Fix thinking tag leakage by converting unsigned blocks to plain text

Closes #302

packages/ai/CHANGELOG.md

@@ -1,5 +1,11 @@
 # Changelog
 
+## [Unreleased]
+
+### Fixed
+
+- **Thinking tag leakage**: Fixed Claude mimicking literal `</thinking>` tags in responses. Unsigned thinking blocks (from aborted streams) are now converted to plain text without `<thinking>` tags. The TUI still displays them as thinking blocks. ([#302](https://github.com/badlogic/pi-mono/pull/302) by [@nicobailon](https://github.com/nicobailon))
+
 ## [0.25.1] - 2025-12-21
 
 ### Added

packages/ai/src/providers/anthropic.ts

@@ -463,11 +463,12 @@ function convertMessages(messages: Message[], model: Model<"anthropic-messages">
 				} else if (block.type === "thinking") {
 					if (block.thinking.trim().length === 0) continue;
 					// If thinking signature is missing/empty (e.g., from aborted stream),
-					// convert to text block to avoid API rejection
+					// convert to plain text block without <thinking> tags to avoid API rejection
+					// and prevent Claude from mimicking the tags in responses
 					if (!block.thinkingSignature || block.thinkingSignature.trim().length === 0) {
 						blocks.push({
 							type: "text",
-							text: sanitizeSurrogates(`<thinking>\n${block.thinking}\n</thinking>`),
+							text: sanitizeSurrogates(block.thinking),
 						});
 					} else {
 						blocks.push({