fix(coding-agent): preserve custom fallback thinking

closes #5552

packages/coding-agent/CHANGELOG.md

@@ -6,6 +6,10 @@
 
 - Added an experimental first-time setup flow behind `PI_EXPERIMENTAL=1` that asks for a dark/light theme choice (preselecting the detected appearance) and opt-in analytics data sharing on first launch with the default agent directory; opting in stores a `trackingId` in `settings.json`.
 
+### Fixed
+
+- Fixed custom fallback model IDs with `:<thinking>` suffixes to preserve the requested thinking level when the provider template model does not advertise reasoning ([#5552](https://github.com/earendil-works/pi/issues/5552)).
+
 ## [0.79.1] - 2026-06-09
 
 ### New Features

packages/coding-agent/src/core/model-resolver.ts

@@ -340,7 +340,7 @@ export interface ResolveCliModelResult {
 export function resolveCliModel(options: {
 	cliProvider?: string;
 	cliModel?: string;
-	cliThinking?: string;
+	cliThinking?: ThinkingLevel;
 	modelRegistry: ModelRegistry;
 }): ResolveCliModelResult {
 	const { cliProvider, cliModel, cliThinking, modelRegistry } = options;
@@ -470,10 +470,13 @@ export function resolveCliModel(options: {
 
 		const fallbackModel = buildFallbackModel(provider, fallbackPattern, availableModels);
 		if (fallbackModel) {
+			const requestedThinking = cliThinking ?? fallbackThinking;
+			const model =
+				requestedThinking && requestedThinking !== "off" ? { ...fallbackModel, reasoning: true } : fallbackModel;
 			const fallbackWarning = warning
 				? `${warning} Model "${fallbackPattern}" not found for provider "${provider}". Using custom model id.`
 				: `Model "${fallbackPattern}" not found for provider "${provider}". Using custom model id.`;
-			return { model: fallbackModel, thinkingLevel: fallbackThinking, warning: fallbackWarning, error: undefined };
+			return { model, thinkingLevel: fallbackThinking, warning: fallbackWarning, error: undefined };
 		}
 	}
 

packages/coding-agent/test/model-resolver.test.ts

@@ -403,6 +403,7 @@ describe("resolveCliModel", () => {
 			expect(result.model?.provider).toBe("neuralwatt");
 			// The :high suffix must NOT leak into the model id sent to the API
 			expect(result.model?.id).toBe("zai-org/GLM-5.1-FP8");
+			expect(result.model?.reasoning).toBe(true);
 			expect(result.thinkingLevel).toBe("high");
 		});