fix(browser): guard LLM response content against None in snapshot and vision (#3642)

Salvage of PR #3532 (binhnt92). Guards browser_tool.py against None content from reasoning-only models (DeepSeek-R1, QwQ). Follow-up to #3449.

Co-Authored-By: binhnt92 <binhnt92@users.noreply.github.com>

tests/tools/test_browser_content_none_guard.py

@@ -0,0 +1,109 @@
+"""Tests for None guard on browser_tool LLM response content.
+
+browser_tool.py has two call sites that access response.choices[0].message.content
+without checking for None — _extract_relevant_content (line 996) and
+browser_vision (line 1626). When reasoning-only models (DeepSeek-R1, QwQ)
+return content=None, these produce null snapshots or null analysis.
+
+These tests verify both sites are guarded.
+"""
+
+import types
+from unittest.mock import MagicMock, patch
+
+import pytest
+
+
+# ── helpers ────────────────────────────────────────────────────────────────
+
+def _make_response(content):
+    """Build a minimal OpenAI-compatible ChatCompletion response stub."""
+    message = types.SimpleNamespace(content=content)
+    choice = types.SimpleNamespace(message=message)
+    return types.SimpleNamespace(choices=[choice])
+
+
+# ── _extract_relevant_content (line 996) ──────────────────────────────────
+
+class TestExtractRelevantContentNoneGuard:
+    """tools/browser_tool.py — _extract_relevant_content()"""
+
+    def test_none_content_falls_back_to_truncated(self):
+        """When LLM returns None content, should fall back to truncated snapshot."""
+        with patch("tools.browser_tool.call_llm", return_value=_make_response(None)), \
+             patch("tools.browser_tool._get_extraction_model", return_value="test-model"):
+            from tools.browser_tool import _extract_relevant_content
+            result = _extract_relevant_content("This is a long snapshot text", "find the button")
+
+        assert result is not None
+        assert isinstance(result, str)
+        assert len(result) > 0
+
+    def test_normal_content_returned(self):
+        """Normal string content should pass through."""
+        with patch("tools.browser_tool.call_llm", return_value=_make_response("Extracted content here")), \
+             patch("tools.browser_tool._get_extraction_model", return_value="test-model"):
+            from tools.browser_tool import _extract_relevant_content
+            result = _extract_relevant_content("snapshot text", "task")
+
+        assert result == "Extracted content here"
+
+    def test_empty_string_content_falls_back(self):
+        """Empty string content should also fall back to truncated."""
+        with patch("tools.browser_tool.call_llm", return_value=_make_response("   ")), \
+             patch("tools.browser_tool._get_extraction_model", return_value="test-model"):
+            from tools.browser_tool import _extract_relevant_content
+            result = _extract_relevant_content("This is a long snapshot text", "task")
+
+        assert result is not None
+        assert len(result) > 0
+
+
+# ── browser_vision (line 1626) ────────────────────────────────────────────
+
+class TestBrowserVisionNoneGuard:
+    """tools/browser_tool.py — browser_vision() analysis extraction"""
+
+    def test_none_content_produces_fallback_message(self):
+        """When LLM returns None content, analysis should have a fallback message."""
+        response = _make_response(None)
+        analysis = (response.choices[0].message.content or "").strip()
+        fallback = analysis or "Vision analysis returned no content."
+
+        assert fallback == "Vision analysis returned no content."
+
+    def test_normal_content_passes_through(self):
+        """Normal analysis content should pass through unchanged."""
+        response = _make_response("  The page shows a login form.  ")
+        analysis = (response.choices[0].message.content or "").strip()
+        fallback = analysis or "Vision analysis returned no content."
+
+        assert fallback == "The page shows a login form."
+
+
+# ── source line verification ──────────────────────────────────────────────
+
+class TestBrowserSourceLinesAreGuarded:
+    """Verify the actual source file has the fix applied."""
+
+    @staticmethod
+    def _read_file() -> str:
+        import os
+        base = os.path.dirname(os.path.dirname(os.path.dirname(__file__)))
+        with open(os.path.join(base, "tools", "browser_tool.py")) as f:
+            return f.read()
+
+    def test_extract_relevant_content_guarded(self):
+        src = self._read_file()
+        # The old unguarded pattern should NOT exist
+        assert "return response.choices[0].message.content\n" not in src, (
+            "browser_tool.py _extract_relevant_content still has unguarded "
+            ".content return — apply None guard"
+        )
+
+    def test_browser_vision_guarded(self):
+        src = self._read_file()
+        assert "analysis = response.choices[0].message.content\n" not in src, (
+            "browser_tool.py browser_vision still has unguarded "
+            ".content assignment — apply None guard"
+        )

tools/browser_tool.py

@@ -993,7 +993,7 @@ def _extract_relevant_content(
         if model:
             call_kwargs["model"] = model
         response = call_llm(**call_kwargs)
-        return response.choices[0].message.content
+        return (response.choices[0].message.content or "").strip() or _truncate_snapshot(snapshot_text)
     except Exception:
         return _truncate_snapshot(snapshot_text)
 
@@ -1623,10 +1623,10 @@ def browser_vision(question: str, annotate: bool = False, task_id: Optional[str]
             call_kwargs["model"] = vision_model
         response = call_llm(**call_kwargs)
         
-        analysis = response.choices[0].message.content
+        analysis = (response.choices[0].message.content or "").strip()
         response_data = {
             "success": True,
-            "analysis": analysis,
+            "analysis": analysis or "Vision analysis returned no content.",
             "screenshot_path": str(screenshot_path),
         }
         # Include annotation data if annotated screenshot was taken