agent: Fix issue with Anthropic thinking models (#33317)

cc @osyvokon 

We were seeing a bunch of errors in our backend when people were using
Claude models with thinking enabled.

In the logs we would see
> an error occurred while interacting with the Anthropic API:
invalid_request_error: messages.x.content.0.type: Expected `thinking` or
`redacted_thinking`, but found `text`. When `thinking` is enabled, a
final `assistant` message must start with a thinking block (preceeding
the lastmost set of `tool_use` and `tool_result` blocks). We recommend
you include thinking blocks from previous turns. To avoid this
requirement, disable `thinking`. Please consult our documentation at
https://docs.anthropic.com/en/docs/build-with-claude/extended-thinking

However, this issue did not occur frequently and was not easily
reproducible. Turns out it was triggered by us not correctly handling
[Redacted Thinking
Blocks](https://docs.anthropic.com/en/docs/build-with-claude/extended-thinking#thinking-redaction).

I could constantly reproduce this issue by including this magic string:
`ANTHROPIC_MAGIC_STRING_TRIGGER_REDACTED_THINKING_46C9A13E193C177646C7398A98432ECCCE4C1253D5E2D82641AC0E52CC2876CB
` in the request, which forces `claude-3-7-sonnet` to emit redacted
thinking blocks (confusingly the magic string does not seem to be
working for `claude-sonnet-4`). As soon as we hit a tool call Anthropic
would return an error.

Thanks to @osyvokon for pointing me in the right direction 😄!


Release Notes:

- agent: Fixed an issue where Anthropic models would sometimes return an
error when thinking was enabled

crates/language_model/src/language_model.rs

@@ -67,6 +67,9 @@ pub enum LanguageModelCompletionEvent {
         text: String,
         signature: Option<String>,
     },
+    RedactedThinking {
+        data: String,
+    },
     ToolUse(LanguageModelToolUse),
     StartMessage {
         message_id: String,
@@ -359,6 +362,7 @@ pub trait LanguageModel: Send + Sync {
                                 Ok(LanguageModelCompletionEvent::StartMessage { .. }) => None,
                                 Ok(LanguageModelCompletionEvent::Text(text)) => Some(Ok(text)),
                                 Ok(LanguageModelCompletionEvent::Thinking { .. }) => None,
+                                Ok(LanguageModelCompletionEvent::RedactedThinking { .. }) => None,
                                 Ok(LanguageModelCompletionEvent::Stop(_)) => None,
                                 Ok(LanguageModelCompletionEvent::ToolUse(_)) => None,
                                 Ok(LanguageModelCompletionEvent::UsageUpdate(token_usage)) => {

crates/language_model/src/request.rs

@@ -303,7 +303,7 @@ pub enum MessageContent {
         text: String,
         signature: Option<String>,
     },
-    RedactedThinking(Vec<u8>),
+    RedactedThinking(String),
     Image(LanguageModelImage),
     ToolUse(LanguageModelToolUse),
     ToolResult(LanguageModelToolResult),