diff --git a/packages/ai/CHANGELOG.md b/packages/ai/CHANGELOG.md
index 46465b66f..cd6b0eda0 100644
--- a/packages/ai/CHANGELOG.md
+++ b/packages/ai/CHANGELOG.md
@@ -19,6 +19,7 @@
 - Added a runtime chat-model check to the `Models` stream entry points so non-chat models fail with a clear `ModelsError` instead of a missing-api stream error.
 - Added array-based `models.all.json` and `providers/{id}.all.json` variants to the generated and published JSON catalog, allowing the same upstream ID once per model type; the existing keyed `models.json` and `providers/{id}.json` stay chat-only for released clients.
 - Added `onProviderStreamEvent` to observe parsed provider stream events before normalization, including provider-specific fields not retained in assistant messages ([#9784](https://github.com/earendil-works/pi/issues/9784)).
+- Added Claude Sonnet 5.5 to the built-in Anthropic model catalog with adaptive thinking, mid-conversation effort, 1M context, and official pricing metadata.
 
 ### Fixed
 
diff --git a/packages/ai/scripts/generate-models.ts b/packages/ai/scripts/generate-models.ts
index 96300189e..93ba057b5 100644
--- a/packages/ai/scripts/generate-models.ts
+++ b/packages/ai/scripts/generate-models.ts
@@ -591,6 +591,7 @@ function supportsAnthropicMidConvoEffort(modelId: string): boolean {
 	const id = modelId.toLowerCase().replace(/^~?anthropic\//, "");
 	return (
 		/^claude-opus-(?:5|5[.-]5)(?:-\d{8})?$/.test(id) ||
+		/^claude-sonnet-5[.-]5(?:-\d{8})?$/.test(id) ||
 		/^claude-(?:fable|mythos)-5(?:[.-]1)(?:-\d{8})?$/.test(id)
 	);
 }
@@ -598,6 +599,7 @@ function supportsAnthropicMidConvoEffort(modelId: string): boolean {
 function supportsAnthropicMidConvoSystemMessages(modelId: string): boolean {
 	return (
 		/^claude-opus-(?:4[.-]8|5(?:[.-]5)?)(?:-\d{8})?$/.test(modelId) ||
+		/^claude-sonnet-5[.-]5(?:-\d{8})?$/.test(modelId) ||
 		/^claude-(?:fable|mythos)-5(?:[.-]1)?(?:-\d{8})?$/.test(modelId)
 	);
 }
@@ -629,7 +631,9 @@ function isAnthropicTemperatureUnsupportedModel(modelId: string): boolean {
 		id.includes("opus-4-8") ||
 		id.includes("opus-4.8") ||
 		id.includes("opus-5") ||
-		id.includes("opus.5")
+		id.includes("opus.5") ||
+		id.includes("sonnet-5-5") ||
+		id.includes("sonnet-5.5")
 	);
 }
 
@@ -2720,6 +2724,32 @@ async function generateModels() {
 		});
 	}
 
+	// Add Claude Sonnet 5.5 until models.dev includes it.
+	// https://platform.claude.com/docs/en/models/sonnet-5-5/overview
+	if (!allModels.some((model) => model.provider === "anthropic" && model.id === "claude-sonnet-5-5")) {
+		allModels.push({
+			id: "claude-sonnet-5-5",
+			name: "Claude Sonnet 5.5",
+			api: "anthropic-messages",
+			provider: "anthropic",
+			baseUrl: "https://api.anthropic.com",
+			reasoning: true,
+			thinkingLevelMap: {
+				off: null,
+				minimal: null,
+				low: "low",
+				medium: "medium",
+				high: "high",
+				xhigh: "xhigh",
+				max: "max",
+			},
+			input: ["text", "image"],
+			cost: { input: 2, output: 10, cacheRead: 0.2, cacheWrite: 2.5 },
+			contextWindow: 1000000,
+			maxTokens: 128000,
+		});
+	}
+
 	// The authenticated Copilot catalog advertised these models on 2026-09-22,
 	// but models.dev did not include them yet.
 	const missingCopilotModels: Model<Api>[] = [
@@ -2771,9 +2801,10 @@ async function generateModels() {
 			candidate.contextWindow = 1000000;
 		}
 
-		// models.dev may list Opus 5.5 before its effort metadata is complete.
+		// models.dev may list Opus 5.5 and Sonnet 5.5 before their effort metadata is complete.
 		if (
-			(candidate.provider === "anthropic" && candidate.id === "claude-opus-5-5") ||
+			(candidate.provider === "anthropic" &&
+				(candidate.id === "claude-opus-5-5" || candidate.id === "claude-sonnet-5-5")) ||
 			(candidate.provider === "github-copilot" && candidate.id === "claude-opus-5.5")
 		) {
 			mergeThinkingLevelMap(candidate, {
diff --git a/packages/ai/test/supports-xhigh.test.ts b/packages/ai/test/supports-xhigh.test.ts
index 90bf3b8ae..5c3a61a21 100644
--- a/packages/ai/test/supports-xhigh.test.ts
+++ b/packages/ai/test/supports-xhigh.test.ts
@@ -39,6 +39,23 @@ describe("getSupportedThinkingLevels", () => {
 		expect(getSupportedThinkingLevels(model)).toEqual(["low", "medium", "high", "xhigh", "max"]);
 	});
 
+	it("includes Claude Sonnet 5.5 with managed effort levels and official pricing", () => {
+		const model = getModel("anthropic", "claude-sonnet-5-5");
+		expect(model).toMatchObject({
+			cost: { input: 2, output: 10, cacheRead: 0.2, cacheWrite: 2.5 },
+			contextWindow: 1_000_000,
+			maxTokens: 128_000,
+			compat: {
+				forceAdaptiveThinking: true,
+				supportsMidConvoEffort: true,
+				supportsMidConvoSystemMessages: true,
+				supportsMidConvoToolChanges: true,
+				supportsTemperature: false,
+			},
+		});
+		expect(getSupportedThinkingLevels(model)).toEqual(["low", "medium", "high", "xhigh", "max"]);
+	});
+
 	it("includes max but not xhigh for Anthropic Sonnet 4.6 on anthropic-messages API", () => {
 		const model = getModel("anthropic", "claude-sonnet-4-6");
 		expect(model).toBeDefined();
diff --git a/packages/coding-agent/CHANGELOG.md b/packages/coding-agent/CHANGELOG.md
index 08d40d3a9..5766db7a9 100644
--- a/packages/coding-agent/CHANGELOG.md
+++ b/packages/coding-agent/CHANGELOG.md
@@ -10,6 +10,7 @@
 - Added `types=chat,image,classifier` to pi.dev model catalog requests so remote refreshes overlay every supported model type; entries of unknown model types are ignored.
 - Added the `provider_stream_event` extension event for observing parsed provider events before normalization, with an opt-in `/debug-provider` example viewer ([#9784](https://github.com/earendil-works/pi/issues/9784)).
 - Added a show/hide toggle (`H`) in HTML exports for custom messages marked `display: false`. Messages remain hidden by default and can also be revealed from the sidebar ([#8896](https://github.com/earendil-works/pi/issues/8896)).
+- Added inherited Claude Sonnet 5.5 support for Anthropic with adaptive thinking and a 1M context window.
 
 ### Changed