ToParam() serialization broken for web_search_tool_result and web_fetch_tool_result in multi-turn

[jshiv]

Bug

Message.ToParam() produces invalid request bodies for multi-turn conversations that include server-side tool results (web_search_tool_result and web_fetch_tool_result). The first turn succeeds (the API executes the tool and returns results), but the second turn fails with a 400 when the conversation history containing those results is sent back.

SDK version

v1.45.0 (also reproduced on v1.41.0)

Errors

web_search:

messages.1.content.1.web_search_tool_result.content.list[RequestWebSearchResultBlock]: Input should be a valid array

web_fetch:

messages.1.content.2.web_fetch_tool_result.content: Field required

Reproduction

Minimal Go program that triggers it (using the agent loop pattern from the SDK README):

// Any multi-turn agent loop that:
// 1. Sends a message with web_search or web_fetch tools enabled
// 2. Gets a response with server-side tool results
// 3. Calls msg.ToParam() to append the response to conversation history
// 4. Sends the next request with that history
// will fail on step 4.

I'm hitting this via cronicle's agent loop, which does:

conversation = append(conversation, msg.ToParam())  // line 323 of agent.go
// ... dispatch tool_use blocks ...
conversation = append(conversation, anthropic.NewUserMessage(results...))
// next iteration: Create request with conversation → 400

Root cause (from reading the SDK source)

WebSearchToolResultBlockContentUnion.ToParam() (messageutil.go:254):
The response union has OfWebSearchResultBlockArray which gets mapped to OfWebSearchToolResultBlockItem on the param side. The param struct serializes via param.MarshalUnion, but the API expects the content field to contain {"list": [...]} wrapping. The current serialization appears to produce a shape the API rejects.

WebFetchToolResultBlock.ToParam():
The content field is not being set at all in the param, so the API returns content: Field required.

Workaround

None currently — any agent loop that uses web_search or web_fetch and needs more than one turn will fail. The tools work fine on a single-turn request.

Environment

• Go 1.24
• macOS (also reproduced on Debian bookworm in k8s)
• Model: claude-sonnet-4-6
• Both web_search and web_fetch affected independently

[jshiv]

Here's a self-contained reproduction script that demonstrates both bugs without needing an API key. It simulates realistic API responses, calls ToParam(), and shows the malformed JSON.

Pass -live with ANTHROPIC_API_KEY set to reproduce the 400 against the real API.

Bug 1 — web_search_tool_result.content: The content round-trips through ToParam() as a bare JSON array, but the API's request schema rejects it. The WebSearchToolResultBlockContentUnion.ToParam() at messageutil.go:254 populates OfWebSearchToolResultBlockItem correctly, but MarshalUnion marshals the slice variant directly — the resulting shape doesn't match what the request-side API validation expects.

Bug 2 — web_fetch_tool_result.content: The Content and Caller fields are simply not copied in WebFetchToolResultBlock.ToParam() at messageutil.go:300-305. Only Type and ToolUseID are set.

Output of go run main.go on v1.41.0:

=== Bug 1: web_search_tool_result.content serialization ===

ToParam() produces:
{
  "content": [
    {
      "content": [
        {
          "encrypted_content": "dGVzdA==",
          "title": "Example Article",
          "url": "https://example.com/article",
          "page_age": "2 days ago",
          "type": "web_search_result"
        }
      ],
      "tool_use_id": "toolu_search_123",
      "type": "web_search_tool_result"
    }
  ],
  "role": "assistant"
}

=== Bug 2: web_fetch_tool_result.content missing entirely ===

ToParam() produces:
{
  "content": [
    {
      "tool_use_id": "toolu_fetch_456",
      "type": "web_fetch_tool_result"
    }
  ],
  "role": "assistant"
}

BUG: content field is missing — API returns 'content: Field required'