Bug Description

In providers/deepseek.rs, the TryFrom<message::Message> for Vec<Message> implementation splits a single rig Message::Assistant (containing Reasoning + ToolCall content) into two separate DeepSeek Message::Assistant entries:

1. First: reasoning_content = Some(...), tool_calls = vec![]
2. Second: tool_calls = [...], reasoning_content = None

This causes DeepSeek API to return 400 Bad Request:

Missing reasoning_content field in the assistant message at message index ...

PR #1333 added a workaround that merges the last reasoning into the last assistant message during request construction. But this only fixes the last assistant message — in multi-turn tool loops, intermediate messages still lack reasoning_content.

Root Cause

The core issue is that the conversion layer discards the reasoning_content from the message that carries tool_calls:

// line 339-345: tool_calls message always gets reasoning_content: None
if !tool_calls.is_empty() {
    messages.push(Message::Assistant {
        content: "".to_string(),
        tool_calls,
        reasoning_content: None,  // ← always None, reasoning is lost
    });
}

Suggested Fix

The SDK should not try to cache or manage reasoning content itself. The caller (application code managing the tool loop) is responsible for passing AssistantContent::Reasoning back in chat_history. The SDK just needs to faithfully serialize it.

The fix is straightforward — put reasoning_content on the message that carries tool_calls, since that's what DeepSeek expects:

message::Message::Assistant { content, .. } => {
    let mut text_content = String::new();
    let mut reasoning_content = String::new();
    let mut tool_calls = Vec::new();

    for item in content.iter() {
        match item {
            message::AssistantContent::Text(text) => {
                text_content.push_str(text.text());
            }
            message::AssistantContent::Reasoning(reasoning) => {
                reasoning_content.push_str(&reasoning.display_text());
            }
            message::AssistantContent::ToolCall(tool_call) => {
                tool_calls.push(ToolCall::from(tool_call.clone()));
            }
            _ => {}
        }
    }

    let reasoning = if reasoning_content.is_empty() {
        None
    } else {
        Some(reasoning_content)
    };

    // Single message — reasoning_content stays with tool_calls
    Ok(vec![Message::Assistant {
        content: text_content,
        name: None,
        tool_calls,
        reasoning_content: reasoning,
    }])
}

This also makes the merge workaround in DeepseekCompletionRequest::try_from (lines 506-534) unnecessary — it can be reverted to a simple full_history.extend(chat_history).

Reproduction

1. Use deepseek-reasoner with tools enabled
2. Trigger a prompt that causes 2+ rounds of tool calls
3. Round 2+ fails with 400: Missing reasoning_content field

Environment

• rig-core: 0.31.0
• Model: deepseek-reasoner (thinking mode + tool use)