deepset-ai/haystack · error · PipelineValidationError

'{type(instance)}' doesn't seem to be a component. Is this c

Error message

'{type(instance)}' doesn't seem to be a component. Is this class decorated with @component?

What it means

Pipeline components must be instances of classes decorated with @component (i.e. implement the Component protocol). If the object passed to add_component is not such an instance, PipelineValidationError is raised so the mistake is caught at wiring time instead of at run time.

Source

Thrown at haystack/core/pipeline/base.py:490

    def _validate_component(self, name: str, instance: Component) -> bool:
        """Validate a component before adding it, returning whether it needs to be added."""
        # Component names are unique
        if name in self.graph.nodes:
            if self.graph.nodes[name]["instance"] is instance:
                return False
            raise ValueError(f"A component named '{name}' already exists in this pipeline: choose another name.")

        # Components can't be named `_debug`
        if name == "_debug":
            raise ValueError("'_debug' is a reserved name for debug output. Choose another name.")

        # Component names can't have "."
        if "." in name:
            raise ValueError(f"{name} is an invalid component name, cannot contain '.' (dot) characters.")

        # Component instances must be components
        if not isinstance(instance, Component):
            raise PipelineValidationError(
                f"'{type(instance)}' doesn't seem to be a component. Is this class decorated with @component?"
            )

        if owning_pipeline := getattr(instance, "__haystack_added_to_pipeline__", None):
            if owning_pipeline is self:
                existing_name = self.get_component_name(instance)
                msg = (
                    f"Component has already been added to this Pipeline under the name '{existing_name}'. "
                    "A component instance can only be added once."
                )
            else:
                msg = (
                    "Component has already been added in another Pipeline. "
                    "Components can't be shared between Pipelines. Create a new instance instead."
                )
            raise PipelineError(msg)

        return True

View on GitHub (pinned to e318778c9b)

Solutions

1. Instantiate the class: pipe.add_component("c", MyComp())
2. Decorate the custom class with @component and implement run()
3. Verify the object's type has the __haystack_supports_can_run__ / Component protocol before adding

Example fix

// before
pipe.add_component("converter", TextConverter)  # class, not instance
// after
pipe.add_component("converter", TextConverter())

from haystack.core.component import Component

def is_component_instance(obj) -> bool:
    return isinstance(obj, Component) and not isinstance(obj, type)

def is_component(obj: object) -> bool:
    return isinstance(obj, Component)

from haystack.core.errors import PipelineValidationError
try:
    pipe.add_component(name, comp)
except PipelineValidationError as e:
    logger.error("Not a component: %s", e)
    raise

When it happens

Trigger: Passing a class instead of an instance: pipe.add_component("c", MyComp) without parentheses; passing an undecorated class instance; passing None or a dict of components by mistake.

Common situations: Forgetting to instantiate the component class; forgetting the @component decorator on a custom component after a refactor; swapping in a mock or plain object in tests.

Related errors

• Component instance cannot be added to the pipeline more than
• Component has already been added to this Pipeline under the
• There is no component named '{name}' in the pipeline. The va
• Connecting a Component to itself is not supported.
• Component named {sender_component_name} not found in the pip

AI-assisted analysis of deepset-ai/haystack@e318778c9b (2026-08-30). Data as JSON: /api/errors/0e1372c56c1fc60b. Report an issue: GitHub.