🤖 feat: promote Claude Opus 5.1 as the opus model

.github/workflows/auto-cleanup-fixup.yml

@@ -118,6 +118,6 @@ jobs:
           FAILED_RUN_ID: ${{ github.event.workflow_run.id }}
         run: |
           bunx mux@next run \
-            --model anthropic:claude-opus-5 \
+            --model anthropic:claude-opus-5-1 \
             --thinking xhigh \
             < .github/prompts/auto-cleanup-fixup.md

.github/workflows/auto-cleanup.yml

@@ -71,6 +71,6 @@ jobs:
           GH_TOKEN: ${{ steps.app-token.outputs.token }}
         run: |
           bunx @coder/xum@next run \
-            --model anthropic:claude-opus-5 \
+            --model anthropic:claude-opus-5-1 \
             --thinking xhigh \
             < .github/prompts/auto-cleanup.md

.github/workflows/nightly-terminal-bench.yml

@@ -10,7 +10,7 @@ on:
   workflow_dispatch:
     inputs:
       models:
-        description: 'Models to test (comma-separated, or "all" for opus-5 + gpt-5.6-sol + google/gemini-3-pro-preview + google/gemini-3-flash-preview + google/gemini-3.7-flash)'
+        description: 'Models to test (comma-separated, or "all" for opus-5-1 + gpt-5.6-sol + google/gemini-3-pro-preview + google/gemini-3-flash-preview + google/gemini-3.7-flash)'
         required: false
         default: "all"
         type: string
@@ -129,7 +129,7 @@ jobs:
           INPUT_MODELS: ${{ inputs.models }}
         run: |
           if [ "$INPUT_MODELS" = "all" ] || [ -z "$INPUT_MODELS" ]; then
-            echo 'models=["anthropic/claude-opus-5","openai/gpt-5.6-sol","google/gemini-3-pro-preview","google/gemini-3-flash-preview","google/gemini-3.7-flash"]' >> "$GITHUB_OUTPUT"
+            echo 'models=["anthropic/claude-opus-5-1","openai/gpt-5.6-sol","google/gemini-3-pro-preview","google/gemini-3-flash-preview","google/gemini-3.7-flash"]' >> "$GITHUB_OUTPUT"
           else
             # Convert comma-separated to JSON array
             models_json=$(echo "$INPUT_MODELS" | jq -R -s -c 'split(",") | map(gsub("^\\s+|\\s+$"; ""))')

.github/workflows/terminal-bench.yml

@@ -4,7 +4,7 @@ on:
   workflow_call:
     inputs:
       model_name:
-        description: "Model to use (e.g., anthropic/claude-opus-5, openai/gpt-5.6-sol)"
+        description: "Model to use (e.g., anthropic/claude-opus-5-1, openai/gpt-5.6-sol)"
         required: false
         type: string
       dataset:
@@ -98,7 +98,7 @@ on:
         required: false
         type: string
       model_name:
-        description: "Model to use (e.g., anthropic/claude-opus-5, openai/gpt-5.6-sol)"
+        description: "Model to use (e.g., anthropic/claude-opus-5-1, openai/gpt-5.6-sol)"
         required: false
         type: string
       mux_run_args:

benchmarks/terminal_bench/prepare_leaderboard_submission.py

@@ -123,6 +123,13 @@
         "model_org_display_name": "Anthropic",
         "folder_name": "Claude-Opus-5",
     },
+    "anthropic/claude-opus-5-1": {
+        "model_name": "claude-opus-5-1",
+        "model_provider": "anthropic",
+        "model_display_name": "Claude Opus 5.1",
+        "model_org_display_name": "Anthropic",
+        "folder_name": "Claude-Opus-5.1",
+    },
     # Keep historical GPT metadata alongside the current GPT-5.6 Sol bench target
     # so mixed or older artifact sets still map to the canonical leaderboard names.
     "openai/gpt-5.2": {

docs/config/models.mdx

@@ -13,9 +13,9 @@ Xum ships with curated models kept up to date with the frontier. Use any custom
 
 | Model                  | ID                            | Aliases                                                      | Default |
 | ---------------------- | ----------------------------- | ------------------------------------------------------------ | ------- |
-| Fable 5                | anthropic:claude-fable-5      | `fable`                                                      |         |
+| Fable 5.1              | anthropic:claude-fable-5-1    | `fable`                                                      |         |
 | Mythos 5               | anthropic:claude-mythos-5     | `mythos`                                                     |         |
-| Opus 5                 | anthropic:claude-opus-5       | `opus`                                                       | ✓       |
+| Opus 5.1               | anthropic:claude-opus-5-1     | `opus`                                                       | ✓       |
 | Sonnet 5               | anthropic:claude-sonnet-5     | `sonnet`                                                     |         |
 | Haiku 4.5              | anthropic:claude-haiku-4-5    | `haiku`                                                      |         |
 | GPT-5.6 Sol            | openai:gpt-5.6-sol            | `gpt`, `sol`                                                 |         |

