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/app/src/context/server-session-v2-reducer.test.ts

@@ -69,6 +69,63 @@ describe("v2 session reducer", () => {
     })
   })
 
+  test("prefers durable selection predecessors and derives them for older events", () => {
+    const source: SessionMessageInfo[] = [
+      { id: "msg_previous_agent", type: "agent-switched", agent: "build", time: { created: 1 } },
+      {
+        id: "msg_previous_model",
+        type: "model-switched",
+        model: { id: "old", providerID: "provider" },
+        time: { created: 1 },
+      },
+    ]
+    const reducer = createV2SessionReducer()
+
+    const agent = reducer.reduce(
+      source,
+      event({
+        ...base,
+        id: "evt_agent",
+        type: "session.agent.selected",
+        data: { sessionID: "ses_1", agent: "plan", previous: "review" },
+      }),
+    )
+    const model = reducer.reduce(
+      source,
+      event({
+        ...base,
+        id: "evt_model",
+        type: "session.model.selected",
+        data: {
+          sessionID: "ses_1",
+          model: { id: "new", providerID: "provider" },
+          previous: { id: "durable", providerID: "provider" },
+        },
+      }),
+    )
+    const legacyAgent = reducer.reduce(
+      source,
+      event({
+        ...base,
+        id: "evt_legacy_agent",
+        type: "session.agent.selected",
+        data: { sessionID: "ses_1", agent: "plan" },
+      }),
+    )
+
+    expect(agent?.messages.at(-1)).toMatchObject({ type: "agent-switched", agent: "plan", previous: "review" })
+    expect(model?.messages.at(-1)).toMatchObject({
+      type: "model-switched",
+      model: { id: "new" },
+      previous: { id: "durable" },
+    })
+    expect(legacyAgent?.messages.at(-1)).toMatchObject({
+      type: "agent-switched",
+      agent: "plan",
+      previous: "build",
+    })
+  })
+
   test("folds tool, retry, and completion events", () => {
     const reducer = createV2SessionReducer()
     let messages: SessionMessageInfo[] = []

packages/app/src/context/server-session-v2-reducer.ts

@@ -61,6 +61,12 @@ export function createV2SessionReducer() {
           type: "agent-switched",
           metadata: event.metadata,
           agent: event.data.agent,
+          previous:
+            event.data.previous ??
+            source.findLast(
+              (item): item is Extract<SessionMessageInfo, { type: "agent-switched" | "assistant" }> =>
+                item.type === "agent-switched" || item.type === "assistant",
+            )?.agent,
           time: { created: event.created },
         })
       case "session.model.selected":
@@ -69,10 +75,12 @@ export function createV2SessionReducer() {
           type: "model-switched",
           metadata: event.metadata,
           model: event.data.model,
-          previous: source.findLast(
-            (item): item is Extract<SessionMessageInfo, { type: "model-switched" | "assistant" }> =>
-              item.type === "model-switched" || item.type === "assistant",
-          )?.model,
+          previous:
+            event.data.previous ??
+            source.findLast(
+              (item): item is Extract<SessionMessageInfo, { type: "model-switched" | "assistant" }> =>
+                item.type === "model-switched" || item.type === "assistant",
+            )?.model,
           time: { created: event.created },
         })
       case "session.synthetic":

packages/client/src/effect/api/api.ts

@@ -339,7 +339,11 @@ export type Endpoint5_31Output =
           readonly type: "session.agent.selected"
           readonly durable: { readonly aggregateID: string; readonly seq: Event.Seq; readonly version: Event.Version }
           readonly location?: Location.Ref | undefined
-          readonly data: { readonly sessionID: Session.ID; readonly agent: Agent.ID }
+          readonly data: {
+            readonly sessionID: Session.ID
+            readonly agent: Agent.ID
+            readonly previous?: Agent.ID | undefined
+          }
         }
       | {
           readonly id: Event.ID
@@ -348,7 +352,11 @@ export type Endpoint5_31Output =
           readonly type: "session.model.selected"
           readonly durable: { readonly aggregateID: string; readonly seq: Event.Seq; readonly version: Event.Version }
           readonly location?: Location.Ref | undefined
-          readonly data: { readonly sessionID: Session.ID; readonly model: Model.Ref }
+          readonly data: {
+            readonly sessionID: Session.ID
+            readonly model: Model.Ref
+            readonly previous?: Model.Ref | undefined
+          }
         }
       | {
           readonly id: Event.ID

packages/client/src/promise/generated/types.ts

@@ -436,7 +436,7 @@ export type SessionAgentSelected = {
   type: "session.agent.selected"
   durable: { aggregateID: string; seq: number; version: 1 }
   location?: LocationRef
-  data: { sessionID: string; agent: string }
+  data: { sessionID: string; agent: string; previous?: string }
 }
 
 export type SessionModelSelected = {
@@ -446,7 +446,7 @@ export type SessionModelSelected = {
   type: "session.model.selected"
   durable: { aggregateID: string; seq: number; version: 1 }
   location?: LocationRef
-  data: { sessionID: string; model: ModelRef }
+  data: { sessionID: string; model: ModelRef; previous?: ModelRef }
 }
 
