Outside 24h: template selection required UI must reflect the session status correctly Critical Requirements Must pass official HubSpot Marketplace certification App must be publicly listed Complete handover of source code and technical documentation UX/UI must align seamlessly with HubSpot design standards Robust error handling (including API outage handling) All necessary technical steps required for certification are included in the scope Required Expertise You MUST have: Proven experience publishing HubSpot Marketplace Apps Deep knowledge of: HubSpot Public Apps (OAuth, scopes, webhooks) CRM Extensions (CRM Cards, iFrames) Timeline Events API Workflow Extensions API Custom Channels API (required) Conversations API Experience handling Marketplace security review & certification process Strong backend architecture experience (Node.js / TypeScript preferred) Experience with scalable webhook and sync systems This is senior-level work. Not suitable for junior or mid-level developers. Deliverables Fully functional HubSpot Marketplace app Successful certification & approval Public listing Source code ownership transferred Technical setup & maintenance documentation Clean, well-commented codebase Timeline Target project start: Within 3 weeks Completion target: Aggressive timeline — please propose a realistic delivery schedule based on experience. When Applying, Please Include Links to HubSpot Marketplace apps you have built Specific experience with Custom Channels API Experience with Workflow Extensions Estimated timeline Estimated fixed-price cost Your proposed technical stack Only apply if you have direct, production-level experience with HubSpot Marketplace apps. We are looking for a senior, reliable partner who understands both the technical and certification complexity involved.