fix(run_agent): handle unescaped control chars in tool_call arguments (#15356)

Extends _repair_tool_call_arguments() to cover the most common local-model
JSON corruption pattern: llama.cpp/Ollama backends emit literal tabs and
newlines inside JSON string values (memory save summaries, file contents,
etc.). Previously fell through to '{}' replacement, losing the call.

Adds two repair passes:
  - Pass 0: json.loads(strict=False) + re-serialise to canonical wire form
  - Pass 4: escape 0x00-0x1F control chars inside string values, then retry

Ports the core utility from #12068 / PR #12093 without the larger plumbing
change (that PR also replaced json.loads at 8 call sites; current main's
_repair_tool_call_arguments is already the single chokepoint, so the
upgrade happens transparently for every existing caller).

Credit: @truenorth-lj for the original utility design.

4 new regression tests covering literal newlines, tabs, re-serialisation
to strict=True-valid output, and the trailing-comma + control-char
combination case.

run_agent.py

@@ -502,6 +502,48 @@ def _sanitize_messages_surrogates(messages: list) -> bool:
     return found
 
 
+def _escape_invalid_chars_in_json_strings(raw: str) -> str:
+    """Escape unescaped control chars inside JSON string values.
+
+    Walks the raw JSON character-by-character, tracking whether we are
+    inside a double-quoted string. Inside strings, replaces literal
+    control characters (0x00-0x1F) that aren't already part of an escape
+    sequence with their ``\\uXXXX`` equivalents. Pass-through for everything
+    else.
+
+    Ported from #12093 — complements the other repair passes in
+    ``_repair_tool_call_arguments`` when ``json.loads(strict=False)`` is
+    not enough (e.g. llama.cpp backends that emit literal apostrophes or
+    tabs alongside other malformations).
+    """
+    out: list[str] = []
+    in_string = False
+    i = 0
+    n = len(raw)
+    while i < n:
+        ch = raw[i]
+        if in_string:
+            if ch == "\\" and i + 1 < n:
+                # Already-escaped char — pass through as-is
+                out.append(ch)
+                out.append(raw[i + 1])
+                i += 2
+                continue
+            if ch == '"':
+                in_string = False
+                out.append(ch)
+            elif ord(ch) < 0x20:
+                out.append(f"\\u{ord(ch):04x}")
+            else:
+                out.append(ch)
+        else:
+            if ch == '"':
+                in_string = True
+            out.append(ch)
+        i += 1
+    return "".join(out)
+
+
 def _repair_tool_call_arguments(raw_args: str, tool_name: str = "?") -> str:
     """Attempt to repair malformed tool_call argument JSON.
 
@@ -523,6 +565,23 @@ def _repair_tool_call_arguments(raw_args: str, tool_name: str = "?") -> str:
         logger.warning("Sanitized Python-None tool_call arguments for %s", tool_name)
         return "{}"
 
+    # Repair pass 0: llama.cpp backends sometimes emit literal control
+    # characters (tabs, newlines) inside JSON string values. json.loads
+    # with strict=False accepts these and lets us re-serialise the
+    # result into wire-valid JSON without any string surgery. This is
+    # the most common local-model repair case (#12068).
+    try:
+        parsed = json.loads(raw_stripped, strict=False)
+        reserialised = json.dumps(parsed, separators=(",", ":"))
+        if reserialised != raw_stripped:
+            logger.warning(
+                "Repaired unescaped control chars in tool_call arguments for %s",
+                tool_name,
+            )
+        return reserialised
+    except (json.JSONDecodeError, TypeError, ValueError):
+        pass
+
     # Attempt common JSON repairs
     fixed = raw_stripped
     # 1. Strip trailing commas before } or ]
@@ -557,6 +616,21 @@ def _repair_tool_call_arguments(raw_args: str, tool_name: str = "?") -> str:
     except json.JSONDecodeError:
         pass
 
+    # Repair pass 4: escape unescaped control chars inside JSON strings,
+    # then retry. Catches cases where strict=False alone fails because
+    # other malformations are present too.
+    try:
+        escaped = _escape_invalid_chars_in_json_strings(fixed)
+        if escaped != fixed:
+            json.loads(escaped)
+            logger.warning(
+                "Repaired control-char-laced tool_call arguments for %s: %s → %s",
+                tool_name, raw_stripped[:80], escaped[:80],
+            )
+            return escaped
+    except (json.JSONDecodeError, TypeError, ValueError):
+        pass
+
     # Last resort: replace with empty object so the API request doesn't
     # crash the entire session.
     logger.warning(

tests/run_agent/test_repair_tool_call_arguments.py

@@ -105,3 +105,39 @@ class TestRepairToolCallArguments:
         result = _repair_tool_call_arguments(raw, "terminal")
         # Should at least be valid JSON, even if background is lost
         json.loads(result)
+
+    # -- Stage 0: strict=False (literal control chars in strings) --
+    # llama.cpp backends sometimes emit literal tabs/newlines inside JSON
+    # string values. strict=False accepts these; we re-serialise to the
+    # canonical wire form (#12068).
+
+    def test_literal_newline_inside_string_value(self):
+        raw = '{"summary": "line one\nline two"}'
+        result = _repair_tool_call_arguments(raw, "t")
+        parsed = json.loads(result)
+        assert parsed == {"summary": "line one\nline two"}
+
+    def test_literal_tab_inside_string_value(self):
+        raw = '{"summary": "col1\tcol2"}'
+        result = _repair_tool_call_arguments(raw, "t")
+        parsed = json.loads(result)
+        assert parsed == {"summary": "col1\tcol2"}
+
+    def test_literal_control_char_reserialised_to_wire_form(self):
+        """After repair, the output must parse under strict=True."""
+        raw = '{"msg": "has\tliteral\ttabs"}'
+        result = _repair_tool_call_arguments(raw, "t")
+        # strict=True must now accept this
+        parsed = json.loads(result)
+        assert parsed["msg"] == "has\tliteral\ttabs"
+
+    # -- Stage 4: control-char escape fallback --
+
+    def test_control_chars_with_trailing_comma(self):
+        """strict=False fails due to trailing comma, but brace-count pass
+        + control-char escape rescues it."""
+        raw = '{"msg": "line\none",}'
+        result = _repair_tool_call_arguments(raw, "t")
+        parsed = json.loads(result)
+        assert "line" in parsed["msg"]
+