fix(core): preserve fullStream output after startup probe

[omeraplak]

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• The commit message follows our guidelines: https://voltagent.dev/docs/community/contributing/#commit-convention

Bugs / Features

• Related issue(s) linked
• Tests for the changes have been added
• Docs have been added / updated
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What is the current behavior?

For some ReadableStream-based providers, stream startup probing could consume/cancel the original fullStream.

In practice this could result in reasoning events being emitted but final answer text-delta chunks not reaching consumers before finish.

What is the new behavior?

Startup probing now uses a tee'd probe branch and preserves a passthrough stream for downstream consumption.

Agent.streamText() and Agent.streamObject() both now continue forwarding full stream output correctly after probing.

A regression test was added in packages/core/src/agent/agent.spec.ts to verify reasoning-delta, text-delta, and finish are all present after probe.

fixes (issue)

N/A

Notes for reviewers

• Changeset: .changeset/green-lizards-wave.md
• Validations run:
  • pnpm --filter @voltagent/core test:single src/agent/agent.spec.ts -t "Stream Text"
  • pnpm --filter @voltagent/core typecheck

---

Summary by cubic

Fixes stream startup probing to preserve fullStream on ReadableStream-based providers. Prevents early interruption so consumers receive reasoning, text, and finish events.

• Bug Fixes
  • Probe uses ReadableStream.tee to read from a probe branch while passing through the main stream.
  • Added regression test to ensure reasoning-delta, text-delta, and finish are emitted after probing.

^{Written for commit e5cf231. Summary will update on new commits.}

[changeset-bot]

🦋 Changeset detected

Latest commit: e5cf231

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
@voltagent/core	Patch

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[cubic-dev-ai]

No issues found across 3 files

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