diff --git a/packages/opencode/src/server/routes/instance/httpapi/groups/session.ts b/packages/opencode/src/server/routes/instance/httpapi/groups/session.ts
index bc26a9e597..e1e93b7d7c 100644
--- a/packages/opencode/src/server/routes/instance/httpapi/groups/session.ts
+++ b/packages/opencode/src/server/routes/instance/httpapi/groups/session.ts
@@ -10,7 +10,6 @@ import { SessionSummary } from "@/session/summary"
 import { Todo } from "@/session/todo"
 import { MessageID, PartID, SessionID } from "@/session/schema"
 import { Snapshot } from "@/snapshot"
-import { NonNegativeInt } from "@/util/schema"
 import { Schema, SchemaGetter, Struct } from "effect"
 import { HttpApi, HttpApiEndpoint, HttpApiError, HttpApiGroup, HttpApiSchema, OpenApi } from "effect/unstable/httpapi"
 import { Authorization } from "../middleware/authorization"
@@ -45,7 +44,7 @@ export const UpdatePayload = Schema.Struct({
   permission: Schema.optional(Permission.Ruleset),
   time: Schema.optional(
     Schema.Struct({
-      archived: Schema.optional(NonNegativeInt),
+      archived: Schema.optional(Schema.Number),
     }),
   ),
 })
diff --git a/packages/opencode/src/session/session.ts b/packages/opencode/src/session/session.ts
index 1be5dfffd4..a3db293ef3 100644
--- a/packages/opencode/src/session/session.ts
+++ b/packages/opencode/src/session/session.ts
@@ -146,7 +146,7 @@ const Time = Schema.Struct({
   created: NonNegativeInt,
   updated: NonNegativeInt,
   compacting: optionalOmitUndefined(NonNegativeInt),
-  archived: optionalOmitUndefined(NonNegativeInt),
+  archived: optionalOmitUndefined(Schema.Number),
 })
 
 const Revert = Schema.Struct({
@@ -215,7 +215,7 @@ export const SetTitleInput = Schema.Struct({ sessionID: SessionID, title: Schema
 )
 export const SetArchivedInput = Schema.Struct({
   sessionID: SessionID,
-  time: Schema.optional(NonNegativeInt),
+  time: Schema.optional(Schema.Number),
 }).pipe(withStatics((s) => ({ zod: zod(s) })))
 export const SetPermissionInput = Schema.Struct({
   sessionID: SessionID,
@@ -244,7 +244,7 @@ const UpdatedTime = Schema.Struct({
   created: Schema.optional(Schema.NullOr(NonNegativeInt)),
   updated: Schema.optional(Schema.NullOr(NonNegativeInt)),
   compacting: Schema.optional(Schema.NullOr(NonNegativeInt)),
-  archived: Schema.optional(Schema.NullOr(NonNegativeInt)),
+  archived: Schema.optional(Schema.NullOr(Schema.Number)),
 })
 
 const UpdatedInfo = Schema.Struct({
diff --git a/packages/opencode/test/server/httpapi-session.test.ts b/packages/opencode/test/server/httpapi-session.test.ts
index 593f9765c7..8e1987cedf 100644
--- a/packages/opencode/test/server/httpapi-session.test.ts
+++ b/packages/opencode/test/server/httpapi-session.test.ts
@@ -18,9 +18,9 @@ void Log.init({ print: false })
 
 const original = Flag.OPENCODE_EXPERIMENTAL_HTTPAPI
 
-function app() {
-  Flag.OPENCODE_EXPERIMENTAL_HTTPAPI = true
-  return Server.Default().app
+function app(experimental = true) {
+  Flag.OPENCODE_EXPERIMENTAL_HTTPAPI = experimental
+  return experimental ? Server.Default().app : Server.Legacy().app
 }
 
 function runSession<A, E>(fx: Effect.Effect<A, E, Session.Service>) {
@@ -76,6 +76,10 @@ function request(path: string, init?: RequestInit) {
   return Effect.promise(async () => app().request(path, init))
 }
 
+function requestWithBackend(experimental: boolean, path: string, init?: RequestInit) {
+  return Effect.promise(async () => app(experimental).request(path, init))
+}
+
 function json<T>(response: Response) {
   return Effect.promise(async () => {
     if (response.status !== 200) throw new Error(await response.text())
@@ -217,6 +221,34 @@ describe("session HttpApi", () => {
     ),
   )
 
+  it.live(
+    "matches legacy archived timestamp validation",
+    withTmp({ git: true, config: { formatter: false, lsp: false } }, (tmp) =>
+      Effect.gen(function* () {
+        const headers = { "x-opencode-directory": tmp.path, "content-type": "application/json" }
+        const legacy = yield* createSession(tmp.path, { title: "legacy" })
+        const effect = yield* createSession(tmp.path, { title: "effect" })
+        const body = JSON.stringify({ time: { archived: -1 } })
+
+        const legacyResponse = yield* requestWithBackend(false, pathFor(SessionPaths.update, { sessionID: legacy.id }), {
+          method: "PATCH",
+          headers,
+          body,
+        })
+        expect(legacyResponse.status).toBe(200)
+        expect((yield* json<Session.Info>(legacyResponse)).time.archived).toBe(-1)
+
+        const effectResponse = yield* requestWithBackend(true, pathFor(SessionPaths.update, { sessionID: effect.id }), {
+          method: "PATCH",
+          headers,
+          body,
+        })
+        expect(effectResponse.status).toBe(legacyResponse.status)
+        expect((yield* json<Session.Info>(effectResponse)).time.archived).toBe(-1)
+      }),
+    ),
+  )
+
   it.live(
     "serves message mutation routes through Hono bridge",
     withTmp({ git: true, config: { formatter: false, lsp: false } }, (tmp) =>