diff --git a/packages/client/src/generated/types.ts b/packages/client/src/generated/types.ts
index 1999704bcc..74e75390a3 100644
--- a/packages/client/src/generated/types.ts
+++ b/packages/client/src/generated/types.ts
@@ -3914,6 +3914,19 @@ export type EventSubscribeOutput =
         readonly recent: string
       }
     }
+  | {
+      readonly id: string
+      readonly metadata?: { readonly [x: string]: unknown }
+      readonly type: "session.next.compaction.failed"
+      readonly durable?: { readonly aggregateID: string; readonly seq: number; readonly version: number }
+      readonly location?: { readonly directory: string; readonly workspaceID?: string }
+      readonly data: {
+        readonly timestamp: number
+        readonly sessionID: string
+        readonly messageID: string
+        readonly reason: "auto" | "manual"
+      }
+    }
   | {
       readonly id: string
       readonly metadata?: { readonly [x: string]: unknown }
diff --git a/packages/core/src/session/compaction.ts b/packages/core/src/session/compaction.ts
index 99c45cbb52..4bd35841d8 100644
--- a/packages/core/src/session/compaction.ts
+++ b/packages/core/src/session/compaction.ts
@@ -237,7 +237,17 @@ const make = (dependencies: Dependencies) => {
         Effect.catchTag("LLM.Error", () => Effect.succeed(false)),
       )
     const summary = chunks.join("")
-    if (!summarized || failed || !summary.trim()) return false
+    if (!summarized || failed || !summary.trim()) {
+      // Clients react to Started (for example by marking the session as running),
+      // so a failed summarization still needs a terminal event.
+      yield* dependencies.events.publish(SessionEvent.Compaction.Failed, {
+        sessionID: input.sessionID,
+        messageID,
+        timestamp: yield* DateTime.now,
+        reason: input.reason,
+      })
+      return false
+    }
     yield* dependencies.events.publish(SessionEvent.Compaction.Ended, {
       sessionID: input.sessionID,
       messageID,
diff --git a/packages/core/src/session/message-updater.ts b/packages/core/src/session/message-updater.ts
index e12c24162c..1272e0f2cf 100644
--- a/packages/core/src/session/message-updater.ts
+++ b/packages/core/src/session/message-updater.ts
@@ -389,6 +389,7 @@ export function update(adapter: Adapter, event: SessionEvent.Event) {
       "session.next.retried": () => Effect.void,
       "session.next.compaction.started": () => Effect.void,
       "session.next.compaction.delta": () => Effect.void,
+      "session.next.compaction.failed": () => Effect.void,
       "session.next.compaction.ended": (event) => {
         return adapter.appendMessage(
           SessionMessage.Compaction.make({
diff --git a/packages/core/test/session-compaction.test.ts b/packages/core/test/session-compaction.test.ts
index 4a7bc426b7..1d1683ee7d 100644
--- a/packages/core/test/session-compaction.test.ts
+++ b/packages/core/test/session-compaction.test.ts
@@ -19,11 +19,12 @@ import { SessionV2 } from "@opencode-ai/core/session"
 import { Project } from "@opencode-ai/core/project"
 import { ProjectTable } from "@opencode-ai/core/project/sql"
 import { AbsolutePath } from "@opencode-ai/core/schema"
-import { DateTime, Effect, Layer, Stream } from "effect"
+import { DateTime, Effect, Fiber, Layer, Stream } from "effect"
 import { asc, eq } from "drizzle-orm"
 import { testEffect } from "./lib/effect"
 
 let requests: LLMRequest[] = []
+let summaryText = "manual summary"
 const model = Model.make({
   id: "summary-model",
   provider: "test",
@@ -33,7 +34,7 @@ const client = Layer.mock(LLMClient.Service)({
   prepare: () => Effect.die("unused"),
   stream: (request: LLMRequest) => {
     requests.push(request)
-    return Stream.make(LLMEvent.textDelta({ id: "summary", text: "manual summary" }))
+    return Stream.make(LLMEvent.textDelta({ id: "summary", text: summaryText }))
   },
   generate: () => Effect.die("unused"),
 })
@@ -66,9 +67,81 @@ test("compaction describes tool media without embedding base64", () => {
   expect(serialized).not.toContain(base64)
 })
 
