fix(stream): robust reasoning/content split + flush un-tagged output

The incremental </think> search missed tags split across tokens, inverting
reasoning/content classification. Detect the first </think> on the full
buffer and track the content boundary as a byte offset. If the model never
closes </think>, flush the buffered text as content so clients always
receive the response. Chat UI now renders reasoning_content too.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

src/application/chat/mod.rs

@@ -1,6 +1,6 @@
 pub mod use_cases;
 
 pub use use_cases::{
-    build_prompt, build_sampler, clean_text, parse_tool_calls, split_stream_chunk, validate_model,
-    SamplerParams,
+    build_prompt, build_sampler, clean_text, parse_tool_calls, split_stream_chunk, strip_markup,
+    validate_model, SamplerParams,
 };

src/application/chat/use_cases.rs

@@ -335,7 +335,7 @@ pub fn parse_tool_calls(text: &str) -> (String, Vec<ToolCall>) {
 /// Handles both fixed tags (`<|im_end|>`, `<think>`, `<tool_call>`, …) and the
 /// attribute-bearing openers used by this model's tool format (`<function=…>`,
 /// `<parameter=…>`), even when a tag straddles a token boundary.
-fn strip_markup(text: &str) -> String {
+pub fn strip_markup(text: &str) -> String {
     let mut out = String::with_capacity(text.len());
     let mut rest = text;
 
@@ -383,22 +383,42 @@ fn strip_markup(text: &str) -> String {
     out
 }
 
-/// Split an incremental streamed text fragment into `(reasoning, content)`
-/// deltas for SSE.
+/// Compute `(reasoning, content)` deltas for an incremental streamed fragment.
 ///
-/// * `think_done` means the `</think>` boundary was already crossed **before**
-///   this fragment (i.e. it is not the chunk containing the first `</think>`).
-/// * Whitespace **inside** a fragment is preserved — only the whitespace
-///   sitting immediately around the `</think>` boundary is trimmed, so
-///   reasoning does not end with a dangling newline and content does not begin
-///   with one. (Trimming every fragment corrupted inter-word spaces.)
-/// * Before the first `</think>`, everything is emitted as `reasoning_content`;
-///   after it, as `content`. A stray second `</think>` is stripped, not split.
-pub fn split_stream_chunk(new_text: &str, think_done: bool) -> (Option<String>, String) {
-    if !think_done {
-        if let Some(pos) = new_text.find("</think>") {
-            let mut before = strip_markup(&new_text[..pos]);
-            let mut after = strip_markup(&new_text[pos + 8..]);
+/// * `new_text` — the not-yet-emitted fragment (`text_buf[sent_len..]`).
+/// * `sent_len` — byte offset in the full buffer where `new_text` begins.
+/// * `content_start` — byte offset in the full buffer where the content phase
+///   begins (immediately after the first `</think>`); `None` while still
+///   thinking.
+///
+/// The boundary is a position in the *full* buffer, not a string search in the
+/// fragment — this stays correct even when `</think>` is split across tokens.
+/// Whitespace inside a fragment is preserved; only the edges around the
+/// boundary are trimmed. A stray second `</think>` is stripped, not re-split.
+pub fn split_stream_chunk(
+    new_text: &str,
+    sent_len: usize,
+    content_start: Option<usize>,
+) -> (Option<String>, String) {
+    match content_start {
+        // Still thinking — everything is reasoning.
+        None => {
+            let cleaned = strip_markup(new_text);
+            let reasoning = if cleaned.is_empty() {
+                None
+            } else {
+                Some(cleaned)
+            };
+            (reasoning, String::new())
+        }
+        // Boundary already emitted — everything is content.
+        Some(cs) if cs <= sent_len => (None, strip_markup(new_text)),
+        // Boundary falls inside this fragment (or beyond it, defensively).
+        Some(cs) => {
+            let rel = (cs - sent_len).min(new_text.len());
+            let (before, after) = new_text.split_at(rel);
+            let mut before = strip_markup(before);
+            let mut after = strip_markup(after);
 
             while before.ends_with(['\n', ' ', '\t']) {
                 before.pop();
@@ -413,19 +433,7 @@ pub fn split_stream_chunk(new_text: &str, think_done: bool) -> (Option<String>,
                 Some(before)
             };
             (reasoning, after)
-        } else {
-            // Still thinking — everything is reasoning.
-            let cleaned = strip_markup(new_text);
-            let reasoning = if cleaned.is_empty() {
-                None
-            } else {
-                Some(cleaned)
-            };
-            (reasoning, String::new())
         }
-    } else {
-        // Think phase already ended — everything is content; strip stray markup.
-        (None, strip_markup(new_text))
     }
 }
 
