fix(core): tighten plan switch reminder copy

Rewrite enter and leave synthetics to state the read-only constraint
and its removal without extra workflow instructions.

packages/core/src/plugin/agent.ts

@@ -101,16 +101,6 @@ export const Plugin = define({
         item.permissions.push({ action: "question", resource: "*", effect: "allow" })
       })
 
-      draft.update(Agent.ID.make("plan"), (item) => {
-        item.name = Agent.Name.make("Plan")
-        item.description = "Plan mode. Disallows all edit tools."
-        item.mode = "primary"
-        item.permissions.push(
-          { action: "question", resource: "*", effect: "allow" },
-          { action: "edit", resource: "*", effect: "deny" },
-        )
-      })
-
       draft.update(Agent.ID.make("general"), (item) => {
         item.name = Agent.Name.make("General")
         item.description =

packages/core/src/plugin/internal.ts

@@ -60,6 +60,7 @@ import { WellKnown } from "../wellknown"
 import { WriteTool } from "../tool/plugin/write"
 import { AgentPlugin } from "./agent"
 import { CommandPlugin } from "./command"
+import { PlanPlugin } from "./plan"
 import { ModelsDevPlugin } from "./models-dev"
 import { ProviderPlugins } from "./provider"
 import { WebSearchPlugins } from "./websearch"
@@ -192,6 +193,7 @@ export type InternalPlugin = Plugin<Requirements | Scope.Scope>
 const pre = [
   WellKnownPlugin.Plugin,
   AgentPlugin.Plugin,
+  PlanPlugin.Plugin,
   CommandPlugin.Plugin,
   SkillPlugin.Plugin,
   ...SystemPromptPlugin.Plugins,

packages/core/src/plugin/plan.ts

@@ -0,0 +1,55 @@
+export * as PlanPlugin from "./plan"
+
+import { ToolFailure } from "@opencode-ai/ai"
+import { define } from "@opencode-ai/plugin/effect/plugin"
+import { Effect, Stream } from "effect"
+import { Agent } from "../agent"
+
+const plan = Agent.ID.make("plan")
+
+const enter = `<system-reminder>
+You are in Plan mode. This is a READ-ONLY environment. You are not allowed to edit files, and you may not ask a subagent to edit them either.
+</system-reminder>`
+
+const leave = `<system-reminder>
+You are no longer in Plan mode. The previous read-only restrictions no longer apply. You may edit files again.
+</system-reminder>`
+
+export const Plugin = define({
+  id: "opencode.plan",
+  effect: Effect.fn(function* (ctx) {
+    yield* ctx.agent.transform((draft) => {
+      draft.update(plan, (item) => {
+        item.name = Agent.Name.make("Plan")
+        item.description = "Read-only agent for exploring the codebase and planning work before implementation."
+        item.mode = "primary"
+        item.permissions.push({ action: "question", resource: "*", effect: "allow" })
+      })
+    })
+
+    yield* ctx.tool.hook("execute.before", (event) => {
+      if (event.agent !== plan) return Effect.void
+      if (event.tool !== "edit" && event.tool !== "write" && event.tool !== "patch") return Effect.void
+      return new ToolFailure({
+        message: `Cannot use ${event.tool} in Plan mode. You are in a read-only mode and must not modify files.`,
+      })
+    })
+
+    yield* ctx.event.subscribe().pipe(
+      Stream.filter((event) => event.type === "session.agent.selected"),
+      Stream.runForEach((event) => {
+        if (event.data.agent === event.data.previous) return Effect.void
+        const text = event.data.agent === plan ? enter : event.data.previous === plan ? leave : undefined
+        if (!text) return Effect.void
+        return ctx.session
+          .synthetic({
+            sessionID: event.data.sessionID,
+            text,
+            resume: false,
+          })
+          .pipe(Effect.catch(() => Effect.void))
+      }),
+      Effect.forkScoped({ startImmediately: true }),
+    )
+  }),
+})

packages/core/test/agent.test.ts

@@ -165,7 +165,6 @@ describe("Agent", () => {
         "compaction",
         "explore",
         "general",
-        "plan",
         "summary",
         "title",
       ])