diff --git a/packages/opencode/src/cli/cmd/tui/routes/session/sidebar.tsx b/packages/opencode/src/cli/cmd/tui/routes/session/sidebar.tsx
index 4ffe91558ed..1bfe10beeed 100644
--- a/packages/opencode/src/cli/cmd/tui/routes/session/sidebar.tsx
+++ b/packages/opencode/src/cli/cmd/tui/routes/session/sidebar.tsx
@@ -41,7 +41,7 @@ export function Sidebar(props: { sessionID: string; overlay?: boolean }) {
   )
 
   const cost = createMemo(() => {
-    const total = messages().reduce((sum, x) => sum + (x.role === "assistant" ? x.cost : 0), 0)
+    const total = session().cost ?? messages().reduce((sum, x) => sum + (x.role === "assistant" ? x.cost : 0), 0)
     return new Intl.NumberFormat("en-US", {
       style: "currency",
       currency: "USD",
diff --git a/packages/opencode/src/session/index.ts b/packages/opencode/src/session/index.ts
index 87cf3a0820a..99ac112f93d 100644
--- a/packages/opencode/src/session/index.ts
+++ b/packages/opencode/src/session/index.ts
@@ -71,6 +71,7 @@ export namespace Session {
         .optional(),
       title: z.string(),
       version: z.string(),
+      cost: z.number().optional(),
       time: z.object({
         created: z.number(),
         updated: z.number(),
@@ -218,6 +219,7 @@ export namespace Session {
       directory: input.directory,
       parentID: input.parentID,
       title: input.title ?? createDefaultTitle(!!input.parentID),
+      cost: 0,
       permission: input.permission,
       time: {
         created: Date.now(),
@@ -262,6 +264,11 @@ export namespace Session {
     return Storage.read<ShareInfo>(["share", id])
   })
 
+  export const getCost = fn(Identifier.schema("session"), async (id) => {
+    const read = await Storage.read<Info>(["session", Instance.project.id, id])
+    return read.cost
+  })
+
   export const share = fn(Identifier.schema("session"), async (id) => {
     const cfg = await Config.get()
     if (cfg.share === "disabled") {
@@ -308,6 +315,13 @@ export namespace Session {
     return result
   }
 
+  export async function addCost(sessionID: string, amount: number) {
+    if (amount === 0) return
+    await update(sessionID, (draft) => {
+      draft.cost = (draft.cost ?? 0) + amount
+    })
+  }
+
   export const diff = fn(Identifier.schema("session"), async (sessionID) => {
     const diffs = await Storage.read<Snapshot.FileDiff[]>(["session_diff", sessionID])
     return diffs ?? []
diff --git a/packages/opencode/src/session/processor.ts b/packages/opencode/src/session/processor.ts
index 27071056180..7f823c6aece 100644
--- a/packages/opencode/src/session/processor.ts
+++ b/packages/opencode/src/session/processor.ts
@@ -253,6 +253,7 @@ export namespace SessionProcessor {
                     cost: usage.cost,
                   })
                   await Session.updateMessage(input.assistantMessage)
+                  await Session.addCost(input.sessionID, usage.cost)
                   if (snapshot) {
                     const patch = await Snapshot.patch(snapshot)
                     if (patch.files.length) {
diff --git a/packages/opencode/src/storage/storage.ts b/packages/opencode/src/storage/storage.ts
index 18f2d67e7ac..7d4dd006c96 100644
--- a/packages/opencode/src/storage/storage.ts
+++ b/packages/opencode/src/storage/storage.ts
@@ -139,6 +139,53 @@ export namespace Storage {
         )
       }
     },
+    async (dir) => {
+      log.info("migrating session costs")
+      const startTime = Date.now()
+      let migratedCount = 0
+
+      for await (const sessionPath of new Bun.Glob("session/*/*.json").scan({
+        cwd: dir,
+        absolute: true,
+      })) {
+        const session = await Bun.file(sessionPath).json()
+        if (!session) continue
+        if (session.cost != null) continue
+
+        let totalCost = 0
+        const messageDir = path.join(dir, "message", session.id)
+
+        const stats = await fs.stat(messageDir).catch(() => undefined)
+        if (stats?.isDirectory()) {
+          for await (const messagePath of new Bun.Glob("*.json").scan({
+            cwd: messageDir,
+            absolute: true,
+          })) {
+            const message = await Bun.file(messagePath).json()
+            if (message.role !== "assistant") continue
+            totalCost += message.cost ?? 0
+          }
+        }
+
+        await Bun.file(sessionPath).write(
+          JSON.stringify(
+            {
+              ...session,
+              cost: totalCost,
+            },
+            null,
+            2,
+          ),
+        )
+
+        migratedCount++
+      }
+
+      log.info("cost migration complete", {
+        sessionsMigrated: migratedCount,
+        elapsedMs: Date.now() - startTime,
+      })
+    },
   ]
 
   const state = lazy(async () => {
diff --git a/packages/sdk/js/src/v2/gen/types.gen.ts b/packages/sdk/js/src/v2/gen/types.gen.ts
index cb2f586775a..d95edbc2d7a 100644
--- a/packages/sdk/js/src/v2/gen/types.gen.ts
+++ b/packages/sdk/js/src/v2/gen/types.gen.ts
@@ -770,6 +770,7 @@ export type Session = {
   }
   title: string
   version: string
+  cost?: number
   time: {
     created: number
     updated: number
diff --git a/packages/sdk/openapi.json b/packages/sdk/openapi.json
index 4be0a87f980..9da187b7088 100644
--- a/packages/sdk/openapi.json
+++ b/packages/sdk/openapi.json
@@ -7936,6 +7936,9 @@
           "version": {
             "type": "string"
           },
+          "cost": {
+            "type": "number"
+          },
           "time": {
             "type": "object",
             "properties": {