sf: route Gemini family models to google-gemini-cli by default

resolveModelId now prefers google-gemini-cli over google (direct API) for
bare Gemini/Gemma IDs, matching the operational default after the CLI-core
re-platform. google-vertex is still honoured when it's the current provider.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/resources/extensions/sf/auto-model-selection.ts

@@ -479,6 +479,9 @@ export function resolveModelId<T extends { id: string; provider: string }>(
   if (candidates.length === 0) return undefined;
   if (candidates.length === 1) return candidates[0];
 
+  const lowerModelId = modelId.toLowerCase();
+  const isGeminiFamily = lowerModelId.startsWith("gemini-") || lowerModelId.startsWith("gemma-");
+
   // When the user's current provider is claude-code (set by startup migration
   // or explicit selection), honour it for bare IDs.  Routing back to anthropic
   // would undo the migration and hit the third-party subscription block (#3772).
@@ -487,6 +490,22 @@ export function resolveModelId<T extends { id: string; provider: string }>(
     if (ccMatch) return ccMatch;
   }
 
+  // Google Gemini routing should converge on the operational Google default
+  // backend for bare IDs. Keep Vertex explicit, but prefer the CLI-core
+  // backend over the direct API backend when both expose the same Gemini
+  // family model. The direct API path remains available as an explicit or
+  // fallback route, but is not the default operational surface.
+  if (isGeminiFamily && currentProvider === "google-vertex") {
+    const vertexMatch = candidates.find(m => m.provider === "google-vertex");
+    if (vertexMatch) return vertexMatch;
+  }
+  if (isGeminiFamily) {
+    const googleCliMatch = candidates.find(m => m.provider === "google-gemini-cli");
+    if (googleCliMatch) return googleCliMatch;
+    const googleApiMatch = candidates.find(m => m.provider === "google");
+    if (googleApiMatch) return googleApiMatch;
+  }
+
   // Extension / CLI-wrapper providers that should not win bare-ID resolution
   // when a first-class API provider also offers the same model AND the user
   // has not explicitly chosen the extension provider.

src/resources/extensions/sf/tests/auto-model-selection.test.ts

@@ -199,6 +199,38 @@ test("resolveModelId: bare ID with claude-code as only provider still resolves",
   assert.equal(result.provider, "claude-code");
 });
 
+test("resolveModelId: bare Gemini ID prefers cli-core backend over direct google api", () => {
+  const availableModels = [
+    { id: "gemini-2.5-pro", provider: "google-gemini-cli" },
+    { id: "gemini-2.5-pro", provider: "google" },
+  ];
+
+  const result = resolveModelId("gemini-2.5-pro", availableModels, "google-gemini-cli");
+  assert.ok(result, "should resolve a Gemini model");
+  assert.equal(result.provider, "google-gemini-cli", "bare Gemini IDs should use CLI-core when api and cli backends coexist");
+});
+
+test("resolveModelId: bare Gemini ID preserves google-vertex when that is the current provider", () => {
+  const availableModels = [
+    { id: "gemini-2.5-pro", provider: "google" },
+    { id: "gemini-2.5-pro", provider: "google-vertex" },
+  ];
+
+  const result = resolveModelId("gemini-2.5-pro", availableModels, "google-vertex");
+  assert.ok(result, "should resolve a Gemini model");
+  assert.equal(result.provider, "google-vertex", "vertex should remain an explicit enterprise route");
+});
+
+test("resolveModelId: bare Gemini ID falls back to direct google api when cli-core backend is absent", () => {
+  const availableModels = [
+    { id: "gemini-2.5-pro", provider: "google" },
+  ];
+
+  const result = resolveModelId("gemini-2.5-pro", availableModels, "google-gemini-cli");
+  assert.ok(result, "should resolve a Gemini model");
+  assert.equal(result.provider, "google");
+});
+
 // ─── selectAndApplyModel verbose-gating tests ──────────────────────────
 
 test("model change notify in selectAndApplyModel is gated behind verbose flag", () => {