docs/guides/github-actions.mdx

@@ -116,7 +116,7 @@ jobs:
           GH_TOKEN: ${{ steps.app-token.outputs.token }}
         run: |
           bunx @coder/xum@next run \
-            --model anthropic:claude-opus-5 \
+            --model anthropic:claude-opus-5-1 \
             --thinking xhigh \
             < .github/prompts/auto-cleanup.md
 ```

src/browser/features/ChatInput/useCreationWorkspace.test.tsx

@@ -1360,7 +1360,7 @@ describe("useCreationWorkspace", () => {
     });
     expect(onWorkspaceCreated.mock.calls[0][1]).toEqual({
       autoNavigate: true,
-      pendingStreamModel: "anthropic:claude-opus-5",
+      pendingStreamModel: "anthropic:claude-opus-5-1",
       markPendingInitialSend: false,
     });
   });
@@ -1838,7 +1838,7 @@ describe("useCreationWorkspace", () => {
     expect(onWorkspaceCreated.mock.calls.length).toBe(1);
     expect(onWorkspaceCreated.mock.calls[0][1]).toEqual({
       autoNavigate: true,
-      pendingStreamModel: "anthropic:claude-opus-5",
+      pendingStreamModel: "anthropic:claude-opus-5-1",
       markPendingInitialSend: true,
     });
   });

src/cli/run.test.ts

@@ -171,7 +171,7 @@ describe("xum CLI", () => {
     test("shows default model as opus", async () => {
       const result = await runCli(["run", "--help"]);
       expect(result.exitCode).toBe(0);
-      expect(result.stdout).toContain("anthropic:claude-opus-5");
+      expect(result.stdout).toContain("anthropic:claude-opus-5-1");
     });
 
     test("--service-tier has no default auto", async () => {

src/common/config/schemas/appConfigOnDisk.ts

@@ -117,6 +117,19 @@ export const AppConfigMigrationsSchema = z
     userPreferencesInitialized: z.boolean().optional(),
     /** One-time seed of DEFAULT_MODEL_FALLBACKS; not re-applied while true. */
     defaultModelFallbacksSeeded: z.boolean().optional(),
+    /**
+     * One-time re-run of the fallback seed after the fable alias moved to
+     * Fable 5.1: configs seeded before the promotion lack a chain for the new
+     * source key.
+     */
+    defaultModelFallbacksSeededFable51: z.boolean().optional(),
+    /**
+     * One-time chain-target migration after the opus alias moved to Opus 5.1:
+     * chains still exactly equal to the superseded shipped default
+     * (Fable 5.1 → Opus 5) move to the current default (Fable 5.1 → Opus 5.1);
+     * any user-edited chain is left untouched.
+     */
+    defaultModelFallbacksSeededOpus51: z.boolean().optional(),
     /** One-time migration from the legacy auto-delete default to persistent sub-agents. */
     persistentSubagentsDefaulted: z.boolean().optional(),
   })

src/common/constants/knownModels.test.ts

@@ -3,7 +3,11 @@
  */
 
 import { describe, test, expect } from "@jest/globals";
-import { KNOWN_MODELS, MODEL_ABBREVIATIONS } from "@/common/constants/knownModels";
+import {
+  KNOWN_MODELS,
+  MODEL_ABBREVIATIONS,
+  TOKENIZER_MODEL_OVERRIDES,
+} from "@/common/constants/knownModels";
 import modelsJson from "@/common/utils/tokens/models.json";
 import { findMissingKnownModels } from "@/common/utils/tokens/updateModelsData";
 
@@ -19,6 +23,16 @@ describe("Known Models Integration", () => {
     }
   });
 
+  test("opus alias tracks Opus 5.1 and retired ids keep tokenizer overrides", () => {
+    expect(MODEL_ABBREVIATIONS.opus).toBe("anthropic:claude-opus-5-1");
+    expect(KNOWN_MODELS.OPUS.id).toBe("anthropic:claude-opus-5-1");
+    // Exact-id lookup for retired-but-documented custom model strings must keep
+    // resolving to the Opus 4.5 approximation instead of falling back (with a
+    // warning) to the generic per-provider tokenizer.
+    expect(TOKENIZER_MODEL_OVERRIDES["anthropic:claude-opus-5"]).toBe("anthropic/claude-opus-4.5");
