diff --git a/packages/opencode/src/provider/transform.ts b/packages/opencode/src/provider/transform.ts
index d86fe9022..4089dde53 100644
--- a/packages/opencode/src/provider/transform.ts
+++ b/packages/opencode/src/provider/transform.ts
@@ -3,6 +3,7 @@ import { unique } from "remeda"
 import type { JSONSchema } from "zod/v4/core"
 import type { Provider } from "./provider"
 import type { ModelsDev } from "./models"
+import type { MessageV2 } from "@/session/message-v2"
 
 type Modality = NonNullable<ModelsDev.Model["modalities"]>["input"][number]
 
@@ -241,6 +242,38 @@ export namespace ProviderTransform {
     return undefined
   }
 
+  export function thinking(model: Provider.Model, thinking: MessageV2.Thinking) {
+    if (!model.capabilities.reasoning) return undefined
+
+    switch (model.api.npm) {
+      case "@openrouter/ai-sdk-provider":
+        return {
+          reasoning: { effort: thinking.effort },
+        }
+      case "@ai-sdk/openai-compatible":
+        const result: Record<string, any> = {
+          reasoningEffort: thinking.effort,
+        }
+
+        if (model.providerID === "baseten") {
+          result["chat_template_args"] = { enable_thinking: true }
+        }
+
+        return result
+
+      case "@ai-sdk/openai":
+        return {
+          reasoningEffort: thinking.effort,
+          reasoningSummary: "auto",
+          include: ["reasoning.encrypted_content"],
+        }
+
+      case "@ai-sdk/anthropic":
+        // TODO: map to thinking budgets
+        return {}
+    }
+  }
+
   export function options(
     model: Provider.Model,
     sessionID: string,
diff --git a/packages/opencode/src/session/message-v2.ts b/packages/opencode/src/session/message-v2.ts
index 14542669e..77858fa03 100644
--- a/packages/opencode/src/session/message-v2.ts
+++ b/packages/opencode/src/session/message-v2.ts
@@ -287,6 +287,9 @@ export namespace MessageV2 {
     sessionID: z.string(),
   })
 
+  export const Thinking = z.object({ effort: z.enum(["low", "medium", "high"]) })
+  export type Thinking = z.infer<typeof Thinking>
+
   export const User = Base.extend({
     role: z.literal("user"),
     time: z.object({
@@ -306,6 +309,7 @@ export namespace MessageV2 {
     }),
     system: z.string().optional(),
     tools: z.record(z.string(), z.boolean()).optional(),
+    thinking: Thinking.optional(),
   }).meta({
     ref: "UserMessage",
   })
diff --git a/packages/sdk/js/src/v2/gen/types.gen.ts b/packages/sdk/js/src/v2/gen/types.gen.ts
index 1372765e3..899dd7c22 100644
--- a/packages/sdk/js/src/v2/gen/types.gen.ts
+++ b/packages/sdk/js/src/v2/gen/types.gen.ts
@@ -90,6 +90,9 @@ export type UserMessage = {
   tools?: {
     [key: string]: boolean
   }
+  thinking?: {
+    effort: "low" | "medium" | "high"
+  }
 }
 
 export type ProviderAuthError = {
diff --git a/packages/sdk/openapi.json b/packages/sdk/openapi.json
index 588b130b9..999a89ecb 100644
--- a/packages/sdk/openapi.json
+++ b/packages/sdk/openapi.json
@@ -5228,6 +5228,16 @@
             "additionalProperties": {
               "type": "boolean"
             }
+          },
+          "thinking": {
+            "type": "object",
+            "properties": {
+              "effort": {
+                "type": "string",
+                "enum": ["low", "medium", "high"]
+              }
+            },
+            "required": ["effort"]
           }
         },
         "required": ["id", "sessionID", "role", "time", "agent", "model"]