fix: re-trace system prompt on agent switch

[VJ-yadav]

Summary

• The step === 1 guard in prompt.ts only traced the first agent's system prompt because step increments continuously and is never reset on agent switches
• Added a tracedAgent variable to track the last-traced agent name, so each agent's system prompt is logged when it first becomes active
• Fixed the inaccurate comment that claimed agent switches reset step to 0

Fixes #291

Test Plan

• Verify typecheck passes on prompt.ts (pre-existing test file failures are unrelated)
• Start a session, switch agents mid-conversation, confirm both agents' system prompts appear in trace output
• Confirm single-agent sessions still trace the system prompt exactly once

Checklist

• Change is minimal and focused on the reported issue
• No functional behavior change — tracing/observability only
• Comment updated to accurately describe the logic

Summary by CodeRabbit

• Refactor
  • Improved session tracing accuracy to emit tracing spans when a new agent begins execution or on initial step, reducing unnecessary trace emissions and providing clearer agent transition visibility in session logs.

[claude]

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[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Repository UI

Review profile: CHILL

Plan: Pro

Run ID: 24008581-d1e1-4ab0-b105-5deff75be41c

📥 Commits

Reviewing files that changed from the base of the PR and between 33c331b and 89b8cbc.

📒 Files selected for processing (1)

• packages/opencode/src/session/prompt.ts

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📝 Walkthrough

Walkthrough

Modified system prompt tracing in the session loop to track the current agent and re-trace whenever the agent changes, rather than only on the first step. This fixes an issue where subsequent agents in multi-agent sessions had their system prompts skipped during tracing.

Changes

Cohort / File(s)	Summary
Agent Tracing Logic packages/opencode/src/session/prompt.ts	Introduced tracedAgent variable to track the last-traced agent name. Changed system prompt tracing condition from step === 1 to `step === 1

Estimated code review effort

🎯 1 (Trivial) | ⏱️ ~5 minutes

Poem

🐰 Hop by hop, each rabbit's traced,
No more agents left erased!
When the builder hands the wheel,
The analyst's song we now can feel.
One variable to remember true,
Which furry friend was logged through.

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly and accurately summarizes the main change: re-tracing system prompts when agents switch mid-session.
Description check	✅ Passed	The description covers all template sections with clear explanations of the problem, solution, test plan, and confirms minimal focused changes.
Linked Issues check	✅ Passed	The PR fully addresses issue #291 by implementing the first proposed fix: tracking the last-traced agent and re-tracing when the agent changes.
Out of Scope Changes check	✅ Passed	All changes are focused on fixing the system prompt tracing bug in prompt.ts; no out-of-scope modifications detected.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.

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✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

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