+    expect(TOKENIZER_MODEL_OVERRIDES[KNOWN_MODELS.OPUS.id]).toBe("anthropic/claude-opus-4.5");
+  });
+
   test("gemini-flash resolves to the stable Gemini 3.7 Flash model", () => {
     expect(MODEL_ABBREVIATIONS["gemini-flash"]).toBe("google:gemini-3.7-flash");
   });

src/common/constants/knownModels.ts

@@ -27,13 +27,13 @@ interface KnownModel extends KnownModelDefinition {
 // Model definitions. Note we avoid listing legacy models here. These represent the focal models
 // of the community.
 const MODEL_DEFINITIONS = {
-  // Claude Fable 5 - Mythos-class model (a tier above Opus) released June 9, 2026.
-  // It is the generally-available variant of the Mythos 5 model, shipped with safeguards
-  // enabled (a small fraction of flagged requests fall back to Opus 4.8 server-side, which
-  // is transparent to API clients). API id `claude-fable-5`; $10/M input, $50/M output.
+  // Claude Fable 5.1 - Mythos-class model (a tier above Opus), successor to Fable 5
+  // (released June 9, 2026) as the generally-available safeguarded variant, at unchanged
+  // pricing ($10/M input, $50/M output). API id `claude-fable-5-1`; Fable 5 stays usable
+  // as the custom model string `anthropic:claude-fable-5`.
   FABLE: {
     provider: "anthropic",
-    providerModelId: "claude-fable-5",
+    providerModelId: "claude-fable-5-1",
     aliases: ["fable"],
     warm: true,
     // Fable/Mythos use the newer Opus 4.7+ tokenizer, which isn't published upstream;
@@ -56,15 +56,15 @@ const MODEL_DEFINITIONS = {
     // approximate counting; real usage can run ~1.0-1.3x higher.
     tokenizerOverride: "anthropic/claude-opus-4.5",
   },
-  // Claude Opus 5 - released July 24, 2026. Successor to Opus 4.8 at the same pricing
-  // ($5/M input, $25/M output). API id `claude-opus-5`; Opus 4.8 stays usable as the
-  // custom model string `anthropic:claude-opus-4-8`.
+  // Claude Opus 5.1 - successor to Opus 5 (released July 24, 2026) at unchanged pricing
+  // ($5/M input, $25/M output). API id `claude-opus-5-1`; Opus 5 stays usable as the
+  // custom model string `anthropic:claude-opus-5`.
   OPUS: {
     provider: "anthropic",
-    providerModelId: "claude-opus-5",
+    providerModelId: "claude-opus-5-1",
     aliases: ["opus"],
     warm: true,
-    // Opus 5 uses the newer Opus 4.7+ tokenizer (~30% more tokens for the same text),
+    // Opus 5.1 uses the newer Opus 4.7+ tokenizer (~30% more tokens for the same text),
     // which isn't published upstream; reuse Opus 4.5 for approximate counting. Real
     // usage can run ~1.0-1.3x higher than this estimate (same situation as FABLE above).
     tokenizerOverride: "anthropic/claude-opus-4.5",
@@ -275,11 +275,24 @@ export const MODEL_ABBREVIATIONS: Record<string, string> = Object.fromEntries(
     .sort(([a], [b]) => a.localeCompare(b))
 );
 
-export const TOKENIZER_MODEL_OVERRIDES: Record<string, string> = Object.fromEntries(
-  Object.values(KNOWN_MODELS)
-    .filter((model) => Boolean(model.tokenizerOverride))
-    .map((model) => [model.id, model.tokenizerOverride!])
-);
+// Retired first-class models stay documented as custom model strings (see the
+// FABLE/OPUS comments); keep their approximate-tokenizer overrides so exact-id
+// lookup does not fall back to the generic per-provider tokenizer.
+const LEGACY_TOKENIZER_MODEL_OVERRIDES: Record<string, string> = {
+  "anthropic:claude-fable-5": "anthropic/claude-opus-4.5",
+  "anthropic:claude-opus-5": "anthropic/claude-opus-4.5",
+  "anthropic:claude-opus-4-8": "anthropic/claude-opus-4.5",
+};
+
+export const TOKENIZER_MODEL_OVERRIDES: Record<string, string> = {
+  ...LEGACY_TOKENIZER_MODEL_OVERRIDES,
+  // Spread current models last so a returning id always wins over its legacy entry.
+  ...Object.fromEntries(
