huggingface/smolagents · error · ValueError

Only 'markdown' is supported for a string argument to `code_

Error message

Only 'markdown' is supported for a string argument to `code_block_tags`.

What it means

CodeAgent.__init__ accepts code_block_tags as either a string or a tuple. The only accepted string is the sentinel 'markdown' (which maps to ('```python', '```')); any other string is rejected — custom tags must be passed as an explicit (opening, closing) tuple.

Source

Thrown at src/smolagents/agents.py:1557

        use_structured_outputs_internally: bool = False,
        code_block_tags: str | tuple[str, str] | None = None,
        **kwargs,
    ):
        self.additional_authorized_imports = additional_authorized_imports if additional_authorized_imports else []
        self.authorized_imports = sorted(set(BASE_BUILTIN_MODULES) | set(self.additional_authorized_imports))
        self.max_print_outputs_length = max_print_outputs_length
        self._use_structured_outputs_internally = use_structured_outputs_internally
        if self._use_structured_outputs_internally:
            prompt_templates = prompt_templates or yaml.safe_load(
                importlib.resources.files("smolagents.prompts").joinpath("structured_code_agent.yaml").read_text()
            )
        else:
            prompt_templates = prompt_templates or yaml.safe_load(
                importlib.resources.files("smolagents.prompts").joinpath("code_agent.yaml").read_text()
            )

        if isinstance(code_block_tags, str) and not code_block_tags == "markdown":
            raise ValueError("Only 'markdown' is supported for a string argument to `code_block_tags`.")
        self.code_block_tags = (
            code_block_tags
            if isinstance(code_block_tags, tuple)
            else ("```python", "```")
            if code_block_tags == "markdown"
            else ("<code>", "</code>")
        )

        super().__init__(
            tools=tools,
            model=model,
            prompt_templates=prompt_templates,
            planning_interval=planning_interval,
            **kwargs,
        )
        self.stream_outputs = stream_outputs
        if self.stream_outputs and not hasattr(self.model, "generate_stream"):
            raise ValueError(

View on GitHub (pinned to 30bb116109)

Solutions

Pass a tuple: code_block_tags=('<code>', '</code>') for custom tags.
Or use the string 'markdown' if you want the default ```python fences.
Or omit the parameter entirely to accept the default <code> tags.

Example fix

# before
agent = CodeAgent(model=model, tools=[], code_block_tags='xml')

# after
agent = CodeAgent(model=model, tools=[], code_block_tags=('<code>', '</code>'))

Defensive patterns

Strategy: validation

Validate before calling

assert isinstance(code_block_tags, tuple) or code_block_tags == 'markdown'
agent = CodeAgent(model=model, tools=[], code_block_tags=code_block_tags)

Type guard

def valid_code_block_tags(v) -> bool:
    return isinstance(v, tuple) and len(v) == 2 or v == 'markdown'

Prevention

Pass code_block_tags as an explicit (open, close) tuple for custom formats.
Reserve the string form for the 'markdown' sentinel only.
Omit the parameter to use defaults when unsure.

When it happens

Trigger: Constructing CodeAgent(model=..., code_block_tags='xml') or any string other than 'markdown'.

Common situations: Users assuming code_block_tags takes a format name like 'xml' or 'html'; migrating configurations where a single string tag was expected.

Related errors

AI-assisted analysis of huggingface/smolagents@30bb116109 (2026-08-28). Data as JSON: /api/errors/a5dbe14a6ffdf35f. Report an issue: GitHub.