diff --git a/apps/api/src/modules/runtime/infrastructure/runtime-sandbox-provisioning/runtime-vendor-proxy-env.builder.ts b/apps/api/src/modules/runtime/infrastructure/runtime-sandbox-provisioning/runtime-vendor-proxy-env.builder.ts
index c940a0eb..64e0db52 100644
--- a/apps/api/src/modules/runtime/infrastructure/runtime-sandbox-provisioning/runtime-vendor-proxy-env.builder.ts
+++ b/apps/api/src/modules/runtime/infrastructure/runtime-sandbox-provisioning/runtime-vendor-proxy-env.builder.ts
@@ -208,6 +208,16 @@ function resolveOpenCodeModelId(vendor: RuntimeCatalogVendor, model: string): st
   return model;
 }
 
+function resolveOpenCodeProviderModelId(vendor: RuntimeCatalogVendor, model: string): string {
+  const openCodeProviderId = resolveOpenCodeProviderId(vendor);
+  const openCodeModel = resolveOpenCodeModelId(vendor, model);
+  const providerPrefix = `${openCodeProviderId}/`;
+
+  return openCodeModel.startsWith(providerPrefix)
+    ? openCodeModel.slice(providerPrefix.length)
+    : openCodeModel;
+}
+
 function resolveOpenCodeProxyBaseUrl(vendor: RuntimeCatalogVendor, proxyUrl: string): string {
   // OpenCode's native providers resolve request paths against a base that
   // already contains the SDK path prefix. @ai-sdk/anthropic defaults to
@@ -225,17 +235,20 @@ function buildOpenCodeProviderConfig(input: OpenCodeProviderConfigInput): OpenCo
   const options: Record<string, string> = {
     apiKey: `{env:${input.vendor.apiKeyEnvVar}}`,
   };
-  const models =
-    input.credential.models === null
-      ? undefined
-      : Object.fromEntries(input.credential.models.map((modelId) => [modelId, { name: modelId }]));
+  // OpenCode removes configured providers that have no models. An unrestricted
+  // Mosoo credential still needs the active model rendered for ACP startup.
+  const declaredModelIds = new Set(input.credential.models ?? []);
+  declaredModelIds.add(resolveOpenCodeProviderModelId(input.vendor, input.model));
+  const models = Object.fromEntries(
+    [...declaredModelIds].map((modelId) => [modelId, { name: modelId }]),
+  );
   const provider = input.vendor.openCodeProvider;
 
   if (provider === undefined) {
     options["baseURL"] = resolveOpenCodeProxyBaseUrl(input.vendor, input.proxyUrl);
 
     return {
-      ...(models === undefined ? {} : { models }),
+      models,
       options,
     };
   }
@@ -246,7 +259,7 @@ function buildOpenCodeProviderConfig(input: OpenCodeProviderConfigInput): OpenCo
   );
 
   return {
-    ...(models === undefined ? {} : { models }),
+    models,
     name: provider.name,
     npm: provider.npmPackage,
     options,
diff --git a/apps/api/tests/runtime-vendor-env-vars.test.ts b/apps/api/tests/runtime-vendor-env-vars.test.ts
index cb41f6dd..7df88824 100644
--- a/apps/api/tests/runtime-vendor-env-vars.test.ts
+++ b/apps/api/tests/runtime-vendor-env-vars.test.ts
@@ -469,6 +469,11 @@ describe("runtime vendor proxy env vars", () => {
         small_model: `${providerId}/${model}`,
         provider: {
           [providerId]: {
+            models: {
+              [model]: {
+                name: model,
+              },
+            },
             name,
             npm,
             options: {
@@ -548,4 +553,35 @@ describe("runtime vendor proxy env vars", () => {
       },
     });
   });
+
+  test("declares the selected custom OpenCode model for an unrestricted credential", async () => {
+    const envVars = await buildVendorProxyEnvVars({
+      bindings: BINDINGS,
+      driverGeneration: DRIVER_GENERATION,
+      driverInstanceId: DRIVER_INSTANCE_ID,
+      profile: {
+        model: "deepseek-v4-flash",
+        runtimeId: "acp-fallback",
+        vendorCredential: vendorCredential({
+          apiBase: "https://api.deepseek.com",
+          vendorId: "openai-compatible",
+        }),
+      },
+      requestUrl: REQUEST_URL,
+    });
+
+    expect(parseOpenCodeConfig(envVars)).toMatchObject({
+      enabled_providers: ["openai-compatible"],
+      model: "openai-compatible/deepseek-v4-flash",
+      provider: {
+        "openai-compatible": {
+          models: {
+            "deepseek-v4-flash": {
+              name: "deepseek-v4-flash",
+            },
+          },
+        },
+      },
+    });
+  });
 });