feat: add Mistral Small reasoning variant support (issue #19479) (#23735)

2 files changed, 42 insertions, 3 deletions

diff --git a/packages/opencode/src/provider/transform.ts b/packages/opencode/src/provider/transform.ts
index 863f01273..1d84c7c93 100644
--- a/packages/opencode/src/provider/transform.ts
+++ b/packages/opencode/src/provider/transform.ts
@@ -408,7 +408,6 @@ export function variants(model: Provider.Model): Record<string, Record<string, a
     id.includes("deepseek") ||
     id.includes("minimax") ||
     id.includes("glm") ||
-    id.includes("mistral") ||
     id.includes("kimi") ||
     id.includes("k2p") ||
     id.includes("qwen") ||
@@ -713,7 +712,14 @@ export function variants(model: Provider.Model): Record<string, Record<string, a
 
     case "@ai-sdk/mistral":
       // https://v5.ai-sdk.dev/providers/ai-sdk-providers/mistral
-      return {}
+      // https://docs.mistral.ai/capabilities/reasoning/adjustable
+      if (!model.capabilities.reasoning) return {}
+      // Only Mistral Small 4 supports reasoning (mistral-small-2603, mistral-small-latest)
+      const mistralId = model.api.id.toLowerCase()
+      if (!mistralId.includes("mistral-small-2603") && !mistralId.includes("mistral-small-latest")) return {}
+      return {
+        high: { reasoningEffort: "high" },
+      }
 
     case "@ai-sdk/cohere":
       // https://v5.ai-sdk.dev/providers/ai-sdk-providers/cohere
diff --git a/packages/opencode/test/provider/transform.test.ts b/packages/opencode/test/provider/transform.test.ts
index 7a7631710..791fcdedc 100644
--- a/packages/opencode/test/provider/transform.test.ts
+++ b/packages/opencode/test/provider/transform.test.ts
@@ -2113,7 +2113,39 @@ describe("ProviderTransform.variants", () => {
     expect(result).toEqual({})
   })
 
-  test("mistral returns empty object", () => {
+  test("mistral with reasoning returns variants", () => {
+    const model = createMockModel({
+      id: "mistral/mistral-small-latest",
+      providerID: "mistral",
+      api: {
+        id: "mistral-small-latest",
+        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",
+      providerID: "mistral",
+      api: {
+        id: "mistral-large-latest",
+        url: "https://api.mistral.com",
+        npm: "@ai-sdk/mistral",
+      },
+      capabilities: { reasoning: false },
+    })
+    const result = ProviderTransform.variants(model)
+    expect(result).toEqual({})
+  })
+
+  test("mistral large with reasoning returns empty object (only small supports reasoning)", () => {
     const model = createMockModel({
       id: "mistral/mistral-large",
       providerID: "mistral",
@@ -2122,6 +2154,7 @@ describe("ProviderTransform.variants", () => {
         url: "https://api.mistral.com",
         npm: "@ai-sdk/mistral",
       },
+      capabilities: { reasoning: true },
     })
     const result = ProviderTransform.variants(model)
     expect(result).toEqual({})