diff --git a/plugins/web-ui/src/model-options.ts b/plugins/web-ui/src/model-options.ts
index 5ee2c799..1b5752c5 100644
--- a/plugins/web-ui/src/model-options.ts
+++ b/plugins/web-ui/src/model-options.ts
@@ -78,7 +78,7 @@ function buildOption(
   id: string,
   harnessId = "pi",
   qualified = false,
-  catalog: Readonly<Record<string, { name: string; provider: string }>> = {},
+  catalog: Readonly<Record<string, { name: string; provider: string; api?: string }>> = {},
 ): ModelOption | null {
   try {
     const dynamic = catalog[id];
@@ -101,7 +101,7 @@ function buildOptions(
   ids: readonly string[],
   harnessId = "pi",
   qualified = false,
-  catalog: Readonly<Record<string, { name: string; provider: string }>> = {},
+  catalog: Readonly<Record<string, { name: string; provider: string; api?: string }>> = {},
 ): ModelOption[] {
   const seen = new Set<string>();
   const out: ModelOption[] = [];
@@ -154,7 +154,7 @@ export function applyPickerModelIds(ids: readonly string[] | null | undefined, b
 export function runtimeModelOptions(
   approvedHarnesses: readonly string[],
   modelsByHarness: Readonly<Record<string, readonly string[]>>,
-  catalog: Readonly<Record<string, { name: string; provider: string }>> = {},
+  catalog: Readonly<Record<string, { name: string; provider: string; api?: string }>> = {},
 ): ModelOption[] {
   const options = approvedHarnesses.flatMap((harnessId) => {
     const configured = buildOptions(modelsByHarness[harnessId] ?? [], harnessId, true, catalog);
@@ -170,7 +170,7 @@ export function applyRuntimeOptions(
   approvedHarnesses: readonly string[],
   modelsByHarness: Readonly<Record<string, readonly string[]>>,
   effective: { harnessId: string; modelId: string },
-  catalog: Readonly<Record<string, { name: string; provider: string }>> = {},
+  catalog: Readonly<Record<string, { name: string; provider: string; api?: string }>> = {},
 ): void {
   const options = runtimeModelOptions(approvedHarnesses, modelsByHarness, catalog);
   const applied = { options, defaultValue: `${effective.harnessId}:${effective.modelId}` };
diff --git a/plugins/web-ui/src/pi-models.ts b/plugins/web-ui/src/pi-models.ts
index ca2128a3..3b770166 100644
--- a/plugins/web-ui/src/pi-models.ts
+++ b/plugins/web-ui/src/pi-models.ts
@@ -22,7 +22,7 @@ function builtinModel(id: string): PiModel | undefined {
   return undefined;
 }
 
-export function getBaseModel(id: string, fallback?: { name: string; provider: string }): PiModel {
+export function getBaseModel(id: string, fallback?: { name: string; provider: string; api?: string }): PiModel {
   const builtin = builtinModel(id);
   if (builtin) return builtin;
   const clone = CLONE_TEMPLATES[id];
@@ -30,9 +30,16 @@ export function getBaseModel(id: string, fallback?: { name: string; provider: st
     const template = builtinModel(clone.template);
     if (template) return cloneModel(template, id, clone.name);
   }
-  if (fallback?.provider === "openrouter") {
-    const template = getModel("openrouter", "openrouter/auto" as Parameters<typeof getModel>[1]) as PiModel | undefined;
-    if (template) return cloneModel(template, id, fallback.name);
+  if (fallback) {
+    const templateId = fallback.api === "anthropic-messages" ? "claude-sonnet-4-6" : "gpt-5.5";
+    const template =
+      fallback.provider === "openrouter"
+        ? (getModel("openrouter", "openrouter/auto" as Parameters<typeof getModel>[1]) as PiModel | undefined)
+        : builtinModel(templateId);
+    if (template) {
+      const cloned = cloneModel(template, id, fallback.name);
+      return { ...cloned, provider: fallback.provider, ...(fallback.api ? { api: fallback.api } : {}) } as PiModel;
+    }
   }
   throw new Error(`Unsupported model: ${id}`);
 }
diff --git a/plugins/web-ui/test/pi-models.test.ts b/plugins/web-ui/test/pi-models.test.ts
index 8f586d46..476ba813 100644
--- a/plugins/web-ui/test/pi-models.test.ts
+++ b/plugins/web-ui/test/pi-models.test.ts
@@ -40,3 +40,25 @@ test("fast-mode support is fed from core's runtime config, not a hardcoded clien
   assert.equal(modelSupportsFastMode(null, "claude-haiku-4-5"), false);
   assert.equal(modelSupportsFastMode(null, undefined), false);
 });
+
+test("web UI resolves custom-provider models regardless of provider id", () => {
+  const openaiProtocol = getBaseModel("acme-large", {
+    name: "Acme Large",
+    provider: "acme",
+    api: "openai-completions",
+  });
+  assert.equal(openaiProtocol.id, "acme-large");
+  assert.equal(openaiProtocol.name, "Acme Large");
+  assert.equal(openaiProtocol.provider, "acme");
+  assert.equal(openaiProtocol.api, "openai-completions");
+
+  const anthropicProtocol = getBaseModel("vendor-messages-model", {
+    name: "Vendor Messages Model",
+    provider: "vendor",
+    api: "anthropic-messages",
+  });
+  assert.equal(anthropicProtocol.provider, "vendor");
+  assert.equal(anthropicProtocol.api, "anthropic-messages");
+
+  assert.throws(() => getBaseModel("still-unknown"), /Unsupported/);
+});
diff --git a/src/api/routes/surface.ts b/src/api/routes/surface.ts
index 3fbe270a..f957c76e 100644
--- a/src/api/routes/surface.ts
+++ b/src/api/routes/surface.ts
@@ -1164,10 +1164,12 @@ async function runtimeConfigBody(ctx: ApiCtx, scope: ScopeId): Promise<Record<st
   const advertisedModelIds = new Set(Object.values(modelsByHarness).flat());
   const modelCatalog = Object.fromEntries(
     [...advertisedModelIds].flatMap((id) => {
-      const model = catalog.find((candidate) => candidate.id === id);
-      if (model) return [[id, { name: model.name, provider: model.provider }]];
       const resolved = resolveModel(id);
-      return resolved ? [[id, { name: resolved.name, provider: resolved.provider }]] : [];
+      const model = catalog.find((candidate) => candidate.id === id);
+      const name = model?.name ?? resolved?.name;
+      const provider = model?.provider ?? resolved?.provider;
+      if (!name || !provider) return [];
+      return [[id, { name, provider, ...(resolved?.api ? { api: resolved.api } : {}) }]];
     }),
   );
   return {