From 717cdeed41662719fb52fc93b7317c96eec8f5ee Mon Sep 17 00:00:00 2001
From: Aiden Cline <rekram1-node@users.noreply.github.com>
Date: Sun, 9 Aug 2026 21:27:23 +0000
Subject: [PATCH] feat(core): expose previous agent on selection events

---
 packages/client/src/effect/api/api.ts          | 6 +++++-
 packages/client/src/promise/generated/types.ts | 2 +-
 packages/core/src/session.ts                   | 3 ++-
 packages/core/test/session-create.test.ts      | 4 ++--
 packages/schema/src/session-event.ts           | 1 +
 5 files changed, 11 insertions(+), 5 deletions(-)

diff --git a/packages/client/src/effect/api/api.ts b/packages/client/src/effect/api/api.ts
index e0867bedd9..f102d42e14 100644
--- a/packages/client/src/effect/api/api.ts
+++ b/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
diff --git a/packages/client/src/promise/generated/types.ts b/packages/client/src/promise/generated/types.ts
index 090811bc07..45d5e468fe 100644
--- a/packages/client/src/promise/generated/types.ts
+++ b/packages/client/src/promise/generated/types.ts
@@ -439,7 +439,7 @@ 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 = {
diff --git a/packages/core/src/session.ts b/packages/core/src/session.ts
index fb49b19cb8..bbc0ee9c8e 100644
--- a/packages/core/src/session.ts
+++ b/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) {
diff --git a/packages/core/test/session-create.test.ts b/packages/core/test/session-create.test.ts
index 56db03a2c0..65cc321a2b 100644
--- a/packages/core/test/session-create.test.ts
+++ b/packages/core/test/session-create.test.ts
@@ -648,14 +648,14 @@ describe("Session.create", () => {
   it.effect("switches the selected agent through the durable Session event", () =>
     Effect.gen(function* () {
       const session = yield* Session.Service
-      const created = yield* session.create({ location })
+      const created = yield* session.create({ location, agent: Agent.defaultID })
 
       yield* session.switchAgent({ sessionID: created.id, agent: Agent.ID.make("plan") })
 
       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" } }])
     }),
   )
 
diff --git a/packages/schema/src/session-event.ts b/packages/schema/src/session-event.ts
index 2ae3f7c405..1513e6fc37 100644
--- a/packages/schema/src/session-event.ts
+++ b/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