#8363 – Preserve tail bond length after Arrange as a Ring

[Copilot]

• Run black formatting check on arrange_ring_tail_length.py (black missing, installed)
• Apply black formatting to arrange_ring_tail_length.py
• Run targeted arrange_ring_tail_length integration test
• Update HELM string concatenation for clarity
• Run code review + CodeQL

Original prompt

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This section details on the original issue you should resolve

<issue_title>Bond lenth become wrong after Arrange as a Ring option applied</issue_title>
<issue_description>Steps to Reproduce

1. Open Macromolecules mode - Flex canvas (clean canvas)
2. Load following HELM: PEPTIDE1{C.C.C.C.C.C.C.C.C.C.C.C.C}$PEPTIDE1,PEPTIDE1,5:R3-8:R3$$$V2.0

3. Select ring and right tail of the chain

4. Call context menu and click Arrange as a Ring option

Actual behavior
System layout monomer in the ring as expected but distance (e.g. bond length) between monomer in the tail is less than standard monomer size

Expected behavior
System layout monomer in the ring as expected, distance (e.g. bond length) between monomer has the same standard length

Related issue: epam/ketcher#8363

Environment details:

• Ketcher Version 3.12.0-rc.1 Build at 2026-01-08; 17:35:48
• Indigo Version 1.40.0-rc.1.0-g63d8f2068-wasm32-wasm-clang-19.0.0
• Chrome Version 143.0.7499.170 (Official Build) (64-bit)
• Win10</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes Bond length become wrong after Arrange as a Ring option applied #3449

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[AlexeyGirin]

@copilot please format changes in api/tests/integration/tests/layout/arrange_ring_tail_length.py file in black way

[Copilot]

@copilot please format changes in api/tests/integration/tests/layout/arrange_ring_tail_length.py file in black way

Applied black formatting to arrange_ring_tail_length.py and kept it formatted after the latest tweaks (commit 582f01e).