 export type SessionMoved = {

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/src/session.ts

@@ -716,10 +716,11 @@ const layer = Layer.effect(
             .pipe(Effect.ignore, Effect.forkIn(scope, { startImmediately: true }), Effect.asVoid)
       }),
       switchAgent: Effect.fn("Session.switchAgent")(function* (input) {
-        yield* result.get(input.sessionID)
+        const session = yield* result.get(input.sessionID)
         yield* bus.publish(SessionEvent.AgentSelected, {
           sessionID: input.sessionID,
           agent: input.agent,
+          previous: session.agent,
         })
       }),
       switchModel: Effect.fn("Session.switchModel")(function* (input) {
@@ -733,6 +734,7 @@ const layer = Layer.effect(
         yield* bus.publish(SessionEvent.ModelSelected, {
           sessionID: input.sessionID,
           model: input.model,
+          previous: session.model,
         })
       }),
       rename: Effect.fn("Session.rename")(function* (input) {

packages/core/src/session/message-updater.ts

@@ -61,7 +61,7 @@ export function update(adapter: Adapter, event: SessionEvent.DurableEvent) {
       "session.usage.recorded": () => Effect.void,
       "session.agent.selected": (event) => {
         return Effect.gen(function* () {
-          const previous = yield* adapter.getAgent()
+          const previous = event.data.previous ?? (yield* adapter.getAgent())
           yield* adapter.appendMessage(
             SessionMessage.AgentSelected.make({
               id: SessionMessage.ID.fromEvent(event.id),
@@ -76,7 +76,7 @@ export function update(adapter: Adapter, event: SessionEvent.DurableEvent) {
       },
       "session.model.selected": (event) => {
         return Effect.gen(function* () {
-          const previous = yield* adapter.getModel()
+          const previous = event.data.previous ?? (yield* adapter.getModel())
           yield* adapter.appendMessage(
             SessionMessage.ModelSelected.make({
               id: SessionMessage.ID.fromEvent(event.id),

packages/core/test/agent.test.ts

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

packages/core/test/session-create.test.ts

@@ -654,7 +654,7 @@ describe("Session.create", () => {
       expect(yield* session.get(created.id)).toMatchObject({ agent: "plan" })
       expect(
         Array.from(yield* logEvents(session, created.id, true).pipe(Stream.drop(1), Stream.take(1), Stream.runCollect)),
-      ).toMatchObject([{ type: "session.agent.selected", data: { agent: "plan" } }])
+      ).toMatchObject([{ type: "session.agent.selected", data: { agent: "plan", previous: "build" } }])
       expect(yield* session.messages({ sessionID: created.id, order: "asc" })).toMatchObject([
         { type: "agent-switched", agent: "plan", previous: "build" },
       ])
@@ -678,7 +678,12 @@ describe("Session.create", () => {
   it.effect("switches the selected model through the durable Session event", () =>
     Effect.gen(function* () {
       const session = yield* Session.Service
-      const created = yield* session.create({ location })
+      const previous = Model.Ref.make({
+        id: Model.ID.make("haiku"),
+        providerID: Provider.ID.anthropic,
+        variant: Model.VariantID.make("default"),
+      })
+      const created = yield* session.create({ location, model: previous })
       const model = Model.Ref.make({
         id: Model.ID.make("sonnet"),
         providerID: Provider.ID.anthropic,
@@ -692,7 +697,10 @@ describe("Session.create", () => {
         yield* logEvents(session, created.id, true).pipe(Stream.drop(1), Stream.take(1), Stream.runCollect),
       )
       expect(bus).toMatchObject([{ type: "session.model.selected" }])
-      expect(bus[0]?.data).toEqual({ sessionID: created.id, model })
+      expect(bus[0]?.data).toEqual({ sessionID: created.id, model, previous })
+      expect(yield* session.messages({ sessionID: created.id, order: "asc" })).toMatchObject([
+        { type: "model-switched", model, previous },
+      ])
     }),
   )
 

packages/protocol/openapi.json

@@ -14300,9 +14300,15 @@
               "agent": {
                 "type": "string"
               },
+              "previous": {
+                "type": "string"
+              },
               "model": {
                 "$ref": "#/components/schemas/Model.Ref"
               },
+              "previous": {
+                "$ref": "#/components/schemas/Model.Ref"
+              },
               "version": {
                 "type": "string"
               }

packages/schema/src/session-event.ts

@@ -69,6 +69,7 @@ export const AgentSelected = Event.durable({
   schema: {
     ...Base,
     agent: Agent.ID,
+    previous: Agent.ID.pipe(optional),
   },
 })
 export type AgentSelected = typeof AgentSelected.Type
@@ -79,6 +80,7 @@ export const ModelSelected = Event.durable({
   schema: {
     ...Base,
     model: Model.Ref,
+    previous: Model.Ref.pipe(optional),
   },
 })
 export type ModelSelected = typeof ModelSelected.Type

packages/www/openapi.json

@@ -14300,9 +14300,15 @@
               "agent": {
                 "type": "string"
               },
+              "previous": {
+                "type": "string"
+              },
               "model": {
                 "$ref": "#/components/schemas/Model.Ref"
               },
+              "previous": {
+                "$ref": "#/components/schemas/Model.Ref"
+              },
               "version": {
                 "type": "string"
               }

packages/www/public/openapi.json

@@ -14300,9 +14300,15 @@
               "agent": {
                 "type": "string"
               },
+              "previous": {
+                "type": "string"
+              },
               "model": {
                 "$ref": "#/components/schemas/Model.Ref"
               },
+              "previous": {
+                "$ref": "#/components/schemas/Model.Ref"
+              },
               "version": {
                 "type": "string"
               }