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Release · Fx · vercel-labs

Fx v0.0.13

What's new in Fx v0.0.13, released Oct 4, 2026 (). Newer: v0.0.14 (Oct 16, 2026)

Official release notes ↗ Fx changelog

Release notes

Fx v0.0.13

fx now supports Ultrafast on supported OpenAI models and launches up to 23× faster. Shell calls are up to 8.6× faster, and exits are up to 44× faster in matched benchmarks.

Breaking Changes
  • Terminals launched through the shell tool now stop when fx exits instead of staying alive for the next resume.
  • New libfx checkpoints cannot be restored by older versions. Existing checkpoints still load.
  • Invalid Base64 image input now throws directly from prompt() instead of failing the turn later.
New Features
  • Enable Ultrafast with /ultrafast on or --ultrafast on supported OpenAI models through AI Gateway. It uses a higher-priced inference tier.
  • Subagents and ACP sessions support Ultrafast. libfx supports it through model.ultrafast.
  • /mcp add slack now configures Slack, opens sign-in, and connects your account in one command.
  • Set auto_compact_percent from 10 to 80 to choose when automatic compaction starts.
  • libfx accepts raw image bytes and adds a resizeImage hook for preparing images before sending.
  • libfx image references survive checkpoints, so your app's tools can recover originals later.
  • ACP clients can steer running turns, supply session instructions and workspaces, and host MCP servers over the ACP connection.
Improvements
  • Launch benchmarks show 1.6–23× faster median first-screen rendering, depending on the terminal. Sign-in and skills finish loading afterward.
  • Shell calls are up to 8.6× faster with heavy startup files, dropping from 238.8 ms to 27.9 ms. Startup files load once per fx process; /shell reload refreshes them.
  • Exiting after a reply is up to 44× faster while a large usage history is loading, dropping from 1,003 ms to 23 ms.
  • Exiting with MCP servers is 2.8× faster in the matched macOS benchmark, dropping from 20.6 ms to 7.4 ms.
  • The shell overhaul removed 236–279 KB from native binaries in matched builds.
  • Compaction preserves original messages and final replies where they fit, while older turns and tool results remain searchable.
  • The Grok model picker now shows all supported subscription models.
  • Images can reach the model at up to 8,000 pixels per side when the request contains 20 images or fewer.
Bug Fixes
  • Steering sent after a tool result now survives cancellation and resume.
  • Subagents now start when the parent has more than 256 MCP tools.
  • Compaction no longer incorrectly flags answers to questions as unsupported rules.
  • Sessions v2 now retain assistant text around interrupted tool calls and generate titles after a crashed first turn.
Security
  • Reloading shell configuration clears remembered command approvals. Auto mode reviews routine commands that use captured aliases or functions.
  • ACP sessions use profile MCP servers only when the client explicitly opts in.