diff --git a/docs/config/models.mdx b/docs/config/models.mdx
index 79f56e53876..7741775e98a 100644
--- a/docs/config/models.mdx
+++ b/docs/config/models.mdx
@@ -16,7 +16,7 @@ Xum ships with curated models kept up to date with the frontier. Use any custom
 | 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`                                                     |         |
+| Sonnet 5.1             | anthropic:claude-sonnet-5-1   | `sonnet`                                                     |         |
 | Haiku 4.5              | anthropic:claude-haiku-4-5    | `haiku`                                                      |         |
 | GPT-5.6 Sol            | openai:gpt-5.6-sol            | `gpt`, `sol`                                                 |         |
 | GPT-5.6 Terra          | openai:gpt-5.6-terra          | `terra`                                                      |         |
diff --git a/src/browser/utils/slashCommands/parser.test.ts b/src/browser/utils/slashCommands/parser.test.ts
index d74c9fba1d5..a69c5dc611b 100644
--- a/src/browser/utils/slashCommands/parser.test.ts
+++ b/src/browser/utils/slashCommands/parser.test.ts
@@ -80,7 +80,7 @@ describe("commandParser", () => {
     });
 
     it("should parse /model with full provider:model format", () => {
-      expectModelSet("/model anthropic:claude-sonnet-5", KNOWN_MODELS.SONNET.id);
+      expectModelSet("/model anthropic:claude-sonnet-5-1", KNOWN_MODELS.SONNET.id);
     });
 
     it("should parse /compact -m with alias", () => {
diff --git a/src/common/constants/knownModels.ts b/src/common/constants/knownModels.ts
index 5a4a3f90c9d..a5e34815b20 100644
--- a/src/common/constants/knownModels.ts
+++ b/src/common/constants/knownModels.ts
@@ -69,18 +69,19 @@ const MODEL_DEFINITIONS = {
     // usage can run ~1.0-1.3x higher than this estimate (same situation as FABLE above).
     tokenizerOverride: "anthropic/claude-opus-4.5",
   },
-  // Claude Sonnet 5 - released June 30, 2026. The most agentic Sonnet yet (native 1M context,
-  // 128K max output, adaptive thinking + effort including native xhigh). Standard pricing matches
-  // Sonnet 4.6 ($3/M in, $15/M out); introductory $2/$10 applies through Aug 31, 2026. API id
-  // `claude-sonnet-5`. The bare `sonnet` alias tracks the latest Sonnet tier.
+  // Claude Sonnet 5.1 - successor to Sonnet 5 (released June 30, 2026) with the same
+  // envelope (native 1M context, 128K max output, adaptive thinking + effort including
+  // native xhigh) at unchanged standard pricing ($3/M in, $15/M out). API id
+  // `claude-sonnet-5-1`; Sonnet 5 stays usable as the custom model string
+  // `anthropic:claude-sonnet-5`. The bare `sonnet` alias tracks the latest Sonnet tier.
   SONNET: {
     provider: "anthropic",
-    providerModelId: "claude-sonnet-5",
+    providerModelId: "claude-sonnet-5-1",
     aliases: ["sonnet"],
     warm: true,
-    // Sonnet 5 ships an updated tokenizer (same kind of change introduced with Opus 4.7) that
-    // isn't published upstream yet; reuse Sonnet 4.5 for approximate counting. Real usage can run
-    // ~1.0-1.35x higher than this estimate depending on content type.
+    // Sonnet 5/5.1 ship an updated tokenizer (same kind of change introduced with Opus 4.7)
+    // that isn't published upstream yet; reuse Sonnet 4.5 for approximate counting. Real usage
+    // can run ~1.0-1.35x higher than this estimate depending on content type.
     tokenizerOverride: "anthropic/claude-sonnet-4.5",
   },
   HAIKU: {
@@ -276,11 +277,12 @@ export const MODEL_ABBREVIATIONS: Record<string, string> = Object.fromEntries(
 );
 
