deepset-ai/haystack · error · ValueError

Invalid template for output: {output!r} (type: {type(output)

Error message

Invalid template for output: {output!r} (type: {type(output).__name__}). Output must be a string representing a valid Jinja2 template. For example, use {str(output)!r} instead of {output!r}.

What it means

ConditionalRouter validates each route's 'output' as a Jinja2 template string in _validate_routes (called from __init__) unless output_passthrough is set. A non-string output (or an invalid template string) raises this ValueError with the value and its type.

Source

Thrown at haystack/components/routers/conditional_router.py:527

            # Condition is always a Jinja2 template — validate it
            if not self._validate_template(self._env, route["condition"]):
                condition_value = route["condition"]
                if not isinstance(condition_value, str):
                    raise ValueError(
                        f"Invalid template for condition: {condition_value!r} (type: {type(condition_value).__name__})."
                        f"Condition must be a string representing a valid Jinja2 template. "
                        f"For example, use {str(condition_value)!r} instead of {condition_value!r}."
                    )
                raise ValueError(f"Invalid template for condition: {condition_value}")

            # Only validate output as Jinja2 template when output_passthrough is False (default)
            output_passthrough = route.get("output_passthrough", False)
            if not output_passthrough:
                for output in outputs:
                    if not self._validate_template(self._env, output):
                        if not isinstance(output, str):
                            raise ValueError(
                                f"Invalid template for output: {output!r} (type: {type(output).__name__}). "
                                f"Output must be a string representing a valid Jinja2 template. "
                                f"For example, use {str(output)!r} instead of {output!r}."
                            )
                        raise ValueError(f"Invalid template for output: {output}")

    @staticmethod
    def _extract_variables(env: Environment, templates: list[str]) -> set[str]:
        """
        Extracts all variables from a list of Jinja template strings.

        :param env: A Jinja environment.
        :param templates: A list of Jinja template strings.
        :returns: A set of variable names.
        """
        variables = set()
        for template in templates:
            assigned_variables, template_variables = _extract_template_variables_and_assignments(

View on GitHub (pinned to e318778c9b)

Solutions

1. Make the route output a string, e.g. '{{ result }}' instead of the raw value
2. Use the str() representation shown in the error message
3. Set output_passthrough=True only if you intentionally want non-templated output

Example fix

# before
{"condition": "{{ ok }}", "output": 42, "output_name": "out", "output_type": int}
# after
{"condition": "{{ ok }}", "output": "{{ result }}", "output_name": "out", "output_type": int}

outputs = [r.get("output") for r in routes if not r.get("output_passthrough", False)]
assert all(isinstance(o, str) for o in outputs), "route output must be a Jinja2 template string"

def is_valid_output(output) -> bool:
    return isinstance(output, str)

try:
    router = ConditionalRouter(routes=routes)
except ValueError as e:
    if "Invalid template for output" in str(e):
        routes = [{**r, "output": str(r["output"])} for r in routes if not r.get("output_passthrough", False)] + [r for r in routes if r.get("output_passthrough", False)]
        router = ConditionalRouter(routes=routes)
    else:
        raise

When it happens

Trigger: Passing a non-string as route 'output', e.g. {'output': 42} or {'output': None}, while output_passthrough is False (default).

Common situations: Programmatic route construction where output is mistakenly a number or object; deserialized configs with wrong types.

Related errors

• Invalid template for condition: {condition_value}
• Invalid template for output: {output}
• The list of mime types cannot be empty.
• At least one of 'mime_type_meta_field' or 'file_path_meta_fi
• The list of mime types cannot be empty.

AI-assisted analysis of deepset-ai/haystack@e318778c9b (2026-08-30). Data as JSON: /api/errors/820b5df40d14906b. Report an issue: GitHub.