diff --git a/packages/client/src/effect/api/api.ts b/packages/client/src/effect/api/api.ts
index 38758b0a7997..5ca4c5463bd2 100644
--- a/packages/client/src/effect/api/api.ts
+++ b/packages/client/src/effect/api/api.ts
@@ -986,6 +986,15 @@ export type SessionLogOutput =
               | undefined
             readonly text: string
             readonly recent: string
+            readonly cost?: (number & Brand.Brand<"Money.USD">) | undefined
+            readonly tokens?:
+              | {
+                  readonly input: number
+                  readonly output: number
+                  readonly reasoning: number
+                  readonly cache: { readonly read: number; readonly write: number }
+                }
+              | undefined
           }
         }
       | {
@@ -1000,6 +1009,15 @@ export type SessionLogOutput =
             readonly reason: "auto" | "manual"
             readonly error: { readonly type: string; readonly message: string; readonly status?: number | undefined }
             readonly inputID?: SessionMessage.ID | undefined
+            readonly cost?: (number & Brand.Brand<"Money.USD">) | undefined
+            readonly tokens?:
+              | {
+                  readonly input: number
+                  readonly output: number
+                  readonly reasoning: number
+                  readonly cache: { readonly read: number; readonly write: number }
+                }
+              | undefined
           }
         }
       | {
diff --git a/packages/client/src/promise/generated/types.ts b/packages/client/src/promise/generated/types.ts
index 7f8c229e9ff8..fac9f7659cda 100644
--- a/packages/client/src/promise/generated/types.ts
+++ b/packages/client/src/promise/generated/types.ts
@@ -523,6 +523,8 @@ export type SessionMessageCompactionFailed = {
   status: "failed"
   reason: "auto" | "manual"
   error: SessionStructuredError
+  cost?: MoneyUSD
+  tokens?: TokenUsageInfo
 }
 
 export type SessionProviderContext = { version: 1; provenance: SessionProviderContextProvenance; messages: JsonValue }
@@ -808,7 +810,14 @@ export type SessionCompactionFailed = {
   type: "session.compaction.failed"
   durable: { aggregateID: string; seq: number; version: 1 }
   location?: LocationRef
-  data: { sessionID: string; reason: "auto" | "manual"; error: SessionStructuredError; inputID?: string }
+  data: {
+    sessionID: string
+    reason: "auto" | "manual"
+    error: SessionStructuredError
+    inputID?: string
+    cost?: MoneyUSD
+    tokens?: TokenUsageInfo
+  }
 }
 
 export type SessionRevertCleared = {
@@ -1738,6 +1747,8 @@ export type SessionMessageCompactionCompleted = {
   summary: string
   recent: string
   providerContext?: SessionProviderContext
+  cost?: MoneyUSD
+  tokens?: TokenUsageInfo
 }
 
 export type SessionCompactionEnded = {
@@ -1755,6 +1766,8 @@ export type SessionCompactionEnded = {
     providerContext?: SessionProviderContext
     text: string
     recent: string
+    cost?: MoneyUSD
+    tokens?: TokenUsageInfo
   }
 }
 
