fix(frontend): keep inline-code <think> pairs in content and restore copy for reasoning-only turns (#4647)

Two related fixes in the reasoning extraction path of
core/messages/utils.ts:

1. splitInlineReasoning's first pass stripped every closed
   <think>...</think> pair unconditionally, so a message that discusses
   the tag literally in markdown inline code (e.g. "Wrap your reasoning
   in `<think>...</think>`") had its code span hollowed out and the
   inner text shipped to the Reasoning panel. The streaming pass already
   guards backtick-adjacent openers; apply the same guard to the
   closed-pair pass so both passes agree on what counts as literal tag
   talk.

2. getAssistantTurnCopyData fell back to reasoning via
   `content ?? reasoning`, but extractContentFromMessage never returns
   null, so the fallback was dead code and a reasoning-only turn (e.g.
   stopped mid-thinking) rendered no copy button at all - inconsistent
   with getMessageCopyData, which does copy reasoning in that case. Use
   the same empty-string check it uses.

frontend/src/core/messages/utils.ts

@@ -431,8 +431,13 @@ export function getAssistantTurnCopyData(
       .reverse()
       .filter((message) => message.type === "ai")
       .map((message) => {
+        // extractContentFromMessage never returns null, so fall back to
+        // reasoning on empty text (same rule as getMessageCopyData) —
+        // otherwise a reasoning-only turn loses its copy button entirely.
         const content = extractContentFromMessage(message);
-        return content ?? extractReasoningContentFromMessage(message) ?? "";
+        return content.length > 0
+          ? content
+          : (extractReasoningContentFromMessage(message) ?? "");
       })
       .find((content) => content.length > 0) ?? null
   );
@@ -483,14 +488,23 @@ function splitInlineReasoning(content: string): InlineReasoningSplit {
   const reasoningParts: string[] = [];
 
   // First pass: strip every fully closed `<think>...</think>` pair and
-  // collect its body as reasoning.
-  let cleaned = content.replace(THINK_TAG_RE, (_, reasoning: string) => {
-    const normalized = reasoning.trim();
-    if (normalized) {
-      reasoningParts.push(normalized);
-    }
-    return "";
-  });
+  // collect its body as reasoning. A pair whose opener sits right after a
+  // backtick is the model talking about the tag literally inside markdown
+  // inline code (same guard as the streaming pass below) — leave it in the
+  // rendered content instead of hollowing out the code span.
+  let cleaned = content.replace(
+    THINK_TAG_RE,
+    (match: string, reasoning: string, offset: number) => {
+      if (content[offset - 1] === "`") {
+        return match;
+      }
+      const normalized = reasoning.trim();
+      if (normalized) {
+        reasoningParts.push(normalized);
+      }
+      return "";
+    },
+  );
 
   // Streaming-safe pass: a `<think>` opener whose `</think>` has not arrived
   // yet means the rest of the chunk is reasoning in flight. Route it into the

frontend/tests/unit/core/messages/utils.test.ts

@@ -456,6 +456,30 @@ describe("inline <think> tag splitting", () => {
     expect(hasReasoning(message)).toBe(false);
   });
 
+  test("a closed <think> pair inside markdown inline code stays in content", () => {
+    // The model talking about the tag literally, e.g. when explaining prompt
+    // engineering. Hollowing the code span out into the reasoning panel
+    // corrupts the visible answer.
+    const message = aiMessage(
+      "Wrap your reasoning in `<think>your reasoning</think>` before answering.",
+    );
+    expect(extractContentFromMessage(message)).toBe(
+      "Wrap your reasoning in `<think>your reasoning</think>` before answering.",
+    );
+    expect(extractReasoningContentFromMessage(message)).toBeNull();
+    expect(hasReasoning(message)).toBe(false);
+  });
+
+  test("a closed <think> pair outside code is still stripped when another sits in inline code", () => {
+    const message = aiMessage(
+      "<think>real reasoning</think>Use `<think>text</think>` markers.",
+    );
+    expect(extractContentFromMessage(message)).toBe(
+      "Use `<think>text</think>` markers.",
+    );
+    expect(extractReasoningContentFromMessage(message)).toBe("real reasoning");
+  });
+
   test("a backtick-prefixed <think> mid-stream is not split into reasoning", () => {
     // Simulates the moment the model has emitted the opening backtick and
     // `<think>` for a literal documentation reference, before the closing
@@ -713,6 +737,22 @@ test("hides assistant copy data while that turn is streaming", () => {
   expect(getAssistantTurnCopyData(messages, { isStreaming: true })).toBeNull();
 });
 
+test("falls back to reasoning for a reasoning-only assistant turn's copy data", () => {
+  // A turn can end with reasoning but no answer text (e.g. stopped during
+  // thinking). getMessageCopyData already copies the reasoning in that case;
+  // the turn-level copy button must not disappear instead.
+  const messages = [
+    {
+      id: "ai-1",
+      type: "ai",
+      content: "",
+      additional_kwargs: { reasoning_content: "the actual reasoning" },
+    },
+  ] as Message[];
+
+  expect(getAssistantTurnCopyData(messages)).toBe("the actual reasoning");
+});
+
 test("marks the latest assistant message as streaming", () => {
   const messages = [
     {