The Anthropic announcement train ran for 26 days. Inside Fazm, every stop arrived through three lines of Swift.

The path the announcement takes, from press release to your chat bubble

Fazm runs on macOS. The chat surface is native Swift. The model conversation runs through a Node.js subprocess called the ACP bridge, spawned by a Process started in Desktop/Sources/Chat/ACPBridge.swift. The bridge speaks the Agent Client Protocol over stdio, the Swift side talks to it over pipes, and the bridge in turn talks to whatever host is on the other end of an Anthropic SDK client. That last hop is where the news lands.

Five actors. One direction of arrows on the way out, the same actors in reverse on the way back. The interesting part is which host the arrow points at, and that decision is made before the bridge process ever starts.

The three lines of Swift the whole month rests on

Open Desktop/Sources/Chat/ACPBridge.swift, scroll to line 383, and the entire conditional is right there. The comment is verbatim from the source, and the logic is unforgiving in how plain it is. If the toggle is off, the conditional never enters, ANTHROPIC_BASE_URL is never set on the spawned process, and the Anthropic SDK inside the bridge falls back to its built-in default host. If the toggle is on and the field has anything in it, that string becomes the env value, and from that moment forward the bridge subprocess sends every Anthropic-shaped request to that host.

// Default route, Custom API Endpoint toggle off
// /Users/matthewdi/fazm/Desktop/Sources/Chat/ACPBridge.swift

var env = ProcessInfo.processInfo.environment
env["NODE_NO_WARNINGS"] = "1"

switch mode {
case .personalOAuth:
  env.removeValue(forKey: "ANTHROPIC_API_KEY")
case .bundledKey(let apiKey):
  env["ANTHROPIC_API_KEY"] = apiKey
}

// ANTHROPIC_BASE_URL is never set.
// The Anthropic SDK inside the bridge falls back
// to its hardcoded default: https://api.anthropic.com
// Opus 4.7, Sonnet 4.6, Haiku 4.5, every model the
// April announcements introduced, all arrive here.

proc.environment = env

The diff is one if-let and one assignment. There is no second code path, no SDK swap, no separate transport. The Anthropic SDK inside the Node bridge does the rest: it reads ANTHROPIC_BASE_URL when it constructs its HTTP client, it joins the base with the request path, and it sends the same body it would have sent to api.anthropic.com. The proxy, gateway, or local bridge on the receiving end is responsible for whatever it does with the request before forwarding it upstream.

The toggle that flips it

The user-facing surface lives at Desktop/Sources/MainWindow/Pages/SettingsPage.swift inside a settingsCard with the internal id aichat.endpoint. The card runs from line 906 to line 952. It has a header row with a server-rack glyph and a Switch toggle, a TextField with placeholder https://your-proxy:8766, and one help line that reads, verbatim:

Route API calls through a custom endpoint (e.g. local LLM bridge, corporate proxy, or GitHub Copilot bridge). Leave empty to use the default Anthropic API.

The bindings are the interesting part. The toggle is a derived binding: its getter returns true if either an internal showCustomEndpoint flag is on, or the stored customApiEndpoint string is non-empty. The setter only takes action when flipping off: it clears the field and awaits chatProvider?.restartBridgeForEndpointChange(), which kills the running ACP subprocess and respawns it with the new env. The TextField has the same restart bound to onSubmit. The next chat message after either action goes through a fresh process, either with ANTHROPIC_BASE_URL set to the new value, or with it unset.

The shape of that restart matters because none of the four big April announcements changed it. Opus 4.7 GA on April 22 was a model id rename and a pricing confirmation. Claude Design on April 17 was a new product surface, not a new transport. ACP 0.29.2 on April 20 reshaped the model picker payload but kept the env contract. Mythos Preview on April 7 was a research-grade preview, gated, not a public host change. The toggle did not need to know about any of them. It only needs to know what host string to set, and the rest of the app handles the announcement-shaped contents.

What changes when you flip it

Two parallel realities, depending on a single boolean. Read each column knowing the entire body of the request is identical. Same model id, same ACP frames, same tools, same accessibility-tree-shaped inputs that the Mac side feeds into Claude. Only the host changes.

The same env var, four April announcements

Here is what the month looked like at the boundary of that one env var. None of these events changed the conditional. Some of them changed the body of what flowed through it. Almost all of the companion Fazm releases ride the same pipe.

Notice the rhythm. New protocol version, same env var. New model id, same env var. New product surface from Anthropic, same env var. The shape that does not move is the contract between Fazm and whatever host responds. That stability is why a corporate proxy or a local LLM bridge written before April still works after Opus 4.7 GA on April 22, as long as the host on the other end serves the Anthropic-shaped responses the SDK expects.

Why this is the unusual shape, not the obvious one

The obvious shape, and the one most desktop AI apps ship, is to bake a vendor SDK directly into the native binary, hardcode the host, and pin the model list to whatever shipped at app build time. New Anthropic announcement, new app build. Custom endpoint, no. Air-gapped local LLM, hard. Corporate compliance proxy, contact support.

Fazm took the unusual shape because the bridge process is the only process that talks to the model. Its environment is the only thing that decides which host that conversation hits. That is why one env var carries the load. The Swift side stays declarative: a Settings card, a TextField, a toggle. The Node side stays vendor-agnostic in terms of host: it speaks Anthropic protocol, but it does not require api.anthropic.com on the other end. The result is that an Anthropic announcement can change what tokens are available to the user without changing the app, and a user can redirect those tokens through any Anthropic-compatible host without changing the app.

Two independent vectors, one shared boundary. That is the actual design choice the April news cycle stress-tested.

The bridge is one input among many

The endpoint is the most consequential env var, but not the only one. The bridge picks up its environment from a small handful of settings the Swift side writes. Each one steers a different part of how the bridge runs. None of them, except ANTHROPIC_BASE_URL, change which host the April announcements arrive from.

The diagram exaggerates the symmetry, the source code does not. Of the env vars feeding the bridge, only ANTHROPIC_BASE_URL is the one users wire themselves. The others are wired by the app. That asymmetry is intentional. Auth mode, browser-control path, voice path, and tool timeouts are local concerns. The host the bridge talks to is a deployment decision, sometimes a regulatory decision, sometimes a pricing decision, and the one that benefits most from being a one-line setting rather than a code change.

What you can verify yourself in 90 seconds

The Fazm desktop repo is open source. None of the claims on this page require trusting the writer. To confirm the env var lives in exactly one place:

1. Clone the public repo from github.com/m13v/fazm.
2. Run grep -nR "ANTHROPIC_BASE_URL" Desktop/Sources.
3. One match. ACPBridge.swift line 385. Same env value, same source of truth.
4. Run grep -nR "customApiEndpoint" Desktop/Sources.
5. Three matches. ACPBridge.swift line 384 (the read), SettingsPage.swift line 840 (the AppStorage declaration), SettingsPage.swift line 921 and 925 (the toggle binding).
6. Open the app, hit Cmd+, to open Settings, scroll to the AI Chat group, find Custom API Endpoint, type any URL, then watch the ACP bridge process restart in Activity Monitor.

That is the whole audit. The April announcements are interesting regardless of where you read about them. Where they actually arrive on a real machine is, in this case, this one line.