feat: Antigravity CLI (CLI error: bubbletea: error opening TTY: bubbletea: could not open TTY: open /dev/tty: no such device or address) engine support — Gemini CLI deprecates June 18

[downwind7clawd-ctrl]

Summary

Google announced at I/O 2026 that Gemini CLI will stop serving requests for individual/free users on June 18, 2026, replaced by Antigravity CLI (agy v1.0.0, Go-based). This issue tracks the work needed to add Antigravity CLI as a supported engine in Untether.

Context

• Announcement: Transitioning Gemini CLI to Antigravity CLI (Google Developers Blog, May 19, 2026)
• Migration guide: https://antigravity.google/docs/cli/migration
• Antigravity CLI download: https://antigravity.google/download (install via curl -fsSL https://antigravity.google/cli/install.sh | bash)
• Binary name: agy (installed to ~/.local/bin/agy)

Interface Incompatibilities

The current GeminiRunner (untether/runners/gemini.py) uses CLI flags that do not exist in Antigravity CLI:

Gemini CLI flag	Untether usage	Antigravity CLI equivalent
--output-format stream-json	Structured JSONL stream parsing	❌ No equivalent flag
--resume <id>	Session resumption (line 341-343)	--continue / --conversation <id>
--approval-mode yolo	Auto-approve tools (line 351-353)	--dangerously-skip-permissions
--model <model>	Model selection (line 349)	❌ Config file only (~/.gemini/antigravity-cli/settings.json)
--prompt=<text>	Non-interactive mode (line 354)	--print / -p

Critical blocker: stream-json output format

Untether parses stream-json output for events: Init, Message, ToolUse, ToolResult, Result, Error. Antigravity CLI does not expose a --output-format flag in v1.0.0. This means either:

1. Antigravity CLI has an undocumented JSON output mode (needs investigation)
2. A new output parser needs to be written for whatever format agy --print produces
3. Google adds --output-format support in a future version

What still works (shared context)

• Context files: GEMINI.md, AGENTS.md — Antigravity CLI reads the same files
• MCP servers: Supported, but config location changed to mcp_config.json (field: serverUrl instead of url)
• Skills: ~/.gemini/skills/ (global) / .agents/skills/ (workspace)
• Plugins: agy plugin import gemini migrates extensions
• Authentication: OAuth-based, same Google account

Suggested approach

1. Investigate agy --print output format — determine if it produces parseable structured output
2. Create untether/runners/antigravity.py as a new engine runner (do not modify existing gemini.py for backward compat)
3. Map the flag equivalences above
4. Handle the MCP config location migration (settings.json → mcp_config.json)

Environment

• Antigravity CLI: agy v1.0.0 (Go binary, linux_amd64)
• Current Gemini CLI: gemini v0.42.0 (Node.js)
• Untether: 0.35.2
• OS: Ubuntu 24.04 (Linux)