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

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

@@ -339,11 +339,7 @@ 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 previous?: Agent.ID | undefined
-          }
+          readonly data: { readonly sessionID: Session.ID; readonly agent: Agent.ID }
         }
       | {
           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; previous?: string }
+  data: { sessionID: string; agent: string }
 }
 
 export type SessionModelSelected = {

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/plan.ts

@@ -2,6 +2,7 @@ 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"
 
@@ -37,18 +38,18 @@ export const Plugin = define({
 
     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({
+      Stream.runForEach((event) =>
+        Effect.gen(function* () {
+          const message = yield* ctx.session.message({
             sessionID: event.data.sessionID,
-            text,
-            resume: false,
+            messageID: SessionMessage.ID.fromEvent(event.id),
           })
-          .pipe(Effect.catch(() => Effect.void))
-      }),
+          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/src/session.ts

@@ -716,11 +716,10 @@ const layer = Layer.effect(
             .pipe(Effect.ignore, Effect.forkIn(scope, { startImmediately: true }), Effect.asVoid)
       }),
       switchAgent: Effect.fn("Session.switchAgent")(function* (input) {
-        const session = yield* result.get(input.sessionID)
+        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) {

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 = event.data.previous ?? (yield* adapter.getAgent())
+          const previous = yield* adapter.getAgent()
           yield* adapter.appendMessage(
             SessionMessage.AgentSelected.make({
               id: SessionMessage.ID.fromEvent(event.id),

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/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", previous: "build" } }])
+      ).toMatchObject([{ type: "session.agent.selected", data: { agent: "plan" } }])
       expect(yield* session.messages({ sessionID: created.id, order: "asc" })).toMatchObject([
         { type: "agent-switched", agent: "plan", previous: "build" },
       ])

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>
 }

packages/protocol/openapi.json

@@ -14373,9 +14373,6 @@
               },
               "agent": {
                 "type": "string"
-              },
-              "previous": {
-                "type": "string"
               }
             },
             "required": ["sessionID", "agent"],

packages/schema/src/session-event.ts

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

packages/www/openapi.json

@@ -14373,9 +14373,6 @@
               },
               "agent": {
                 "type": "string"
-              },
-              "previous": {
-                "type": "string"
               }
             },
             "required": ["sessionID", "agent"],

packages/www/public/openapi.json

@@ -14373,9 +14373,6 @@
               },
               "agent": {
                 "type": "string"
-              },
-              "previous": {
-                "type": "string"
               }
             },
             "required": ["sessionID", "agent"],