 // Retired first-class models stay documented as custom model strings (see the
-// FABLE/OPUS comments); keep their approximate-tokenizer overrides so exact-id
+// FABLE/OPUS/SONNET 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",
+  "anthropic:claude-sonnet-5": "anthropic/claude-sonnet-4.5",
 };
 
 export const TOKENIZER_MODEL_OVERRIDES: Record<string, string> = {
diff --git a/src/common/utils/ai/modelDisplay.test.ts b/src/common/utils/ai/modelDisplay.test.ts
index f5e4cc59287..4115a483d7c 100644
--- a/src/common/utils/ai/modelDisplay.test.ts
+++ b/src/common/utils/ai/modelDisplay.test.ts
@@ -19,6 +19,7 @@ describe("formatModelDisplayName", () => {
     test("formats Sonnet models", () => {
       expect(formatModelDisplayName("claude-sonnet-4-5")).toBe("Sonnet 4.5");
       expect(formatModelDisplayName("claude-sonnet-4")).toBe("Sonnet 4");
+      expect(formatModelDisplayName("claude-sonnet-5-1")).toBe("Sonnet 5.1");
     });
 
     test("formats Opus models", () => {
diff --git a/src/common/utils/ai/models.test.ts b/src/common/utils/ai/models.test.ts
index 8abea904a54..9d250586648 100644
--- a/src/common/utils/ai/models.test.ts
+++ b/src/common/utils/ai/models.test.ts
@@ -159,12 +159,18 @@ describe("Anthropic 1M context classification", () => {
     expect(hasNative1MContext("anthropic:claude-sonnet-4-6")).toBe(true);
   });
 
-  it("treats Sonnet 5 as a native 1M model", () => {
+  it("treats Sonnet 5 / Sonnet 5.1 as native 1M models", () => {
     expect(getAnthropic1MContextMode("anthropic:claude-sonnet-5")).toBe("native");
     expect(getAnthropic1MContextMode("anthropic:claude-sonnet-5-20260630")).toBe("native");
     expect(getAnthropic1MContextMode("mux-gateway:anthropic/claude-sonnet-5")).toBe("native");
     expect(supports1MContext("anthropic:claude-sonnet-5")).toBe(false);
     expect(hasNative1MContext("anthropic:claude-sonnet-5")).toBe(true);
+    // The claude-sonnet-5 pattern only tolerates date suffixes, so 5.1 needs
+    // its own pattern (same trap as claude-fable-5-1).
+    expect(getAnthropic1MContextMode("anthropic:claude-sonnet-5-1")).toBe("native");
+    expect(getAnthropic1MContextMode("mux-gateway:anthropic/claude-sonnet-5-1")).toBe("native");
+    expect(supports1MContext("anthropic:claude-sonnet-5-1")).toBe(false);
+    expect(hasNative1MContext("anthropic:claude-sonnet-5-1")).toBe(true);
   });
 
   it("treats Mythos-class Fable 5 / Fable 5.1 / Mythos 5 as native 1M models", () => {
diff --git a/src/common/utils/ai/models.ts b/src/common/utils/ai/models.ts
index a58d7432b98..a03dfd2d1f5 100644
--- a/src/common/utils/ai/models.ts
+++ b/src/common/utils/ai/models.ts
@@ -186,6 +186,7 @@ const ANTHROPIC_NATIVE_1M_PATTERNS = [
   new RegExp(`^claude-opus-4-7${OPTIONAL_VERSION_SUFFIX}$`, "i"),
   new RegExp(`^claude-opus-4-6${OPTIONAL_VERSION_SUFFIX}$`, "i"),
   new RegExp(`^claude-sonnet-5${OPTIONAL_VERSION_SUFFIX}$`, "i"),
+  new RegExp(`^claude-sonnet-5-1${OPTIONAL_VERSION_SUFFIX}$`, "i"),
   new RegExp(`^claude-sonnet-4-6${OPTIONAL_VERSION_SUFFIX}$`, "i"),
 ];
 const ANTHROPIC_BETA_1M_PATTERNS = [
diff --git a/src/common/utils/ai/providerOptions.test.ts b/src/common/utils/ai/providerOptions.test.ts
index 6fe8c7658b3..348c1bb0c76 100644
--- a/src/common/utils/ai/providerOptions.test.ts
+++ b/src/common/utils/ai/providerOptions.test.ts
@@ -139,7 +139,14 @@ describe("buildProviderOptions - Anthropic", () => {
   // Native-xhigh models (Opus 4.7+ / Sonnet 5+): xhigh is a distinct native
   // effort and adaptive thinking requires `display: "summarized"` to return
   // thinking content.
-  for (const model of ["claude-opus-4-7", "claude-opus-5", "claude-sonnet-5"] as const) {
+  // Sonnet 5.1 rides the same Sonnet 5+ wildcard matcher; unlike Mythos-class
+  // models it still sends `thinking: { type: "disabled" }` when off.
+  for (const model of [
+    "claude-opus-4-7",
+    "claude-opus-5",
+    "claude-sonnet-5",
+    "claude-sonnet-5-1",
+  ] as const) {
     describe(`${model} (native xhigh effort + summarized display)`, () => {
       for (const { thinking, expectedThinking, effort } of [
         {
@@ -2056,6 +2063,7 @@ describe("buildRequestHeaders", () => {
   test("does not emit any Xum-internal effort header for native-xhigh models", () => {
     expect(buildRequestHeaders("anthropic:claude-opus-4-7")).toBeUndefined();
     expect(buildRequestHeaders("anthropic:claude-sonnet-5")).toBeUndefined();
+    expect(buildRequestHeaders("anthropic:claude-sonnet-5-1")).toBeUndefined();
   });
 