@@ -471,7 +479,7 @@ mod tests {
 
     #[test]
     fn split_chunk_before_think_is_reasoning() {
-        let (reasoning, content) = split_stream_chunk("Hello ", false);
+        let (reasoning, content) = split_stream_chunk("Hello ", 0, None);
         assert_eq!(reasoning.as_deref(), Some("Hello "));
         assert_eq!(content, "");
     }
@@ -479,21 +487,23 @@ mod tests {
     #[test]
     fn split_chunk_preserves_internal_spaces() {
         // Regression: trimming every fragment used to eat inter-word spaces.
-        let (r1, _) = split_stream_chunk("Hello", false);
-        let (r2, _) = split_stream_chunk(" world", false);
+        let (r1, _) = split_stream_chunk("Hello", 0, None);
+        let (r2, _) = split_stream_chunk(" world", 5, None);
         assert_eq!(format!("{}{}", r1.unwrap(), r2.unwrap()), "Hello world");
     }
 
     #[test]
     fn split_chunk_boundary_trims_only_edges() {
-        let (reasoning, content) = split_stream_chunk("question\n</think>\n\nAnswer ", false);
+        // text_buf = "question\n</think>\n\nAnswer "; boundary right after the
+        // tag at byte 17.
+        let (reasoning, content) = split_stream_chunk("question\n</think>\n\nAnswer ", 0, Some(17));
         assert_eq!(reasoning.as_deref(), Some("question"));
         assert_eq!(content, "Answer ");
     }
 
     #[test]
     fn split_chunk_after_think_is_content() {
-        let (reasoning, content) = split_stream_chunk(" answer", true);
+        let (reasoning, content) = split_stream_chunk(" answer", 0, Some(0));
         assert_eq!(reasoning, None);
         assert_eq!(content, " answer");
     }
@@ -502,16 +512,29 @@ mod tests {
     fn split_chunk_stray_think_tag_is_stripped_not_split() {
         // A second </think> (already past the boundary) must not restart
         // reasoning classification.
-        let (reasoning, content) = split_stream_chunk("...</think>more", true);
+        let (reasoning, content) = split_stream_chunk("...</think>more", 0, Some(0));
         assert_eq!(reasoning, None);
         assert_eq!(content, "...more");
     }
 
+    #[test]
+    fn split_chunk_boundary_straddling_tokens() {
+        // `</think>` split as "</think" + ">" across two fragments: the boundary
+        // is detected on the full buffer, so the answer still becomes content.
+        let (r1, _) = split_stream_chunk("reasoning...</think", 0, None);
+        assert!(r1.is_some());
+        let (r2, c2) = split_stream_chunk("\n\n2 + 2 = 4.", 18, Some(18));
+        assert_eq!(r2, None);
+        assert_eq!(c2, "\n\n2 + 2 = 4.");
+    }
+
     #[test]
     fn split_chunk_strips_special_and_markup() {
+        // boundary at byte 30 (right after "</think>").
         let (reasoning, content) = split_stream_chunk(
             "<|im_end|><think>Hello</think>\n<tool_call><function=get_weather>",
-            false,
+            0,
+            Some(30),
         );
         assert_eq!(reasoning.as_deref(), Some("Hello"));
         assert_eq!(content, "");

src/presentation/handler/chat-ui/index.html

@@ -72,6 +72,13 @@
     display: flex; align-items: center; gap: 6px;
   }
   .msg .tool-call::before { content: '\1F527'; }
+  .msg .reasoning {
+    font-size: 12px; font-style: italic;
+    color: var(--text2);
+    white-space: pre-wrap; word-break: break-word;
+    border-bottom: 1px solid var(--border);
+    padding-bottom: 8px; margin-bottom: 8px;
+  }
   .msg.error {
     background: #2a1818; border-color: #4a2828; color: #f08080;
   }
@@ -186,8 +193,33 @@ async function send() {
     const decoder = new TextDecoder();
     let buffer = '';
     let full = '';
+    let reasoning = '';
     let toolCalls = null;
 
