diff --git a/packages/core/src/plugin/plan.ts b/packages/core/src/plugin/plan.ts
index 1c5efeaedfd..6aae61869a0 100644
--- a/packages/core/src/plugin/plan.ts
+++ b/packages/core/src/plugin/plan.ts
@@ -1,7 +1,7 @@
 export * as PlanPlugin from "./plan.js"
 
 import { ToolFailure } from "@opencode-ai/ai"
-import { define } from "@opencode-ai/plugin/effect/plugin"
+import { define, type Context } from "@opencode-ai/plugin/effect/plugin"
 import { Effect, Stream } from "effect"
 import { Agent } from "../agent.js"
 import { SessionEvent } from "../session/event.js"
@@ -40,35 +40,47 @@ export const Plugin = define({
 
     yield* ctx.event.subscribe().pipe(
       Stream.filter(
-        (event): event is SessionEvent.Created | SessionEvent.AgentSelected =>
-          event.type === "session.created" || event.type === "session.agent.selected",
+        (event): event is SessionEvent.Created | SessionEvent.AgentSelected | SessionEvent.Compaction.Ended =>
+          event.type === "session.created" ||
+          event.type === "session.agent.selected" ||
+          event.type === "session.compaction.ended",
       ),
-      Stream.runForEach((event) => {
-        const text = reminder(event)
-        if (!text) return Effect.void
-        return ctx.session
-          .synthetic({
+      Stream.runForEach((event) =>
+        Effect.gen(function* () {
+          const text = yield* reminder(ctx.session, event)
+          if (!text) return
+          yield* ctx.session.synthetic({
             sessionID: event.data.sessionID,
             text,
             resume: false,
           })
-          .pipe(
-            Effect.catchCause((cause) =>
-              Effect.logWarning("failed to inject Plan mode reminder", { sessionID: event.data.sessionID, cause }),
-            ),
-          )
-      }),
+        }).pipe(
+          Effect.catchCause((cause) =>
+            Effect.logWarning("failed to inject Plan mode reminder", { sessionID: event.data.sessionID, cause }),
+          ),
+        ),
+      ),
       Effect.forkScoped({ startImmediately: true }),
     )
   }),
 })
 
