fix: send reasoning content as reasoning field in assistant message history

[mchenco]

Summary

• Fixes a bug in convert-to-workersai-chat-messages.ts where reasoning parts in assistant messages were concatenated into the content string instead of being sent as a separate field
• When reasoning is smeared into content, models like @cf/moonshotai/kimi-k2.5 and @cf/zai-org/glm-4.7-flash receive their own internal reasoning as if it were spoken text, corrupting the conversation history and causing empty text responses or leaked special tokens on subsequent turns
• Reasoning parts are now accumulated separately and sent as the reasoning field on the assistant message object — the field name Chat Completions spec expects on input for reasoning models

Root cause

In multi-turn agentic conversations (e.g. using generateText with tools), the ai-sdk stores reasoning content as reasoning parts on assistant messages. When building the message history for the next turn, the provider was doing:

case "reasoning": {
  // Reasoning is passed through to text for the message conversion,
  // since Workers AI doesn't have a separate reasoning field in messages
  text += part.text;  // BUG: smears reasoning into content string
}

This sends hundreds of tokens of internal model reasoning back to the model as if it were the assistant's spoken reply, which confuses the chat template and causes the model to send back content in reasoning.

Fix

case "reasoning": {
  reasoning += part.text;  // accumulate separately
}

// then when building the message:
messages.push({
  content: text,
  role: "assistant",
  ...(reasoning ? { reasoning } : {}),  // sidecar field Chat Completions spec expects
  tool_calls: ...,
});

The reasoning field name is what Chat Completions spec CustomChatCompletionMessageParam defines for input — reasoning_content exists on ConversationMessage but is deprecated in favour of reasoning.

Testing

Updated existing unit tests and added 4 new cases in convert-to-workersai-chat-messages.test.ts covering:

• reasoning sent as reasoning field, not concatenated into content
• reasoning field omitted when absent
• multiple reasoning parts accumulated into one field
• tool calls preserved alongside reasoning in multi-turn conversations

Full suite: 213 tests passing.

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🦋 Changeset detected

Latest commit: b12704b

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

This PR includes changesets to release 1 package

Name	Type
workers-ai-provider	Patch

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