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.