test: add variant inheritance tests for subagent task flow

Cover both paths of the task.ts inheritance logic:
- subagent without own variant inherits parent's variant
- subagent with own variant skips inheritance

Co-authored-by: Cursor <[email protected]>

Author: mugnimaestra

packages/opencode/test/session/prompt-variant.test.ts

@@ -2,6 +2,7 @@ import { describe, expect, test } from "bun:test"
 import { Instance } from "../../src/project/instance"
 import { Session } from "../../src/session"
 import { SessionPrompt } from "../../src/session/prompt"
+import { Agent } from "../../src/agent/agent"
 import { tmpdir } from "../fixture/fixture"
 
 describe("session.prompt agent variant", () => {
@@ -84,4 +85,99 @@ describe("session.prompt agent variant", () => {
       },
     })
   })
+
+  test("subagent inherits variant from parent session user message", async () => {
+    await using tmp = await tmpdir({
+      git: true,
+      config: {},
+    })
+
+    await Instance.provide({
+      directory: tmp.path,
+      fn: async () => {
+        // 1. Create a parent session and send a user message with variant
+        const parent = await Session.create({})
+        const parentMsg = await SessionPrompt.prompt({
+          sessionID: parent.id,
+          variant: "high",
+          noReply: true,
+          parts: [{ type: "text", text: "use @general to explore the codebase" }],
+        })
+        if (parentMsg.info.role !== "user") throw new Error("expected user message")
+        expect(parentMsg.info.variant).toBe("high")
+
+        // 2. Simulate what task.ts does: read back the parent user message
+        //    and compute the inherited variant for the subagent
+        const agent = await Agent.get("general")
+        const resolved = !agent.variant && parentMsg.info.role === "user" ? parentMsg.info.variant : undefined
+        expect(resolved).toBe("high")
+
+        // 3. Create a child session (like task.ts does) and prompt with inherited variant
+        const child = await Session.create({ parentID: parent.id })
+        const childMsg = await SessionPrompt.prompt({
+          sessionID: child.id,
+          agent: "general",
+          variant: resolved,
+          noReply: true,
+          parts: [{ type: "text", text: "explore the project structure" }],
+        })
+        if (childMsg.info.role !== "user") throw new Error("expected user message")
+        expect(childMsg.info.variant).toBe("high")
+
+        await Session.remove(child.id)
+        await Session.remove(parent.id)
+      },
+    })
+  })
+
+  test("subagent does not inherit variant when agent has own variant", async () => {
+    await using tmp = await tmpdir({
+      git: true,
+      config: {
+        agent: {
+          build: {
+            model: "openai/gpt-5.2",
+            variant: "xhigh",
+          },
+        },
+      },
+    })
+
+    await Instance.provide({
+      directory: tmp.path,
+      fn: async () => {
+        // 1. Parent message with variant="high"
+        const parent = await Session.create({})
+        const parentMsg = await SessionPrompt.prompt({
+          sessionID: parent.id,
+          variant: "high",
+          noReply: true,
+          parts: [{ type: "text", text: "use @build to compile" }],
+        })
+        if (parentMsg.info.role !== "user") throw new Error("expected user message")
+        expect(parentMsg.info.variant).toBe("high")
+
+        // 2. Agent "build" has its own variant="xhigh", so inheritance should NOT happen
+        const agent = await Agent.get("build")
+        const resolved = !agent.variant && parentMsg.info.role === "user" ? parentMsg.info.variant : undefined
+        expect(resolved).toBeUndefined()
+
+        // 3. When variant=undefined, the child session uses agent's own variant via prompt.ts
+        const child = await Session.create({ parentID: parent.id })
+        const childMsg = await SessionPrompt.prompt({
+          sessionID: child.id,
+          agent: "build",
+          variant: resolved,
+          noReply: true,
+          parts: [{ type: "text", text: "compile the project" }],
+        })
+        if (childMsg.info.role !== "user") throw new Error("expected user message")
+        // agent.variant="xhigh" is applied by prompt.ts since we're using the agent's model
+        expect(childMsg.info.variant).toBe("xhigh")
+
+        await Session.remove(child.id)
+        await Session.remove(parent.id)
+      },
+    })
+  })
 })