From b2f7238a03357f2d928784a8619edb01cd2f86f9 Mon Sep 17 00:00:00 2001
From: roboomp <omp@can.ac>
Date: Sun, 5 Jul 2026 19:21:08 +0000
Subject: [PATCH] fix(coding-agent): allowed mixed fallback chains

- Allowed implicit default fallback resolution when other role fallback chains are configured.
- Covered the mixed-role first-run fallback case.

Fixes #4533
---
 packages/coding-agent/src/session/agent-session.ts    | 11 ++---------
 .../test/agent-session-retry-fallback.test.ts         |  6 ++++--
 2 files changed, 6 insertions(+), 11 deletions(-)

diff --git a/packages/coding-agent/src/session/agent-session.ts b/packages/coding-agent/src/session/agent-session.ts
index e44bd7d81..b57ccbca0 100644
--- a/packages/coding-agent/src/session/agent-session.ts
+++ b/packages/coding-agent/src/session/agent-session.ts
@@ -13122,10 +13122,7 @@ export class AgentSession {
 		if (
 			Array.isArray(defaultChain) &&
 			defaultChain.length > 0 &&
-			this.#getRetryFallbackPrimarySelector("default") === undefined &&
-			Object.entries(chains).every(
-				([role, chain]) => role === "default" || !Array.isArray(chain) || chain.length === 0,
-			)
+			this.#getRetryFallbackPrimarySelector("default") === undefined
 		) {
 			return "default";
 		}
@@ -13155,11 +13152,7 @@ export class AgentSession {
 			if (
 				Array.isArray(defaultChain) &&
 				defaultChain.length > 0 &&
-				this.#getRetryFallbackPrimarySelector("default") === undefined &&
-				Object.entries(chains).every(
-					([chainRole, roleChain]) =>
-						chainRole === "default" || !Array.isArray(roleChain) || roleChain.length === 0,
-				)
+				this.#getRetryFallbackPrimarySelector("default") === undefined
 			) {
 				const seen = new Set<string>([parsedCurrent.raw]);
 				chain = [parsedCurrent];
diff --git a/packages/coding-agent/test/agent-session-retry-fallback.test.ts b/packages/coding-agent/test/agent-session-retry-fallback.test.ts
index 1adeda0ec..345d42df3 100644
--- a/packages/coding-agent/test/agent-session-retry-fallback.test.ts
+++ b/packages/coding-agent/test/agent-session-retry-fallback.test.ts
@@ -219,10 +219,11 @@ describe("AgentSession retry fallback", () => {
 		]);
 	});
 
-	it("uses the active initial model as the default fallback primary when the default role is unset", async () => {
+	it("uses the active initial model as the default fallback primary when other role fallback chains are configured", async () => {
 		const primaryModel = getBundledModel("anthropic", "claude-sonnet-4-5");
 		const fallbackModel = getBundledModel("openai", "gpt-4o-mini");
-		if (!primaryModel || !fallbackModel) {
+		const otherRoleFallbackModel = getBundledModel("openai", "gpt-4o");
+		if (!primaryModel || !fallbackModel || !otherRoleFallbackModel) {
 			throw new Error("Expected bundled test models to exist");
 		}
 
@@ -235,6 +236,7 @@ describe("AgentSession retry fallback", () => {
 			"retry.maxRetries": 1,
 			"retry.fallbackChains": {
 				default: [`${fallbackModel.provider}/${fallbackModel.id}`],
+				smol: [`${otherRoleFallbackModel.provider}/${otherRoleFallbackModel.id}`],
 			},
 		});