diff --git a/packages/cli/tests/15-provider.test.ts b/packages/cli/tests/15-provider.test.ts
index 3081ee3584..c4731e1de6 100644
--- a/packages/cli/tests/15-provider.test.ts
+++ b/packages/cli/tests/15-provider.test.ts
@@ -62,6 +62,11 @@ const EXPECTED_CLAUDE_MODELS = [
     model: "Opus 5",
     descriptionFragment: "Previous release",
   },
+  {
+    id: "claude-mythos-5-1",
+    model: "Mythos 5.1",
+    descriptionFragment: "Requires verified access",
+  },
   {
     id: "claude-fable-5-1",
     model: "Fable 5.1",
diff --git a/packages/server/src/server/agent/providers/claude/agent.test.ts b/packages/server/src/server/agent/providers/claude/agent.test.ts
index 9f7430b103..dc90b742d3 100644
--- a/packages/server/src/server/agent/providers/claude/agent.test.ts
+++ b/packages/server/src/server/agent/providers/claude/agent.test.ts
@@ -438,6 +438,7 @@ describe("ClaudeAgentClient.fetchCatalog", () => {
 
       expect(models.map((m) => m.id)).toEqual([
         "claude-opus-5",
+        "claude-mythos-5-1",
         "claude-fable-5-1",
         "claude-fable-5",
         "claude-fable-5[1m]",
@@ -509,6 +510,14 @@ describe("ClaudeAgentClient.fetchCatalog", () => {
       };
 
       expect(getThinkingIds("claude-opus-5")).toContain("ultracode");
+      expect(getThinkingIds("claude-mythos-5-1")).toEqual([
+        "low",
+        "medium",
+        "high",
+        "xhigh",
+        "max",
+        "ultracode",
+      ]);
       expect(getThinkingIds("claude-fable-5-1")).toContain("ultracode");
       expect(getThinkingIds("claude-fable-5")).toContain("ultracode");
       expect(getThinkingIds("claude-opus-4-8[1m]")).toContain("ultracode");
@@ -1232,6 +1241,7 @@ describe("ClaudeAgentSession features", () => {
 
   test.each([
     ["supported model", "claude-opus-4-8", { type: "disabled" }, undefined],
+    ["Mythos 5.1", "claude-mythos-5-1", { type: "adaptive", display: "summarized" }, "high"],
     ["Haiku 5.5", "claude-haiku-5-5", { type: "disabled" }, undefined],
     ["unsupported model", "claude-fable-5", { type: "adaptive", display: "summarized" }, "high"],
     ["custom model", "openrouter/anthropic/claude-opus-4-8", undefined, undefined],
diff --git a/packages/server/src/server/agent/providers/claude/model-manifest.ts b/packages/server/src/server/agent/providers/claude/model-manifest.ts
index 398fcb1ccd..f77c7de2fb 100644
--- a/packages/server/src/server/agent/providers/claude/model-manifest.ts
+++ b/packages/server/src/server/agent/providers/claude/model-manifest.ts
@@ -57,6 +57,13 @@ export const CLAUDE_MODEL_MANIFEST = [
     supportsThinkingDisabled: true,
     supportsFastMode: true,
   },
+  {
+    id: "claude-mythos-5-1",
+    label: "Mythos 5.1",
+    description: "Mythos 5.1 · Requires verified access",
+    contextWindowMaxTokens: 1_000_000,
+    effortLevels: CLAUDE_EFFORT_LEVELS.xhigh,
+  },
   {
     id: "claude-fable-5-1",
     label: "Fable 5.1",
@@ -359,7 +366,7 @@ export function normalizeClaudeManifestModelId(value: string | null | undefined)
   }
 
   const singleSegmentMatch = trimmed.match(
-    /^(?:claude[-_ ])?(fable|opus|sonnet|haiku)[-_ ]+(\d+)(?:\[1m\])?(?:[-_ ]+\d{8})?(?:\[1m\])?$/i,
+    /^(?:claude[-_ ])?(mythos|fable|opus|sonnet|haiku)[-_ ]+(\d+)(?:\[1m\])?(?:[-_ ]+\d{8})?(?:\[1m\])?$/i,
   );
   if (singleSegmentMatch) {
     return normalizeSingleSegmentClaudeModelId(
@@ -370,7 +377,7 @@ export function normalizeClaudeManifestModelId(value: string | null | undefined)
   }
 
