diff --git a/src/shared/components/ModelSelectModal.tsx b/src/shared/components/ModelSelectModal.tsx
index c6360bc920..e44dddecf4 100644
--- a/src/shared/components/ModelSelectModal.tsx
+++ b/src/shared/components/ModelSelectModal.tsx
@@ -3,6 +3,7 @@
 import { useState, useMemo, useEffect } from "react";
 import { useTranslations } from "next-intl";
 import Modal from "./Modal";
+import { buildPassthroughAliasModels } from "./modelSelectModalHelpers";
 import { getModelsByProviderId, PROVIDER_ID_TO_ALIAS } from "@/shared/constants/models";
 import { getCompatibleFallbackModels } from "@/lib/providers/managedAvailableModels";
 import {
@@ -235,14 +236,14 @@ export default function ModelSelectModal({
       const providerCustomModels = customModels[providerId] || [];
 
       if (providerInfo.passthroughModels) {
-        const aliasModels = Object.entries(modelAliases as Record<string, string>)
-          .filter(([, fullModel]: [string, string]) => fullModel.startsWith(`${alias}/`))
-          .map(([aliasName, fullModel]: [string, string]) => ({
-            id: fullModel.replace(`${alias}/`, ""),
-            name: aliasName,
-            value: fullModel,
-            source: "alias",
-          }));
+        // Passthrough aliases are stored prefixed by the canonical providerId
+        // (e.g. "github/gpt-4"), not the public alias (e.g. "gh/"), so we must
+        // filter/strip by providerId — matching the sibling custom-provider
+        // branch below. (port: decolua/9router#485)
+        const aliasModels = buildPassthroughAliasModels(
+          modelAliases as Record<string, string>,
+          providerId
+        );
 
         // Merge custom models for passthrough providers
         const customEntries = providerCustomModels
diff --git a/src/shared/components/modelSelectModalHelpers.ts b/src/shared/components/modelSelectModalHelpers.ts
new file mode 100644
index 0000000000..01d7f26011
--- /dev/null
+++ b/src/shared/components/modelSelectModalHelpers.ts
@@ -0,0 +1,37 @@
+// Pure helpers extracted from ModelSelectModal so the grouping logic is
+// unit-testable (the component itself lives inside a useMemo and is not
+// directly exercisable by node:test). Keep these free of React imports.
+
+export type PassthroughAliasModel = {
+  id: string;
+  name: string;
+  value: string;
+  source: "alias";
+};
+
+/**
+ * Build the alias-derived model rows for a passthrough provider.
+ *
+ * `modelAliases` maps an alias name → the fully-qualified model string, which
+ * is prefixed by the provider's *canonical id* (e.g. `github/gpt-4`), NOT by
+ * its public alias (e.g. `gh`). Filtering/stripping must therefore use the
+ * `providerId`, mirroring the sibling custom-provider branch. Using the alias
+ * here meant aliases registered under a providerId whose alias differs (the
+ * common case) never resolved.
+ *
+ * Inspired by upstream PR decolua/9router#485 (Anurag Saxena).
+ */
+export function buildPassthroughAliasModels(
+  modelAliases: Record<string, string>,
+  providerId: string
+): PassthroughAliasModel[] {
+  const prefix = `${providerId}/`;
+  return Object.entries(modelAliases || {})
+    .filter(([, fullModel]) => typeof fullModel === "string" && fullModel.startsWith(prefix))
+    .map(([aliasName, fullModel]) => ({
+      id: fullModel.replace(prefix, ""),
+      name: aliasName,
+      value: fullModel,
+      source: "alias" as const,
+    }));
+}
diff --git a/tests/unit/model-select-passthrough-alias-485.test.ts b/tests/unit/model-select-passthrough-alias-485.test.ts
new file mode 100644
index 0000000000..95cc0d2ecf
--- /dev/null
+++ b/tests/unit/model-select-passthrough-alias-485.test.ts
@@ -0,0 +1,49 @@
+import test from "node:test";
+import assert from "node:assert/strict";
+import { buildPassthroughAliasModels } from "../../src/shared/components/modelSelectModalHelpers.ts";
+
+// Regression guard for port of decolua/9router#485 (Anurag Saxena).
+//
+// In ModelSelectModal, passthrough-provider model aliases are stored in
+// `modelAliases` prefixed by the provider's CANONICAL id (e.g. "github/gpt-4"),
+// while the public alias may differ (e.g. providerId "github" → alias "gh").
+// The passthrough branch previously filtered/stripped by `${alias}/`, so any
+// provider whose alias differed from its id resolved to ZERO models. The fix
+// filters by `${providerId}/`, mirroring the sibling custom-provider branch.
+
+test("buildPassthroughAliasModels: resolves aliases keyed by providerId even when the public alias differs", () => {
+  // providerId "github", public alias would be "gh" — aliases are stored under the id.
+  const modelAliases = {
+    "GPT 4o": "github/gpt-4o",
+    "GPT 4o mini": "github/gpt-4o-mini",
+    Sonnet: "anthropic/claude-3-5-sonnet",
+  };
+
+  const result = buildPassthroughAliasModels(modelAliases, "github");
+
+  // The bug (filtering by alias "gh/") would yield [] here. The fix yields the
+  // two github-prefixed entries, with the providerId prefix stripped from `id`.
+  assert.deepEqual(result, [
+    { id: "gpt-4o", name: "GPT 4o", value: "github/gpt-4o", source: "alias" },
+    { id: "gpt-4o-mini", name: "GPT 4o mini", value: "github/gpt-4o-mini", source: "alias" },
+  ]);
+});
+
+test("buildPassthroughAliasModels: only matches the requested providerId prefix", () => {
+  const modelAliases = {
+    a: "openrouter/model-a",
+    b: "github/model-b",
+  };
+  const result = buildPassthroughAliasModels(modelAliases, "openrouter");
+  assert.deepEqual(result, [
+    { id: "model-a", name: "a", value: "openrouter/model-a", source: "alias" },
+  ]);
+});
+
+test("buildPassthroughAliasModels: tolerates empty / malformed maps", () => {
+  assert.deepEqual(buildPassthroughAliasModels({}, "github"), []);
+  assert.deepEqual(
+    buildPassthroughAliasModels({ x: undefined as unknown as string }, "github"),
+    []
+  );
+});