Dynamic autosuggestions for slash command arguments

Problem

The argument-hint frontmatter for slash commands renders a static label after the command name. Useful for documentation, not interactive — no way to surface live, contextual values (ticket IDs, file paths, recent symbols, MCP-driven completions).

For users with custom workflow commands tied to issue trackers (Linear, GitHub Issues, Jira), this means typing the ticket ID by hand every time, or building a two-step "picker" command that breaks the input flow.

Proposal

Inline autosuggestions in the prompt area, fish/zsh-autosuggestions style:

1. History-based — as the user types, ghost-text completes from prompt history.
2. Dynamic via callback — slash-command frontmatter declares a completions field:

• Shell command (completions: !./scripts/list-tickets.sh) returning newline-separated suggestions, OR
• MCP tool reference (completions: mcp__linear__list_issues) the runtime calls with current input as filter.

Returned strings render as a navigable dropdown.

1. Keyboard nav — accept with → or Tab, cycle with ↑/↓.

Concrete use case

Custom /build command. After typing /build , surface live Linear tickets inline (filtered as you type), pick with arrows, Enter. Today this requires either a two-step picker command or knowing the ticket ID by heart.

Why

The MCP ecosystem is the right home for live data (tickets, files, symbols), but there's no UX bridge between MCP and the prompt input. A completions callback closes the gap.

Acceptable simpler version

History-only ghost text (no MCP callback) would already be valuable on its own.