🤖 Skip [CONTINUE] sentinel when user message follows

[ammar-agent]

Problem

Currently we always add the [CONTINUE] sentinel after partial messages:

[user1] → [partial assistant] → [CONTINUE] → [user2]

But when a user message already follows the partial, the sentinel is redundant - the user message itself provides the continuation signal.

Solution

Conditionally skip the sentinel when unnecessary:

// Before: Always add sentinel
[user] → [partial] → [CONTINUE] → [user] → ...

// After: Only add when needed
[user] → [partial] → [user] → ...              // Skip - user follows
[user] → [partial] → [CONTINUE]                // Keep - last message
[user] → [partial] → [CONTINUE] → [assistant]  // Keep - assistant follows

Implementation:

if (msg.role === "assistant" && msg.metadata?.partial) {
  const nextMsg = messages[i + 1];
  
  // Only add sentinel if NO user message follows
  if (!nextMsg || nextMsg.role !== "user") {
    result.push(createSentinel());
  }
}

Benefits

• ✅ Saves tokens - One fewer message per resume-with-user-message (common case!)
• ✅ More natural - Models handle "user interrupts then continues" well
• ✅ Cleaner history - No redundant synthetic messages
• ✅ Stateless logic - Just looks at message array structure

When Sentinel Is Added

• Partial is last message (empty resume)
• Partial followed by assistant message (rare but possible)

When Sentinel Is Skipped

• Partial followed by user message (most common!)
  • User interrupts stream and adds follow-up
  • This is the typical resume pattern

Testing

Updated tests:

• ✅ Multi-partial test now expects 5 messages (not 6)
• ✅ New test specifically for skip-sentinel behavior
• ✅ All 298 unit tests pass

Test coverage:

1. Sentinel added when partial is last message ✅
2. Sentinel skipped when user follows partial ✅
3. No synthetic messages when sentinel skipped ✅

Risk Assessment

Very low risk:

• Only affects case where sentinel was already redundant
• Models naturally understand "user continues conversation"
• Existing behavior unchanged for empty resumes
• Stateless transform, no history dependencies

Related

Builds on #170 which changed sentinel text from [INTERRUPTED] to [CONTINUE].

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