fix(telegram): prevent message truncation when mentioning think tags

When users discuss <think> tags in prose (e.g., "Use the <think> tag to
enable thinking"), the text was being truncated because the unclosed tag
caused all trailing content to be dropped.

This fix adds a heuristic to differentiate between:
1. Model thinking output (tag at start) - strip as before
2. Prose mentioning tags (content before tag) - preserve the text

Fixes #3952

Co-Authored-By: Claude <noreply@anthropic.com>

src/shared/text/reasoning-tags.test.ts

@@ -0,0 +1,147 @@
+import { describe, expect, it } from "vitest";
+
+import { stripReasoningTagsFromText } from "./reasoning-tags.js";
+
+describe("stripReasoningTagsFromText", () => {
+  describe("basic functionality", () => {
+    it("returns empty string as-is", () => {
+      expect(stripReasoningTagsFromText("")).toBe("");
+    });
+
+    it("returns text without think tags as-is", () => {
+      expect(stripReasoningTagsFromText("Hello world")).toBe("Hello world");
+    });
+
+    it("strips complete <think>...</think> blocks", () => {
+      const input = "Before <think>reasoning content</think> After";
+      expect(stripReasoningTagsFromText(input)).toBe("Before  After");
+    });
+
+    it("strips complete <thinking>...</thinking> blocks", () => {
+      const input = "Before <thinking>reasoning content</thinking> After";
+      expect(stripReasoningTagsFromText(input)).toBe("Before  After");
+    });
+
+    it("strips multiple think blocks", () => {
+      const input = "A <think>x</think> B <think>y</think> C";
+      expect(stripReasoningTagsFromText(input)).toBe("A  B  C");
+    });
+
+    it("strips nested content (flat parse)", () => {
+      const input = "Start <think>outer <think>inner</think> more</think> End";
+      expect(stripReasoningTagsFromText(input)).toBe("Start  more End");
+    });
+  });
+
+  describe("unclosed tag handling - issue #3952", () => {
+    it("preserves content when <think> appears in prose (not at start)", () => {
+      const input = "Use the <think> tag to enable thinking. Then do X.";
+      // Content before <think> tag - treat as prose mentioning the tag
+      expect(stripReasoningTagsFromText(input)).toBe(
+        "Use the <think> tag to enable thinking. Then do X.",
+      );
+    });
+
+    it("preserves content when <thinking> appears in prose", () => {
+      const input = "The <thinking> mode is useful. Try it!";
+      expect(stripReasoningTagsFromText(input)).toBe("The <thinking> mode is useful. Try it!");
+    });
+
+    it("preserves code block containing <think> tag", () => {
+      const input = "Example:\n```\n<think>\nprint('hello')\n```\nMore text";
+      expect(stripReasoningTagsFromText(input)).toBe(
+        "Example:\n```\n<think>\nprint('hello')\n```\nMore text",
+      );
+    });
+
+    it("strips paired tags even when discussing them", () => {
+      const input = "You can use <think> and </think> to wrap reasoning.";
+      // Tags are properly paired, so content between is stripped
+      expect(stripReasoningTagsFromText(input)).toBe("You can use  to wrap reasoning.");
+    });
+
+    it("preserves multiple unclosed tags when in prose", () => {
+      const input = "First <think> then <thinking> more text";
+      expect(stripReasoningTagsFromText(input)).toBe("First <think> then <thinking> more text");
+    });
+
+    it("strips unclosed tag at start (model thinking output)", () => {
+      // When <think> is at the very start, treat as model thinking that was cut off
+      const input = "<think>Pensando sobre el problema...";
+      expect(stripReasoningTagsFromText(input)).toBe("");
+    });
+
+    it("strips unclosed tag at start with whitespace", () => {
+      const input = "  <think>Some thinking content...";
+      expect(stripReasoningTagsFromText(input)).toBe("");
+    });
+  });
+
+  describe("preserve mode", () => {
+    it("strips thinking tags and preserves trailing content in preserve mode", () => {
+      // In preserve mode with unclosed tags, it still strips but preserves trailing
+      const input = "Before <think>content without close";
+      const result = stripReasoningTagsFromText(input, { mode: "preserve" });
+      // preserve mode strips the tag but keeps trailing content
+      expect(result).toBe("Before content without close");
+    });
+
+    it("strips properly paired tags in preserve mode", () => {
+      const input = "Before <think>hidden</think> After";
+      const result = stripReasoningTagsFromText(input, { mode: "preserve" });
+      expect(result).toBe("Before  After");
+    });
+  });
+
+  describe("trim options", () => {
+    it("trims both ends by default", () => {
+      const input = "  <think>x</think>  result  ";
+      expect(stripReasoningTagsFromText(input)).toBe("result");
+    });
+
+    it("respects trim: none", () => {
+      const input = "  <think>x</think>  result  ";
+      expect(stripReasoningTagsFromText(input, { trim: "none" })).toBe("    result  ");
+    });
+
+    it("respects trim: start", () => {
+      const input = "  <think>x</think>  result  ";
+      // trim: start only trims from the start, not end
+      expect(stripReasoningTagsFromText(input, { trim: "start" })).toBe("result  ");
+    });
+  });
+
+  describe("final tag handling", () => {
+    it("strips <final> tags", () => {
+      const input = "Before <final>answer</final> After";
+      expect(stripReasoningTagsFromText(input)).toBe("Before answer After");
+    });
+
+    it("strips only the tags, not content inside <final>", () => {
+      const input = "<final>The answer is 42</final>";
+      expect(stripReasoningTagsFromText(input)).toBe("The answer is 42");
+    });
+  });
+
+  describe("edge cases", () => {
+    it("handles whitespace in tags", () => {
+      const input = "Before < think >content< / think > After";
+      expect(stripReasoningTagsFromText(input)).toBe("Before  After");
+    });
+
+    it("handles case variations", () => {
+      const input = "Before <THINK>content</THINK> After";
+      expect(stripReasoningTagsFromText(input)).toBe("Before  After");
+    });
+
+    it("handles <thought> variant", () => {
+      const input = "Before <thought>content</thought> After";
+      expect(stripReasoningTagsFromText(input)).toBe("Before  After");
+    });
+
+    it("handles <antthinking> variant", () => {
+      const input = "Before <antthinking>content</antthinking> After";
+      expect(stripReasoningTagsFromText(input)).toBe("Before  After");
+    });
+  });
+});

