deepset-ai/haystack · error · PipelineConnectError

'${sender}' does not exist. Output connections of ${sender_c

Error message

'${sender}' does not exist. Output connections of ${sender_component_name} are: {}

What it means

Pipeline.connect() raises PipelineConnectError when an explicitly given sender socket name (in 'component.socket' syntax) does not match any output socket of the sender component. The error lists the actual available output connections with their types.

Source

Thrown at haystack/core/pipeline/base.py:644

            raise ValueError(f"Component named {sender_component_name} not found in the pipeline.") from exc
        try:
            receiver_sockets = self.graph.nodes[receiver_component_name]["input_sockets"]
        except KeyError as exc:
            raise ValueError(f"Component named {receiver_component_name} not found in the pipeline.") from exc

        if not sender_sockets:
            raise PipelineConnectError(
                f"'{sender_component_name}' does not have any output connections. "
                f"Please check that the output types of '{sender_component_name}.run' are set, "
                f"for example by using the '@component.output_types' decorator."
            )

        # If the name of either socket is given, get the socket
        sender_socket: OutputSocket | None = None
        if sender_socket_name:
            sender_socket = sender_sockets.get(sender_socket_name)
            if not sender_socket:
                raise PipelineConnectError(
                    f"'{sender}' does not exist. "
                    f"Output connections of {sender_component_name} are: "
                    + ", ".join([f"{name} (type {_type_name(socket.type)})" for name, socket in sender_sockets.items()])
                )

        receiver_socket: InputSocket | None = None
        if receiver_socket_name:
            receiver_socket = receiver_sockets.get(receiver_socket_name)
            if not receiver_socket:
                raise PipelineConnectError(
                    f"'{receiver} does not exist. "
                    f"Input connections of {receiver_component_name} are: "
                    + ", ".join(
                        [f"{name} (type {_type_name(socket.type)})" for name, socket in receiver_sockets.items()]
                    )
                )

        # Look for a matching connection among the possible ones.

View on GitHub (pinned to e318778c9b)

Solutions

Read the 'Output connections of X are: ...' list in the error and use one of those exact socket names
Inspect the sender's run() @component.output_types to see valid socket names
Check haystack changelog if a component's output names changed between versions
Drop the '.socket' suffix and let Haystack auto-match if only one output exists

Example fix

// before
pipeline.connect('llm.replie', 'prompt_builder')
// after
pipeline.connect('llm.replies', 'prompt_builder')

Defensive patterns

Strategy: validation

Validate before calling

comp = pipeline.get_component(sender)
valid = set(comp.__haystack_output__._sockets_dict)
assert socket_name in valid, f"{socket_name!r} not an output of {sender}; valid: {valid}"

Type guard

def output_exists(pipeline, sender: str, socket: str) -> bool:
    return socket in pipeline.get_component(sender).__haystack_output__._sockets_dict

Try / catch

try:
    pipeline.connect(f'{sender}.{socket}', receiver)
except PipelineConnectError as e:
    logger.error('%s; valid outputs listed in error: %s', e, e)

Prevention

Read the available output names from the error message
Check the component's @component.output_types declaration
Pin haystack versions and review changelogs for socket renames

When it happens

Trigger: Calling pipeline.connect('sender.wrong_socket', 'receiver') where 'wrong_socket' is not a key in the sender's output_sockets dict, usually a typo or an outdated socket name.

Common situations: Typo in the socket name; assuming a socket name that differs from the dict key returned by run(); library upgrade changed a component's output socket names; auto-generated type names like 'documents' vs 'replies'.

Related errors

AI-assisted analysis of deepset-ai/haystack@e318778c9b (2026-08-30). Data as JSON: /api/errors/6fe4d473b40fbba6. Report an issue: GitHub.