fix(agent): emit a reasoning start when a v2 delta finds no span open

[kwakayama]

Closes veryfront/veryfront-issue-inbox#408.

Bug

On the v2 stream-lifecycle path (VF_STREAM_LIFECYCLE_MODE), a reasoning_content arriving with no span open mints a span id but emits only Content:

const continueReasoning = (): string => activeReasoningMessageId ?? startReasoning();

No ReasoningMessageStart is ever emitted for that id. Per veryfront/veryfront-issue-inbox#152, the API canonicalizer and Studio stream decoder key strictly on the reasoning lifecycle, so content whose start never arrived is dropped — silently missing reasoning text.

browser-encoder.ts (v1) already synthesizes the missing start. This makes v2 match.

Fix

One case in lifecycle-browser-adapter.ts: emit the start first when a delta finds no span open. No other paths touched.

Tests

One test, opens a reasoning span visibly when a delta arrives with none open, asserting the full Start → Content → End sequence on one id. Confirmed red before the fix, green after.

src/agent/ag-ui/ 29 passed / 158 steps. deno check / fmt / lint / docs check clean.

Note

Needs a provider that emits reasoning content without a preceding start; I have not confirmed one does. This fixes a real v1/v2 divergence, not a proven live incident.

Summary by CodeRabbit

• Bug Fixes

  • Reasoning content received without an active reasoning span now starts one automatically.
  • Reasoning messages consistently emit start, content, and end events with a stable message identifier.
  • Automatically opened reasoning spans close correctly when the corresponding end event arrives.

• Tests

  • Added coverage for orphan reasoning content events and their complete lifecycle.

[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 Plus

Run ID: 558139cf-5af9-40ab-a609-07564150f6c0

📥 Commits

Reviewing files that changed from the base of the PR and between 396477b and 2f8e83f.

📒 Files selected for processing (2)

• src/agent/ag-ui/lifecycle-browser-adapter.test.ts
• src/agent/ag-ui/lifecycle-browser-adapter.ts

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📝 Walkthrough

Walkthrough

The lifecycle adapter now opens a reasoning span when content arrives without an active span. Tests verify the start, content, and end events and the stable generated message ID.

Changes

Reasoning lifecycle

Layer / File(s)	Summary
Implicit reasoning span opening src/agent/ag-ui/lifecycle-browser-adapter.ts, src/agent/ag-ui/lifecycle-browser-adapter.test.ts	The adapter emits a reasoning start before orphan reasoning content. Tests verify event ordering, message ID reuse, and the matching reasoning end event.

Estimated code review effort: 2 (Simple) | ~10 minutes

Possibly related PRs

• veryfront/veryfront-code#3438: Handles related reasoning-stream lifecycle and ordering behavior.
• veryfront/veryfront-code#3444: Updates related reasoning span lifecycle and start/end state handling.
• veryfront/veryfront-code#3471: Covers related reasoning-span handling in the same lifecycle adapter.

Suggested reviewers: kojiwakayama

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly and concisely describes the main change: emitting a reasoning start when a v2 delta has no open span.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

✨ Finishing Touches

📝 Generate docstrings

• Create stacked PR
• Commit on current branch

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch fix/ag-ui-v2-orphan-reasoning-delta

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