Revert "🐛 fix(opencode): strip thinking blocks from last assistant message"

This reverts commit 18b9492.

Author: praxstack

docs/09-temp/thinking-block-fix-design.md

@@ -702,23 +702,6 @@ export namespace MessageV2 {
       }
     }
 
-    // Always strip reasoning/thinking parts from the last assistant message.
-    // Claude API enforces that thinking blocks in the latest assistant message
-    // must be byte-identical to the original response. Since OpenCode reconstructs
-    // them from stored parts, they may not match exactly. Stripping is safe because
-    // Claude doesn't need its own thinking blocks to continue the conversation.
-    {
-      const lastAssistantIdx = result.findLastIndex((msg) => msg.role === "assistant")
-      if (lastAssistantIdx !== -1) {
-        const filtered = result[lastAssistantIdx].parts.filter((part) => part.type !== "reasoning")
-        if (filtered.length > 0 && !filtered.every((p) => p.type === "step-start")) {
-          result[lastAssistantIdx].parts = filtered
-        } else {
-          result.splice(lastAssistantIdx, 1)
-        }
-      }
-    }
-
     const tools = Object.fromEntries(Array.from(toolNames).map((toolName) => [toolName, { toModelOutput }]))
 
     return convertToModelMessages(