[6uclz1]

Problem

arrangement clip create TRACK --start S --length L --audio-path ABS ignored --length on audio tracks. Live 12 sizes a new arrangement audio clip to the file's warped length: a 3 s WAV at 120 BPM gives an 8-beat clip. The old code tried clip.length = L. Live raises on that because length is read-only, so the code fell back to clip.end_marker = L. It then returned success with length_set: false, end_marker_set: true, while the clip still covered 8 beats.

What Live accepted (Live 12 Suite, 2026-10-24, scratch track)

Write	Result
clip.length = L	raises (read-only)
end_marker = L with looping on	span unchanged; only the marker moves
end_marker = L with looping off	span unchanged
loop_end = loop_start + L with looping on	span changes to L, both shorter and longer (8→3, 8→4, 8→6, 8→12, 8→16 beats); length reads back as L. A single write works on a fresh clip.
loop_end on an unwarped clip	measured in seconds and clamped to the file duration (3 s file: loop_end stayed 3.0, length 6 beats)

The arrangement span follows loop_end - loop_start.

Change

• After track.create_audio_clip, the backend writes loop_end = loop_start + length and reads back length. If they don't match, it fails with INVALID_ARGUMENT. details gives track, index, requested_length, actual_length, warping, looping and, when the write raised, write_error. The hint says how to recover (warp the clip, then arrangement clip loop set, or delete it).
• The success result no longer carries requested_length / length_set / end_marker_set. Its length is the requested length, which the read-back check guarantees.
• The new clip is found by start time with fix: arrangement clip create targets the new clip; clarify set-display #133's _created_arrangement_clip_index, and the span is set on that clip, so creating a clip before an existing one leaves the existing clip alone.
• arrangement from-session gets the fix too, since it places audio through the same path.
• Test stub _ArrangementAudioClip models Live: length is a read-only property equal to loop_end - loop_start, and a new clip starts at the warped length of 8 beats. _LoopEndIgnoringArrangementAudioClip models a write that Live accepts but ignores.
• README: one paragraph on how --length is applied to audio clips and when it fails.

Public contract impact

Small, intentional. Three undocumented diagnostic fields are removed from the arrangement clip create audio result, and a length mismatch is now an error instead of ok: true. The command surface snapshot and generated skill docs are unchanged. docs/computer-use/cli-gaps.md is unchanged because the CLI can do this without the GUI.

Validation

• uv run python -m ableton_cli.dev_checks, ruff check ., ruff format --check ., pytest (1526 passed, rebased on main 8d55f09), quality-harness dev_checks_enforce: all pass.
• tools/generate_skill_docs.py and tools/update_public_contract_snapshot.py: no diff.
• New tests failed before the implementation (red → green). fix: arrangement clip create targets the new clip; clarify set-display #133's out-of-order audio test now gives the existing clip 12 beats, not the default 8, so it only passes if the new clip alone is resized: [(0.0, 4.0), (16.0, 12.0)].
• Live: installed this branch's Remote Script, restarted Live, appended a scratch audio track, and created clips from a 3 s WAV with --length 4, 12 and 8. arrangement clip props get read back length 4.0 / 12.0 / 8.0, loop_end 4.0 / 12.0 / 8.0, looping true.
• Live, after rebasing onto fix: arrangement clip create targets the new clip; clarify set-display #133: reinstalled the Remote Script, restarted Live, and created clips on an appended scratch track out of start-time order (start 32 / len 12, then 0 / 4, then 16 / 6). arrangement clip list gives [(0, 0.0, 4.0), (1, 16.0, 6.0), (2, 32.0, 12.0)], and props get gives loop_end 4 / 6 / 12. Each clip has its requested length.
• Not exercised in Live: the mismatch error path. A new clip is auto-warped, so create_audio_clip didn't produce an unwarped clip. That path is covered by the stub test.

Interaction with #133

Rebased onto main after #133 merged. The audio branch uses #133's start-time lookup instead of the old clips[before_count].

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