Fix reasoning chunks in approval continuations

[whoiskatrin]

Summary

Fixes #1480.

When a needsApproval tool is approved with autoContinueAfterToolResult enabled, the auto-continuation stream can resume from an assistant message that already contains a completed reasoning part.

Two related issues were happening in that path:

1. The continuation's reasoning-start could be suppressed before reaching the client, so AI SDK v6 saw reasoning-delta/reasoning-end without a preceding reasoning-start and threw:

   AI_UIMessageStreamError: Received reasoning-delta for missing reasoning part

2. After forwarding reasoning-start to fix the stream protocol, completed prior reasoning could still cause the server-side continuation builder to merge new reasoning into the existing reasoning block. That made the live stream look correct, but the final persisted/rehydrated assistant message did not keep the continuation reasoning as its own persisted reasoning part, so the reasoning block disappeared after the response finished.

This PR keeps the client stream and persisted message in sync:

• the client stream receives a valid reasoning-start → reasoning-delta → reasoning-end sequence
• interrupted/streaming reasoning continuations still resume the existing reasoning part
• continuations after an already-completed reasoning block create and persist a new reasoning part, so rehydrated messages preserve the reasoning shown during streaming

Changes

• Forward reasoning-start during approval continuations so AI SDK v6 receives a valid reasoning lifecycle.
• Only skip server-side reasoning-start application when resuming an existing state: "streaming" reasoning part.
• Preserve continuation reasoning in the final persisted assistant message when the previous reasoning part was already done.
• Extend the addToolApprovalResponse: Ensure a "reasoning-start" chunk is sent before any "reasoning-delta" chunks. #1480 regression test to assert both the streamed chunk order and the final SQLite-persisted reasoning parts.
• Add/update a patch changeset for @cloudflare/ai-chat.

Testing

cd packages/ai-chat && npm run test:workers -- src/tests/client-tools-continuation.test.ts -t "#1480"
cd packages/ai-chat && npm run test:workers -- src/tests/client-tools-continuation.test.ts
npm run check

npm run check completes successfully. It still prints existing sherif warnings for package-less workspace directories, unrelated to this change.

[changeset-bot]

🦋 Changeset detected

Latest commit: 6c20048

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

This PR includes changesets to release 1 package

Name	Type
@cloudflare/ai-chat	Patch

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[pkg-pr-new]

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agents

npm i https://pkg.pr.new/agents@1481

@cloudflare/ai-chat

npm i https://pkg.pr.new/@cloudflare/ai-chat@1481

@cloudflare/codemode

npm i https://pkg.pr.new/@cloudflare/codemode@1481

hono-agents

npm i https://pkg.pr.new/hono-agents@1481

@cloudflare/shell

npm i https://pkg.pr.new/@cloudflare/shell@1481

@cloudflare/think

npm i https://pkg.pr.new/@cloudflare/think@1481

@cloudflare/voice

npm i https://pkg.pr.new/@cloudflare/voice@1481

@cloudflare/worker-bundler

npm i https://pkg.pr.new/@cloudflare/worker-bundler@1481

commit: 6c20048

[devin-ai-integration]

✅ Devin Review: No Issues Found

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