diff --git a/packages/cli/src/services/service-config.ts b/packages/cli/src/services/service-config.ts
index 57c3102993..5c643437db 100644
--- a/packages/cli/src/services/service-config.ts
+++ b/packages/cli/src/services/service-config.ts
@@ -5,6 +5,7 @@ import { Service } from "@opencode-ai/client/effect/service"
 import { Effect, FileSystem, Option, Schema } from "effect"
 import { randomBytes } from "crypto"
 import path from "path"
+import semver from "semver"
 import { selfCommand } from "../util/process"
 
 // The CLI's service configuration file, plus the Service.EnsureOptions binding that
@@ -104,10 +105,21 @@ export const options = Effect.fnUntraced(function* () {
   return {
     file,
     version: OPENCODE_VERSION,
+    canReplace: (version: string | undefined) => canReplaceVersion(version),
     command: [...selfCommand(), "serve", "--service"],
   }
 })
 
+export function canReplaceVersion(serverVersion: string | undefined, clientVersion = OPENCODE_VERSION) {
+  if (serverVersion === undefined) return true
+  // Preview versions end in `<channel>-<build>[.<attempt>]`. Convert the build
+  // to a numeric semver identifier so next-15000 sorts after next-9999.
+  const server = serverVersion.replace(/-(\d+)(?=(?:\.\d+)?$)/, ".$1")
+  const client = clientVersion.replace(/-(\d+)(?=(?:\.\d+)?$)/, ".$1")
+  if (!semver.valid(server) || !semver.valid(client)) return true
+  return semver.lt(server, client)
+}
+
 export const read = Effect.fn("cli.service-config.read")(function* () {
   const { fs, configFile, legacyConfigFile } = yield* paths
   if (legacyConfigFile) yield* migrateConfig(legacyConfigFile, configFile)
diff --git a/packages/cli/test/service.test.ts b/packages/cli/test/service.test.ts
index 3e29d6e443..a1d3f97509 100644
--- a/packages/cli/test/service.test.ts
+++ b/packages/cli/test/service.test.ts
@@ -47,6 +47,15 @@ test("service filenames share release channels and identify preview channels", (
   expect(ServiceConfig.versionBelongsToChannel("1.2.3", "preview-a")).toBe(false)
 })
 
+test("only newer clients replace managed service versions", () => {
+  expect(ServiceConfig.canReplaceVersion("1.2.3", "1.2.4")).toBe(true)
+  expect(ServiceConfig.canReplaceVersion("1.2.4", "1.2.3")).toBe(false)
+  expect(ServiceConfig.canReplaceVersion("1.2.3", "1.2.3")).toBe(false)
+  expect(ServiceConfig.canReplaceVersion("0.0.0-next-9999", "0.0.0-next-15000")).toBe(true)
+  expect(ServiceConfig.canReplaceVersion("0.0.0-next-15000", "0.0.0-next-9999")).toBe(false)
+  expect(ServiceConfig.canReplaceVersion(undefined, "1.2.3")).toBe(true)
+})
+
 test("service config migrates from the hashed channel filename", async () => {
   const root = await fs.mkdtemp(path.join(os.tmpdir(), "opencode-service-config-migration-"))
   const legacy = path.join(root, ServiceConfig.legacyFilename("preview-a")!)
diff --git a/packages/client/src/effect/service.ts b/packages/client/src/effect/service.ts
index f27b273620..751a94ae72 100644
--- a/packages/client/src/effect/service.ts
+++ b/packages/client/src/effect/service.ts
@@ -3,7 +3,13 @@ import { Effect, FileSystem, Option, Schedule, Schema } from "effect"
 import { spawn, type ChildProcess } from "node:child_process"
 import { homedir } from "node:os"
 import { join } from "node:path"
-import type { DiscoverOptions, Endpoint, EnsureOptions, StopOptions } from "../service.js"
+import {
+  VersionMismatchError,
+  type DiscoverOptions,
+  type Endpoint,
+  type EnsureOptions,
+  type StopOptions,
+} from "../service.js"
 
