LLM new model release April 2026: the three-substring picker that absorbs every release without a new build.

The April 2026 release wave every roundup is tracking: Anthropic shipped the Claude 4 family on April 2 (Opus 4 as flagship for extended autonomous coding, Sonnet and Haiku tiers), previewed Claude Mythos on April 7 through Project Glasswing to roughly 50 partners, OpenAI shipped GPT-5 Turbo on April 7 with native image and audio inside one model, and Google released four Gemma 4 variants under Apache 2.0 on April 2. Open-source side: Meta Llama 4 Scout (10M context MoE), Alibaba Qwen 3 (Apache 2.0, 128K), Zhipu GLM-5.1 (MIT, 200K, 754B MoE), Mistral Medium 3 (open weights), DeepSeek V3.2 (128K), Arcee Trinity (400B, Apache 2.0). Every recap lists the same spec-sheet columns: parameters, context window, license, benchmark score.

They miss the code a shipping consumer agent runs when a new model ID arrives on its ACP SDK response. A desktop app like Fazm does not get to redeploy the binary every time Anthropic ships a new snapshot. So it either hardcodes a model list (and breaks the day Claude Mythos lands) or it builds a dynamic detection path. Fazm does the second. The adaptation layer is about 60 lines of Swift plus a 10-line bridge forwarder, and it is the thing a reader cannot see from a spec-sheet table because it lives inside the shipping app, not in the Anthropic release notes.

Two files. First, /Users/matthewdi/fazm/Desktop/Sources/FloatingControlBar/ShortcutSettings.swift, lines 150 to 212. Line 151 is defaultModels (three fallback ModelOptions). Line 159 is the modelFamilyMap with three substring keys: haiku, sonnet, opus. Line 169 is normalizeModelId. Line 178 is updateModels(_:) which receives the ACP SDK's availableModels array. Line 184 is the matched-family label template: (short) (family, latest). Line 189 is the unknown-model fallback that shows the raw API name. Second, /Users/matthewdi/fazm/acp-bridge/src/index.ts, lines 1245 to 1281. Line 1245 pins the default to claude-sonnet-4-6. Line 1271 is emitModelsIfChanged which re-broadcasts session/new.models.availableModels to the Swift side every time the ACP SDK reports a new list.

Five concrete steps, each pinned to a file and line. 1. acp-bridge calls acpRequest("session/new", sessionParams) at index.ts line 1488. 2. The ACP SDK returns a models.availableModels array with the new model ID included. 3. emitModelsIfChanged at line 1271 filters out the "default" pseudo-model (line 1274), diffs against lastEmittedModelsJson, and sends a { type: "models_available", models } message to Swift. 4. ACPBridge.swift hits the "models_available" case at line 1131 and dispatches to the handler registered at line 233 via setModelsAvailableHandler. 5. ShortcutSettings.updateModels at line 178 substring-matches the new ID against (haiku, sonnet, opus), produces a ModelOption, and re-publishes @Published var availableModels at line 166. The Settings page at SettingsPage.swift line 2025 iterates shortcutSettings.availableModels and renders the button. No app update, no redeploy.

Three lines at ShortcutSettings.swift 187 to 189: '// Unknown model family: use the API name directly' then 'let displayName = model.name.isEmpty ? modelId : model.name' then 'return (ModelOption(id: modelId, label: displayName, shortLabel: displayName), 99)'. The 99 is a deliberate sort order key so any unknown family sits after the three known ones. That branch is what turns claude-mythos-preview-20260407 (or whatever future ID Anthropic picks) into a selectable button labeled Claude Mythos Preview drawn from the SDK's model.name.

Because Anthropic ships snapshot versions constantly. In the April 2026 window alone, claude-sonnet-4-6 is the default at index.ts line 1245, claude-opus-4-6 shows up in user accounts (CHANGELOG.json v2.0.7 on 2026-04-05: "Fixed chat failing silently when personal Claude account lacks access to the selected model"), and claude-haiku-4-5 has been in the mix since late 2025. A full list grows and rots. A three-keyword substring match at line 183 (modelId.contains(match.substring)) absorbs every future Haiku, Sonnet, or Opus snapshot. The (latest) label at line 184 is not a marketing promise, it is a consequence of the model family always resolving to whatever the ACP SDK reports today.

ShortcutSettings.swift line 184: 'let label = "\(match.short) (\(match.family), latest)"'. With the three hardcoded entries at lines 160-162, this renders as "Scary (Haiku, latest)", "Fast (Sonnet, latest)", "Smart (Opus, latest)". The short and family strings are pinned in the modelFamilyMap tuple. The order integer at position 3 of each tuple (0 for haiku, 1 for sonnet, 2 for opus) feeds the .sorted(by:) at line 191.

The default lives at one line: acp-bridge/src/index.ts line 1245, 'const DEFAULT_MODEL = "claude-sonnet-4-6";'. Line 1426 picks it up as the fallback when a prompt arrives without a modelId: 'const requestedModel = msg.model || DEFAULT_MODEL;'. For the case where the user's personal Claude account does not have access to the requested model, the v2.0.7 changelog entry on 2026-04-05 describes the auto-fallback to the built-in account. That is a separate code path from the picker: the picker shows what the SDK reports; the account-fallback kicks in after a 402 or access-denied on the request itself.

Yes, at index.ts line 1277: 'if (json === lastEmittedModelsJson) return;'. Without that guard, every session/new call (main session, floating session, observer session, each is pre-warmed separately) would emit a duplicate models_available message to the Swift side, which would retrigger ShortcutSettings.updateModels, which would re-compute the ModelOption array and re-publish availableModels. The SwiftUI ForEach at SettingsPage.swift line 2025 would re-render the picker every few seconds. The JSON-string diff at line 1276 keeps the UI steady.

Two greps. First, 'grep -n modelFamilyMap /Users/matthewdi/fazm/Desktop/Sources/FloatingControlBar/ShortcutSettings.swift' shows line 159. Read lines 159 to 175 inline to see the three substrings and the normalize function. Second, 'grep -n emitModelsIfChanged /Users/matthewdi/fazm/acp-bridge/src/index.ts' shows lines 1271 and 1281. Read lines 1271 to 1281 to see the filter-out-default, diff-against-last, send-to-Swift pipeline. Every claim on this page resolves to those two passages.

The bridge process wraps ClaudeAcpAgent, so the ACP SDK is the source of truth for what modelId strings are valid. The substring-family map (haiku, sonnet, opus) is Claude-specific by design; anything outside those three families hits the unknown-model fallback at line 189 and shows up in the picker with its API-reported name. The accessibility-tree capture, the floating bar UI, and the prompt-to-session plumbing are model-agnostic. The picker is not.

Because the difference between a consumer agent that survives a model release and one that breaks is a handful of lines. The April 2026 releases (Claude 4 family, Claude Mythos preview, GPT-5 Turbo, Gemma 4, Llama 4 Scout, Qwen 3, GLM-5.1) are all shipping behind evolving SDK surface areas. An agent that hardcodes a model list ships a new binary every time a provider bumps a version. An agent that dynamically discovers models (via session/new.models.availableModels in the ACP case) stays on the user's Mac through the whole release wave. The 60-line picker and the 10-line bridge forwarder are the entire delta between those two worlds.