+    // Re-render the assistant bubble: reasoning (muted) above the answer.
+    function render() {
+      el.innerHTML = '';
+      if (reasoning) {
+        const r = document.createElement('div');
+        r.className = 'reasoning';
+        r.textContent = reasoning;
+        el.appendChild(r);
+      }
+      if (full) {
+        const c = document.createElement('div');
+        c.textContent = full;
+        el.appendChild(c);
+      }
+      if (toolCalls?.length) {
+        for (const tc of toolCalls) {
+          const t = document.createElement('div');
+          t.className = 'tool-call';
+          t.textContent = 'Calling tool: ' + (tc.function?.name || 'tool');
+          el.appendChild(t);
+        }
+      }
+    }
+
     while (true) {
       const { done, value } = await reader.read();
       if (done) break;
@@ -206,18 +238,20 @@ async function send() {
           const delta = chunk.choices?.[0]?.delta;
           const finish = chunk.choices?.[0]?.finish_reason;
 
+          if (delta?.reasoning_content) {
+            reasoning += delta.reasoning_content;
+            render();
+          }
           if (delta?.content) {
             full += delta.content;
-            el.textContent = full;
+            render();
           }
           if (delta?.tool_calls) {
             toolCalls = delta.tool_calls;
+            render();
           }
-          if (finish === 'tool_calls' && toolCalls) {
-            const t = document.createElement('div');
-            t.className = 'tool-call';
-            t.textContent = 'Calling tool: ' + toolCalls.map(tc => tc.function?.name).join(', ');
-            el.appendChild(t);
+          if (finish === 'tool_calls') {
+            render();
           }
         } catch (e) { /* skip malformed chunk */ }
       }

src/presentation/handler/chat.rs

@@ -176,7 +176,9 @@ async fn handle_streaming(
         let mut sampler = SendSampler(chat::build_sampler(&params));
         let mut text_buf = String::new();
         let mut sent_len: usize = 0;
-        let mut think_done = false;
+        // Byte offset in text_buf where the content phase begins (right after
+        // the first `</think>`); None while still thinking.
+        let mut content_start: Option<usize> = None;
 
         let outcome = engine.generate(
             &input_tokens,
@@ -187,11 +189,13 @@ async fn handle_streaming(
             &mut |_token, piece| {
                 text_buf.push_str(piece);
 
-                // `was_thinking` is passed to split_stream_chunk so the chunk
-                // containing the first </think> is treated as the boundary.
-                let was_thinking = !think_done;
-                if !think_done && text_buf.contains("</think>") {
-                    think_done = true;
+                // Robust boundary detection on the *full* buffer — a `</think>`
+                // tag may be split across tokens, which would defeat a search
+                // over the incremental fragment only.
+                if content_start.is_none() {
+                    if let Some(pos) = text_buf.find("</think>") {
+                        content_start = Some(pos + 8);
+                    }
                 }
 
                 let new_text = &text_buf[sent_len..];
@@ -199,7 +203,8 @@ async fn handle_streaming(
                     return true;
                 }
 
-                let (reasoning, content) = chat::split_stream_chunk(new_text, was_thinking);
+                let (reasoning, content) =
+                    chat::split_stream_chunk(new_text, sent_len, content_start);
 
                 if let Some(reasoning) = reasoning {
                     let chunk = SseChunk::delta(
@@ -251,6 +256,28 @@ async fn handle_streaming(
                     total_tokens: prompt_tokens + completion_tokens,
                 };
 
+                // If the model never emitted `</think>`, everything was streamed
+                // as reasoning_content. Flush it as content so the client always
+                // receives the response text.
+                if content_start.is_none() && !text_buf.is_empty() {
+                    let cleaned = chat::strip_markup(&text_buf);
+                    if !cleaned.is_empty() {
+                        let chunk = SseChunk::delta(
+                            chat_id.clone(),
+                            created,
+                            model_name.clone(),
+                            SseDelta {
+                                role: None,
+                                content: Some(cleaned),
+                                tool_calls: None,
+                                reasoning_content: None,
+                            },
+                        );
+                        let event = serde_json::to_string(&chunk).unwrap();
+                        let _ = tx.blocking_send(Ok(Event::default().data(event)));
+                    }
+                }
+
                 // Single-shot tool-calls delta (this model emits whole blocks).
                 let mut sent_tool_calls = false;
                 if outcome.finish == FinishReason::ToolCalls {