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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 onor--ultrafaston 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 slacknow configures Slack, opens sign-in, and connects your account in one command.- Set
auto_compact_percentfrom 10 to 80 to choose when automatic compaction starts. - libfx accepts raw image bytes and adds a
resizeImagehook 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 reloadrefreshes 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.