unslothai/unsloth · error · TypeError

no attempt rendered the continuation prefix

Error message

no attempt rendered the continuation prefix

What it means

Raised (as TypeError) by _render_continuation_manually when none of the kwarg attempts could even render the prefix (apply_chat_template on swept[:-1] with add_generation_prompt=True). It only runs after the primary _render failed with TypeError while continuing a final assistant message — so the tokenizer cannot render a generation prompt under any swept configuration.

Source

Thrown at studio/backend/core/inference/chat_template_helpers.py:2573

        raise RuntimeError("apply_chat_template_for_generation: no attempt produced a result")

    def _render_continuation_manually(msgs: list) -> str:
        """For tokenizers predating ``continue_final_message`` (TypeError above).

        Prefix and partial come from the SAME swept copy: an attempt that drops the tools
        kwarg re-sweeps for the default template, whose markup would otherwise survive raw.
        """
        for kwargs in attempts:
            swept = _swept_for(kwargs, msgs)
            try:
                prefix = tokenizer.apply_chat_template(
                    swept[:-1], tokenize = False, add_generation_prompt = True, **kwargs
                )
            except TypeError:
                continue
            partial = trailing_assistant_text(swept) or _continue_text
            return f"{strip_open_reasoning_prefill(prefix)}{partial}"
        raise TypeError("no attempt rendered the continuation prefix")

    def _render_with_fallback(msgs: list) -> str:
        try:
            return _render(msgs)
        except TypeError:
            if not _continuing:
                raise
            return _render_continuation_manually(msgs)

    try:
        return _render_with_fallback(messages)
    except Exception:
        # Retry with repairs applied cumulatively. Originals render first, so
        # working templates stay byte-identical.
        candidates: list = []
        normalized = _normalize_tool_call_arguments(messages)
        if normalized is not messages:
            candidates.append(normalized)

View on GitHub (pinned to 203007d190)

Solutions

1. Upgrade transformers to a version supporting continue_final_message so the primary _render path succeeds
2. Validate the messages list structure (roles/content present, no None) before calling generation
3. If using a custom tokenizer subclass, ensure apply_chat_template accepts add_generation_prompt and tokenize kwargs

def messages_renderable(tokenizer, msgs) -> bool:
    try:
        tokenizer.apply_chat_template(msgs[:-1], tokenize=False, add_generation_prompt=True)
        return True
    except TypeError:
        return False

try:
    prompt = apply_chat_template_for_generation(tokenizer, messages)
except TypeError as e:
    if "continuation prefix" in str(e):
        upgrade_transformers_hint()
        prompt = f"{tokenizer.apply_chat_template(messages[:-1], tokenize=False)}{trailing_text}"

When it happens

Trigger: Tokenizer's apply_chat_template always raises TypeError regardless of kwargs (custom subclass, ancient transformers); the swept copy differs from the template's expected input schema (e.g. role/content structure) so every render fails; continue_final_message unsupported AND the manual prefix render also impossible.

Common situations: Old transformers without continue_final_message paired with a template that requires it; malformed messages (missing content keys, None values) surviving the sweep; custom tokenizer overriding apply_chat_template.

Related errors

• apply_chat_template_for_generation: no attempt produced a re
• Model '{self.active_model_name}' has no chat_template set in
• the template produced an empty prompt
• apply_chat_template returned None — tokenizer may be incompa
• VirusTotal returned HTTP {status} for {_redact_url(url)}

AI-assisted analysis of unslothai/unsloth@203007d190 (2026-08-15). Data as JSON: /api/errors/37e92bd9155ae972. Report an issue: GitHub.