fix(streaming): strip Gemma 4 thinking token delimiter in all paths — closes #607

Fixes <|turn|>thinking delimiter (was wrong as <|turn>thinking) in api/streaming.py, static/messages.js, and static/ui.js. Adds 13 regression tests. Independent review by @nesquena.

tests/test_issue607.py

@@ -0,0 +1,98 @@
+"""Tests for PR #648 — Gemma 4 thinking token stripping (closes #607)."""
+import re
+import pathlib
+import pytest
+
+
+# ---------------------------------------------------------------------------
+# _strip_thinking_markup tests
+# ---------------------------------------------------------------------------
+
+from api.streaming import _strip_thinking_markup, _looks_invalid_generated_title
+
+
+class TestGemma4ThinkingTokenStrip:
+    """Verify that <|turn|>thinking\n...\n<turn|> blocks are stripped."""
+
+    def test_strip_gemma4_basic(self):
+        """Basic Gemma 4 thinking block stripped, answer kept."""
+        raw = "<|turn|>thinking\nSome internal reasoning\n<turn|>Final answer"
+        result = _strip_thinking_markup(raw)
+        assert result == "Final answer"
+
+    def test_strip_gemma4_multiline_reasoning(self):
+        """Multi-line reasoning block stripped cleanly."""
+        raw = "<|turn|>thinking\nLine 1\nLine 2\nLine 3\n<turn|>Answer here"
+        result = _strip_thinking_markup(raw)
+        assert result == "Answer here"
+
+    def test_strip_gemma4_no_thinking_passthrough(self):
+        """Normal response without thinking tokens passes through unchanged."""
+        raw = "Normal response without thinking tokens"
+        result = _strip_thinking_markup(raw)
+        assert result == raw
+
+    def test_strip_gemma4_with_leading_whitespace(self):
+        """Leading whitespace before the thinking block is handled."""
+        raw = "\n\n<|turn|>thinking\nReasoning\n<turn|>Answer"
+        result = _strip_thinking_markup(raw)
+        assert result == "Answer"
+
+    def test_strip_gemma4_empty_reasoning(self):
+        """Empty reasoning block (just delimiters) is stripped."""
+        raw = "<|turn|>thinking\n<turn|>Response"
+        result = _strip_thinking_markup(raw)
+        assert result == "Response"
+
+    def test_strip_gemma4_case_insensitive(self):
+        """Pattern is case-insensitive (though Gemma 4 uses fixed case)."""
+        raw = "<|TURN|>THINKING\nreasoning\n<TURN|>answer"
+        result = _strip_thinking_markup(raw)
+        # The regex uses re.IGNORECASE — should strip uppercase variant too
+        assert "THINKING" not in result
+        assert "reasoning" not in result
+
+    def test_existing_think_tag_still_works(self):
+        """Ensure <think>...</think> still stripped (no regression)."""
+        raw = "<think>inner reasoning</think>Final"
+        result = _strip_thinking_markup(raw)
+        assert result == "Final"
+
+    def test_existing_channel_tag_still_works(self):
+        """Ensure <|channel|>thought...</channel|> still stripped."""
+        raw = "<|channel|>thoughtSome reasoning<channel|>Answer"
+        result = _strip_thinking_markup(raw)
+        assert result == "Answer"
+
+
+class TestGemma4TitleLeakDetection:
+    """Verify _looks_invalid_generated_title catches Gemma 4 leak."""
+
+    def test_detects_gemma4_leak_in_title(self):
+        raw = "<|turn|>thinking\nUser asked about X\n<turn|>Session Title"
+        assert _looks_invalid_generated_title(raw) is True
+
+    def test_clean_title_not_flagged(self):
+        assert _looks_invalid_generated_title("Python debugging session") is False
+
+
+class TestGemma4MessagesJsThinkPairs:
+    """Verify static/messages.js contains the correct Gemma 4 pair."""
+
+    def test_messages_js_has_correct_gemma4_open(self):
+        js = pathlib.Path("static/messages.js").read_text()
+        # Must have double-pipe format: <|turn|>thinking
+        assert "<|turn|>thinking" in js, (
+            "messages.js is missing correct Gemma 4 open delimiter '<|turn|>thinking'"
+        )
+
+    def test_messages_js_no_wrong_gemma4_open(self):
+        js = pathlib.Path("static/messages.js").read_text()
+        # Must NOT have single-pipe wrong format: <|turn>thinking
+        assert "<|turn>thinking" not in js, (
+            "messages.js still contains wrong Gemma 4 delimiter '<|turn>thinking' (missing |)"
+        )
+
+    def test_messages_js_has_gemma4_close(self):
+        js = pathlib.Path("static/messages.js").read_text()
+        assert "<turn|>" in js, "messages.js missing Gemma 4 close delimiter '<turn|>'"