+it.effect("manual compaction failure publishes a terminal failed event", () =>
+  Effect.scoped(
+    Effect.gen(function* () {
+      requests = []
+      summaryText = ""
+      const db = (yield* Database.Service).db
+      const compaction = yield* SessionCompaction.Service
+      const store = yield* SessionStore.Service
+      const events = yield* EventV2.Service
+      const sessionID = SessionV2.ID.make("ses_failed_compaction")
+      yield* db
+        .insert(ProjectTable)
+        .values({ id: Project.ID.global, worktree: AbsolutePath.make("/project"), sandboxes: [] })
+        .onConflictDoNothing()
+        .run()
+        .pipe(Effect.orDie)
+      yield* db
+        .insert(SessionTable)
+        .values({
+          id: sessionID,
+          project_id: Project.ID.global,
+          slug: "failed-compaction",
+          directory: "/project",
+          title: "Failed compaction",
+          version: "test",
+        })
+        .run()
+        .pipe(Effect.orDie)
+      const failed = yield* events
+        .subscribe(SessionEvent.Compaction.Failed)
+        .pipe(Stream.take(1), Stream.runCollect, Effect.forkScoped)
+      yield* Effect.yieldNow
+
+      const session = yield* store
+        .get(sessionID)
+        .pipe(
+          Effect.flatMap((session) =>
+            session ? Effect.succeed(session) : Effect.die("failed compaction test session missing"),
+          ),
+        )
+      expect(
+        yield* compaction.compactManual({
+          session,
+          messages: [
+            {
+              id: SessionMessage.ID.create(),
+              type: "user" as const,
+              text: "Summarization for this conversation will fail.",
+              time: { created: DateTime.makeUnsafe(0) },
+            },
+          ],
+        }),
+      ).toBe(false)
+
+      expect(Array.from(yield* Fiber.join(failed))[0]?.data).toMatchObject({ sessionID, reason: "manual" })
+      expect(yield* store.context(sessionID)).toEqual([])
+      // The failed event is live-only; only Started is durably recorded.
+      expect(
+        yield* db
+          .select({ type: EventTable.type })
+          .from(EventTable)
+          .where(eq(EventTable.aggregate_id, sessionID))
+          .orderBy(asc(EventTable.seq))
+          .all()
+          .pipe(Effect.orDie),
+      ).toEqual([{ type: EventV2.versionedType(SessionEvent.Compaction.Started.type, 1) }])
+      summaryText = "manual summary"
+    }),
+  ),
+)
+
 it.effect("manual compaction summarizes short context instead of no-op", () =>
   Effect.gen(function* () {
     requests = []
+    summaryText = "manual summary"
     const db = (yield* Database.Service).db
     const compaction = yield* SessionCompaction.Service
     const store = yield* SessionStore.Service
diff --git a/packages/schema/src/session-event.ts b/packages/schema/src/session-event.ts
index 5013dddee9..ad4882694b 100644
--- a/packages/schema/src/session-event.ts
+++ b/packages/schema/src/session-event.ts
@@ -466,6 +466,18 @@ export namespace Compaction {
     },
   })
   export type Ended = typeof Ended.Type
+
+  // Live-only: a failed compaction produces no durable message, but clients that
+  // reacted to Started need a terminal signal.
+  export const Failed = Event.define({
+    type: "session.next.compaction.failed",
+    schema: {
+      ...Base,
+      messageID: SessionMessage.ID,
+      reason: Started.data.fields.reason,
+    },
+  })
+  export type Failed = typeof Failed.Type
 }
 
 export namespace RevertEvent {
@@ -549,6 +561,7 @@ export const Definitions = Event.inventory(
   Compaction.Started,
   Compaction.Delta,
   Compaction.Ended,
+  Compaction.Failed,
   RevertEvent.Staged,
   RevertEvent.Cleared,
   RevertEvent.Committed,
diff --git a/packages/schema/test/event-manifest.test.ts b/packages/schema/test/event-manifest.test.ts
index 5243fb69e8..88fb646b59 100644
--- a/packages/schema/test/event-manifest.test.ts
+++ b/packages/schema/test/event-manifest.test.ts
@@ -9,11 +9,11 @@ import { WorkspaceEvent } from "../src/workspace-event.js"
 
