fix(coding-agent): limit @upstream fuzzy bypass to aggregators

The routing bypass in matchModel() returned early for any provider whose
post-slash id contained a valid @slug, so fuzzy provider-qualified patterns
over non-aggregator ids that legitimately end in @ (e.g. google-vertex/opus@default
-> claude-opus-4-8@default) resolved to nothing. Gate the bypass on
providerModels.some(supportsUpstreamRouting) so only OpenRouter / Vercel Gateway
short-circuit to the routing fallback. Addresses Codex review feedback on #2710.

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

@@ -481,8 +481,11 @@ function matchModel(
 			// The prefix is not a known provider in this candidate set, so treat the
 			// slash as part of the raw model ID and continue with generic matching.
 		} else {
-			// Let the routing fallback apply `@upstream` before fuzzy matching can consume the slug.
-			if (splitUpstreamRouting(modelId)) {
+			// Let the routing fallback apply `@upstream` before fuzzy matching can consume the
+			// slug — but only for aggregator providers (OpenRouter / Vercel Gateway). Other
+			// providers have ids that legitimately end in `@` (Vertex `claude-opus-4-8@default`),
+			// and the fallback never routes them, so they must keep fuzzy matching.
+			if (splitUpstreamRouting(modelId) && providerModels.some(supportsUpstreamRouting)) {
 				return undefined;
 			}
 			const scored = providerModels

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

@@ -1020,6 +1020,28 @@ describe("provider routing selector (@upstream)", () => {
 		expect(openRouterOnly(result.model)).toBeUndefined();
 	});
 
+	test("keeps fuzzy matching a non-aggregator provider id that ends in @ (Vertex)", () => {
+		const vertexModel: Model<"anthropic-messages"> = buildModel({
+			id: "claude-opus-4-8@default",
+			name: "Claude Opus 4.8",
+			api: "anthropic-messages",
+			provider: "google-vertex",
+			baseUrl: "https://us-aiplatform.googleapis.com",
+			reasoning: true,
+			input: ["text"],
+			cost: { input: 15, output: 75, cacheRead: 1.5, cacheWrite: 18.75 },
+			contextWindow: 200000,
+			maxTokens: 32000,
+		});
+		// `opus@default` is a fuzzy provider-qualified pattern: the `@upstream` bypass must not
+		// swallow it, because google-vertex is not an aggregator and the routing fallback would
+		// never resolve it, leaving the selector unmatched.
+		const result = parseModelPattern("google-vertex/opus@default", [vertexModel]);
+		expect(result.model?.id).toBe("claude-opus-4-8@default");
+		expect(result.upstream).toBeUndefined();
+		expect(openRouterOnly(result.model)).toBeUndefined();
+	});
+
 	test("ignores @slug on a non-aggregator model (no silent routing)", () => {
 		const result = parseModelPattern("gpt-4o@cerebras", allModels);
 		expect(result.model).toBeUndefined();