fix(core): derive previous agent in projection (#41685)

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/host.ts

@@ -349,6 +349,7 @@ export const make = Effect.fn("PluginHost.make")(function* (plugin: import("../p
             input?.location ?? Location.Ref.make({ directory: location.directory, workspaceID: location.workspaceID }),
         }),
       get: (input) => runtime.session.get(input.sessionID),
+      message: runtime.session.message,
       prompt: runtime.session.prompt,
       generate: (input) => runtime.session.generate(input).pipe(Effect.map((text) => ({ text }))),
       command: runtime.session.command,

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 +1,56 @@
+export * as PlanPlugin from "./plan"
+
+import { ToolFailure } from "@opencode-ai/ai"
+import { define } from "@opencode-ai/plugin/effect/plugin"
+import { SessionMessage } from "@opencode-ai/schema/session-message"
+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) =>
+        Effect.gen(function* () {
+          const message = yield* ctx.session.message({
+            sessionID: event.data.sessionID,
+            messageID: SessionMessage.ID.fromEvent(event.id),
+          })
+          if (message?.type !== "agent-switched" || message.agent === message.previous) return
+          const text = message.agent === plan ? enter : message.previous === plan ? leave : undefined
+          if (!text) return
+          yield* ctx.session.synthetic({ sessionID: event.data.sessionID, text, resume: false })
+        }).pipe(Effect.catch(() => Effect.void)),
+      ),
+      Effect.forkScoped({ startImmediately: true }),
+    )
+  }),
+})

packages/core/src/plugin/runtime.ts

@@ -14,6 +14,7 @@ export interface Interface {
     | "get"
     | "create"
     | "messages"
+    | "message"
     | "prompt"
     | "generate"
     | "command"
@@ -59,6 +60,7 @@ export const layerWithCell = (cell: Cell) =>
         get: (sessionID) => require(cell, (runtime) => runtime.session.get(sessionID)),
         create: (input) => require(cell, (runtime) => runtime.session.create(input)),
         messages: (input) => require(cell, (runtime) => runtime.session.messages(input)),
+        message: (input) => require(cell, (runtime) => runtime.session.message(input)),
         prompt: (input) => require(cell, (runtime) => runtime.session.prompt(input)),
         generate: (input) => require(cell, (runtime) => runtime.session.generate(input)),
         command: (input) => require(cell, (runtime) => runtime.session.command(input)),

packages/core/test/agent.test.ts

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

packages/core/test/plugin/host.ts

@@ -102,6 +102,7 @@ export function host(overrides: Overrides = {}): Plugin.Context {
       hook: overrides.session?.hook ?? (() => Effect.die("unused session.hook")),
       create: overrides.session?.create ?? (() => Effect.die("unused session.create")),
       get: overrides.session?.get ?? (() => Effect.die("unused session.get")),
+      message: overrides.session?.message ?? (() => Effect.die("unused session.message")),
       prompt: overrides.session?.prompt ?? (() => Effect.die("unused session.prompt")),
       generate: overrides.session?.generate ?? (() => Effect.die("unused session.generate")),
       command: overrides.session?.command ?? (() => Effect.die("unused session.command")),

packages/plugin/src/effect/session.ts

@@ -3,7 +3,8 @@ import type { Message, SystemPart } from "@opencode-ai/ai"
 import type { Agent } from "@opencode-ai/schema/agent"
 import type { Model } from "@opencode-ai/schema/model"
 import type { Session } from "@opencode-ai/schema/session"
-import type { JsonSchema } from "effect"
+import type { SessionMessage } from "@opencode-ai/schema/session-message"
+import type { Effect, JsonSchema } from "effect"
 import type { Hooks } from "./registration.js"
 
 export interface SessionContext {
@@ -40,5 +41,9 @@ export type SessionDomain = Pick<
   SessionApi<unknown>,
   "create" | "get" | "prompt" | "generate" | "command" | "synthetic" | "interrupt" | "rename" | "wait"
 > & {
+  readonly message: (input: {
+    readonly sessionID: Session.ID
+    readonly messageID: SessionMessage.ID
+  }) => Effect.Effect<SessionMessage.Info | undefined, unknown>
   readonly hook: Hooks<SessionHooks>
 }