 describe("public event manifest", () => {
   test("owns the complete public event surface", () => {
-    expect(EventManifest.ServerDefinitions.filter((definition) => definition.type !== "agent.updated").length).toBe(63)
+    expect(EventManifest.ServerDefinitions.filter((definition) => definition.type !== "agent.updated").length).toBe(66)
     expect(EventManifest.ServerDefinitions.filter((definition) => definition.type === "agent.updated")).toEqual([
       Agent.Event.Updated,
     ])
-    expect(EventManifest.Definitions.filter((definition) => definition.type !== "agent.updated").length).toBe(93)
+    expect(EventManifest.Definitions.filter((definition) => definition.type !== "agent.updated").length).toBe(97)
     expect(EventManifest.Definitions.filter((definition) => definition.type === "agent.updated")).toEqual([
       Agent.Event.Updated,
     ])
@@ -29,7 +29,7 @@ describe("public event manifest", () => {
       SessionV1.Event.Diff,
       SessionV1.Event.Error,
     ])
-    expect(Array.from(EventManifest.Latest.keys()).filter((type) => type !== "agent.updated").length).toBe(93)
+    expect(Array.from(EventManifest.Latest.keys()).filter((type) => type !== "agent.updated").length).toBe(97)
     expect(EventManifest.Latest.get("agent.updated")).toBe(Agent.Event.Updated)
     expect(Agent.Event.Updated.durable).toBeUndefined()
     expect(EventManifest.Durable.has("agent.updated")).toBe(false)
diff --git a/packages/sdk/js/src/v2/gen/sdk.gen.ts b/packages/sdk/js/src/v2/gen/sdk.gen.ts
index 145ae005b3..1a82fca91c 100644
--- a/packages/sdk/js/src/v2/gen/sdk.gen.ts
+++ b/packages/sdk/js/src/v2/gen/sdk.gen.ts
@@ -399,6 +399,8 @@ import type {
   V2SessionSwitchAgentResponses,
   V2SessionSwitchModelErrors,
   V2SessionSwitchModelResponses,
+  V2SessionSyntheticErrors,
+  V2SessionSyntheticResponses,
   V2SessionWaitErrors,
   V2SessionWaitResponses,
   V2ShellCreateErrors,
@@ -5816,6 +5818,47 @@ export class Session3 extends HeyApiClient {
     })
   }
 
+  /**
+   * Add synthetic message
+   *
+   * Append a synthetic message to a session and resume execution.
+   */
+  public synthetic<ThrowOnError extends boolean = false>(
+    parameters: {
+      sessionID: string
+      text?: string
+      description?: string
+      metadata?: {
+        [key: string]: unknown
+      }
+    },
+    options?: Options<never, ThrowOnError>,
+  ) {
+    const params = buildClientParams(
+      [parameters],
+      [
+        {
+          args: [
+            { in: "path", key: "sessionID" },
+            { in: "body", key: "text" },
+            { in: "body", key: "description" },
+            { in: "body", key: "metadata" },
+          ],
+        },
+      ],
+    )
+    return (options?.client ?? this.client).post<V2SessionSyntheticResponses, V2SessionSyntheticErrors, ThrowOnError>({
+      url: "/api/session/{sessionID}/synthetic",
+      ...options,
+      ...params,
+      headers: {
+        "Content-Type": "application/json",
+        ...options?.headers,
+        ...params.headers,
+      },
+    })
+  }
+
   /**
    * Compact session
    *
diff --git a/packages/sdk/js/src/v2/gen/types.gen.ts b/packages/sdk/js/src/v2/gen/types.gen.ts
index 55eac01f48..9b32479a22 100644
--- a/packages/sdk/js/src/v2/gen/types.gen.ts
+++ b/packages/sdk/js/src/v2/gen/types.gen.ts
@@ -49,6 +49,7 @@ export type Event =
   | EventSessionNextCompactionStarted
   | EventSessionNextCompactionDelta
   | EventSessionNextCompactionEnded
+  | EventSessionNextCompactionFailed
   | EventSessionNextRevertStaged
   | EventSessionNextRevertCleared
   | EventSessionNextRevertCommitted
@@ -942,6 +943,9 @@ export type GlobalEvent = {
           messageID: string
           text: string
           description?: string
+          metadata?: {
+            [key: string]: unknown
+          }
         }
       }
     | {
@@ -1229,6 +1233,16 @@ export type GlobalEvent = {
           recent: string
         }
       }
+    | {
+        id: string
+        type: "session.next.compaction.failed"
+        properties: {
+          timestamp: number
+          sessionID: string
+          messageID: string
+          reason: "auto" | "manual"
+        }
+      }
     | {
         id: string
         type: "session.next.revert.staged"
@@ -3032,6 +3046,7 @@ export type V2Event =
   | SessionNextCompactionStarted
   | SessionNextCompactionDelta
   | SessionNextCompactionEnded
+  | SessionNextCompactionFailed
   | SessionNextRevertStaged
   | SessionNextRevertCleared
   | SessionNextRevertCommitted
@@ -3618,6 +3633,9 @@ export type SyncEventSessionNextSynthetic = {
       messageID: string
       text: string
       description?: string
+      metadata?: {
+        [key: string]: unknown
+      }
     }
   }
 }
@@ -4583,6 +4601,9 @@ export type SessionNextSynthetic = {
     messageID: string
     text: string
     description?: string
+    metadata?: {
+      [key: string]: unknown
+    }
   }
 }
 
@@ -5664,6 +5685,26 @@ export type SessionNextCompactionDelta = {
   }
 }
 
+export type SessionNextCompactionFailed = {
+  id: string
+  metadata?: {
+    [key: string]: unknown
+  }
+  type: "session.next.compaction.failed"
+  durable?: {
+    aggregateID: string
+    seq: number
+    version: number
+  }
+  location?: LocationRef
+  data: {
+    timestamp: number
+    sessionID: string
+    messageID: string
+    reason: "auto" | "manual"
+  }
+}
+
 export type MessagePartDelta = {
   id: string
   metadata?: {
@@ -6806,6 +6847,9 @@ export type EventSessionNextSynthetic = {
     messageID: string
     text: string
     description?: string
+    metadata?: {
+      [key: string]: unknown
+    }
   }
 }
 
