🤖 feat: promote Claude Fable 5.1 as the fable model

docs/config/models.mdx

@@ -13,7 +13,7 @@ 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`                                                       | ✓       |
 | Sonnet 5               | anthropic:claude-sonnet-5     | `sonnet`                                                     |         |

src/common/config/schemas/appConfigOnDisk.ts

@@ -117,6 +117,12 @@ 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 migration from the legacy auto-delete default to persistent sub-agents. */
     persistentSubagentsDefaulted: z.boolean().optional(),
   })

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;
@@ -275,11 +275,23 @@ 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-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

@@ -27,6 +27,7 @@ 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 +16,9 @@ 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)
+ * — 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 +27,25 @@ 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 = {
+  "anthropic:claude-fable-5": { models: [KNOWN_MODELS.OPUS.id] },
+};
 // 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.freeze(entry);
   Object.freeze(entry.models);
 }

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

@@ -167,12 +167,14 @@ 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

@@ -179,6 +179,7 @@ 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-4-8${OPTIONAL_VERSION_SUFFIX}$`, "i"),

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

@@ -180,6 +180,12 @@ describe("buildProviderOptions - Anthropic", () => {
       // the summarized display flag.
       expect(anthropic.thinking).toEqual({ type: "adaptive", display: "summarized" });
       expect(anthropic.effort).toBe("medium");
+      // Fable 5.1 rides the same Mythos-class wildcard matcher.
+      const anthropic51 = anthropicProviderOptions(
+        buildProviderOptions("anthropic:claude-fable-5-1", "medium")
+      );
+      expect(anthropic51.thinking).toEqual({ type: "adaptive", display: "summarized" });
+      expect(anthropic51.effort).toBe("medium");
     });
 
     test("omits thinking instead of sending disabled when off", () => {
@@ -189,6 +195,9 @@ describe("buildProviderOptions - Anthropic", () => {
       expect(buildProviderOptions("anthropic:claude-fable-5", "off")).toEqual({
         anthropic: { ...baseAnthropicOptions, effort: "low" },
       });
+      expect(buildProviderOptions("anthropic:claude-fable-5-1", "off")).toEqual({
+        anthropic: { ...baseAnthropicOptions, effort: "low" },
+      });
     });
   });
 

src/common/utils/thinking/policy.test.ts

@@ -443,6 +443,13 @@ describe("getThinkingPolicyForModel", () => {
       "xhigh",
       "max",
     ]);
+    expect(getThinkingPolicyForModel("anthropic:claude-fable-5-1")).toEqual([
+      "low",
+      "medium",
+      "high",
+      "xhigh",
+      "max",
+    ]);
     expect(getThinkingPolicyForModel("anthropic:claude-mythos-5")).toEqual([
       "low",
       "medium",
@@ -455,6 +462,7 @@ describe("getThinkingPolicyForModel", () => {
   test("clamps 'off' up to 'low' for Mythos-class models", () => {
     // A stored/legacy "off" selection must not reach the wire as disabled thinking.
     expect(enforceThinkingPolicy("anthropic:claude-fable-5", "off")).toBe("low");
+    expect(enforceThinkingPolicy("anthropic:claude-fable-5-1", "off")).toBe("low");
     expect(enforceThinkingPolicy("anthropic:claude-mythos-5", "off")).toBe("low");
   });
 

src/common/utils/tokens/models-extra.ts

@@ -135,6 +135,25 @@ export const modelsExtra: Record<string, ModelData> = {
     supports_pdf_input: true,
   },
 
+  // Claude Fable 5.1 - successor to Fable 5 in the Mythos-class tier at the same
+  // pricing/shape: $10/M input, $50/M output, cache write 1.25x input / cache read
+  // 0.1x input, native 1M context, 128K max output, native xhigh effort level.
+  "claude-fable-5-1": {
+    max_input_tokens: 1000000,
+    max_output_tokens: 128000,
+    input_cost_per_token: 0.00001, // $10 per million input tokens
+    output_cost_per_token: 0.00005, // $50 per million output tokens
+    cache_creation_input_token_cost: 0.0000125, // $12.50 per million tokens (1.25× input)
+    cache_read_input_token_cost: 0.000001, // $1.00 per million tokens (0.1× input)
+    litellm_provider: "anthropic",
+    mode: "chat",
+    supports_function_calling: true,
+    supports_vision: true,
+    supports_pdf_input: true,
+    supports_reasoning: true,
+    supports_response_schema: true,
+  },
+
   // Claude Fable 5 / Mythos 5 - Released June 9, 2026
   // Mythos-class model (a tier above Opus). Fable 5 (`claude-fable-5`) is the
   // generally-available variant with safeguards; Mythos 5 (`claude-mythos-5`) is the same

src/common/utils/tools/tools.test.ts

@@ -72,6 +72,7 @@ describe("supportsAnthropicNativeWebFetch", () => {
     ["claude-fable-5", true],
     ["claude-mythos-5", true],
     // Two-segment IDs at/after the 4.6 cutoff.
+    ["claude-fable-5-1", true],
     ["claude-sonnet-4-6", true],
     ["claude-opus-4-6", true],
     ["claude-opus-4-8", true],