Repair truncated LLM proposal JSON

src/aituner/llm.py

@@ -128,15 +128,50 @@ def validate_proposal(proposal: Proposal, study: StudySpec) -> Proposal:
     return proposal
 
 
+def _repair_json_text(text: str) -> str:
+    candidate = text.strip()
+    if not candidate:
+        return candidate
+    stack: list[str] = []
+    in_string = False
+    escape = False
+    for char in candidate:
+        if in_string:
+            if escape:
+                escape = False
+            elif char == "\\":
+                escape = True
+            elif char == '"':
+                in_string = False
+            continue
+        if char == '"':
+            in_string = True
+        elif char == "{":
+            stack.append("}")
+        elif char == "[":
+            stack.append("]")
+        elif char in {"}", "]"} and stack and stack[-1] == char:
+            stack.pop()
+    if in_string:
+        candidate += '"'
+    if stack:
+        candidate += "".join(reversed(stack))
+    return candidate
+
+
 def _parse_json_object_text(text: str) -> dict[str, Any]:
     try:
         payload = json.loads(text)
     except json.JSONDecodeError:
         start = text.find("{")
-        end = text.rfind("}")
+        if start < 0:
-        if start < 0 or end < start:
             raise
-        payload = json.loads(text[start : end + 1])
+        end = text.rfind("}")
+        candidate = text[start:] if end < start else text[start : end + 1]
+        try:
+            payload = json.loads(candidate)
+        except json.JSONDecodeError:
+            payload = json.loads(_repair_json_text(candidate))
     if not isinstance(payload, dict):
         raise SpecError("proposal payload must be a JSON object")
     return payload

tests/test_core_flow.py

@@ -317,6 +317,31 @@ class CoreFlowTests(unittest.TestCase):
             self.assertIn('"failure_reason": "engine_process_exited_before_ready exit_code=1"', prompt)
             self.assertIn('"VLLM_ATTENTION_BACKEND": "FLASHINFER"', prompt)
 
+    def test_parse_proposal_text_repairs_truncated_json(self) -> None:
+        with tempfile.TemporaryDirectory() as tmp:
+            tmp_path = Path(tmp)
+            study = load_study_spec(_write_study_assets(tmp_path))
+            proposal = parse_proposal_text(
+                """
+                {
+                  "observation": "obs",
+                  "diagnosis": "diag",
+                  "config_patch": {
+                    "env_patch": {},
+                    "flag_patch": {
+                      "max-num-seqs": 24
+                    }
+                  },
+                  "expected_effects": [
+                    "faster batching"
+                  ],
+                  "why_not_previous_failures": "none"
+                """,
+                study,
+            )
+            self.assertEqual(proposal.diagnosis, "diag")
+            self.assertEqual(proposal.config_patch.flag_patch["max-num-seqs"], 24)
+
     def test_length_only_trace_rows_are_synthesized(self) -> None:
         with tempfile.TemporaryDirectory() as tmp:
             tmp_path = Path(tmp)