src/shared/text/reasoning-tags.ts

@@ -32,12 +32,46 @@ export function stripReasoningTagsFromText(
     FINAL_TAG_RE.lastIndex = 0;
   }
 
+  // Collect all tag matches to check for proper pairing
   THINKING_TAG_RE.lastIndex = 0;
+  const matches = [...cleaned.matchAll(THINKING_TAG_RE)];
+
+  // Check if tags are properly paired (every open tag has a matching close)
+  let openCount = 0;
+  let firstOpenIdx: number | null = null;
+  for (const match of matches) {
+    const isClose = match[1] === "/";
+    if (isClose) {
+      if (openCount > 0) {
+        openCount--;
+      }
+    } else {
+      if (firstOpenIdx === null) {
+        firstOpenIdx = match.index ?? 0;
+      }
+      openCount++;
+    }
+  }
+
+  // If we end with unclosed tags and there's content before the first tag,
+  // treat as literal text (user prose mentioning tags). This prevents data loss
+  // when users discuss <think> tags in prose or code.
+  // But if the first open tag is at the start (after trimming), treat as model
+  // thinking output that was interrupted and strip it.
+  if (openCount > 0 && mode !== "preserve") {
+    const textBeforeFirstTag = firstOpenIdx !== null ? cleaned.slice(0, firstOpenIdx).trim() : "";
+    if (textBeforeFirstTag.length > 0) {
+      // Has content before the tag - likely prose mentioning the tag
+      return applyTrim(cleaned, trimMode);
+    }
+    // Tag at start - likely model thinking output, continue with stripping
+  }
+
   let result = "";
   let lastIndex = 0;
   let inThinking = false;
 
-  for (const match of cleaned.matchAll(THINKING_TAG_RE)) {
+  for (const match of matches) {
     const idx = match.index ?? 0;
     const isClose = match[1] === "/";