diff --git a/packages/ai/CHANGELOG.md b/packages/ai/CHANGELOG.md
index aa97a6b7f..f757493c8 100644
--- a/packages/ai/CHANGELOG.md
+++ b/packages/ai/CHANGELOG.md
@@ -2,6 +2,10 @@
 
 ## [Unreleased]
 
+### Fixed
+
+- Fixed Claude Fable 5 thinking-off requests to omit Anthropic's unsupported `thinking.type: "disabled"` payload ([#5567](https://github.com/earendil-works/pi/pull/5567) by [@tmustier](https://github.com/tmustier)).
+
 ## [0.79.1] - 2026-06-09
 
 ### Added
diff --git a/packages/ai/scripts/generate-models.ts b/packages/ai/scripts/generate-models.ts
index 3253de64a..5e5fff94d 100644
--- a/packages/ai/scripts/generate-models.ts
+++ b/packages/ai/scripts/generate-models.ts
@@ -299,7 +299,7 @@ function applyThinkingLevelMetadata(model: Model<any>): void {
 		(model.api === "anthropic-messages" || model.api === "bedrock-converse-stream") &&
 		model.id.includes("fable-5")
 	) {
-		mergeThinkingLevelMap(model, { xhigh: "xhigh" });
+		mergeThinkingLevelMap(model, { off: null, xhigh: "xhigh" });
 	}
 	if (model.api === "anthropic-messages" && isAnthropicAdaptiveThinkingModel(model.id)) {
 		mergeAnthropicMessagesCompat(model, { forceAdaptiveThinking: true });
diff --git a/packages/ai/src/providers/anthropic.ts b/packages/ai/src/providers/anthropic.ts
index 158311b07..efe4bd757 100644
--- a/packages/ai/src/providers/anthropic.ts
+++ b/packages/ai/src/providers/anthropic.ts
@@ -972,7 +972,7 @@ function buildParams(
 					display,
 				};
 			}
-		} else if (options?.thinkingEnabled === false) {
+		} else if (options?.thinkingEnabled === false && model.thinkingLevelMap?.off !== null) {
 			params.thinking = { type: "disabled" };
 		}
 	}
diff --git a/packages/ai/test/anthropic-thinking-disable.test.ts b/packages/ai/test/anthropic-thinking-disable.test.ts
index 5ee89b1ef..13d333e57 100644
--- a/packages/ai/test/anthropic-thinking-disable.test.ts
+++ b/packages/ai/test/anthropic-thinking-disable.test.ts
@@ -132,6 +132,13 @@ describe("Anthropic thinking disable payload", () => {
 		expect(payload.output_config).toBeUndefined();
 	});
 
+	it("omits thinking.type=disabled for Claude Fable 5 when thinking is off", async () => {
+		const payload = await capturePayload(getModel("anthropic", "claude-fable-5"));
+
+		expect(payload.thinking).toBeUndefined();
+		expect(payload.output_config).toBeUndefined();
+	});
+
 	it("uses adaptive thinking for Claude Opus 4.8 when reasoning is enabled", async () => {
 		const payload = await capturePayload(getModel("anthropic", "claude-opus-4-8"), { reasoning: "high" });
 
diff --git a/packages/ai/test/supports-xhigh.test.ts b/packages/ai/test/supports-xhigh.test.ts
index 80b3e683c..ec6dc87b5 100644
--- a/packages/ai/test/supports-xhigh.test.ts
+++ b/packages/ai/test/supports-xhigh.test.ts
@@ -20,10 +20,11 @@ describe("getSupportedThinkingLevels", () => {
 		expect(getSupportedThinkingLevels(model!)).toContain("xhigh");
 	});
 
-	it("includes xhigh for Anthropic Claude Fable 5 on anthropic-messages API", () => {
+	it("includes xhigh but not off for Anthropic Claude Fable 5 on anthropic-messages API", () => {
 		const model = getModel("anthropic", "claude-fable-5");
 		expect(model).toBeDefined();
 		expect(getSupportedThinkingLevels(model!)).toContain("xhigh");
+		expect(getSupportedThinkingLevels(model!)).not.toContain("off");
 	});
 
 	it("does not include xhigh for Claude Sonnet 4.5", () => {
@@ -86,9 +87,10 @@ describe("getSupportedThinkingLevels", () => {
 		expect(getSupportedThinkingLevels(model!)).toContain("xhigh");
 	});
 
-	it("includes xhigh for Bedrock Claude Fable 5", () => {
+	it("includes xhigh but not off for Bedrock Claude Fable 5", () => {
 		const model = getModel("amazon-bedrock", "global.anthropic.claude-fable-5");
 		expect(model).toBeDefined();
 		expect(getSupportedThinkingLevels(model!)).toContain("xhigh");
+		expect(getSupportedThinkingLevels(model!)).not.toContain("off");
 	});
 });