   const runtimeMatch = trimmed.match(
-    /^(?:claude[-_ ])?(fable|opus|sonnet|haiku)[-_ ]+(\d+)[-.](\d+)(?:\[1m\])?(?:[-_ ]+\d{8})?(?:\[1m\])?$/i,
+    /^(?:claude[-_ ])?(mythos|fable|opus|sonnet|haiku)[-_ ]+(\d+)[-.](\d+)(?:\[1m\])?(?:[-_ ]+\d{8})?(?:\[1m\])?$/i,
   );
   if (!runtimeMatch) {
     return null;
@@ -403,7 +410,7 @@ export function normalizeClaudeRuntimeModelId(value: string | null | undefined):
   }
 
   const runtimeMatch = trimmed.match(
-    /claude[-_ ](fable|opus|sonnet|haiku)[-_ ]+(\d+)[-.](\d+)(\[1m\])?/i,
+    /claude[-_ ](mythos|fable|opus|sonnet|haiku)[-_ ]+(\d+)[-.](\d+)(\[1m\])?/i,
   );
   if (runtimeMatch) {
     const normalizedModelId = normalizeMajorMinorClaudeModelId(
@@ -418,7 +425,7 @@ export function normalizeClaudeRuntimeModelId(value: string | null | undefined):
   }
 
   const singleSegmentMatch = trimmed.match(
-    /claude[-_ ](fable|opus|sonnet|haiku)[-_ ]+(\d+)(\[1m\])?/i,
+    /claude[-_ ](mythos|fable|opus|sonnet|haiku)[-_ ]+(\d+)(\[1m\])?/i,
   );
   if (!singleSegmentMatch) {
     return null;
diff --git a/packages/server/src/server/agent/providers/claude/models.test.ts b/packages/server/src/server/agent/providers/claude/models.test.ts
index ed7db8841c..ce8c583917 100644
--- a/packages/server/src/server/agent/providers/claude/models.test.ts
+++ b/packages/server/src/server/agent/providers/claude/models.test.ts
@@ -52,6 +52,7 @@ describe("getClaudeModels", () => {
     expect(models.map((m) => m.id)).toEqual([
       "claude-opus-5-5",
       "claude-opus-5",
+      "claude-mythos-5-1",
       "claude-fable-5-1",
       "claude-fable-5",
       "claude-fable-5[1m]",
@@ -87,6 +88,7 @@ describe("getClaudeModels", () => {
       new Map([
         ["claude-opus-5-5", 1_000_000],
         ["claude-opus-5", 1_000_000],
+        ["claude-mythos-5-1", 1_000_000],
         ["claude-fable-5-1", 1_000_000],
         ["claude-fable-5", 1_000_000],
         ["claude-fable-5[1m]", 1_000_000],
@@ -724,3 +726,50 @@ describe("claudeManifestModelSupportsFastMode", () => {
     expect(claudeManifestModelSupportsFastMode("claude-fable-5-1")).toBe(false);
   });
 });
+
+describe("Claude Mythos 5.1 catalog", () => {
+  it("offers one 1M model with high default effort and always-on thinking", () => {
+    const models = getClaudeModels().filter((model) => model.id.startsWith("claude-mythos"));
+    expect(models).toHaveLength(1);
+    expect(models[0]).toMatchObject({
+      id: "claude-mythos-5-1",
+      label: "Mythos 5.1",
+      contextWindowMaxTokens: 1_000_000,
+      defaultThinkingOptionId: "high",
+    });
+    expect(models[0].thinkingOptions?.map((option) => option.id)).toEqual([
+      "low",
+      "medium",
+      "high",
+      "xhigh",
+      "max",
+      CLAUDE_ULTRACODE_THINKING_OPTION_ID,
+    ]);
+    expect(
+      models[0].thinkingOptions?.filter((option) => option.isDefault).map((option) => option.id),
+    ).toEqual(["high"]);
+    expect(resolveClaudeDisabledThinkingForModel("claude-mythos-5-1")).toEqual({
+      supported: false,
+      fallbackThinkingOptionId: "high",
+    });
+    expect(claudeManifestModelSupportsFastMode("claude-mythos-5-1")).toBe(false);
+    expect(getClaudeModels().find((model) => model.isDefault)?.id).toBe("claude-opus-5-5");
+  });
+
+  it.each([
+    "claude-mythos-5-1[1m]",
+    "claude-mythos-5-1-20260901",
+    "claude-mythos-5-1-20260901[1m]",
+    "Mythos 5.1",
+  ])("resolves %s to the single catalog entry", (id) => {
+    expect(findClaudeModel(id)?.id).toBe("claude-mythos-5-1");
+  });
+
+  it.each(["anthropic/claude-mythos-5-1", "us.anthropic.claude-mythos-5-1-v1:0"])(
+    "normalizes runtime identity without granting first-party capabilities: %s",
+    (id) => {
+      expect(normalizeClaudeRuntimeModelId(id)).toBe("claude-mythos-5-1");
+      expect(normalizeClaudeManifestModelId(id)).toBeNull();
+    },
+  );
+});