fix(ai): route GitHub Copilot Fable 5 through Anthropic Messages

closes #8961

packages/ai/CHANGELOG.md

@@ -2,6 +2,10 @@
 
 ## [Unreleased]
 
+### Fixed
+
+- Fixed GitHub Copilot Claude Fable 5 requests to use the Anthropic Messages adapter so selected reasoning levels are sent ([#8961](https://github.com/earendil-works/pi/issues/8961)).
+
 ## [0.84.4] - 2026-08-28
 
 ### Added

packages/ai/scripts/generate-models.ts

@@ -2059,7 +2059,7 @@ async function loadModelsDevData(): Promise<Model<any>[]> {
 				if (m.status === "deprecated") continue;
 
 				// Claude 4.x and 5.x models route to Anthropic Messages API
-				const isCopilotClaude = /^claude-(haiku|sonnet|opus)-[45]([.\-]|$)/.test(modelId);
+				const isCopilotClaude = /^claude-(haiku|sonnet|opus|fable)-[45]([.\-]|$)/.test(modelId);
 				// Grok, gpt-5, oswe, and MAI-Code models are only served through
 				// the Copilot /responses endpoint.
 				const needsResponsesApi =