deepset-ai/haystack · error · PipelineConnectError

Connection must have both sender and receiver: ${sender_repr

Error message

Connection must have both sender and receiver: ${sender_repr} -> ${receiver_repr}

What it means

Pipeline.connect() raises PipelineConnectError when only one side of the connection could be resolved — i.e. the sender or receiver portion resolves to 'input'/'output' but not both components with sockets. A valid connection needs both a resolved sender (component+socket) and a resolved receiver (component+socket).

Source

Thrown at haystack/core/pipeline/base.py:757

            # Get the only possible match
            sender_socket = name_matches[0][0]
            receiver_socket = name_matches[0][1]
            conversion_strategy = name_matches[0][2]

        # Connection must be valid on both sender/receiver sides
        if not sender_socket or not receiver_socket or not sender_component_name or not receiver_component_name:
            if sender_component_name and sender_socket:
                sender_repr = f"{sender_component_name}.{sender_socket.name} ({_type_name(sender_socket.type)})"
            else:
                sender_repr = "input needed"

            if receiver_component_name and receiver_socket:
                receiver_repr = f"({_type_name(receiver_socket.type)}) {receiver_component_name}.{receiver_socket.name}"
            else:
                receiver_repr = "output"
            msg = f"Connection must have both sender and receiver: {sender_repr} -> {receiver_repr}"
            raise PipelineConnectError(msg)

        logger.debug(
            "Connecting '{sender_component}.{sender_socket_name}' to '{receiver_component}.{receiver_socket_name}'",
            sender_component=sender_component_name,
            sender_socket_name=sender_socket.name,
            receiver_component=receiver_component_name,
            receiver_socket_name=receiver_socket.name,
        )

        if receiver_component_name in sender_socket.receivers and sender_component_name in receiver_socket.senders:
            # This is already connected, nothing to do
            return self

        if receiver_socket.senders:
            receiver_socket = self._make_socket_auto_variadic(
                component_name=receiver_component_name, receiver_socket=receiver_socket, error_type=PipelineConnectError
            )

View on GitHub (pinned to e318778c9b)

Solutions

Provide full 'component.socket' names for BOTH sender and receiver so both sides resolve
Remove any literal 'input'/'output' placeholders copied from 1.x pipelines
Verify both component names and socket names exist on their components

Example fix

// before
pipeline.connect('retriever', 'output')
// after
pipeline.connect('retriever.documents', 'writer.documents')

Defensive patterns

Strategy: validation

Validate before calling

parts_s, parts_r = sender_spec.split('.'), receiver_spec.split('.')
assert all(len(p) == 2 and p[0] not in ('input', 'output') for p in (parts_s, parts_r)), 'Provide full component.socket names for both sides'

Type guard

def is_full_socket_spec(spec: str) -> bool:
    comp, _, sock = spec.partition('.')
    return bool(comp and sock) and comp not in ('input', 'output')

Try / catch

try:
    pipeline.connect(sender_spec, receiver_spec)
except PipelineConnectError as e:
    if 'must have both sender and receiver' in str(e):
        raise ValueError('Use full component.socket notation on both sides') from e

Prevention

Avoid 1.x 'input'/'output' endpoint conventions in 2.x pipelines
Always use component.socket on both sides when partially specifying
Migrate legacy pipeline definitions systematically

When it happens

Trigger: Using the legacy 'component.socket' shorthand with 'input'/'output' literals such as pipeline.connect('comp.input', 'output') or similar partial notations that resolve only one side, in a haystack version that still accepts the input/output keyword forms but finds the other side missing.

Common situations: Mixing old deepset-ai/haystack 1.x pipeline.connect('a', 'b.c') conventions with 2.x semantics; passing 'input'/'output' placeholder strings copied from old code or docs; partially commenting out one side of a connection.

Related errors

AI-assisted analysis of deepset-ai/haystack@e318778c9b (2026-08-30). Data as JSON: /api/errors/e24f37ec7aa3e2d6. Report an issue: GitHub.