Description

When the autonumber feature is enabled in a sequence diagram, bidirectional arrows (<<->>) do not render as expected. The arrowhead at the start of the message is missing, and the sequence number is only placed at the beginning of the interaction. This makes the bidirectional flow appear as a one-directional message, which can cause confusion when documenting protocols or data flows that are inherently two-way.

Expected Behavior

A line with arrowheads at both ends should be drawn between participant A and participant B. The sequence number (e.g., "1") should be clearly associated with the entire bidirectional message.

Steps to reproduce

The issue can be reproduced with the following minimal code:

sequenceDiagram
    autonumber
    participant A
    participant B
    A<<->>B: This is a bidirectional message

Screenshots

Code Sample

Setup

• Mermaid version: v11.6.0
• Browser and Version: [Chrome, Edge, Firefox]

Suggested Solutions

No response

Additional Context

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