[Bugfix] DSV32/V4 defensively unwrap OpenAI-legacy nested-arguments shape in tool parser

[Lambdauv]

Purpose

DeepSeek-V4-Flash occasionally emits a single DSML parameter literally named arguments whose value is the entire arg payload as a JSON string — a leak from training data that included OpenAI's legacy function-call format. The unpatched parser dutifully wraps that into function.arguments, and downstream consumers (which JSON.parse function.arguments once and then access fields) see a doubly-wrapped shape:

{"function": {"name": "exec", "arguments": "{\"arguments\": \"{\\\"command\\\":\\\"echo hello\\\"}\"}"}}

instead of the expected:

{"function": {"name": "exec", "arguments": "{\"command\": \"echo hello\"}"}}

This PR adds defensive normalization at the parser layer so consumers always see the flat shape.

Real-world incident

A production deployment of deepseek-v4-flash on a single GH200, serving an OpenClaw assistant via OpenAI-compatible streaming + tool calling, was intermittently dropping tool dispatches because OpenClaw expected flat args and saw the nested-arguments form. The model emits the legacy shape stochastically (estimated ~1-3% of tool calls in the affected workload). Local bind-mount of this fix on 2026-05-03 eliminated the recurrence.

Implementation

New _maybe_unwrap_legacy_arguments(param_dict, function_name) helper called at both extract paths (non-streaming extract_tool_calls and streaming _extract_delta_tool_calls). Gated on all four of:

1. len(param_dict) == 1
2. that key is named "arguments"
3. the value parses to a non-empty JSON object
4. the tool's own schema does NOT declare "arguments" as a property

Gate 4 is the key safety: a tool that legitimately accepts an arguments parameter (declared in its own schema) is left alone. Implemented via _tool_schema_property_names() which introspects self.tools.

Inner non-string values are stringified to preserve the _parse_invoke_params output contract (string-valued dicts); downstream _convert_params_with_schema continues to handle type coercion via the tool's schema.

DeepSeekV4ToolParser inherits from DeepSeekV32ToolParser so this fix applies to V4 too without code duplication.

Test Plan

tests/tool_parsers/test_deepseekv32_tool_parser.py adds TestLegacyArgumentsUnwrap with 9 cases:

Test	What it covers
test_unwraps_legacy_nested_arguments	Happy path: single arguments key with JSON-object value → unwrapped
test_normal_flat_args_untouched	Regression: flat args unchanged
test_multi_key_with_arguments_untouched	Gate 1: multi-key payload left alone
test_tool_with_arguments_in_schema_untouched	Gate 4: tool legitimately accepting arguments param left alone
test_arguments_value_not_json_untouched	Gate 3: non-JSON values left alone
test_arguments_value_json_array_untouched	Gate 3: JSON arrays (vs objects) left alone
test_arguments_value_empty_object_untouched	Gate 3: empty objects left alone (no useful content)
test_legacy_and_flat_round_trip_to_same_args	End-to-end: legacy and flat forms produce identical function.arguments after type coercion
test_unwraps_in_streaming_path	Streaming path also benefits (uses the existing extract_tool_calls_streaming harness pattern from TestExtractToolCallsStreaming)

Test Result

$ python3 -m pytest tests/tool_parsers/test_deepseekv32_tool_parser.py -v
...
======================== 55 passed, 2 warnings in 5.12s ========================

All 55 tests pass: 9 new + 46 pre-existing. No regressions.

End-to-end smoke against the running production deployment (vllm/vllm-openai:deepseekv4-arm64-cu130 image with this patch bind-mounted):

$ curl -sS http://localhost:8000/v1/chat/completions -H 'Content-Type: application/json' -d '{
    "model":"deepseek-v4-flash",
    "messages":[{"role":"user","content":"What is the weather in Paris? Use the tool."}],
    "tools":[{"type":"function","function":{"name":"get_weather","parameters":{"type":"object","properties":{"city":{"type":"string"}},"required":["city"]}}}],
    "tool_choice":"auto","max_tokens":128
  }' | jq '.choices[0].message.tool_calls[0].function.arguments'
"{\"city\": \"Paris\"}"

Streaming path produces the same flat shape after fragment reassembly.

[claude]

Claude Code Review

This pull request is from a fork — automated review is disabled. A repository maintainer can comment @claude review to run a one-time review.

