AI news for April 16-17, 2026 is a list of weights. The structural change is one JSON file in your home directory.

The 48-hour window covered continued uptake of NVIDIA's Ising open family for quantum error correction, OpenAI's GPT-5.4-Cyber variant hardening in the Trusted Access program, a fresh push of Meta's Llama 4 Maverick checkpoints and Mistral's Codestral 2 (Apache 2.0), a Hugging Face wave of quantized OLMo 2 32B and fine-tuned agent-tool models, arXiv drops on long-context retrieval and tool-use RL, and the Agent Client Protocol tick from 0.25.0 to 0.29.2 which Fazm picked up in the acp-bridge package on April 18. For a Mac user the structurally bigger shift was the tooling side, not the weight side: MCP became the shared catalog format between Claude Code and consumer agents.

Three things. First, a user-writable Claude-Code-compatible MCP server registry at ~/.fazm/mcp-servers.json whose JSON shape is byte-for-byte identical to Claude Code's mcpServers object. Second, a new ACP message type mcp_servers_available that tags each loaded server as builtin:true or builtin:false so the Mac app can render the full tool catalog live in Settings. Third, a jump in the pinned ACP SDK to 0.29.2 (package.json, 2026-04-18) that unlocked setModelsAvailableHandler and dynamic model enumeration. Every MCP tool you already run in Claude Code now plugs into a native AX-tree control surface with zero rewrite.

~/.fazm/mcp-servers.json. Verify it by opening /Users/matthewdi/fazm/Desktop/Sources/MCPServerManager.swift lines 4-5 and 40-44: the init builds the URL from FileManager.default.homeDirectoryForCurrentUser appending .fazm/mcp-servers.json. The schema mirrors Claude Code's mcpServers exactly: a top-level JSON object whose keys are server names, values are { command: string, args?: string[], env?: Record<string,string>, enabled?: boolean }. When enabled is true (the default) the field is omitted on save. The ACP bridge re-reads this file every time buildMcpServers runs inside acp-bridge/src/index.ts at lines 1102-1141.

Hardcoded in acp-bridge/src/index.ts line 1266: BUILTIN_MCP_NAMES = { fazm_tools, playwright, macos-use, whatsapp, google-workspace }. Any name not in that set is flagged as builtin:false when the emitMcpServers function sends the mcp_servers_available ACP message (line 1252). In the app, Settings > MCP Servers renders a built-in section and a custom section side by side. The tagging is pure name lookup, so if you add a user server called 'playwright' the bridge will treat it as built-in. Pick a distinct name.

Because the AI news cycle for April 16-17 is already saturated with weight lists. Every SERP result for these dates ranks arXiv papers and Hugging Face uploads. What nobody writes is the answer to 'the new model shipped, now what?' On a Mac the path from a new model to real desktop automation goes through MCP servers, which is the protocol the model speaks to tools through. Fazm is now the consumer surface for that protocol: paste the same JSON you already have in ~/.claude.json or your Claude Code project config, restart Fazm, and the model you picked can use those tools on your actual Mac.

Two user-visible wires. First, emitModelsIfChanged, which reads the ACP SDK's announced available model list and broadcasts models_available messages so the Smart/Fast toggle, paywall, and shortcut settings can update without an app rebuild (acp-bridge/src/index.ts line 1271). Second, mcp_servers_available, the sibling event that does the same thing for the tool catalog (line 1252). Both messages are deduped by JSON-equality against the last-emitted payload, so the UI only re-renders when the list actually changes. The bump lands in the pinned version in acp-bridge/package.json line 15 (@agentclientprotocol/claude-agent-acp ^0.29.2).

Yes for any stdio MCP server whose command and args refer to absolute paths or path-resolvable binaries. The loader in buildMcpServers at lines 1102-1137 iterates your user servers, skips any with enabled:false, logs a warning if command is missing, and appends them to the same McpServerConfig array the built-ins live in. There is no per-server wrapper or translation layer. Environment variables are passed as name/value pairs. Cwd inheritance follows the session's configured workspace. If your server works with 'claude mcp add --transport stdio', it works with Fazm.

The bridge type McpServerConfigHttp is defined at acp-bridge/src/index.ts lines 984-988 (type: 'http', url, headers). A follow-up commit on April 17 at 12:45 (a3bb8fac) fixes command property resolution for URL-based MCP servers specifically, so the loader no longer errors when a user config supplies url instead of command. If you have a remote MCP server from Claude Code, Linear, or any self-hosted endpoint, you can reference it directly. Authentication headers are forwarded verbatim.

The ACP SDK emits the live available-model list through setModelsAvailableHandler (commit 6b26b408, 2026-04-17). Fazm rebroadcasts it as the models_available ACP message. That list feeds the chat header's Smart/Fast toggle and the model picker in shortcut settings. Combined with the Custom API Endpoint shipped in v2.2.0 on April 11, you can point ANTHROPIC_BASE_URL at a local vLLM serving Codestral 2 or an LiteLLM proxy fronting Llama 4 Maverick, and the exact same MCP catalog from ~/.fazm/mcp-servers.json rides along. The tool surface is identical regardless of which model answers.

Neither Fazm nor its MCP layer is screenshot-based. Desktop control goes through mcp-server-macos-use (a bundled native binary, not a vision model) which talks to AXUIElementCreateApplication and AXUIElementCopyAttributeValue directly. The tool surface any MCP server sees is a text-shaped accessibility tree, not a pixel buffer. So when the April 16-17 news cycle talks about GPT-5.4-Cyber or agent tool-use RL papers, the model is swappable but the tool-call format is stable. You do not retrain against a new screenshot distribution every time a weight update drops.

Anything you already run in Claude Code. Common picks as of April 17, 2026: a Linear MCP server for issue triage, a Postgres or Neon MCP server for your app's DB, a Slack or Linear stdio tool, a GitHub MCP for gh command wrapping, a local Python one for domain-specific automations. The format is { "name": { "command": "/path/to/bin", "args": ["--flag"], "env": {"TOKEN": "..."} } }. Drop the file into place, restart Fazm, watch the Settings > MCP Servers > Custom section populate, and watch mcp_servers_available get emitted in the bridge logs.

Four files, all local and inspectable. /Users/matthewdi/fazm/Desktop/Sources/MCPServerManager.swift (the Swift-side config loader). /Users/matthewdi/fazm/Desktop/Sources/MainWindow/Pages/MCPServerEditSheet.swift (the UI). /Users/matthewdi/fazm/acp-bridge/src/index.ts lines 1102-1141, 1252-1269 (the Node-side merge and event broadcast). /Users/matthewdi/fazm/acp-bridge/package.json line 15 (the ACP SDK pin at ^0.29.2). git log between 2026-04-16 and 2026-04-18 in the fazm repo shows 22 commits landing this work, including 2152e6a3 'Add support for custom MCP servers via ~/.fazm/mcp-servers.json' and 95287a32 'Update @agentclientprotocol/claude-agent-acp to 0.29.2'.