+    Object.values(KNOWN_MODELS)
+      .filter((model) => Boolean(model.tokenizerOverride))
+      .map((model) => [model.id, model.tokenizerOverride!])
+  ),
+};
 
 /** Tooltip-friendly abbreviation examples: show representative shortcuts */
 export const MODEL_ABBREVIATION_EXAMPLES = (["opus", "sonnet"] as const).map((abbrev) => ({

src/common/utils/ai/modelDisplay.test.ts

@@ -10,6 +10,7 @@ describe("formatCompactModelDisplayName", () => {
 
   test("falls back to the full display name when no shorter label is clearer", () => {
     expect(formatCompactModelDisplayName("claude-opus-5")).toBe("Opus 5");
+    expect(formatCompactModelDisplayName("claude-opus-5-1")).toBe("Opus 5.1");
     expect(formatCompactModelDisplayName("gpt-5.3-codex-spark")).toBe("Spark 5.3");
   });
 });
@@ -27,6 +28,7 @@ describe("formatModelDisplayName", () => {
 
     test("formats Mythos-class Fable / Mythos models", () => {
       expect(formatModelDisplayName("claude-fable-5")).toBe("Fable 5");
+      expect(formatModelDisplayName("claude-fable-5-1")).toBe("Fable 5.1");
       expect(formatModelDisplayName("claude-mythos-5")).toBe("Mythos 5");
     });
   });

src/common/utils/ai/modelFallbacks.ts

@@ -16,7 +16,11 @@ export const MODEL_FALLBACK_CHAIN_LIMIT = 3;
  * the sensible out-of-the-box behavior.
  *
  * Seeded into the config exactly once, guarded by
- * migrations.defaultModelFallbacksSeeded — on versions that know the flag,
+ * migrations.defaultModelFallbacksSeeded (plus the one-shot
+ * defaultModelFallbacksSeededFable51 re-seed for the key move to Fable 5.1,
+ * and the one-shot defaultModelFallbacksSeededOpus51 target migration for the
+ * Opus 5.1 promotion, see SUPERSEDED_DEFAULT_MODEL_FALLBACKS)
+ * — on versions that know the flag,
  * user edits or deletions of these chains are never overridden by updates.
  * (Versions predating the flag strip it on save, so a downgrade→save→
  * re-upgrade round-trip re-seeds a deleted chain; bounded to re-adding this
@@ -25,10 +29,43 @@ export const MODEL_FALLBACK_CHAIN_LIMIT = 3;
 export const DEFAULT_MODEL_FALLBACKS: ModelFallbacks = {
   [KNOWN_MODELS.FABLE.id]: { models: [KNOWN_MODELS.OPUS.id] },
 };
+
+/**
+ * Legacy default chain for the pre-5.1 fable id, seeded alongside
+ * DEFAULT_MODEL_FALLBACKS whenever the original seed pass runs (fresh installs
+ * and configs that never completed it). Those configs mark
+ * defaultModelFallbacksSeeded, so a downgrade to a build whose FABLE is
+ * Fable 5 would skip its own seed; carrying this chain keeps the old build's
+ * refusal fallback intact.
+ */
+export const LEGACY_DEFAULT_MODEL_FALLBACKS: ModelFallbacks = {
+  // Target pinned to the literal Opus 5 id (not KNOWN_MODELS.OPUS.id): this
+  // chain exists for downgraded pre-5.1 builds, whose own shipped default was
+  // Fable 5 → Opus 5. Pinning keeps the seeded bytes identical no matter which
+  // build ran the original seed pass.
+  "anthropic:claude-fable-5": { models: ["anthropic:claude-opus-5"] },
+};
+
+/**
+ * Shipped defaults superseded by the Opus 5.1 promotion, keyed like
+ * DEFAULT_MODEL_FALLBACKS. Guarded by migrations.defaultModelFallbacksSeededOpus51,
+ * config load rewrites a chain to the current default exactly once IF the stored
+ * entry still deep-equals the superseded default (same single-model chain, no
+ * enabled/triggers customization). Any deviation is user intent and is never
+ * touched; the legacy `anthropic:claude-fable-5` chain is deliberately not
+ * migrated so downgraded builds keep their expected bytes on disk.
+ */
+export const SUPERSEDED_DEFAULT_MODEL_FALLBACKS: ModelFallbacks = {
+  [KNOWN_MODELS.FABLE.id]: { models: ["anthropic:claude-opus-5"] },
+};
