🤖 feat: promote Claude Mythos 5.1 as the mythos model (DO NOT MERGE until GA)

docs/config/models.mdx

@@ -14,7 +14,7 @@ Xum ships with curated models kept up to date with the frontier. Use any custom
 | Model                  | ID                            | Aliases                                                      | Default |
 | ---------------------- | ----------------------------- | ------------------------------------------------------------ | ------- |
 | Fable 5.1              | anthropic:claude-fable-5-1    | `fable`                                                      |         |
-| Mythos 5               | anthropic:claude-mythos-5     | `mythos`                                                     |         |
+| Mythos 5.1             | anthropic:claude-mythos-5-1   | `mythos`                                                     |         |
 | Opus 5                 | anthropic:claude-opus-5       | `opus`                                                       | ✓       |
 | Sonnet 5               | anthropic:claude-sonnet-5     | `sonnet`                                                     |         |
 | Haiku 4.5              | anthropic:claude-haiku-4-5    | `haiku`                                                      |         |

src/common/constants/knownModels.ts

@@ -42,15 +42,16 @@ const MODEL_DEFINITIONS = {
     // same text, so real usage can run ~1.0-1.3x higher than this estimate.
     tokenizerOverride: "anthropic/claude-opus-4.5",
   },
-  // Claude Mythos 5 - released June 9, 2026 alongside Fable 5. Same underlying model,
-  // specs, and pricing as Fable 5 ($10/M input, $50/M output) but with safeguards lifted
-  // in some areas. Limited availability: restricted to approved Project Glasswing /
-  // trusted-access customers (no self-serve sign-up). API id `claude-mythos-5`.
+  // Claude Mythos 5.1 - successor to Mythos 5 (released June 9, 2026 alongside Fable 5)
+  // as the restricted-access, safeguards-lifted twin of Fable 5.1, assumed at unchanged
+  // pricing ($10/M input, $50/M output) and availability (approved Project Glasswing /
+  // trusted-access customers, no self-serve sign-up). API id `claude-mythos-5-1`;
+  // Mythos 5 stays usable as the custom model string `anthropic:claude-mythos-5`.
   // Not warmed: most users cannot access it, and its tokenizer override is already
   // warmed via FABLE.
   MYTHOS: {
     provider: "anthropic",
-    providerModelId: "claude-mythos-5",
+    providerModelId: "claude-mythos-5-1",
     aliases: ["mythos"],
     // Same tokenizer situation as Fable 5 (see FABLE above): reuse Opus 4.5 for
     // approximate counting; real usage can run ~1.0-1.3x higher.
@@ -280,6 +281,7 @@ export const MODEL_ABBREVIATIONS: Record<string, string> = Object.fromEntries(
 // 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-mythos-5": "anthropic/claude-opus-4.5",
   "anthropic:claude-opus-4-8": "anthropic/claude-opus-4.5",
 };
 

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

@@ -29,6 +29,7 @@ describe("formatModelDisplayName", () => {
       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");
+      expect(formatModelDisplayName("claude-mythos-5-1")).toBe("Mythos 5.1");
     });
   });
 

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

@@ -167,14 +167,18 @@ describe("Anthropic 1M context classification", () => {
     expect(hasNative1MContext("anthropic:claude-sonnet-5")).toBe(true);
   });
 
-  it("treats Mythos-class Fable 5 / Fable 5.1 / Mythos 5 as native 1M models", () => {
+  it("treats Mythos-class Fable 5 / Fable 5.1 / Mythos 5 / Mythos 5.1 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("anthropic:claude-mythos-5-1")).toBe("native");
     expect(getAnthropic1MContextMode("mux-gateway:anthropic/claude-fable-5-1")).toBe("native");
+    expect(getAnthropic1MContextMode("mux-gateway:anthropic/claude-mythos-5-1")).toBe("native");
     expect(hasNative1MContext("anthropic:claude-fable-5")).toBe(true);
     expect(hasNative1MContext("anthropic:claude-fable-5-1")).toBe(true);
+    expect(hasNative1MContext("anthropic:claude-mythos-5-1")).toBe(true);
     expect(supports1MContext("anthropic:claude-fable-5-1")).toBe(false);
+    expect(supports1MContext("anthropic:claude-mythos-5-1")).toBe(false);
   });
 
