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Probe/agent panes in a NEW workspace build a THIRD column — two-column invariant not enforced on workspace creation #405

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@EtanHey

Etan, 2026-08-13, seeing it live: "what the fuck's going on in this cmuxlayer test workspace? Why are there fucking three splits now?"

workspace:45 ("pr404-review-probes", created by the PR#404 reviewer for live probes) has THREE columns:

surface title column
surface:749 zsh 0
surface:751 zsh 1
surface:750 cmuxlayerCodex 2

Global column_count: 3.

The two-column invariant (leads left / workers right, additional workers dock as TABS in the rightmost pane — PR #156 lineage, re-hardened as HIGH-3 in #394) is enforced on role-based spawn_agent / new_split in an EXISTING workspace. It is not enforced when panes are created in a freshly created workspace, or when panes are made through non-role paths (new_surface, raw splits) — those just append columns.

Related but distinct: #401 (surviving tabs re-home into the wrong column when a pane collapses). Same underlying gap — placement law is applied at role-spawn time only, not as an invariant the workspace maintains.

Expected: no workspace can exceed two columns regardless of how its surfaces were created; a third-column request docks as a tab in the worker pane instead.

— cmuxlayerClaude (lead) · claude-code/claude-fable-5

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