@@ -7117,6 +7161,17 @@ export type EventSessionNextCompactionEnded = {
   }
 }
 
+export type EventSessionNextCompactionFailed = {
+  id: string
+  type: "session.next.compaction.failed"
+  properties: {
+    timestamp: number
+    sessionID: string
+    messageID: string
+    reason: "auto" | "manual"
+  }
+}
+
 export type EventSessionNextRevertStaged = {
   id: string
   type: "session.next.revert.staged"
@@ -12284,6 +12339,47 @@ export type V2SessionSkillResponses = {
 
 export type V2SessionSkillResponse = V2SessionSkillResponses[keyof V2SessionSkillResponses]
 
+export type V2SessionSyntheticData = {
+  body: {
+    text: string
+    description?: string
+    metadata?: {
+      [key: string]: unknown
+    }
+  }
+  path: {
+    sessionID: string
+  }
+  query?: never
+  url: "/api/session/{sessionID}/synthetic"
+}
+
+export type V2SessionSyntheticErrors = {
+  /**
+   * InvalidRequestError
+   */
+  400: InvalidRequestError
+  /**
+   * UnauthorizedError
+   */
+  401: UnauthorizedError
+  /**
+   * SessionNotFoundError
+   */
+  404: SessionNotFoundError
+}
+
+export type V2SessionSyntheticError = V2SessionSyntheticErrors[keyof V2SessionSyntheticErrors]
+
+export type V2SessionSyntheticResponses = {
+  /**
+   * <No Content>
+   */
+  204: void
+}
+
+export type V2SessionSyntheticResponse = V2SessionSyntheticResponses[keyof V2SessionSyntheticResponses]
+
 export type V2SessionCompactData = {
   body?: never
   path: {
diff --git a/packages/tui/src/context/data.tsx b/packages/tui/src/context/data.tsx
index 88e3649e63..c1c58fb1a4 100644
--- a/packages/tui/src/context/data.tsx
+++ b/packages/tui/src/context/data.tsx
@@ -535,7 +535,14 @@ export const { use: useData, provider: DataProvider } = createSimpleContext({
           break
         case "session.next.compaction.delta":
           break
+        case "session.next.compaction.failed":
+          // Started marked the session running; without this a failed compaction
+          // leaves the session spinning forever. Mid-run auto compaction is
+          // followed by step events that set the status again.
+          setStore("session", "status", event.data.sessionID, "idle")
+          break
         case "session.next.compaction.ended":
+          setStore("session", "status", event.data.sessionID, "idle")
           message.update(event.data.sessionID, (draft, index) => {
             message.append(draft, index, {
               id: event.data.messageID,
diff --git a/packages/tui/test/cli/tui/data.test.tsx b/packages/tui/test/cli/tui/data.test.tsx
index ed4f407993..e7a2bd98db 100644
--- a/packages/tui/test/cli/tui/data.test.tsx
+++ b/packages/tui/test/cli/tui/data.test.tsx
@@ -248,6 +248,41 @@ test("tracks session status from active sessions and execution events", async ()
       },
     })
     await wait(() => data.session.status("session-failed") === "idle")
+
+    emitEvent(events, {
+      id: "evt_compaction_started",
+      type: "session.next.compaction.started",
+      data: { sessionID: "session-compacting", messageID: "message-compaction", timestamp: 5, reason: "manual" },
+    })
+    await wait(() => data.session.status("session-compacting") === "running")
+
+    emitEvent(events, {
+      id: "evt_compaction_ended",
+      type: "session.next.compaction.ended",
+      data: {
+        sessionID: "session-compacting",
+        messageID: "message-compaction",
+        timestamp: 6,
+        reason: "manual",
+        text: "summary",
+        recent: "",
+      },
+    })
+    await wait(() => data.session.status("session-compacting") === "idle")
+
+    emitEvent(events, {
+      id: "evt_compaction_started_again",
+      type: "session.next.compaction.started",
+      data: { sessionID: "session-compacting", messageID: "message-compaction-2", timestamp: 7, reason: "manual" },
+    })
+    await wait(() => data.session.status("session-compacting") === "running")
+
+    emitEvent(events, {
+      id: "evt_compaction_failed",
+      type: "session.next.compaction.failed",
+      data: { sessionID: "session-compacting", messageID: "message-compaction-2", timestamp: 8, reason: "manual" },
+    })
+    await wait(() => data.session.status("session-compacting") === "idle")
   } finally {
     app.renderer.destroy()
   }