@@ -3111,6 +3124,13 @@ export type SessionImportInput = {
                 }
                 readonly messages: JsonValue
               }
+              readonly cost?: number
+              readonly tokens?: {
+                readonly input: number
+                readonly output: number
+                readonly reasoning: number
+                readonly cache: { readonly read: number; readonly write: number }
+              }
             }
           | {
               readonly type: "compaction"
@@ -3120,6 +3140,13 @@ export type SessionImportInput = {
               readonly status: "failed"
               readonly reason: "auto" | "manual"
               readonly error: { readonly type: string; readonly message: string; readonly status?: number }
+              readonly cost?: number
+              readonly tokens?: {
+                readonly input: number
+                readonly output: number
+                readonly reasoning: number
+                readonly cache: { readonly read: number; readonly write: number }
+              }
             }
         )
     >
@@ -3402,6 +3429,13 @@ export type SessionImportInput = {
                 }
                 readonly messages: JsonValue
               }
+              readonly cost?: number
+              readonly tokens?: {
+                readonly input: number
+                readonly output: number
+                readonly reasoning: number
+                readonly cache: { readonly read: number; readonly write: number }
+              }
             }
           | {
               readonly type: "compaction"
@@ -3411,6 +3445,13 @@ export type SessionImportInput = {
               readonly status: "failed"
               readonly reason: "auto" | "manual"
               readonly error: { readonly type: string; readonly message: string; readonly status?: number }
+              readonly cost?: number
+              readonly tokens?: {
+                readonly input: number
+                readonly output: number
+                readonly reasoning: number
+                readonly cache: { readonly read: number; readonly write: number }
+              }
             }
         )
     >
@@ -3693,6 +3734,13 @@ export type SessionImportInput = {
                 }
                 readonly messages: JsonValue
               }
+              readonly cost?: number
+              readonly tokens?: {
+                readonly input: number
+                readonly output: number
+                readonly reasoning: number
+                readonly cache: { readonly read: number; readonly write: number }
+              }
             }
           | {
               readonly type: "compaction"
@@ -3702,6 +3750,13 @@ export type SessionImportInput = {
               readonly status: "failed"
               readonly reason: "auto" | "manual"
               readonly error: { readonly type: string; readonly message: string; readonly status?: number }
+              readonly cost?: number
+              readonly tokens?: {
+                readonly input: number
+                readonly output: number
+                readonly reasoning: number
+                readonly cache: { readonly read: number; readonly write: number }
+              }
             }
         )
     >
diff --git a/packages/client/src/solid/data.ts b/packages/client/src/solid/data.ts
index 7fb8d710fbb2..7c08fe73a251 100644
--- a/packages/client/src/solid/data.ts
+++ b/packages/client/src/solid/data.ts
@@ -1083,8 +1083,11 @@ export function createData(config: CreateDataInput) {
               reason: event.data.reason,
               model: event.data.model,
               providerState: event.data.providerState,
+              providerContext: event.data.providerContext,
               summary: event.data.text,
               recent: event.data.recent,
+              cost: event.data.cost,
+              tokens: event.data.tokens,
             })
             return
           }
@@ -1095,8 +1098,11 @@ export function createData(config: CreateDataInput) {
             reason: event.data.reason,
             model: event.data.model,
             providerState: event.data.providerState,
+            providerContext: event.data.providerContext,
             summary: event.data.text,
             recent: event.data.recent,
+            cost: event.data.cost,
+            tokens: event.data.tokens,
             time: { created: event.created },
           })
         })
@@ -1116,6 +1122,8 @@ export function createData(config: CreateDataInput) {
               message: "Compaction failed before recording an error",
             },
             metadata: current?.type === "compaction" ? current.metadata : event.metadata,
+            cost: event.data.cost,
+            tokens: event.data.tokens,
             time: current?.type === "compaction" ? current.time : { created: event.created },
           }
           if (current?.type === "compaction") {
diff --git a/packages/client/test/solid-compaction.test.ts b/packages/client/test/solid-compaction.test.ts
index f33596cc5641..1ea40d5d0476 100644
--- a/packages/client/test/solid-compaction.test.ts
+++ b/packages/client/test/solid-compaction.test.ts
@@ -100,13 +100,46 @@ test.each(["started", "cancelled", "failed"])(
       expect(fixture.data.session.message.list(sessionID)).toMatchObject([{ type: "compaction", status: "running" }])
       const model = { providerID: "demo", id: "model" }
       const providerState = { responseId: "summary-response" }
+      const tokens = { input: 10, output: 4, reasoning: 0, cache: { read: 3, write: 0 } }
+      const providerContext = {
+        version: 1 as const,
+        provenance: {
+          providerID: "demo",
+          provider: "demo",
+          modelID: "model",
+          route: "demo-responses",
+          protocol: "demo",
+          endpoint: "digest",
+        },
+        messages: [],
+      }
       fixture.emit({
         ...event,
         type: "session.compaction.ended",
-        data: { sessionID, reason: "manual", model, providerState, text: "Summary", recent: "Recent" },
+        data: {
+          sessionID,
+          reason: "manual",
+          model,
+          providerState,
+          providerContext,
+          text: "Summary",
+          recent: "Recent",
+          cost: 0.01,
+          tokens,
+        },
       })
+      // The live fold carries the provider window and request usage so the label matches a reloaded session.
       expect(fixture.data.session.message.list(sessionID)).toMatchObject([
-        { type: "compaction", status: "completed", summary: "Summary", model, providerState },
+        {
+          type: "compaction",
+          status: "completed",
+          summary: "Summary",
+          model,
+          providerState,
+          providerContext,
+          cost: 0.01,
+          tokens,
+        },
       ])
     }
   },
diff --git a/packages/core/src/session/compaction.ts b/packages/core/src/session/compaction.ts
