ToParam() Method Doesn't Handle WebSearchToolResultBlock

[katiabazzi]

When using the web search tool (web_search_20250305), the Go SDK's Message.ToParam() method fails to properly convert WebSearchToolResultBlock and ServerToolUseBlock content, resulting in empty content blocks that cause API errors in multi-turn conversations.

Steps to reproduce:

1. Enable web search tool in API request
2. Send a message that triggers web search (e.g., "What's the latest news about AI?")
3. Store the assistant's response using message.ToParam()
4. Send a follow-up message in the same conversation
5. API returns error: "messages.1: all messages must have non-empty content except for the optional final assistant message"

Root Cause
The SDK's ToParam() method (in message.go:664-680) only handles:

• TextBlock
• ToolUseBlock
• ThinkingBlock
• RedactedThinkingBlock

It does NOT handle:

• ServerToolUseBlock (used for web search invocation)
• WebSearchToolResultBlock (contains search results)

When encountering these unhandled types, it returns an empty ContentBlockParamUnion{}.

Impact
Applications cannot use web search in multi-turn conversations without implementing custom message conversion logic to filter out or properly handle these blocks.

Suggested Fix
Update the SDK's ToParam() method to properly handle ServerToolUseBlock and WebSearchToolResultBlock

[katiabazzi]

Additional Issues Found with Web Search Integration

After further investigation, I've discovered several more critical issues beyond the empty text blocks:

1. WebSearchToolResultBlock Content is Always Empty

When using web search, the WebSearchToolResultBlock.Content.OfWebSearchResultBlockArray is always empty despite the API returning valid search results.

To reproduce:

• Enable web search in a streaming message request
• Accumulate the response using Message.Accumulate()
• Check the resulting ContentBlockUnion.OfWebSearchToolResult.Content.OfWebSearchResultBlockArray - it will be empty []
• The raw JSON from the API contains the search results, but they're not being deserialized

2. Citations are Stored as Null

Citations from web search results are always null when stored in the conversation, even though the API is sending valid citation data.

To reproduce:

• Make a web search query that returns citations
• Store the accumulated message
• Retrieve and check TextBlock.Citations - they will be [null, null] instead of containing the citation data
• The streaming events show CitationsDelta events with valid data (URLs, titles, etc.) but this data is lost

3. Citations Missing from MessageParam Conversion

When converting a Message with web search citations to MessageParam using ToParam(), citations of type CitationsWebSearchResultLocation and
CitationsSearchResultLocation are silently dropped.

To reproduce:

• Create a Message with TextBlock containing web search citations
• Call message.ToParam()
• The resulting MessageParam will have empty/null citations for any web search results

4. Empty Text Blocks During Web Search (Original Issue)

As originally reported, when using web search, the API sends empty text blocks in the streaming response that it then rejects when sent back in conversation history.

To reproduce:

• Make a web search query
• Store the response
• Send a follow-up message with the conversation history
• Receive error: "text content blocks must be non-empty"