openai/openai-python · error · TypeError

Unexpected list delta entry is not a dictionary: {delta_entr

Error message

Unexpected list delta entry is not a dictionary: {delta_entry}

What it means

Same accumulator check as in _assistants.py, raised by accumulate_delta in _deltas.py (used by Chat Completions and Responses streaming accumulation): a list delta entry that is not a dictionary cannot be index-merged, so it is rejected with this TypeError.

Source

Thrown at src/openai/lib/streaming/_deltas.py:42

            acc[key] = delta_value
            continue

        if isinstance(acc_value, str) and isinstance(delta_value, str):
            acc_value += delta_value
        elif isinstance(acc_value, (int, float)) and isinstance(delta_value, (int, float)):
            acc_value += delta_value
        elif is_dict(acc_value) and is_dict(delta_value):
            acc_value = accumulate_delta(acc_value, delta_value)
        elif is_list(acc_value) and is_list(delta_value):
            # for lists of non-dictionary items we'll only ever get new entries
            # in the array, existing entries will never be changed
            if all(isinstance(x, (str, int, float)) for x in acc_value):
                acc_value.extend(delta_value)
                continue

            for delta_entry in delta_value:
                if not is_dict(delta_entry):
                    raise TypeError(f"Unexpected list delta entry is not a dictionary: {delta_entry}")

                try:
                    index = delta_entry["index"]
                except KeyError as exc:
                    raise RuntimeError(f"Expected list delta entry to have an `index` key; {delta_entry}") from exc

                if not isinstance(index, int):
                    raise TypeError(f"Unexpected, list delta entry `index` value is not an integer; {index}")

                try:
                    acc_entry = acc_value[index]
                except IndexError:
                    acc_value.insert(index, delta_entry)
                else:
                    if not is_dict(acc_entry):
                        raise TypeError("not handled yet")

                    acc_value[index] = accumulate_delta(acc_entry, delta_entry)

View on GitHub (pinned to 9917c6e28e)

Solutions

  1. Upgrade the openai SDK to the latest version
  2. Make every list delta entry a dict with an integer index key
  3. Catch TypeError and fall back to replacing the accumulated list
  4. Report reproducible payloads to OpenAI

Example fix

# before
acc = accumulate_delta(acc, {"choices": ["bad"]})

# after
acc = accumulate_delta(acc, {"choices": [{"index": 0, "delta": {"content": "ok"}}]})
Defensive patterns

Strategy: validation

Validate before calling

def list_delta_ok(value: list) -> bool:
    return all(isinstance(e, dict) for e in value)

Type guard

def is_scalar_list(value: list) -> bool:
    return all(isinstance(x, (str, int, float)) for x in value)

Try / catch

try:
    acc = accumulate_delta(acc, delta)
except TypeError:
    acc[key] = copy.deepcopy(delta[key])

Prevention

When it happens

Trigger: Streaming chat/responses where a list-valued delta contains non-dict entries after the scalar fast-path no longer applies; schema changes in streaming output (e.g. content parts, tool call arguments lists); custom test deltas with scalar entries.

Common situations: Pinned older SDK against a newer API; beta features emitting new list shapes; hand-written delta fixtures.

Related errors

AI-assisted analysis of openai/openai-python@9917c6e28e (2026-08-28). Data as JSON: /api/errors/e9d842510a6d10ed. Report an issue: GitHub.