[FEATURE]: Explicit setting to keep shell output and error/failed output collapsed #55089

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Describe the enhancement you want to request

Problem

Both error output and shell output render expanded in my client, and I want an explicit, easy-to-find, persistent choice to keep them collapsed instead of being locked into a default-expanded state.

Findings from the source (dev, 2026-10-13):

• Error / failed output has no user-facing setting. The timeline display policy (packages/session-ui/src/timeline/detail.ts) only defines a details: collapsed | expanded field for shell, edit and thinking. There is no errors / failures category, and the settings UI (packages/app/src/settings/timeline-detail.tsx) only renders the "Collapse" switch for those three categories. Error output expansion is therefore decided entirely by hard-coded rules (see below) and cannot be configured.
• Failures reported through metadata are not handled as errors. A shell command with a non-zero exit code (shellResultFailed) and Code Mode execute failures (executeToolFailed) keep state.status === "completed", so they do not take the error branch of currentContentDefaultOpen() (packages/session-ui/src/message/current-tool-state.ts) and instead inherit the shell/execute expansion. In addition, SessionShellMessage (packages/session-ui/src/tools/tool-renderer.tsx) always renders a ToolErrorCard for a non-zero exit, independent of the collapse setting.
• The TUI only has transient controls. /details, the tool_details keybind, session.tool_details and the tool_expand_output keybind (feat(tui): add keybind to expand/collapse tool output blocks #52725) are not persisted as a config default, and there is no control for error/failed output at all.

This area has been a recurring source of issues and PRs: #17003 (error tool cards did not respect the expand setting), #6908, #7286, #11942, #34246, #42552, #49474, #52725, plus the still-open #45538 / #45872 / #51229.

Proposal

Make expanded/collapsed a persistent, explicit, per-category default that includes error/failed output, so users can choose (and keep) a compact view.

1. Desktop / web: add an errors / failures category to the timeline display policy (packages/session-ui/src/timeline/detail.ts) with its own details: collapsed | expanded default and a matching "Collapse" switch in the advanced settings, and make metadata-reported failures (shell non-zero exit, Code Mode execute failure) follow it instead of the shell/execute expansion. Illustrative shape:

   {
     "timeline": {
       "shell": { "placement": "separate", "details": "collapsed" },
       "errors": { "placement": "separate", "details": "collapsed" }
     }
   }

2. TUI: add a persistent config default (e.g. under tui.json / cli.json) covering shell output and error/failed output, mirroring the existing session.tool_details / session.thinking options. Illustrative shape:

   {
     "session": {
       "tool_details": "collapse",
       "error_output": "collapse"
     }
   }

3. Keep expanded/collapsed a per-item click/toggle layered on top of the default, and preserve the current defaults for anyone who does not opt in.

Benefits

• Users can keep successful tool activity and error output compact, or expand everything — their choice, persisted across sessions and set independently per category.
• Removes the inconsistency where error output follows hard-coded logic or the shell setting instead of the user's display preferences (App: error tool cards do not respect 'expand tool' setting #17003).
• Consolidates the many ad-hoc toggles into one predictable, documented policy per client.

Alternatives considered

• Per-tool-type display policy (show/collapse/hide) — [FEATURE] Configurable per-tool-type display policy (show/collapse/hide) #35193 (closed); it does not cover the success/failure distinction.
• Auto-collapse completed activity ([FEATURE]: Auto-collapse all completed response activity into one summary row in Desktop #49088, Transcript: auto-collapse executed tool groups + show result in group ticker #54515) — reduces noise after the fact but does not give control over error output.

Related

#35193, #45538, #45872, #51229, #51308, #49088, #54515, #17003 and PRs #52725 / #42552 / #34246.