From 1fc80f4f6cc3db77824223c7bf3ec83515ae7792 Mon Sep 17 00:00:00 2001
From: Armin Ronacher <armin.ronacher@active-4.com>
Date: Fri, 12 Jun 2026 17:26:42 +0200
Subject: [PATCH] fix(coding-agent): preserve custom fallback thinking

closes #5552
---
 packages/coding-agent/CHANGELOG.md                | 4 ++++
 packages/coding-agent/src/core/model-resolver.ts  | 7 +++++--
 packages/coding-agent/test/model-resolver.test.ts | 1 +
 3 files changed, 10 insertions(+), 2 deletions(-)

diff --git a/packages/coding-agent/CHANGELOG.md b/packages/coding-agent/CHANGELOG.md
index f24e827e..4d3ee3c6 100644
--- a/packages/coding-agent/CHANGELOG.md
+++ b/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
diff --git a/packages/coding-agent/src/core/model-resolver.ts b/packages/coding-agent/src/core/model-resolver.ts
index c9afb7b2..292ccf37 100644
--- a/packages/coding-agent/src/core/model-resolver.ts
+++ b/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 };
 		}
 	}
 
diff --git a/packages/coding-agent/test/model-resolver.test.ts b/packages/coding-agent/test/model-resolver.test.ts
index 4b4caf0f..32861a9e 100644
--- a/packages/coding-agent/test/model-resolver.test.ts
+++ b/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");
 		});