BerriAI/litellm · error · ValueError

Chat provider: Invalid function argument delta {parsed_chunk

Error message

Chat provider: Invalid function argument delta {parsed_chunk}

What it means

For response.function_call_arguments.delta events the transformer requires a non-None 'delta' field carrying the argument fragment; if delta is missing or None it raises. The delta is the incremental tool-call argument string streamed by the provider.

Source

Thrown at litellm/completion_extras/litellm_responses_transformation/transformation.py:1315

                    choices=[
                        StreamingChoices(
                            index=0,
                            delta=Delta(
                                tool_calls=[
                                    ChatCompletionToolCallChunk(
                                        id=None,
                                        index=tool_call_index,
                                        type="function",
                                        function=ChatCompletionToolCallFunctionChunk(name=None, arguments=content_part),
                                    )
                                ]
                            ),
                            finish_reason=None,
                        )
                    ]
                )
            else:
                raise ValueError(f"Chat provider: Invalid function argument delta {parsed_chunk}")
        elif event_type == ResponsesAPIStreamEvents.OUTPUT_ITEM_DONE:
            # New output item added
            output_item = parsed_chunk.get("item", {})
            if output_item.get("type") in ("function_call", "custom_tool_call"):
                if tool_call_index_map is None:
                    # Stateless callers (the responses guardrail handler extracting
                    # tool calls from a buffered output_item.done) get the complete
                    # tool call; per-stream callers already received it via
                    # output_item.added and the argument delta events
                    return ModelResponseStream(
                        choices=[  # mutable-ok: ModelResponseStream coerces only list choices
                            StreamingChoices(
                                index=0,
                                delta=Delta(
                                    tool_calls=(
                                        _tool_call_dict_from_output_item(
                                            output_item, parsed_chunk.get("output_index", 0)
                                        ),

View on GitHub (pinned to 6c2dcb801b)

Solutions

1. Filter out delta events with no payload before transformation: skip if event.get('delta') is None
2. If behind a gateway, update it or report the event shape divergence
3. Upgrade litellm in case tolerances for empty deltas have been added

Example fix

# before
for ev in events:
    chunks = transformer.chunk_parser(ev, ...)

# after — tolerate providers that emit delta events without a payload
for ev in events:
    if ev.get("type") == "response.function_call_arguments.delta" and ev.get("delta") is None:
        continue
    chunks = transformer.chunk_parser(ev, ...)

def delta_event_valid(ev: dict) -> bool:
    return ev.get("type") != "response.function_call_arguments.delta" or ev.get("delta") is not None

try:
    out = transformer.chunk_parser(ev, ...)
except ValueError as e:
    if "Invalid function argument delta" in str(e):
        out = None  # tolerate provider emitting empty delta events
    else:
        raise

When it happens

Trigger: A provider or proxy emits response.function_call_arguments.delta events with the delta field omitted or null (e.g. an empty keep-alive delta or a non-conformant gateway rewriting events).

Common situations: Non-OpenAI providers whose Responses API emulation drops empty deltas instead of omitting the event; proxies that re-serialize events and lose null-adjacent fields; older fixtures recorded before the field was required.

Related errors

• Chat provider: Invalid text delta {parsed_chunk}
• Chat provider: Empty parsed_chunk
• Chat provider: Invalid chunk type {type(parsed_chunk)}
• Braintrust API error: {e.response.text}
• Failed to connect to Braintrust API: {str(e)}

AI-assisted analysis of BerriAI/litellm@6c2dcb801b (2026-08-15). Data as JSON: /api/errors/454d8a5eeb9f9a5a. Report an issue: GitHub.