   describe("openaiDirectProviderOptionsAvailable", () => {
diff --git a/src/common/utils/thinking/policy.test.ts b/src/common/utils/thinking/policy.test.ts
index 6c58576bf13..5995b1d3777 100644
--- a/src/common/utils/thinking/policy.test.ts
+++ b/src/common/utils/thinking/policy.test.ts
@@ -532,7 +532,7 @@ describe("getThinkingPolicyForModel", () => {
     expect(getDefaultMinimumThinkingLevel("openai:team-sol")).toBe("off");
   });
 
-  test("returns all 6 levels for Sonnet 5 (native xhigh)", () => {
+  test("returns all 6 levels for Sonnet 5 / Sonnet 5.1 (native xhigh)", () => {
     // Sonnet 5 introduced the native xhigh effort level for the Sonnet tier, so it exposes
     // all 6 levels (unlike Sonnet 4.6, which maps xhigh -> "max" and stops at 5).
     expect(getThinkingPolicyForModel("anthropic:claude-sonnet-5")).toEqual([
@@ -543,6 +543,17 @@ describe("getThinkingPolicyForModel", () => {
       "xhigh",
       "max",
     ]);
+    // Sonnet 5.1 rides the same Sonnet 5+ wildcard matcher. Unlike Mythos-class
+    // models, the Sonnet tier keeps "off" (disabled thinking stays accepted).
+    expect(getThinkingPolicyForModel("anthropic:claude-sonnet-5-1")).toEqual([
+      "off",
+      "low",
+      "medium",
+      "high",
+      "xhigh",
+      "max",
+    ]);
+    expect(enforceThinkingPolicy("anthropic:claude-sonnet-5-1", "off")).toBe("off");
     expect(getThinkingPolicyForModel("anthropic:claude-sonnet-5-20260630")).toEqual([
       "off",
       "low",
diff --git a/src/common/utils/tokens/models-extra.ts b/src/common/utils/tokens/models-extra.ts
index 81beb9a31b3..60d46153766 100644
--- a/src/common/utils/tokens/models-extra.ts
+++ b/src/common/utils/tokens/models-extra.ts
@@ -276,6 +276,26 @@ export const modelsExtra: Record<string, ModelData> = {
     supports_response_schema: true,
   },
 
