diff --git a/packages/opencode/src/provider/provider.ts b/packages/opencode/src/provider/provider.ts
index 220b0144e1..24c5119296 100644
--- a/packages/opencode/src/provider/provider.ts
+++ b/packages/opencode/src/provider/provider.ts
@@ -1439,6 +1439,7 @@ const layer = Layer.effect(
           }
 
           for (const [modelID, model] of Object.entries(provider.models ?? {})) {
+            const catalogModel = existing?.models[model.id ?? modelID]
             const existingModel = parsed.models[model.id ?? modelID]
             const apiID = model.id ?? existingModel?.api.id ?? modelID
             const apiNpm =
@@ -1491,7 +1492,10 @@ const layer = Layer.effect(
                     ? { field: "reasoning_content" }
                     : false),
               },
-              reasoning_options: existingModel?.reasoning_options,
+              // Config models don't define reasoning_options. Inherit them only
+              // when the config model is explicitly based on a catalog model;
+              // pure custom models keep undefined so legacy heuristics still run.
+              reasoning_options: catalogModel?.reasoning_options,
               cost: {
                 input: model?.cost?.input ?? existingModel?.cost?.input ?? 0,
                 output: model?.cost?.output ?? existingModel?.cost?.output ?? 0,
diff --git a/packages/opencode/test/provider/provider.test.ts b/packages/opencode/test/provider/provider.test.ts
index 5c8414b1ab..3222a836fe 100644
--- a/packages/opencode/test/provider/provider.test.ts
+++ b/packages/opencode/test/provider/provider.test.ts
@@ -1644,6 +1644,39 @@ it.instance(
   },
 )
 
+it.instance(
+  "custom config reasoning model without variants uses fallback variants",
+  Effect.gen(function* () {
+    const providers = yield* list
+    const model = providers[ProviderV2.ID.make("custom-reasoning-fallback")].models["reasoning-model"]
+    expect(model.reasoning_options).toBeUndefined()
+    expect(model.variants).toEqual({
+      low: { reasoningEffort: "low" },
+      medium: { reasoningEffort: "medium" },
+      high: { reasoningEffort: "high" },
+    })
+  }),
+  {
+    config: {
+      provider: {
+        "custom-reasoning-fallback": {
+          name: "Custom Reasoning Fallback",
+          npm: "@ai-sdk/openai-compatible",
+          env: [],
+          models: {
+            "reasoning-model": {
+              name: "Reasoning Model",
+              reasoning: true,
+              limit: { context: 128000, output: 16000 },
+            },
+          },
+          options: { apiKey: "test-key" },
+        },
+      },
+    },
+  },
+)
+
 it.instance(
   "custom model with variants enabled and disabled",
   Effect.gen(function* () {