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[gemini-code-assist]

Code Review

This pull request implements a normalization layer in the DeepSeek-V3/V2 tool parser to unwrap legacy nested-arguments, ensuring compatibility with dispatchers expecting flat parameter structures. This logic is applied to both streaming and non-streaming paths and includes a safety check against tool schemas. The review feedback correctly points out that the schema property lookup should be expanded to support FunctionTool objects in addition to ChatCompletionToolsParam to ensure the safety gate functions correctly for all supported tool types.

[gemini-code-assist]

The _tool_schema_property_names method only handles tools that have a .function attribute (such as ChatCompletionToolsParam). However, self.tools can also contain FunctionTool objects, which store name and parameters at the top level. This means the safety gate in _maybe_unwrap_legacy_arguments will fail to correctly identify properties for FunctionTool tools, potentially leading to incorrect unwrapping of legitimate parameters named arguments for those tools.

Consider updating the logic to handle both tool structures, similar to how vllm/tool_parsers/utils.py:_extract_tool_info does it.

    def _tool_schema_property_names(self, function_name: str) -> set:
        """Return parameter names declared by the tool's schema, or empty set."""
        if not self.tools:
            return set()
        for tool in self.tools:
            # Handle both ChatCompletionToolsParam and FunctionTool structures
            if hasattr(tool, "function"):
                name = tool.function.name
                parameters = tool.function.parameters
            else:
                name = getattr(tool, "name", None)
                parameters = getattr(tool, "parameters", None)

            if name == function_name:
                if isinstance(parameters, dict) and isinstance(parameters.get("properties"), dict):
                    return set(parameters["properties"].keys())
                break
        return set()

[Lambdauv]

Thanks @gemini-code-assist — addressed the FunctionTool shape concern.

Updated _tool_schema_property_names to recognize both supported tool shapes:

• ChatCompletionToolsParam (Chat Completions API) — schema under tool.function.{name,parameters}
• FunctionTool (Responses API, openai.types.responses) — name and parameters at top level

While fixing this I noticed _convert_params_with_schema (the type-coercion helper) had the same hasattr-only tool.function access, which would silently fail to find the schema for FunctionTool and default all params to string type. Applied the same shape-distinction pattern there for consistency. Out-of-scope strictly speaking, but it's the same one-line bug class and fixing only one and leaving the other obviously broken would be sloppy.

New regression test test_function_tool_shape_safety_gate exercises the safety gate with a real FunctionTool whose schema legitimately declares an arguments property — verifies the unwrap does NOT fire (the previous version of the safety gate would have unwrapped the payload incorrectly because it couldn't read the FunctionTool's schema). All 10 tests in TestLegacyArgumentsUnwrap pass; all 56 tests in the file pass under pytest 9.0 in the v0.20.1 image.

Force-pushed; pre-commit clean (ruff, ruff-format, typos, mypy-3.10, SPDX, lazy-imports, forbidden-imports, etc.). Ready for re-review.

[gemini-code-assist]

Code Review

This pull request introduces a normalization mechanism to the DeepSeek-V3/V4 tool parser to handle cases where the model emits legacy OpenAI-style nested "arguments" payloads. The implementation includes a safety check to avoid unwrapping if the tool's schema explicitly defines an "arguments" parameter, and it covers both streaming and non-streaming execution paths with comprehensive tests. Feedback was provided to improve the robustness of the tool schema lookup, as the current implementation may fail to correctly identify parameters for certain tool object types like FunctionTool.

I am having trouble creating individual review comments. Click here to see my feedback.

vllm/tool_parsers/deepseekv32_tool_parser.py (176-185)

The current implementation of _tool_schema_property_names only handles tools that have a .function attribute (typically ChatCompletionToolsParam). However, self.tools can also contain FunctionTool objects, which have name and parameters at the top level. This causes the safety gate to fail for FunctionTool objects, which could lead to incorrect unwrapping of a legitimate arguments parameter if it's defined in the tool's schema. Using a more robust check that handles both types is recommended.

        for tool in self.tools:
            # Handle both ChatCompletionToolsParam (has .function) and FunctionTool (direct)
            fn = getattr(tool, "function", tool)
            if (getattr(fn, "name", None) == function_name 
                    and hasattr(fn, "parameters")):
                schema = fn.parameters
                if isinstance(schema, dict) and isinstance(schema.get("properties"), dict):
                    return set(schema["properties"].keys())
                break

[sfeng33]

Closing as fixed in #41801