fix(ollama): robustly parse single-quoted JSON in tool calls (#32109)

**Description:**
This PR makes argument parsing for Ollama tool calls more robust. Some
LLMs—including Ollama—may return arguments as Python-style dictionaries
with single quotes (e.g., `{'a': 1}`), which are not valid JSON and
previously caused parsing to fail.
The updated `_parse_json_string` method in
`langchain_ollama.chat_models` now attempts standard JSON parsing and,
if that fails, falls back to `ast.literal_eval` for safe evaluation of
Python-style dictionaries. This improves interoperability with LLMs and
fixes a common usability issue for tool-based agents.

**Issue:**
Closes #30910

**Dependencies:**
None

**Tests:**
- Added new unit tests for double-quoted JSON, single-quoted dicts,
mixed quoting, and malformed/failure cases.
- All tests pass locally, including new coverage for single-quoted
inputs.

**Notes:**
- No breaking changes.
- No new dependencies introduced.
- Code is formatted and linted (`ruff format`, `ruff check`).
- If maintainers have suggestions for further improvements, I’m happy to
revise!

Thank you for maintaining LangChain! Looking forward to your feedback.

libs/langchain/pyproject.toml

@@ -5,7 +5,7 @@ build-backend = "pdm.backend"
 [project]
 authors = []
 license = { text = "MIT" }
-requires-python = ">=3.9"
+requires-python = ">=3.9, <4.0"
 dependencies = [
     "langchain-core<1.0.0,>=0.3.66",
     "langchain-text-splitters<1.0.0,>=0.3.8",

libs/partners/ollama/langchain_ollama/chat_models.py

@@ -2,6 +2,7 @@
 
 from __future__ import annotations
 
+import ast
 import json
 from collections.abc import AsyncIterator, Iterator, Mapping, Sequence
 from operator import itemgetter
@@ -77,31 +78,43 @@ def _get_usage_metadata_from_generation_info(
 
 def _parse_json_string(
     json_string: str,
+    *,
     raw_tool_call: dict[str, Any],
-    skip: bool,  # noqa: FBT001
+    skip: bool,
 ) -> Any:
     """Attempt to parse a JSON string for tool calling.
 
+    It first tries to use the standard json.loads. If that fails, it falls
+    back to ast.literal_eval to safely parse Python literals, which is more
+    robust against models using single quotes or containing apostrophes.
+
     Args:
         json_string: JSON string to parse.
-        skip: Whether to ignore parsing errors and return the value anyways.
         raw_tool_call: Raw tool call to include in error message.
+        skip: Whether to ignore parsing errors and return the value anyways.
 
     Returns:
-        The parsed JSON string.
+        The parsed JSON string or Python literal.
 
     Raises:
-        OutputParserException: If the JSON string wrong invalid and skip=False.
+        OutputParserException: If the string is invalid and skip=False.
     """
     try:
         return json.loads(json_string)
-    except json.JSONDecodeError as e:
+    except json.JSONDecodeError:
+        try:
+            # Use ast.literal_eval to safely parse Python-style dicts
+            # (e.g. with single quotes)
+            return ast.literal_eval(json_string)
+        except (SyntaxError, ValueError) as e:
+            # If both fail, and we're not skipping, raise an informative error.
             if skip:
                 return json_string
             msg = (
                 f"Function {raw_tool_call['function']['name']} arguments:\n\n"
-            f"{raw_tool_call['function']['arguments']}\n\nare not valid JSON. "
-            f"Received JSONDecodeError {e}"
+                f"{raw_tool_call['function']['arguments']}"
+                "\n\nare not valid JSON or a Python literal. "
+                f"Received error: {e}"
             )
             raise OutputParserException(msg) from e
     except TypeError as e:

libs/partners/ollama/tests/unit_tests/test_chat_models.py

@@ -8,10 +8,15 @@ from unittest.mock import patch
 
 import pytest
 from httpx import Client, Request, Response
+from langchain_core.exceptions import OutputParserException
 from langchain_core.messages import ChatMessage
 from langchain_tests.unit_tests import ChatModelUnitTests
 
-from langchain_ollama.chat_models import ChatOllama, _parse_arguments_from_tool_call
+from langchain_ollama.chat_models import (
+    ChatOllama,
+    _parse_arguments_from_tool_call,
+    _parse_json_string,
+)
 
 MODEL_NAME = "llama3.1"
 
@@ -49,13 +54,11 @@ def test_arbitrary_roles_accepted_in_chatmessages(
     monkeypatch: pytest.MonkeyPatch,
 ) -> None:
     monkeypatch.setattr(Client, "stream", _mock_httpx_client_stream)
-
     llm = ChatOllama(
         model=MODEL_NAME,
         verbose=True,
         format=None,
     )
-
     messages = [
         ChatMessage(
             role="somerandomrole",
@@ -64,7 +67,6 @@ def test_arbitrary_roles_accepted_in_chatmessages(
         ChatMessage(role="control", content="thinking"),
         ChatMessage(role="user", content="What is the meaning of life?"),
     ]
-
     llm.invoke(messages)
 
 
@@ -83,3 +85,58 @@ def test_validate_model_on_init(mock_validate_model: Any) -> None:
     # Test that validate_model is NOT called by default
     ChatOllama(model=MODEL_NAME)
     mock_validate_model.assert_not_called()
+
+
+# Define a dummy raw_tool_call for the function signature
+dummy_raw_tool_call = {
+    "function": {"name": "test_func", "arguments": ""},
+}
+
+
+# --- Regression tests for tool-call argument parsing (see #30910) ---
+
+
+@pytest.mark.parametrize(
+    "input_string, expected_output",
+    [
+        # Case 1: Standard double-quoted JSON
+        ('{"key": "value", "number": 123}', {"key": "value", "number": 123}),
+        # Case 2: Single-quoted string (the original bug)
+        ("{'key': 'value', 'number': 123}", {"key": "value", "number": 123}),
+        # Case 3: String with an internal apostrophe
+        ('{"text": "It\'s a great test!"}', {"text": "It's a great test!"}),
+        # Case 4: Mixed quotes that ast can handle
+        ("{'text': \"It's a great test!\"}", {"text": "It's a great test!"}),
+    ],
+)
+def test_parse_json_string_success_cases(
+    input_string: str, expected_output: Any
+) -> None:
+    """Tests that _parse_json_string correctly parses valid and fixable strings."""
+    raw_tool_call = {"function": {"name": "test_func", "arguments": input_string}}
+    result = _parse_json_string(input_string, raw_tool_call=raw_tool_call, skip=False)
+    assert result == expected_output
+
+
+def test_parse_json_string_failure_case_raises_exception() -> None:
+    """Tests that _parse_json_string raises an exception for truly malformed strings."""
+    malformed_string = "{'key': 'value',,}"
+    raw_tool_call = {"function": {"name": "test_func", "arguments": malformed_string}}
+    with pytest.raises(OutputParserException):
+        _parse_json_string(
+            malformed_string,
+            raw_tool_call=raw_tool_call,
+            skip=False,
+        )
+
+
+def test_parse_json_string_skip_returns_input_on_failure() -> None:
+    """Tests that skip=True returns the original string on parse failure."""
+    malformed_string = "{'not': valid,,,}"
+    raw_tool_call = {"function": {"name": "test_func", "arguments": malformed_string}}
+    result = _parse_json_string(
+        malformed_string,
+        raw_tool_call=raw_tool_call,
+        skip=True,
+    )
+    assert result == malformed_string