diff --git a/.changeset/fix-extended-thinking-blocks.md b/.changeset/fix-extended-thinking-blocks.md
new file mode 100644
index 0000000..52aa94c
--- /dev/null
+++ b/.changeset/fix-extended-thinking-blocks.md
@@ -0,0 +1,5 @@
+---
+"@centralinc/browseragent": patch
+---
+
+Fix extended thinking 400 error by ensuring all assistant messages start with thinking blocks when thinkingBudget is enabled
diff --git a/loop.ts b/loop.ts
index d5c9240..6d6e088 100644
--- a/loop.ts
+++ b/loop.ts
@@ -17,6 +17,7 @@ import {
   injectPromptCaching,
   truncateMessageHistory,
   cleanMessageHistory,
+  ensureThinkingBlocksForExtendedThinking,
   PROMPT_CACHING_BETA_FLAG,
 } from "./utils/message-processing";
 import { makeApiToolResult } from "./utils/tool-results";
@@ -219,6 +220,10 @@ ${capabilityDocs}`,
     // Clean message history to ensure tool_use and tool_result blocks are properly paired
     cleanMessageHistory(messages);
 
+    // Ensure all assistant messages have thinking blocks when extended thinking is enabled
+    // This prevents 400 errors from the API
+    ensureThinkingBlocksForExtendedThinking(messages, !!thinkingBudget);
+
     if (onlyNMostRecentImages) {
       maybeFilterToNMostRecentImages(
         messages,
diff --git a/utils/message-processing.ts b/utils/message-processing.ts
index 7bee476..86b7c85 100644
--- a/utils/message-processing.ts
+++ b/utils/message-processing.ts
@@ -244,3 +244,50 @@ export function cleanMessageHistory(messages: BetaMessageParam[]): void {
     }
   }
 }
+
+/**
+ * Ensure all assistant messages start with thinking blocks when extended thinking is enabled
+ * This prevents the 400 error: "Expected `thinking` or `redacted_thinking`, but found `text`"
+ * 
+ * When thinking is enabled, the API requires that every assistant message must start with
+ * a thinking or redacted_thinking block. This function filters out any assistant messages
+ * that don't meet this requirement.
+ *
+ * @param messages - Array of conversation messages
+ * @param thinkingEnabled - Whether extended thinking is enabled
+ */
+export function ensureThinkingBlocksForExtendedThinking(
+  messages: BetaMessageParam[],
+  thinkingEnabled: boolean,
+): void {
+  if (!thinkingEnabled) {
+    return;
+  }
+
+  // Filter out assistant messages that don't start with a thinking block
+  // Keep user messages as they don't need thinking blocks
+  const indicesToRemove: number[] = [];
+  
+  for (let i = 0; i < messages.length; i++) {
+    const message = messages[i];
+    if (message?.role === "assistant" && Array.isArray(message.content)) {
+      const firstBlock = message.content[0];
+      const hasThinkingBlock = 
+        firstBlock &&
+        typeof firstBlock === "object" &&
+        (firstBlock.type === "thinking" || firstBlock.type === "redacted_thinking");
+      
+      if (!hasThinkingBlock) {
+        indicesToRemove.push(i);
+      }
+    }
+  }
+
+  // Remove messages in reverse order to maintain correct indices
+  for (let i = indicesToRemove.length - 1; i >= 0; i--) {
+    const index = indicesToRemove[i];
+    if (index !== undefined) {
+      messages.splice(index, 1);
+    }
+  }
+}