fix(anthropic): validate thinking block requirement and prevent silent reasoning drop

[giulio-leone]

Summary

Three targeted fixes for Anthropic extended thinking + multi-step tool use (#7729).

Problem

When using Anthropic models with extended thinking enabled and multi-step tool use, several failure modes exist:

1. Cryptic API errors: If the last assistant message doesn't start with a thinking block (Anthropic API requirement), the API returns a vague error. This commonly happens when messages round-trip through the UI layer without preserving providerOptions.

2. Silent reasoning drop: When reasoning parts lack providerOptions/signature metadata, the converter silently drops them with a generic warning — leaving developers confused about why their multi-step calls fail.

3. Ghost reasoning parts: When sendReasoning: false is set, reasoning-start and reasoning-end events were still emitted to the UI stream (only reasoning-delta was gated), creating empty ReasoningUIPart objects on the client.

Root Cause Analysis

Through exhaustive tracing of the entire reasoning/thinking block lifecycle:

• Streaming path: signature_delta → reasoning-delta (with providerMetadata.anthropic.signature) → accumulated in stream-text.ts → toResponseMessages() → converter
• UI round-trip: toUIMessageStream → process-ui-message-stream → convertToModelMessages → converter

The standard multi-step flow actually preserves signatures correctly through all layers. The issues are situational:

• Message storage/retrieval losing providerOptions metadata
• prepareStep slicing modifying message order
• sendReasoning: false creating malformed UI parts

Fixes

1. Pre-flight validation (anthropic-messages-language-model.ts): After prompt conversion, if thinking is enabled, validates that the last assistant message starts with a thinking or redacted_thinking block. Throws a clear, actionable error instead of a cryptic Anthropic API error.

2. Improved drop warnings (convert-to-anthropic-messages-prompt.ts): Warning messages now explain the Anthropic API requirement and suggest preserving providerOptions through the message round-trip.

3. Ghost reasoning prevention (stream-text.ts): All three reasoning events (reasoning-start, reasoning-delta, reasoning-end) are now consistently gated behind sendReasoning, preventing empty reasoning parts on the client.

Tests

• 3 new tests for thinking block validation (enabled + text first → throw, enabled + thinking first → pass, disabled + text first → pass)
• 1 new test for sendReasoning: false ghost reasoning prevention
• Updated snapshot for improved warning message
• 2 consecutive clean passes: 1980 AI tests + 314 Anthropic tests = 2294 total, 0 failures, 0 type errors

Related

Fixes #7729 (14 👍, reported by multiple users including framework contributors)

[giulio-leone]

Intervention note for this PR:

Current blocker appears to be Vercel deployment authorization, not code/test correctness.

Observed pattern:

• Vercel check fails with: "Authorization required to deploy."
• Other checks (e.g., Socket Security / Vercel Agent Review) are passing.

Recommended unblock sequence:

1. Complete the Vercel Git authorization flow from the failing check link.
2. Re-run checks on this PR.
3. If Vercel passes, this PR should move forward with normal review.

If useful, I can follow up with a PR-by-PR status sweep after authorization is completed.

[giulio-leone]

This PR is ready for review — all CI checks pass, no merge conflicts, and all review threads have been resolved. Ready to merge when approved. 🚀