+  // Claude Sonnet 5.1 - successor to Sonnet 5 with the same envelope and standard
+  // pricing: $3/M input, $15/M output, cache write 1.25x input / cache read 0.1x
+  // input, native 1M context, 128K max output, adaptive thinking + effort (incl.
+  // native xhigh).
+  "claude-sonnet-5-1": {
+    max_input_tokens: 1000000,
+    max_output_tokens: 128000,
+    input_cost_per_token: 0.000003, // $3 per million input tokens
+    output_cost_per_token: 0.000015, // $15 per million output tokens
+    cache_creation_input_token_cost: 0.00000375, // $3.75 per million tokens
+    cache_read_input_token_cost: 0.0000003, // $0.30 per million tokens
+    litellm_provider: "anthropic",
+    mode: "chat",
+    supports_function_calling: true,
+    supports_vision: true,
+    supports_pdf_input: true,
+    supports_reasoning: true,
+    supports_response_schema: true,
+  },
+
   // Claude Sonnet 5 - Released June 30, 2026
   // Native 1M context. Standard pricing $3/M input, $15/M output (same as Sonnet 4.6).
   // Introductory pricing of $2/$10 per MTok applies through Aug 31, 2026, but we list the
diff --git a/src/common/utils/tools/tools.test.ts b/src/common/utils/tools/tools.test.ts
index 98f281c733f..cb6413a04c7 100644
--- a/src/common/utils/tools/tools.test.ts
+++ b/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-sonnet-5-1", true],
     ["claude-sonnet-4-6", true],
     ["claude-opus-4-6", true],
     ["claude-opus-4-8", true],
diff --git a/src/node/services/agentSkills/builtInSkillContent.generated.ts b/src/node/services/agentSkills/builtInSkillContent.generated.ts
index 9148b88adf5..2f6a799921c 100644
--- a/src/node/services/agentSkills/builtInSkillContent.generated.ts
+++ b/src/node/services/agentSkills/builtInSkillContent.generated.ts
@@ -3853,7 +3853,7 @@ export const BUILTIN_SKILL_FILES: Record<string, Record<string, string>> = {
       "| 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`                                                     |         |",
+      "| Sonnet 5.1             | anthropic:claude-sonnet-5-1   | `sonnet`                                                     |         |",
       "| Haiku 4.5              | anthropic:claude-haiku-4-5    | `haiku`                                                      |         |",
       "| GPT-5.6 Sol            | openai:gpt-5.6-sol            | `gpt`, `sol`                                                 |         |",
       "| GPT-5.6 Terra          | openai:gpt-5.6-terra          | `terra`                                                      |         |",
diff --git a/src/node/services/mock/mockAiRouter.ts b/src/node/services/mock/mockAiRouter.ts
index 02aa1f8ddf3..7e021899f14 100644
--- a/src/node/services/mock/mockAiRouter.ts
+++ b/src/node/services/mock/mockAiRouter.ts
@@ -396,7 +396,7 @@ function buildReviewShowDocReply(): MockAiRouterReply {
 
 function buildModelStatusReply(): MockAiRouterReply {
   return {
-    assistantText: "Claude Sonnet 5 is now responding with standard reasoning capacity.",
+    assistantText: "Claude Sonnet 5.1 is now responding with standard reasoning capacity.",
   };
 }
 
diff --git a/src/node/services/providerService.test.ts b/src/node/services/providerService.test.ts
index 81a5b3df74f..73a8de086e7 100644
--- a/src/node/services/providerService.test.ts
+++ b/src/node/services/providerService.test.ts
@@ -1649,7 +1649,7 @@ describe("ProviderService.setConfig", () => {
 
       const providersConfig = config.loadProvidersConfig();
       expect(providersConfig?.["mux-gateway"]?.models).toEqual([
-        "anthropic/claude-sonnet-5",
+        "anthropic/claude-sonnet-5-1",
         "anthropic/claude-opus-5",
         "openai/gpt-5.5",
       ]);
diff --git a/src/node/services/providerService.ts b/src/node/services/providerService.ts
index c2528c9a5f6..67ab1640c46 100644
--- a/src/node/services/providerService.ts
+++ b/src/node/services/providerService.ts
@@ -1514,7 +1514,7 @@ export class ProviderService {
           const existingModels = normalizeProviderModelEntries(providerConfig.models);
           if (existingModels.length === 0) {
             providerConfig.models = [
-              "anthropic/claude-sonnet-5",
+              "anthropic/claude-sonnet-5-1",
               "anthropic/claude-opus-5",
               "openai/gpt-5.5",
             ];
diff --git a/src/node/services/tools/task.test.ts b/src/node/services/tools/task.test.ts
index f32ead99e13..8ec60118ec5 100644
--- a/src/node/services/tools/task.test.ts
+++ b/src/node/services/tools/task.test.ts
@@ -477,7 +477,7 @@ describe("task tool", () => {
 
     expect(create).toHaveBeenCalledTimes(1);
     const createArgs = create.mock.calls[0]?.[0];
-    expect(createArgs?.modelString).toBe("anthropic:claude-sonnet-5");
+    expect(createArgs?.modelString).toBe("anthropic:claude-sonnet-5-1");
     expect(createArgs?.thinkingLevel).toBe("high");
     // Parent runtime hint is still forwarded so unspecified fields keep inheriting.
     expect(createArgs?.parentRuntimeAiSettings).toEqual({
diff --git a/tests/e2e/scenarios/slashCommands.spec.ts b/tests/e2e/scenarios/slashCommands.spec.ts
index 38b13579f0d..10684180ce6 100644
--- a/tests/e2e/scenarios/slashCommands.spec.ts
+++ b/tests/e2e/scenarios/slashCommands.spec.ts
@@ -80,18 +80,18 @@ test.describe("slash command flows", () => {
     await expect(modeToggles.getByText("Opus 5", { exact: true })).toBeVisible();
 
