langchain-ai/langchain · error · ValueError

Partial variables are not supported for list of templates.

Error message

Partial variables are not supported for list of templates.

What it means

In BaseChatPromptTemplate.from_template / ChatPromptTemplate.from_messages, when the template is a sequence of message templates (a list), passing non-empty partial_variables raises ValueError. Partials are only wired through the single-string branch (delegated to PromptTemplate.from_template); the list branch has no mechanism to distribute partials across heterogeneous message templates.

Source

Thrown at libs/core/langchain_core/prompts/chat.py:451

        Raises:
            ValueError: If the template is not a string or list of strings.
        """
        prompt: (
            StringPromptTemplate
            | list[StringPromptTemplate | ImagePromptTemplate | DictPromptTemplate]
        )
        if isinstance(template, str):
            prompt = PromptTemplate.from_template(
                template,
                template_format=template_format,
                partial_variables=partial_variables,
            )
            return cls(prompt=prompt, **kwargs)
        if isinstance(template, Sequence):
            if (partial_variables is not None) and len(partial_variables) > 0:
                msg = "Partial variables are not supported for list of templates."
                raise ValueError(msg)
            prompt = []
            for tmpl in template:
                if isinstance(tmpl, str) or (
                    isinstance(tmpl, dict)
                    and "text" in tmpl
                    and set(tmpl.keys()) <= {"type", "text"}
                ):
                    if isinstance(tmpl, str):
                        text: str = tmpl
                    else:
                        text = cast("_TextTemplateParam", tmpl)["text"]  # type: ignore[assignment]
                    prompt.append(
                        PromptTemplate.from_template(
                            text, template_format=template_format
                        )
                    )
                elif (
                    isinstance(tmpl, dict)

View on GitHub (pinned to e32fa9a52e)

Solutions

1. Drop partial_variables from the from_messages/from_template call and call .partial(**vars) on the resulting ChatPromptTemplate instead
2. Or bake the partial values directly into the message template strings

Example fix

# before
ChatPromptTemplate.from_messages(
    ["system: Answer in {lang}", "human: {q}"],
    partial_variables={"lang": "English"},  # ValueError
)

# after
(ChatPromptTemplate.from_messages(
    ["system: Answer in {lang}", "human: {q}"],
).partial(lang="English"))

def build_chat_prompt(messages, partials: dict | None = None):
    if isinstance(messages, (list, tuple)) and partials:
        return ChatPromptTemplate.from_messages(messages).partial(**partials)
    return ChatPromptTemplate.from_messages(messages, partial_variables=partials)

When it happens

Trigger: Calling ChatPromptTemplate.from_messages([...], partial_variables={'lang': 'en'}) or from_template(list_of_templates, partial_variables={...}) with a non-empty dict and a non-None value.

Common situations: Developers trying to share a common partial (like a date or locale) across a multi-message prompt in one call, following the single-string API ergonomics.

Related errors

• Cannot save prompt with partial variables.
• variable {self.variable_name} should be a list of base messa
• Only one format variable allowed per image template.\nGot: {
• Invalid image template: {tmpl}
• jinja2 is unsafe and is not supported for templates expresse

AI-assisted analysis of langchain-ai/langchain@e32fa9a52e (2026-08-14). Data as JSON: /api/errors/f53b55f1b24bde30. Report an issue: GitHub.