feat: add Mistral Medium 3.5 with reasoning support (anomalyco#24996)

Author: rubdos

packages/opencode/src/provider/transform.ts

@@ -760,9 +760,10 @@ export function variants(model: Provider.Model): Record<string, Record<string, a
       // https://v5.ai-sdk.dev/providers/ai-sdk-providers/mistral
       // https://docs.mistral.ai/capabilities/reasoning/adjustable
       if (!model.capabilities.reasoning) return {}
-      // Only Mistral Small 4 supports reasoning (mistral-small-2603, mistral-small-latest)
+      // Only Mistral Small 4 and Medium 3.5 support reasoning
+      const MISTRAL_REASONING_IDS = ["mistral-small-2603", "mistral-small-latest", "mistral-medium-3.5"]
       const mistralId = model.api.id.toLowerCase()
-      if (!mistralId.includes("mistral-small-2603") && !mistralId.includes("mistral-small-latest")) return {}
+      if (!MISTRAL_REASONING_IDS.some((id) => mistralId.includes(id))) return {}
       return {
         high: { reasoningEffort: "high" },
       }

packages/opencode/test/provider/transform.test.ts

@@ -2257,7 +2257,7 @@ describe("ProviderTransform.variants", () => {
     expect(result).toEqual({})
   })
 
-  test("mistral with reasoning returns variants", () => {
+  test("mistral models with reasoning support return variants", () => {
     const model = createMockModel({
       id: "mistral/mistral-small-latest",
       providerID: "mistral",
@@ -2274,6 +2274,23 @@ describe("ProviderTransform.variants", () => {
     })
   })
 
+  test("mistral-medium-3.5 with reasoning returns variants", () => {
+    const model = createMockModel({
+      id: "mistral/mistral-medium-3.5",
+      providerID: "mistral",
+      api: {
+        id: "mistral-medium-3.5",
+        url: "https://api.mistral.com",
+        npm: "@ai-sdk/mistral",
+      },
+      capabilities: { reasoning: true },
+    })
+    const result = ProviderTransform.variants(model)
+    expect(result).toEqual({
+      high: { reasoningEffort: "high" },
+    })
+  })
+
   test("mistral without reasoning returns empty object", () => {
     const model = createMockModel({
       id: "mistral/mistral-large",