index d3d44ee49656..22139a0eaccf 100644
--- a/packages/core/src/session/compaction.ts
+++ b/packages/core/src/session/compaction.ts
@@ -383,12 +383,7 @@ export const layer = Layer.effect(
         },
       }),
     })
-    const failed = Effect.fnUntraced(function* (input: {
-      readonly sessionID: SessionSchema.ID
-      readonly reason: SessionMessage.Compaction["reason"]
-      readonly error: SessionError.Error
-      readonly inputID?: SessionMessage.ID
-    }) {
+    const failed = Effect.fnUntraced(function* (input: SessionEvent.Compaction.Failed["data"]) {
       yield* bus.publish(SessionEvent.Compaction.Failed, input)
       return { status: "failed" as const, error: input.error }
     })
@@ -504,11 +499,12 @@ export const layer = Layer.effect(
               )
             }),
           )
-          if (result.usage)
+          const usage = result.usage ? SessionUsage.record(result.usage, context.model.cost) : undefined
+          if (usage)
             yield* bus.publish(SessionEvent.UsageRecorded, {
               sessionID: context.session.id,
               source: "compaction" as const,
-              ...SessionUsage.record(result.usage, context.model.cost),
+              ...usage,
             })
           yield* bus.publish(SessionEvent.Compaction.Ended, {
             sessionID: context.session.id,
@@ -517,6 +513,7 @@ export const layer = Layer.effect(
             text: "",
             recent: "",
             providerContext: SessionProviderContext.encode(provenance, result.replacement),
+            ...usage,
           })
           return { status: "completed" as const }
         }),
@@ -662,6 +659,7 @@ export const layer = Layer.effect(
           reason: input.reason,
           error,
           inputID: input.inputID,
+          ...usage,
         })
       }
       yield* bus.publish(SessionEvent.Compaction.Ended, {
@@ -671,6 +669,7 @@ export const layer = Layer.effect(
         providerState,
         text: summary,
         recent: history.recent,
+        ...usage,
       })
       return { status: "completed" as const }
     })
diff --git a/packages/core/src/session/message-updater.ts b/packages/core/src/session/message-updater.ts
index 8675690f33b2..0177c095a845 100644
--- a/packages/core/src/session/message-updater.ts
+++ b/packages/core/src/session/message-updater.ts
@@ -415,6 +415,8 @@ export function update(adapter: Adapter, event: SessionEvent.DurableEvent) {
               summary: event.data.text,
               providerContext: event.data.providerContext,
               recent: event.data.recent,
+              cost: event.data.cost,
+              tokens: event.data.tokens,
             })
             return
           }
@@ -430,6 +432,8 @@ export function update(adapter: Adapter, event: SessionEvent.DurableEvent) {
               summary: event.data.text,
               providerContext: event.data.providerContext,
               recent: event.data.recent,
+              cost: event.data.cost,
+              tokens: event.data.tokens,
               time: { created },
             }),
           )
@@ -444,6 +448,8 @@ export function update(adapter: Adapter, event: SessionEvent.DurableEvent) {
             metadata: current?.metadata ?? event.metadata,
             reason: event.data.reason,
             error: event.data.error,
+            cost: event.data.cost,
+            tokens: event.data.tokens,
             time: current?.time ?? { created },
           })
           if (current?.status === "running") return yield* adapter.updateCompaction(failed)
diff --git a/packages/core/test/session-compaction.test.ts b/packages/core/test/session-compaction.test.ts
index 5384eb99efe8..0d4c21991eb8 100644
--- a/packages/core/test/session-compaction.test.ts
+++ b/packages/core/test/session-compaction.test.ts
@@ -401,8 +401,16 @@ it.effect("manual compaction summarizes short context instead of no-op", () =>
     expect(JSON.stringify(requests[0]?.messages)).toContain("Use Effect services and generators.")
     expect(JSON.stringify(requests[0]?.messages)).toContain("User shell pwd completed: /project")
     expect(JSON.stringify(requests[0]?.messages)).not.toContain("display-only-output")
+    // The compaction message carries its own request usage so clients can show what compacting cost.
     expect(yield* store.context(sessionID)).toMatchObject([
-      { type: "compaction", reason: "manual", summary: "## Objective\n- manual summary", recent: "" },
+      {
+        type: "compaction",
+        reason: "manual",
+        summary: "## Objective\n- manual summary",
+        recent: "",
+        cost: 0.0000233,
+        tokens: { input: 10, output: 4, reasoning: 2, cache: { read: 3, write: 2 } },
+      },
     ])
     expect(yield* store.get(sessionID)).toMatchObject({
       cost: 0.0000233,
diff --git a/packages/core/test/session-native-compaction.test.ts b/packages/core/test/session-native-compaction.test.ts
index 7a68439fff1f..872a1212e557 100644
--- a/packages/core/test/session-native-compaction.test.ts
+++ b/packages/core/test/session-native-compaction.test.ts
@@ -240,6 +240,8 @@ const setup = Effect.fnUntraced(function* (endpoint = false) {
       return yield* Effect.die("Missing native checkpoint")
     expect(last.summary).toBe("")
     expect(last.recent).toBe("")
+    // Provider compaction has no summary, so the request usage is the only visible cost of the operation.
+    expect(last.tokens).toMatchObject({ input: 20, output: 4 })
     return last.providerContext
   })
   return {
diff --git a/packages/schema/src/session-event.ts b/packages/schema/src/session-event.ts
index d82ad08a0aa1..ca1b9a7ec381 100644
--- a/packages/schema/src/session-event.ts
+++ b/packages/schema/src/session-event.ts
@@ -590,6 +590,10 @@ export namespace Compaction {
       providerContext: SessionMessage.CompactionCompleted.fields.providerContext,
       text: Schema.String,
       recent: Schema.String,
+      // Repeats the internal `session.usage.recorded` figures: that event never reaches clients, and it
+      // stays the accounting source for session totals and stats.
+      cost: SessionMessage.CompactionCompleted.fields.cost,
+      tokens: SessionMessage.CompactionCompleted.fields.tokens,
     },
   })
   export type Ended = typeof Ended.Type
@@ -602,6 +606,8 @@ export namespace Compaction {
       reason: Started.data.fields.reason,
       error: SessionError.Error,
       inputID: SessionMessage.ID.pipe(optional),
+      cost: SessionMessage.CompactionFailed.fields.cost,
+      tokens: SessionMessage.CompactionFailed.fields.tokens,
     },
   })
   export type Failed = typeof Failed.Type
diff --git a/packages/schema/src/session-message.ts b/packages/schema/src/session-message.ts
index 6bd37eb51746..7e8ce0482ae9 100644
--- a/packages/schema/src/session-message.ts
+++ b/packages/schema/src/session-message.ts
@@ -237,6 +237,12 @@ export const Assistant = Schema.Struct({
 
