refactor: centralize default model to Opus 4.5

- Change default model from Sonnet to Opus 4.5
- Remove isDefault boolean from model definitions (no static guarantee)
- Add DEFAULT_MODEL_KEY const that directly references a KnownModelKey
- Update all hardcoded model strings to use the centralized DEFAULT_MODEL
- Fix WORKSPACE_DEFAULTS to import from knownModels.ts

The default model is now defined in one place:
  src/common/constants/knownModels.ts -> DEFAULT_MODEL_KEY = "OPUS"

_Generated with `mux`_

Author: ammar-agent

src/browser/stories/mockFactory.ts

@@ -16,6 +16,7 @@ import type {
   MuxImagePart,
   MuxToolPart,
 } from "@/common/types/message";
+import { DEFAULT_MODEL } from "@/common/constants/knownModels";
 
 /** Part type for message construction */
 type MuxPart = MuxTextPart | MuxReasoningPart | MuxImagePart | MuxToolPart;
@@ -196,7 +197,7 @@ export function createAssistantMessage(
     metadata: {
       historySequence: opts.historySequence,
       timestamp: opts.timestamp ?? STABLE_TIMESTAMP,
-      model: opts.model ?? "anthropic:claude-sonnet-4-5",
+      model: opts.model ?? DEFAULT_MODEL,
       usage: { inputTokens: 100, outputTokens: 50, totalTokens: 150 },
       duration: 1000,
     },

src/browser/stories/storyHelpers.ts

@@ -14,6 +14,7 @@ import {
   getInputKey,
   getModelKey,
 } from "@/common/constants/storage";
+import { DEFAULT_MODEL } from "@/common/constants/knownModels";
 import {
   createWorkspace,
   groupWorkspacesByProject,
@@ -178,7 +179,7 @@ export function setupStreamingChatStory(opts: StreamingChatSetupOptions): APICli
       createStreamingChatHandler({
         messages: opts.messages,
         streamingMessageId: opts.streamingMessageId,
-        model: opts.model ?? "anthropic:claude-sonnet-4-5",
+        model: opts.model ?? DEFAULT_MODEL,
         historySequence: opts.historySequence,
         streamText: opts.streamText,
         pendingTool: opts.pendingTool,

src/common/constants/knownModels.ts

@@ -15,8 +15,6 @@ interface KnownModelDefinition {
   aliases?: string[];
   /** Preload tokenizer encodings at startup */
   warm?: boolean;
-  /** Use as global default model */
-  isDefault?: boolean;
   /** Optional tokenizer override for ai-tokenizer */
   tokenizerOverride?: string;
 }
@@ -29,12 +27,17 @@ interface KnownModel extends KnownModelDefinition {
 // Model definitions. Note we avoid listing legacy models here. These represent the focal models
 // of the community.
 const MODEL_DEFINITIONS = {
+  OPUS: {
+    provider: "anthropic",
+    providerModelId: "claude-opus-4-5",
+    aliases: ["opus"],
+    warm: true,
+  },
   SONNET: {
     provider: "anthropic",
     providerModelId: "claude-sonnet-4-5",
     aliases: ["sonnet"],
     warm: true,
-    isDefault: true,
     tokenizerOverride: "anthropic/claude-sonnet-4.5",
   },
   HAIKU: {
@@ -43,11 +46,6 @@ const MODEL_DEFINITIONS = {
     aliases: ["haiku"],
     tokenizerOverride: "anthropic/claude-3.5-haiku",
   },
-  OPUS: {
-    provider: "anthropic",
-    providerModelId: "claude-opus-4-5",
-    aliases: ["opus"],
-  },
   GPT: {
     provider: "openai",
     providerModelId: "gpt-5.1",
@@ -119,10 +117,10 @@ export function getKnownModel(key: KnownModelKey): KnownModel {
 // Derived collections
 // ------------------------------------------------------------------------------------
 
-const DEFAULT_MODEL_ENTRY =
-  Object.values(KNOWN_MODELS).find((model) => model.isDefault) ?? KNOWN_MODELS.SONNET;
+/** The default model key - change this single line to update the global default */
+export const DEFAULT_MODEL_KEY: KnownModelKey = "OPUS";
 
-export const DEFAULT_MODEL = DEFAULT_MODEL_ENTRY.id;
+export const DEFAULT_MODEL = KNOWN_MODELS[DEFAULT_MODEL_KEY].id;
 
 export const DEFAULT_WARM_MODELS = Object.values(KNOWN_MODELS)
   .filter((model) => model.warm)

src/constants/workspaceDefaults.test.ts

@@ -1,5 +1,6 @@
 import { describe, test, expect } from "bun:test";
 import { WORKSPACE_DEFAULTS } from "./workspaceDefaults";
+import { DEFAULT_MODEL } from "@/common/constants/knownModels";
 
 type Mutable<T> = { -readonly [P in keyof T]: T[P] };
 
@@ -15,7 +16,7 @@ describe("WORKSPACE_DEFAULTS", () => {
   test("should have correct default values", () => {
     expect(WORKSPACE_DEFAULTS.mode).toBe("exec");
     expect(WORKSPACE_DEFAULTS.thinkingLevel).toBe("off");
-    expect(WORKSPACE_DEFAULTS.model).toBe("anthropic:claude-sonnet-4-5");
+    expect(WORKSPACE_DEFAULTS.model).toBe(DEFAULT_MODEL);
     expect(WORKSPACE_DEFAULTS.autoRetry).toBe(true);
     expect(WORKSPACE_DEFAULTS.input).toBe("");
   });

src/constants/workspaceDefaults.ts

@@ -22,6 +22,7 @@
 
 import type { UIMode } from "@/common/types/mode";
 import type { ThinkingLevel } from "@/common/types/thinking";
+import { DEFAULT_MODEL } from "@/common/constants/knownModels";
 
 /**
  * Hard-coded default values for workspace settings.
@@ -36,9 +37,9 @@ export const WORKSPACE_DEFAULTS = {
 
   /**
    * Default AI model for new workspaces.
-   * This is the TRUE default - not dependent on user's LRU cache.
+   * Uses the centralized default from knownModels.ts.
    */
-  model: "anthropic:claude-sonnet-4-5" as string,
+  model: DEFAULT_MODEL as string,
 
   /** Default auto-retry preference for new workspaces */
   autoRetry: true as boolean,

src/node/services/workspaceService.ts

@@ -31,6 +31,7 @@ import type {
 import type { MuxMessage } from "@/common/types/message";
 import type { RuntimeConfig } from "@/common/types/runtime";
 import { hasSrcBaseDir, getSrcBaseDir } from "@/common/types/runtime";
+import { defaultModel } from "@/common/utils/ai/models";
 import type { StreamEndEvent, StreamAbortEvent } from "@/common/types/stream";
 import type { TerminalService } from "@/node/services/terminalService";
 
@@ -660,7 +661,7 @@ export class WorkspaceService extends EventEmitter {
           if (messageText.trim()) {
             const branchNameResult = await generateWorkspaceName(
               messageText,
-              "anthropic:claude-sonnet-4-5",
+              defaultModel,
               this.aiService
             );
 
@@ -880,7 +881,7 @@ export class WorkspaceService extends EventEmitter {
           projectPath?: string;
           trunkBranch?: string;
         })
-      | undefined = { model: "claude-sonnet-4-5-latest" }
+      | undefined = { model: defaultModel }
   ): Promise<
     | Result<void, SendMessageError>
     | { success: true; workspaceId: string; metadata: FrontendWorkspaceMetadata }