     await ui.chat.sendMessage("/model sonnet");
-    await ui.chat.expectStatusMessageContains("Model changed to anthropic:claude-sonnet-5");
+    await ui.chat.expectStatusMessageContains("Model changed to anthropic:claude-sonnet-5-1");
     // Model is displayed as formatted name
-    await expect(modeToggles.getByText("Sonnet 5", { exact: true })).toBeVisible();
+    await expect(modeToggles.getByText("Sonnet 5.1", { exact: true })).toBeVisible();
 
     const timeline = await ui.chat.captureStreamTimeline(async () => {
       await ui.chat.sendMessage(MOCK_SLASH_COMMAND_PROMPTS.MODEL_STATUS);
     });
 
     const streamStart = timeline.events.find((event) => event.type === "stream-start");
-    expect(streamStart?.model).toBe("anthropic:claude-sonnet-5");
+    expect(streamStart?.model).toBe("anthropic:claude-sonnet-5-1");
     await ui.chat.expectTranscriptContains(
-      "Claude Sonnet 5 is now responding with standard reasoning capacity."
+      "Claude Sonnet 5.1 is now responding with standard reasoning capacity."
     );
   });
 });
diff --git a/tests/ipc/streaming/sendMessage.reasoning.test.ts b/tests/ipc/streaming/sendMessage.reasoning.test.ts
index 2e150d55f66..a921ae915d0 100644
--- a/tests/ipc/streaming/sendMessage.reasoning.test.ts
+++ b/tests/ipc/streaming/sendMessage.reasoning.test.ts
@@ -15,7 +15,17 @@ import {
   withSharedWorkspace,
   configureTestRetries,
 } from "../sendMessageTestHelpers";
-import { KNOWN_MODELS } from "../../../src/common/constants/knownModels";
+// These tests hit the live Anthropic API, and KNOWN_MODELS.SONNET.id now points at
+// claude-sonnet-5-1, which does not exist at the API until Anthropic announces it
+// (this PR is prepared ahead of the drop). Pin the last live Sonnet id so the tests
+// keep exercising real Sonnet-tier reasoning behavior in the meantime.
+//
+// TODO(sonnet-5-1-release): switch back to KNOWN_MODELS.SONNET.id as part of the
+// gated merge, once the id is live — otherwise the promoted production path
+// (adaptive thinking + effort + summarized display on claude-sonnet-5-1) is never
+// exercised by live integration coverage. This is an explicit release-day item in
+// the PR's merge gate.
+const LIVE_SONNET_MODEL = "anthropic:claude-sonnet-5";
 
 // Skip all tests if TEST_INTEGRATION is not set
 const describeIntegration = shouldRunIntegrationTests() ? describe : describe.skip;
@@ -36,7 +46,7 @@ describeIntegration("Anthropic reasoning parameter tests", () => {
     async () => {
       await withSharedWorkspace("anthropic", async ({ env, workspaceId, collector }) => {
         const result = await sendMessage(env, workspaceId, "What is 2+2? Answer in one word.", {
-          model: KNOWN_MODELS.SONNET.id,
+          model: LIVE_SONNET_MODEL,
           thinkingLevel: "low",
         });
         expect(result.success).toBe(true);
@@ -56,7 +66,7 @@ describeIntegration("Anthropic reasoning parameter tests", () => {
     async () => {
       await withSharedWorkspace("anthropic", async ({ env, workspaceId, collector }) => {
         const result = await sendMessage(env, workspaceId, "What is 4+4? Answer in one word.", {
-          model: KNOWN_MODELS.SONNET.id,
+          model: LIVE_SONNET_MODEL,
           thinkingLevel: "low",
         });
         expect(result.success).toBe(true);
@@ -76,7 +86,7 @@ describeIntegration("Anthropic reasoning parameter tests", () => {
     async () => {
       await withSharedWorkspace("anthropic", async ({ env, workspaceId, collector }) => {
         const result = await sendMessage(env, workspaceId, "Explain briefly why 2+2=4", {
-          model: KNOWN_MODELS.SONNET.id,
+          model: LIVE_SONNET_MODEL,
           thinkingLevel: "medium",
         });
         expect(result.success).toBe(true);