-function reminder(event: SessionEvent.Created | SessionEvent.AgentSelected) {
+const reminder = Effect.fnUntraced(function* (
+  session: Context["session"],
+  event: SessionEvent.Created | SessionEvent.AgentSelected | SessionEvent.Compaction.Ended,
+) {
   if (event.type === "session.created") {
     if (event.data.agent !== plan) return
     return enter
   }
+  if (event.type === "session.compaction.ended") {
+    // Compaction cuts the transcript at the checkpoint, dropping any earlier enter
+    // reminder, so re-assert Plan mode when the session is still on the plan agent.
+    const info = yield* session.get({ sessionID: event.data.sessionID })
+    if (info.agent !== plan) return
+    return enter
+  }
   if (event.data.agent === event.data.previous) return
   if (event.data.agent === plan) return enter
   if (event.data.previous === plan) return leave
-}
+})
diff --git a/packages/core/test/plugin/plan.test.ts b/packages/core/test/plugin/plan.test.ts
new file mode 100644
index 00000000000..706359a0ea5
--- /dev/null
+++ b/packages/core/test/plugin/plan.test.ts
@@ -0,0 +1,128 @@
+import { describe, expect } from "bun:test"
+import { DateTime, Effect, Stream } from "effect"
+import { Agent } from "@opencode-ai/core/agent"
+import { Event } from "@opencode-ai/schema/event"
+import { Money } from "@opencode-ai/schema/money"
+import { PlanPlugin } from "@opencode-ai/core/plugin/plan"
+import { AbsolutePath } from "@opencode-ai/core/schema"
+import { Project } from "@opencode-ai/core/project"
+import { Session } from "@opencode-ai/core/session"
+import { SessionEvent } from "@opencode-ai/core/session/event"
+import { SessionInbox } from "@opencode-ai/core/session/inbox"
+import { SessionMessage } from "@opencode-ai/core/session/message"
+import { it } from "../lib/effect"
+import { host } from "./host"
+
+const sessionID = Session.ID.make("ses_plan_test")
+const plan = Agent.ID.make("plan")
+const build = Agent.ID.make("build")
+
+const envelope = () => ({
+  id: Event.ID.create(),
+  created: DateTime.makeUnsafe(0),
+  durable: { aggregateID: sessionID, seq: Event.Seq.make(0), version: Event.Version.make(1) },
+})
+
+const agentSelected = (agent: Agent.ID, previous: Agent.ID): SessionEvent.AgentSelected => ({
+  ...envelope(),
+  type: "session.agent.selected",
+  data: { sessionID, agent, previous },
+})
+
+const compactionEnded = (): SessionEvent.Compaction.Ended => ({
+  ...envelope(),
+  type: "session.compaction.ended",
+  data: { sessionID, reason: "manual", text: "summary", recent: "" },
+})
+
+const sessionInfo = (agent?: Agent.ID): Session.Info => ({
+  id: sessionID,
+  projectID: Project.ID.make("test"),
+  agent,
+  cost: Money.USD.zero,
+  tokens: { input: 0, output: 0, reasoning: 0, cache: { read: 0, write: 0 } },
+  time: { created: DateTime.makeUnsafe(0), updated: DateTime.makeUnsafe(0) },
+  location: { directory: AbsolutePath.make("/") },
+})
+
+/** Runs the plan plugin against stubbed domains and captures injected reminders. */
+const run = Effect.fnUntraced(function* (
+  events: ReadonlyArray<SessionEvent.AgentSelected | SessionEvent.Compaction.Ended>,
+  agent?: Agent.ID,
+) {
+  const captured = new Array<string>()
+  yield* PlanPlugin.Plugin.effect(
+    host({
+      agent: {
+        get: () => Effect.die("unused agent.get"),
+        list: () => Effect.die("unused agent.list"),
+        reload: () => Effect.die("unused agent.reload"),
+        transform: () => Effect.succeed({ dispose: Effect.void }),
+      },
+      tool: {
+        transform: () => Effect.die("unused tool.transform"),
+        hook: () => Effect.succeed({ dispose: Effect.void }),
+      },
+      event: {
+        subscribe: () => Stream.fromIterable(events),
+      },
+      session: {
+        get: () => Effect.succeed(sessionInfo(agent)),
+        synthetic: (input) => {
+          captured.push(input.text)
+          return Effect.succeed(
+            SessionInbox.Synthetic.make({
+              id: SessionMessage.ID.make("msg_plan_test"),
+              sessionID,
+              timeCreated: DateTime.makeUnsafe(0),
+              type: "synthetic",
+              payload: { text: input.text },
+              delivery: "steer",
+            }),
+          )
+        },
+      },
+    }),
+  )
+  return captured
+})
+
+const settle = (captured: ReadonlyArray<string>, expected: number, remaining = 1000): Effect.Effect<void, Error> =>
+  Effect.gen(function* () {
+    if (captured.length >= expected) return
+    if (remaining === 0) {
+      return yield* Effect.fail(new Error(`Timed out waiting for ${expected} reminders, saw ${captured.length}`))
+    }
+    yield* Effect.promise(() => Bun.sleep(1))
+    yield* settle(captured, expected, remaining - 1)
+  })
+
+describe("plan plugin reminders", () => {
+  it.effect("injects enter and leave reminders on agent switches", () =>
+    Effect.gen(function* () {
+      const captured = yield* run([agentSelected(plan, build), agentSelected(build, plan)])
+      yield* settle(captured, 2)
+      expect(captured[0]).toContain("You are in Plan mode")
+      expect(captured[1]).toContain("NO LONGER in Plan mode")
+    }),
+  )
+
+  it.effect("re-injects the enter reminder after compaction while on the plan agent", () =>
+    Effect.gen(function* () {
+      const captured = yield* run([compactionEnded()], plan)
+      yield* settle(captured, 1)
+      expect(captured).toHaveLength(1)
+      expect(captured[0]).toContain("You are in Plan mode")
+    }),
+  )
+
+  it.effect("ignores compaction when the session is not on the plan agent", () =>
+    Effect.gen(function* () {
+      // The trailing switch to plan proves the earlier compaction event was processed.
+      const captured = yield* run([compactionEnded(), agentSelected(plan, build)], build)
+      yield* settle(captured, 1)
+      expect(captured).toHaveLength(1)
+      expect(captured[0]).toContain("You are in Plan mode")
+    }),
+  )
+})