From a76c6af8d9c2c66f84eec53ec8eb8eee8a9bb1fd Mon Sep 17 00:00:00 2001
From: Claude <noreply@anthropic.com>
Date: Fri, 10 Jul 2026 14:47:27 +0000
Subject: [PATCH] fix(llm): make JSON extraction string-aware so braces in text
 cannot drop findings
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parse_llm_json counted braces without tracking string context, so any
'}' inside a JSON string value truncated the object mid-string; the
resulting decode error was swallowed upstream and the whole structured
result (all findings) was silently discarded, leaving only raw text.
Use json.JSONDecoder().raw_decode, trying candidate '{' positions in
order — string/escape aware, and also recovers when a stray '{'
precedes the real object.

Fixes #136

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01W4svt5QTs4WUMSy4HwiVt9
---
 src/medcheck/llm/base.py         | 26 ++++++++++++++++----------
 tests/unit/test_llm/test_base.py | 23 +++++++++++++++++++++++
 2 files changed, 39 insertions(+), 10 deletions(-)

diff --git a/src/medcheck/llm/base.py b/src/medcheck/llm/base.py
index ee1252e..587c613 100644
--- a/src/medcheck/llm/base.py
+++ b/src/medcheck/llm/base.py
@@ -109,17 +109,23 @@ class AnalysisResult:
 
 
 def parse_llm_json(raw: str) -> dict[str, Any]:
-    """Extract and parse the first valid JSON object from *raw* using brace-depth tracking."""
-    start = raw.index("{")
-    depth = 0
-    for i, ch in enumerate(raw[start:], start):
-        if ch == "{":
-            depth += 1
-        elif ch == "}":
-            depth -= 1
-        if depth == 0:
-            result: dict[str, Any] = json.loads(raw[start : i + 1])
+    """Extract and parse the first JSON object embedded in *raw*.
+
+    Uses json's incremental decoder — which is string/escape aware — rather
+    than manual brace counting, so a ``}`` inside a string value (plausible in
+    LLM free-text fields) cannot truncate the object. Candidate ``{`` positions
+    are tried in order until one decodes to an object.
+    """
+    decoder = json.JSONDecoder()
+    pos = raw.find("{")
+    while pos != -1:
+        try:
+            result, _ = decoder.raw_decode(raw, pos)
+        except json.JSONDecodeError:
+            result = None
+        if isinstance(result, dict):
             return result
+        pos = raw.find("{", pos + 1)
     raise ValueError("No valid JSON found")
 
 
diff --git a/tests/unit/test_llm/test_base.py b/tests/unit/test_llm/test_base.py
index a756ce1..afef265 100644
--- a/tests/unit/test_llm/test_base.py
+++ b/tests/unit/test_llm/test_base.py
@@ -121,3 +121,26 @@ def fn():
         call_with_retries(fn, provider="x", attempts=3, base_delay=0)
     # Non-transient error: no retries.
     assert calls["n"] == 1
+
+
+def test_parse_llm_json_braces_inside_string_values():
+    raw = (
+        '{"overall_impression": "Normal study (note: {contrast} was not administered)",'
+        ' "structures": [{"name": "ACL", "status": "intact", "confidence": 0.9}]}'
+    )
+    result = parse_llm_response(raw)
+    assert "not administered" in result.overall_impression
+    assert len(result.structures) == 1
+    assert result.structures[0].name == "ACL"
+
+
+def test_parse_llm_json_object_after_stray_brace():
+    raw = 'Header {not json. {"overall_impression": "ok", "structures": []}'
+    result = parse_llm_response(raw)
+    assert result.overall_impression == "ok"
+
+
+def test_parse_llm_json_markdown_fenced():
+    raw = 'Here is the analysis:\n```json\n{"overall_impression": "unremarkable", "structures": []}\n```\nDone.'
+    result = parse_llm_response(raw)
+    assert result.overall_impression == "unremarkable"