Add (build) indicator next to model name for build mode comments

[julianbenegas]

• Check if comment contains build mode metadata in AgentUIMessage
• Display subtle '(build)' text next to LLM author name
• Uses muted foreground color and normal font weight for subtlety

[vercel]

The latest updates on your projects. Learn more about Vercel for GitHub.

Project	Deployment	Review	Updated (UTC)
forums	Ready	Preview, Comment	Jan 19, 2026 6:28am

[vercel]

Build mode indicator fails to display for subsequent messages in multi-turn conversations because mode metadata is only added to the first message of the first step

View Details

📝 Patch Details

diff --git a/agent/response-agent.ts b/agent/response-agent.ts
index 2f8d0d1..a61fc52 100644
--- a/agent/response-agent.ts
+++ b/agent/response-agent.ts
@@ -366,7 +366,7 @@ async function streamTextStep({
               ...m,
               id: nanoid(),
               metadata:
-                isFirstStep && index === 0 && mode === "build" ? { mode } : {},
+                mode === "build" ? { mode } : {},
             } satisfies AgentUIMessage
           })
         )

Analysis

Bug Explanation

In agent/response-agent.ts around line 367, the code conditionally adds mode metadata only when three conditions are met simultaneously:

• isFirstStep && index === 0 && mode === "build"

The problem manifests in two scenarios:

Scenario 1: Multiple messages in first step
When the AI generates multiple messages (e.g., text + tool use), only the first message (index === 0) gets the mode metadata. Subsequent messages in the same first step won't have it.

Scenario 2: Multi-turn conversations
When stepCount > 0 (happens in the while loop in responseAgent lines 71-86), isFirstStep = newMessages.length === 0 becomes false. This means NO messages in steps 2 and beyond receive the mode metadata, even though they were generated in build mode.

In comment-thread.tsx line 84, the code checks:

const isBuildMode =
  author.isLlm &&
  (comment.content as AgentUIMessage[]).some((m) => m.metadata?.mode === "build")

This check uses .some(), which needs at least ONE message with the mode metadata. The current logic in response-agent makes this unreliable because:

• First step, second+ messages: Won't have mode metadata
• Any messages in steps 2+: Won't have mode metadata

Result: The build mode indicator "(build)" fails to display for real-world multi-turn conversations or when the first response generates multiple messages.

The Fix

Changed line 367 in agent/response-agent.ts from:

metadata: isFirstStep && index === 0 && mode === "build" ? { mode } : {}

To:

metadata: mode === "build" ? { mode } : {}

This ensures that EVERY message generated when mode="build" receives the mode metadata, regardless of step number or message index. Now the .some() check in comment-thread.tsx will reliably find the build mode indication, guaranteeing the indicator displays correctly in all cases.

The fix is minimal, non-breaking, and doesn't require database migrations because it only changes how existing metadata is populated.