fix(gsd): disable dynamic model routing for flat-rate providers

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

@@ -31,6 +31,9 @@ export function resolvePreferredModelConfig(
   const routingConfig = resolveDynamicRoutingConfig();
   if (!routingConfig.enabled || !routingConfig.tier_models) return undefined;
 
+  // Don't synthesize a routing config for flat-rate providers (#3453).
+  if (autoModeStartModel && isFlatRateProvider(autoModeStartModel.provider)) return undefined;
+
   const ceilingModel = routingConfig.tier_models.heavy
     ?? (autoModeStartModel ? `${autoModeStartModel.provider}/${autoModeStartModel.id}` : undefined);
   if (!ceilingModel) return undefined;
@@ -71,6 +74,16 @@ export async function selectAndApplyModel(
     let effectiveModelConfig = modelConfig;
     let routingTierLabel = "";
 
+    // Disable routing for flat-rate providers like GitHub Copilot (#3453).
+    // All models cost the same per request, so downgrading to a cheaper
+    // model provides no cost benefit — it only degrades quality.
+    if (routingConfig.enabled) {
+      const primaryModel = resolveModelId(modelConfig.primary, availableModels, ctx.model?.provider);
+      if (primaryModel && isFlatRateProvider(primaryModel.provider)) {
+        routingConfig.enabled = false;
+      }
+    }
+
     if (routingConfig.enabled) {
       let budgetPct: number | undefined;
       if (routingConfig.budget_pressure !== false) {
@@ -320,3 +333,13 @@ export function resolveModelId<T extends { id: string; provider: string }>(
   // Fall back to first non-extension candidate, or any candidate
   return candidates.find(m => !EXTENSION_PROVIDERS.has(m.provider)) ?? candidates[0];
 }
+
+/**
+ * Flat-rate providers charge the same per request regardless of model.
+ * Dynamic routing provides no cost benefit — it only degrades quality (#3453).
+ */
+const FLAT_RATE_PROVIDERS = new Set(["github-copilot"]);
+
+export function isFlatRateProvider(provider: string): boolean {
+  return FLAT_RATE_PROVIDERS.has(provider);
+}

src/resources/extensions/gsd/tests/flat-rate-routing-guard.test.ts

@@ -0,0 +1,40 @@
+/**
+ * Regression test for #3453: dynamic model routing must be disabled for
+ * flat-rate providers like GitHub Copilot where all models cost the same
+ * per request — routing only degrades quality with no cost benefit.
+ */
+
+import { describe, test } from "node:test";
+import assert from "node:assert/strict";
+import { isFlatRateProvider, resolvePreferredModelConfig } from "../auto-model-selection.ts";
+
+describe("flat-rate provider routing guard (#3453)", () => {
+
+  test("isFlatRateProvider returns true for github-copilot", () => {
+    assert.equal(isFlatRateProvider("github-copilot"), true);
+  });
+
+  test("isFlatRateProvider returns false for anthropic", () => {
+    assert.equal(isFlatRateProvider("anthropic"), false);
+  });
+
+  test("isFlatRateProvider returns false for openai", () => {
+    assert.equal(isFlatRateProvider("openai"), false);
+  });
+
+  test("resolvePreferredModelConfig returns undefined for copilot start model", () => {
+    // When the user's start model is on a flat-rate provider,
+    // resolvePreferredModelConfig should not synthesize a routing
+    // config from tier_models — it should return undefined so the
+    // user's selected model is preserved.
+    const result = resolvePreferredModelConfig("execute-task", {
+      provider: "github-copilot",
+      id: "claude-sonnet-4",
+    });
+
+    // Should be undefined (no routing config created for flat-rate)
+    // Note: this only tests the guard — if explicit per-unit config exists
+    // in preferences, that takes precedence regardless.
+    assert.equal(result, undefined, "Should not create routing config for copilot");
+  });
+});