   it("returns none for models without Anthropic 1M support", () => {

src/common/utils/ai/models.ts

@@ -181,6 +181,7 @@ const ANTHROPIC_NATIVE_1M_PATTERNS = [
   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-mythos-5-1${OPTIONAL_VERSION_SUFFIX}$`, "i"),
   new RegExp(`^claude-opus-5${OPTIONAL_VERSION_SUFFIX}$`, "i"),
   new RegExp(`^claude-opus-4-8${OPTIONAL_VERSION_SUFFIX}$`, "i"),
   new RegExp(`^claude-opus-4-7${OPTIONAL_VERSION_SUFFIX}$`, "i"),

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

@@ -186,6 +186,12 @@ describe("buildProviderOptions - Anthropic", () => {
       );
       expect(anthropic51.thinking).toEqual({ type: "adaptive", display: "summarized" });
       expect(anthropic51.effort).toBe("medium");
+      // Mythos 5.1 does too.
+      const mythos51 = anthropicProviderOptions(
+        buildProviderOptions("anthropic:claude-mythos-5-1", "medium")
+      );
+      expect(mythos51.thinking).toEqual({ type: "adaptive", display: "summarized" });
+      expect(mythos51.effort).toBe("medium");
     });
 
     test("omits thinking instead of sending disabled when off", () => {
@@ -198,6 +204,9 @@ describe("buildProviderOptions - Anthropic", () => {
       expect(buildProviderOptions("anthropic:claude-fable-5-1", "off")).toEqual({
         anthropic: { ...baseAnthropicOptions, effort: "low" },
       });
+      expect(buildProviderOptions("anthropic:claude-mythos-5-1", "off")).toEqual({
+        anthropic: { ...baseAnthropicOptions, effort: "low" },
+      });
     });
   });
 

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

@@ -457,13 +457,21 @@ describe("getThinkingPolicyForModel", () => {
       "xhigh",
       "max",
     ]);
+    expect(getThinkingPolicyForModel("anthropic:claude-mythos-5-1")).toEqual([
+      "low",
+      "medium",
+      "high",
+      "xhigh",
+      "max",
+    ]);
   });
 
   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");
+    expect(enforceThinkingPolicy("anthropic:claude-mythos-5-1", "off")).toBe("low");
   });
 
   test("resolveEffectiveThinkingLevel clamps unset/off for Mythos-class only", () => {

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

@@ -177,6 +177,26 @@ export const modelsExtra: Record<string, ModelData> = {
     supports_response_schema: true,
   },
 
+  // Claude Mythos 5.1 - successor to Mythos 5, the restricted-access twin of
+  // Fable 5.1, assumed 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-mythos-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,
+  },
+
   // Same underlying model as Fable 5 (identical pricing/shape); restricted access.
   "claude-mythos-5": {
     max_input_tokens: 1000000,

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

@@ -73,6 +73,7 @@ describe("supportsAnthropicNativeWebFetch", () => {
     ["claude-mythos-5", true],
     // Two-segment IDs at/after the 4.6 cutoff.
     ["claude-fable-5-1", true],
+    ["claude-mythos-5-1", true],
     ["claude-sonnet-4-6", true],
     ["claude-opus-4-6", true],
     ["claude-opus-4-8", true],

src/node/services/agentSkills/builtInSkillContent.generated.ts

@@ -3851,7 +3851,7 @@ export const BUILTIN_SKILL_FILES: Record<string, Record<string, string>> = {
       "| Model                  | ID                            | Aliases                                                      | Default |",
       "| ---------------------- | ----------------------------- | ------------------------------------------------------------ | ------- |",
       "| Fable 5.1              | anthropic:claude-fable-5-1    | `fable`                                                      |         |",
-      "| Mythos 5               | anthropic:claude-mythos-5     | `mythos`                                                     |         |",
+      "| Mythos 5.1             | anthropic:claude-mythos-5-1   | `mythos`                                                     |         |",
       "| Opus 5                 | anthropic:claude-opus-5       | `opus`                                                       | ✓       |",
       "| Sonnet 5               | anthropic:claude-sonnet-5     | `sonnet`                                                     |         |",
       "| Haiku 4.5              | anthropic:claude-haiku-4-5    | `haiku`                                                      |         |",