fix(opencode): scope agent variant to model (#11410)

packages/opencode/src/agent/agent.ts

@@ -37,6 +37,7 @@ export namespace Agent {
           providerID: z.string(),
         })
         .optional(),
+      variant: z.string().optional(),
       prompt: z.string().optional(),
       options: z.record(z.string(), z.any()),
       steps: z.number().int().positive().optional(),
@@ -214,6 +215,7 @@ export namespace Agent {
           native: false,
         }
       if (value.model) item.model = Provider.parseModel(value.model)
+      item.variant = value.variant ?? item.variant
       item.prompt = value.prompt ?? item.prompt
       item.description = value.description ?? item.description
       item.temperature = value.temperature ?? item.temperature

packages/opencode/src/config/config.ts

@@ -593,6 +593,10 @@ export namespace Config {
   export const Agent = z
     .object({
       model: z.string().optional(),
+      variant: z
+        .string()
+        .optional()
+        .describe("Default model variant for this agent (applies only when using the agent's configured model)."),
       temperature: z.number().optional(),
       top_p: z.number().optional(),
       prompt: z.string().optional(),
@@ -624,6 +628,7 @@ export namespace Config {
       const knownKeys = new Set([
         "name",
         "model",
+        "variant",
         "prompt",
         "description",
         "temperature",

packages/opencode/src/session/prompt.ts

@@ -827,6 +827,17 @@ export namespace SessionPrompt {
 
   async function createUserMessage(input: PromptInput) {
     const agent = await Agent.get(input.agent ?? (await Agent.defaultAgent()))
+
+    const model = input.model ?? agent.model ?? (await lastModel(input.sessionID))
+    const variant =
+      input.variant ??
+      (agent.variant &&
+      agent.model &&
+      model.providerID === agent.model.providerID &&
+      model.modelID === agent.model.modelID
+        ? agent.variant
+        : undefined)
+
     const info: MessageV2.Info = {
       id: input.messageID ?? Identifier.ascending("message"),
       role: "user",
@@ -836,9 +847,9 @@ export namespace SessionPrompt {
       },
       tools: input.tools,
       agent: agent.name,
-      model: input.model ?? agent.model ?? (await lastModel(input.sessionID)),
+      model,
       system: input.system,
-      variant: input.variant,
+      variant,
     }
     using _ = defer(() => InstructionPrompt.clear(info.id))