fix(llm): auto-detect <think> in content stream for unregistered thinking models (#2588)

* fix(llm): auto-detect <think> in content stream for unregistered thinking models

_THINKING_MODEL_PATTERNS only covers known model families by name. Qwen3-derived
models with non-standard names (e.g. Qwopus, custom QwQ forks) are not matched,
so their <think>...</think> content streams through as visible chat text instead
of being routed to the thinking display.

When the first content delta opens with <think> and the model was not already
identified as a thinking model, dynamically flag the stream as a thinking model
for the remainder of the response. This enables the existing </think> repair path
(line below) and ensures the frontend receives the full <think>...</think> wrapper
it needs to split thinking from the final answer.

The check is restricted to the very first content delta (_first_content_sent is
False) to avoid misidentifying models that happen to write "<think>" mid-answer.

Fixes #2225
Related: #2420 (covered by separate PR from @AmmarS-Analyst), #2224 (@RaresKeY)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

* fix(llm): replace inert _thinking_model flag with _in_think_tag state machine

The original auto-detect set _thinking_model=True on the first <think> chunk
but still emitted it as a regular delta and set _first_content_sent=True
immediately, so no subsequent chunk could enter the repair path.

Replace with _in_think_tag bool: enter thinking mode when first content starts
with <think>, route all chunks to the thinking channel until </think> is found,
then the tail becomes the first regular delta. Adds three regression tests.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

* fix(llm): replace _first_content_sent guard with _think_open_stripped

Opening-tag stripping used `not _first_content_sent` as the guard, but
_first_content_sent stays False throughout the entire think block (it only
flips when regular content is emitted). So `find(">")` ran on every
reasoning chunk — not just the first — and silently truncated everything
before the first ">" in any reasoning text containing comparisons, arrows,
or code.

Fix: add `_think_open_stripped = False` alongside `_in_think_tag`. Use it
as the strip guard in both the "still inside <think>" path and the
"</think> found in same chunk" split path. Set it True once the opening
tag is consumed so all subsequent chunks reach the thinking channel
unmolested.

Add regression test: 3-chunk stream where the middle chunk contains
"c > d" — confirms "more c " is not dropped.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

---------

Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>

tests/test_llm_core_reasoning.py

@@ -96,3 +96,79 @@ def test_reasoning_content_field_still_supported(monkeypatch):
     )
     assert any(d.get("thinking") and "older field" in d["delta"] for d in deltas), deltas
     assert any((not d.get("thinking")) and d["delta"] == "Answer" for d in deltas), deltas
+
+
+def test_think_tag_in_content_stream_routes_to_thinking_channel(monkeypatch):
+    # Regression: unregistered model (Qwopus-style) that emits <think>…</think>
+    # directly in the content field. Reasoning must surface as thinking chunks;
+    # only the answer after </think> is a normal delta.
+    deltas = _run_stream(
+        "Qwopus3-9B-custom",  # name not in _THINKING_MODEL_PATTERNS
+        [
+            'data: {"choices":[{"delta":{"content":"<think>step one "}}]}',
+            'data: {"choices":[{"delta":{"content":"step two"}}]}',
+            'data: {"choices":[{"delta":{"content":"</think>Final answer"}}]}',
+            "data: [DONE]",
+        ],
+        monkeypatch,
+    )
+    thinking = [d for d in deltas if d.get("thinking")]
+    regular = [d for d in deltas if not d.get("thinking")]
+    assert thinking, f"expected thinking deltas, got: {deltas}"
+    assert all("Final answer" not in d["delta"] for d in thinking), thinking
+    assert regular, f"expected regular delta after </think>, got: {deltas}"
+    assert any("Final answer" in d["delta"] for d in regular), regular
+
+
+def test_think_tag_and_close_in_same_chunk(monkeypatch):
+    # <think>reasoning</think>answer all arrive in a single content chunk.
+    deltas = _run_stream(
+        "Qwopus3-9B-custom",
+        [
+            'data: {"choices":[{"delta":{"content":"<think>my reasoning</think>my answer"}}]}',
+            "data: [DONE]",
+        ],
+        monkeypatch,
+    )
+    thinking = [d for d in deltas if d.get("thinking")]
+    regular = [d for d in deltas if not d.get("thinking")]
+    assert thinking and "my reasoning" in thinking[0]["delta"], thinking
+    assert regular and "my answer" in regular[0]["delta"], regular
+
+
+def test_think_tag_gt_in_mid_reasoning_not_truncated(monkeypatch):
+    # Regression for _first_content_sent misuse: the opening-tag strip ran on every
+    # chunk (not just the first) because _first_content_sent stays False throughout
+    # the think block. On chunk 2 it did find(">") over reasoning text and silently
+    # dropped everything before the first ">". Repro: 3 chunks, ">" in chunk 2.
+    deltas = _run_stream(
+        "Qwopus3-9B-custom",
+        [
+            'data: {"choices":[{"delta":{"content":"<think>reasoning a "}}]}',
+            'data: {"choices":[{"delta":{"content":"more c > d "}}]}',
+            'data: {"choices":[{"delta":{"content":"</think>answer"}}]}',
+            "data: [DONE]",
+        ],
+        monkeypatch,
+    )
+    thinking = [d for d in deltas if d.get("thinking")]
+    regular = [d for d in deltas if not d.get("thinking")]
+    # "more c " must survive — must not be truncated at the '>'
+    assert any("more c > d" in d["delta"] for d in thinking), thinking
+    assert any("answer" in d["delta"] for d in regular), regular
+
+
+def test_registered_thinking_model_stray_close_tag_repair_unchanged(monkeypatch):
+    # The existing </think> repair for registered models must not regress.
+    # A registered model that starts content with </think> gets <think> prepended.
+    deltas = _run_stream(
+        "qwq-32b",  # registered in _THINKING_MODEL_PATTERNS
+        [
+            'data: {"choices":[{"delta":{"content":"</think>Here is my answer"}}]}',
+            "data: [DONE]",
+        ],
+        monkeypatch,
+    )
+    assert deltas, deltas
+    first = deltas[0]["delta"]
+    assert first.startswith("<think>"), f"expected repair prefix, got: {first!r}"