@@ -606,7 +606,7 @@ function anthropicOpus47OrLater(apiId: string) {
 }
 
 function anthropicAdaptiveEfforts(apiId: string): string[] | null {
-  if (anthropicOpus47OrLater(apiId)) {
+  if (anthropicOpus47OrLater(apiId) || apiId.includes("fable-5")) {
     return ["low", "medium", "high", "xhigh", "max"]
   }
   if (
@@ -619,6 +619,10 @@ function anthropicAdaptiveEfforts(apiId: string): string[] | null {
   return null
 }
 
+function anthropicOmitsThinking(apiId: string) {
+  return anthropicOpus47OrLater(apiId) || apiId.includes("fable-5")
+}
+
 function googleThinkingLevelEfforts(apiId: string) {
   const id = apiId.toLowerCase()
   if (!id.includes("gemini-3")) return ["low", "high"]
@@ -671,7 +675,7 @@ export function variants(model: Provider.Model): Record<string, Record<string, a
       thinking: { thinking: { type: "adaptive" } },
     }
   }
-  const adaptiveOpus = anthropicOpus47OrLater(model.api.id)
+  const adaptiveThinkingOmitted = anthropicOmitsThinking(model.api.id)
   const adaptiveEfforts = anthropicAdaptiveEfforts(model.api.id)
   if (
     id.includes("deepseek-chat") ||
@@ -734,10 +738,10 @@ export function variants(model: Provider.Model): Record<string, Record<string, a
               {
                 thinking: {
                   type: "adaptive",
-                  // Opus 4.7+ flips the API default for `display` to "omitted", which
+                  // Newer adaptive-only models default `display` to "omitted", which
                   // returns empty thinking blocks. Force "summarized" so summaries
                   // survive (4.6/Sonnet 4.6 already default to "summarized").
-                  ...(adaptiveOpus ? { display: "summarized" } : {}),
+                  ...(adaptiveThinkingOmitted ? { display: "summarized" } : {}),
                 },
                 effort,
               },
@@ -884,7 +888,7 @@ export function variants(model: Provider.Model): Record<string, Record<string, a
             {
               thinking: {
                 type: "adaptive",
-                ...(adaptiveOpus ? { display: "summarized" } : {}),
+                ...(adaptiveThinkingOmitted ? { display: "summarized" } : {}),
               },
               effort,
             },
@@ -921,7 +925,7 @@ export function variants(model: Provider.Model): Record<string, Record<string, a
               reasoningConfig: {
                 type: "adaptive",
                 maxReasoningEffort: effort,
-                ...(adaptiveOpus ? { display: "summarized" } : {}),
+                ...(adaptiveThinkingOmitted ? { display: "summarized" } : {}),
               },
             },
           ]),
@@ -1011,7 +1015,7 @@ export function variants(model: Provider.Model): Record<string, Record<string, a
               adaptiveEfforts.map((effort) => [
                 effort,
                 {
-                  thinking: { type: "adaptive", ...(adaptiveOpus ? { display: "summarized" } : {}) },
+                  thinking: { type: "adaptive", ...(adaptiveThinkingOmitted ? { display: "summarized" } : {}) },
                   output_config: { effort },
                 },
               ]),
 
@@ -3486,6 +3486,12 @@ describe("ProviderTransform.variants", () => {
         efforts: ["low", "medium", "high", "xhigh", "max"],
         expectedHigh: { thinking: { type: "adaptive", display: "summarized" }, effort: "high" },
       },
+      {
+        name: "fable 5",
+        apiIds: ["claude-fable-5"],
+        efforts: ["low", "medium", "high", "xhigh", "max"],
+        expectedHigh: { thinking: { type: "adaptive", display: "summarized" }, effort: "high" },
+      },
     ]) {
       for (const apiId of testCase.apiIds) {
         test(`${testCase.name} ${apiId} returns supported reasoning efforts`, () => {