 const CompactionBase = { type: Schema.tag("compaction"), ...Base }
 
+/** Usage of the compaction request itself, not the size of the resulting context. */
+const CompactionUsage = {
+  cost: Money.USD.pipe(optional),
+  tokens: TokenUsage.Info.pipe(optional),
+}
+
 export interface CompactionRunning extends Schema.Schema.Type<typeof CompactionRunning> {}
 export const CompactionRunning = Schema.Struct({
   ...CompactionBase,
@@ -256,6 +262,7 @@ export const CompactionCompleted = Schema.Struct({
   summary: Schema.String,
   recent: Schema.String,
   providerContext: SessionProviderContext.Info.pipe(optional),
+  ...CompactionUsage,
 }).annotate({ identifier: "Session.Message.Compaction.Completed" })
 
 export interface CompactionFailed extends Schema.Schema.Type<typeof CompactionFailed> {}
@@ -264,6 +271,7 @@ export const CompactionFailed = Schema.Struct({
   status: Schema.tag("failed"),
   reason: Schema.Literals(["auto", "manual"]),
   error: SessionError.Error,
+  ...CompactionUsage,
 }).annotate({ identifier: "Session.Message.Compaction.Failed" })
 
 export const Compaction = Schema.Union([CompactionRunning, CompactionCompleted, CompactionFailed]).pipe(
diff --git a/packages/session-ui/src/message/message-content.tsx b/packages/session-ui/src/message/message-content.tsx
index 149e0a60869a..8cc61e80a8c7 100644
--- a/packages/session-ui/src/message/message-content.tsx
+++ b/packages/session-ui/src/message/message-content.tsx
@@ -384,17 +384,38 @@ export function SessionCompactionMessage(props: { message: SessionMessageCompact
     if (props.message.status !== "failed" || props.message.error.type === "aborted") return ""
     return props.error
   }
+  const compact = createMemo(
+    () => new Intl.NumberFormat(i18n.locale(), { notation: "compact", maximumFractionDigits: 1 }),
+  )
+  // Usage of the compaction request itself; the resulting context size only shows on the next assistant step.
+  const usage = () => {
+    if (props.message.status === "running" || !props.message.tokens) return ""
+    const tokens = props.message.tokens
+    const input = tokens.input + tokens.cache.read + tokens.cache.write
+    const output = tokens.output + tokens.reasoning
+    if (input + output <= 0) return ""
+    return i18n.t("ui.messagePart.compaction.usage", {
+      input: compact().format(input),
+      output: compact().format(output),
+    })
+  }
+  const label = createMemo(() =>
+    [
+      i18n.t(
+        props.message.status === "completed" && props.message.providerContext
+          ? "ui.messagePart.providerCompaction"
+          : "ui.messagePart.compaction",
+      ),
+      usage(),
+    ]
+      .filter(Boolean)
+      .join(" · "),
+  )
 