 // Deep-freeze: entries are spread by reference into live configs (fresh-install
 // defaults, seed merge). Accidental in-place mutation must crash fast instead
 // of silently corrupting the process-wide default.
-for (const entry of Object.values(Object.freeze(DEFAULT_MODEL_FALLBACKS))) {
+for (const entry of [
+  ...Object.values(Object.freeze(DEFAULT_MODEL_FALLBACKS)),
+  ...Object.values(Object.freeze(LEGACY_DEFAULT_MODEL_FALLBACKS)),
+  ...Object.values(Object.freeze(SUPERSEDED_DEFAULT_MODEL_FALLBACKS)),
+]) {
   Object.freeze(entry);
   Object.freeze(entry.models);
 }

src/common/utils/ai/models.test.ts

@@ -143,9 +143,15 @@ describe("Anthropic 1M context classification", () => {
     expect(hasNative1MContext("anthropic:claude-sonnet-4-5")).toBe(false);
   });
 
-  it("treats Opus 4.6 to 4.8, Opus 5, and Sonnet 4.6 as native 1M models", () => {
+  it("treats Opus 4.6 to 4.8, Opus 5, Opus 5.1, and Sonnet 4.6 as native 1M models", () => {
     expect(getAnthropic1MContextMode("anthropic:claude-opus-5")).toBe("native");
     expect(getAnthropic1MContextMode("anthropic:claude-opus-5-20260724")).toBe("native");
+    // The claude-opus-5 pattern only tolerates date suffixes, so the 5.1 id
+    // needs (and has) its own pattern.
+    expect(getAnthropic1MContextMode("anthropic:claude-opus-5-1")).toBe("native");
+    expect(getAnthropic1MContextMode("mux-gateway:anthropic/claude-opus-5-1")).toBe("native");
+    expect(hasNative1MContext("anthropic:claude-opus-5-1")).toBe(true);
+    expect(supports1MContext("anthropic:claude-opus-5-1")).toBe(false);
     expect(getAnthropic1MContextMode("anthropic:claude-opus-4-8")).toBe("native");
     expect(getAnthropic1MContextMode("anthropic:claude-opus-4-8-20260528")).toBe("native");
     expect(getAnthropic1MContextMode("anthropic:claude-opus-4-6")).toBe("native");
@@ -167,12 +173,14 @@ describe("Anthropic 1M context classification", () => {
     expect(hasNative1MContext("anthropic:claude-sonnet-5")).toBe(true);
   });
 
-  it("treats Mythos-class Fable 5 / Mythos 5 as native 1M models", () => {
+  it("treats Mythos-class Fable 5 / Fable 5.1 / Mythos 5 as native 1M models", () => {
     expect(getAnthropic1MContextMode("anthropic:claude-fable-5")).toBe("native");
+    expect(getAnthropic1MContextMode("anthropic:claude-fable-5-1")).toBe("native");
     expect(getAnthropic1MContextMode("anthropic:claude-mythos-5")).toBe("native");
-    expect(getAnthropic1MContextMode("mux-gateway:anthropic/claude-fable-5")).toBe("native");
+    expect(getAnthropic1MContextMode("mux-gateway:anthropic/claude-fable-5-1")).toBe("native");
     expect(hasNative1MContext("anthropic:claude-fable-5")).toBe(true);
-    expect(supports1MContext("anthropic:claude-fable-5")).toBe(false);
+    expect(hasNative1MContext("anthropic:claude-fable-5-1")).toBe(true);
+    expect(supports1MContext("anthropic:claude-fable-5-1")).toBe(false);
   });
 
   it("returns none for models without Anthropic 1M support", () => {

src/common/utils/ai/models.ts

@@ -179,8 +179,10 @@ const OPTIONAL_VERSION_SUFFIX = String.raw`(?:-(?:\d{8}|\d{4}-\d{2}-\d{2}))?`;
 const ANTHROPIC_NATIVE_1M_PATTERNS = [
   // Mythos-class models (Fable 5 / Mythos 5) ship 1M context as standard metadata.
   new RegExp(`^claude-fable-5${OPTIONAL_VERSION_SUFFIX}$`, "i"),
+  new RegExp(`^claude-fable-5-1${OPTIONAL_VERSION_SUFFIX}$`, "i"),
   new RegExp(`^claude-mythos-5${OPTIONAL_VERSION_SUFFIX}$`, "i"),
   new RegExp(`^claude-opus-5${OPTIONAL_VERSION_SUFFIX}$`, "i"),
+  new RegExp(`^claude-opus-5-1${OPTIONAL_VERSION_SUFFIX}$`, "i"),
   new RegExp(`^claude-opus-4-8${OPTIONAL_VERSION_SUFFIX}$`, "i"),
   new RegExp(`^claude-opus-4-7${OPTIONAL_VERSION_SUFFIX}$`, "i"),
   new RegExp(`^claude-opus-4-6${OPTIONAL_VERSION_SUFFIX}$`, "i"),