Pre-Arrival Guest Welcome challenge keeps failing — "can't find the merge field {!$Input:Contact.FirstName}" despite correct setup

Hello 

I'm stuck on the "Create a Prompt Template That Delivers a Warm, Personalized Pre-Arrival Message" challenge (Service Agent Customization with Prompt Builder). The check keeps failing with: 

I've verified the following against the challenge steps:

• Template Type: Flex
• Template Name: Pre-Arrival Guest Welcome / API Name: Pre_Arrival_Guest_Welcome
• Input source: Name Contact, API Name Contact, Source Type Object, Object Contact
• Both [Contact First Name] placeholders (opening line and the "Hi" greeting) replaced via Insert Resource → Contact → FirstName — both render as clean Input.Contact.FirstName chips
• [Check-in Date] replaced via Insert Resource → Contact → Next Check-in Date
• Model: OpenAI GPT 4 Omni Mini
• Template is Activated 

What I've already tried:

• Deleted and rebuilt the template from scratch
• Re-inserted all merge fields fresh via the resource picker (not hand-typed)
• Confirmed no leftover placeholder text or dangling @Contact. fragments remain
• Confirmed I'm using the correct connected Dev Edition org

The prompt visibly matches the spec but the grader still can't find the FirstName merge field. Has anyone hit this and found a fix? Is there a known caching/grader issue with this challenge, or something about the Contact input config I might be missing?

Org type: Developer Edition (Prompt Builder + Agentforce special org) 

#Trailhead Challenges  #Agentforce