   return (
     <div data-component="session-compaction-message">
       <div class="py-2">
-        <TimelineSeparator
-          label={i18n.t(
-            props.message.status === "completed" && props.message.providerContext
-              ? "ui.messagePart.providerCompaction"
-              : "ui.messagePart.compaction",
-          )}
-        />
+        <TimelineSeparator label={label()} />
       </div>
       <Show when={summary().trim()}>
         <div data-component="text-part" data-timeline-part-id={props.message.id}>
diff --git a/packages/session-ui/src/storybook/current-session-fixtures.ts b/packages/session-ui/src/storybook/current-session-fixtures.ts
index 8b52c4e0b62f..f29e37a461bb 100644
--- a/packages/session-ui/src/storybook/current-session-fixtures.ts
+++ b/packages/session-ui/src/storybook/current-session-fixtures.ts
@@ -998,6 +998,8 @@ export const compactionDocument = document([
     reason: "auto",
     summary: "The Session timeline now consumes current nested assistant content.",
     recent: "Add deterministic stories and verify Storybook.",
+    cost: 0.0142,
+    tokens: { input: 3_180, output: 412, reasoning: 96, cache: { read: 8_704, write: 0 } },
     time: { created: STORY_TIME + 63_000 },
   },
   assistant({
diff --git a/packages/tui/src/routes/session/index.tsx b/packages/tui/src/routes/session/index.tsx
index dd34e40459cf..f2fa83c68a5e 100644
--- a/packages/tui/src/routes/session/index.tsx
+++ b/packages/tui/src/routes/session/index.tsx
@@ -2098,6 +2098,15 @@ function CompactionMessage(props: { message: Extract<SessionMessageInfo, { type:
     props.message.status === "failed" ? (cancelled() ? "" : props.message.error.message) : props.message.summary
   const content = createMemo(() => text().trim())
   const color = () => (status() === "failed" && !cancelled() ? theme.text.feedback.error.default : theme.text.subdued)
+  // Usage of the compaction request itself; the resulting context size only shows on the next assistant step.
+  const usage = () => {
+    if (props.message.status === "running" || !props.message.tokens) return
+    const tokens = props.message.tokens
+    const input = tokens.input + tokens.cache.read + tokens.cache.write
+    const output = tokens.output + tokens.reasoning
+    if (input + output <= 0) return
+    return `${Locale.number(input)} in · ${Locale.number(output)} out`
+  }
   return (
     <box>
       <box flexDirection="row" alignItems="center">
@@ -2121,6 +2130,9 @@ function CompactionMessage(props: { message: Extract<SessionMessageInfo, { type:
           <Show when={cancelled()}>
             <text fg={color()}>· cancelled</text>
           </Show>
+          <Show when={usage()}>
+            <text fg={color()}>· {usage()}</text>
+          </Show>
         </box>
         <box border={["top"]} borderColor={color()} flexGrow={1} />
       </box>
diff --git a/packages/ui/src/i18n/en.ts b/packages/ui/src/i18n/en.ts
index 24e180694ac6..0f64567f9ecc 100644
--- a/packages/ui/src/i18n/en.ts
+++ b/packages/ui/src/i18n/en.ts
@@ -105,6 +105,7 @@ const source = {
   "ui.messagePart.questions.dismissed": "Questions dismissed",
   "ui.messagePart.compaction": "Session compacted",
   "ui.messagePart.providerCompaction": "Session compacted by provider",
+  "ui.messagePart.compaction.usage": "{{input}} in · {{output}} out",
   "ui.messagePart.context.details": "Details",
   "ui.messagePart.context.read.one": "{{count}} read",
   "ui.messagePart.context.read.other": "{{count}} reads",