 export * from "../service.js"
 /** Contents of the local service registration file. */
@@ -89,6 +95,8 @@ export const ensure = Effect.fn("service.ensure")(function* (options: EnsureOpti
       if (compatible && service.state === "failed")
         return yield* Effect.fail(new Error("Background service failed to start"))
       if (compatible) return Option.none<LocalService>()
+      if (options.canReplace?.(service.version) === false)
+        return yield* Effect.fail(new VersionMismatchError(options.version, service.version))
       yield* announce("version-mismatch", service.version)
       yield* kill(service, options).pipe(Effect.ignore)
       lastSpawn = 0
diff --git a/packages/client/src/promise/service.ts b/packages/client/src/promise/service.ts
index c03f4fe551..06d6a1a3da 100644
--- a/packages/client/src/promise/service.ts
+++ b/packages/client/src/promise/service.ts
@@ -2,7 +2,14 @@ import { readFile } from "node:fs/promises"
 import { spawn, type ChildProcess } from "node:child_process"
 import { homedir } from "node:os"
 import { join } from "node:path"
-import type { DiscoverOptions, Endpoint, Info, EnsureOptions, StopOptions } from "../service.js"
+import {
+  VersionMismatchError,
+  type DiscoverOptions,
+  type Endpoint,
+  type Info,
+  type EnsureOptions,
+  type StopOptions,
+} from "../service.js"
 import type { ServiceHealth, ServiceStopResponse } from "./generated/types.js"
 
 export * from "../service.js"
@@ -70,6 +77,8 @@ export async function ensure(options: EnsureOptions = {}): Promise<Endpoint> {
       if (compatible && service.state === "ready") return service.endpoint
       if (compatible && service.state === "failed") throw new Error("Background service failed to start")
       if (!compatible) {
+        if (options.canReplace?.(service.version) === false)
+          throw new VersionMismatchError(options.version, service.version)
         announce("version-mismatch", service.version)
         await kill(service, options).catch(() => undefined)
         lastSpawn = 0
diff --git a/packages/client/src/service.ts b/packages/client/src/service.ts
index 96d54871f6..5e61da0136 100644
--- a/packages/client/src/service.ts
+++ b/packages/client/src/service.ts
@@ -28,10 +28,24 @@ export type EnsureReason = "missing" | "version-mismatch"
 export type EnsureOptions = DiscoverOptions & {
   /** Service command and arguments. Defaults to `opencode serve --service`. */
   readonly command?: ReadonlyArray<string>
+  /** Decide whether a version-mismatched service may be replaced. Defaults to true. */
+  readonly canReplace?: (version: string | undefined) => boolean
   /** Called once before spawning a new service process. */
   readonly onStart?: (reason: EnsureReason, previousVersion?: string) => void
 }
 
+/** A healthy service exists, but the caller's replacement policy protects it. */
+export class VersionMismatchError extends Error {
+  override readonly name = "VersionMismatchError"
+
+  constructor(
+    readonly clientVersion: string | undefined,
+    readonly serverVersion: string | undefined,
+  ) {
+    super(`Client version ${clientVersion ?? "unknown"} cannot replace server version ${serverVersion ?? "unknown"}`)
+  }
+}
+
 /** Options used to stop the local OpenCode service. */
 export type StopOptions = {
   /** Absolute registration file path. Defaults to the XDG state directory. */
diff --git a/packages/client/test/promise-service.test.ts b/packages/client/test/promise-service.test.ts
index 51744cf50e..7f1331ee91 100644
--- a/packages/client/test/promise-service.test.ts
+++ b/packages/client/test/promise-service.test.ts
@@ -70,6 +70,25 @@ test("reports a failed registered service", async () => {
   )
 })
 
+test("does not replace a version rejected by the caller", async () => {
+  const registration = await setup("graceful")
+  const directory = await temp()
+  const contender = join(directory, "contender.json")
+  const info = await Bun.file(registration).json()
+
+  await expect(
+    Service.ensure({
+      file: registration,
+      version: "old",
+      canReplace: () => false,
+      command: [process.execPath, fixture, contender, "record-start"],
+    }),
+  ).rejects.toThrow("Client version old cannot replace server version test")
+
+  expect(await Bun.file(contender + ".started").exists()).toBe(false)
+  expect(process.kill(info.pid, 0)).toBe(true)
+})
+
 test("requests graceful stop of the exact service instance", async () => {
   const registration = await setup("graceful")
   const info = await Bun.file(registration).json()
diff --git a/packages/client/test/service.test.ts b/packages/client/test/service.test.ts
index 519c2a46df..738a6e1e69 100644
--- a/packages/client/test/service.test.ts
+++ b/packages/client/test/service.test.ts
@@ -107,6 +107,28 @@ test("does not spawn contenders while an incompatible service rejects replacemen
   expect(existing.exitCode).toBe(null)
 })
 
+test("does not replace a version rejected by the caller", async () => {
+  const directory = await temp()
+  const registration = join(directory, "service.json")
+  const contender = join(directory, "contender.json")
+  const existing = spawn(registration, "graceful")
+  await waitForFile(registration)
+
+  await expect(
+    run(
+      Service.ensure({
+        file: registration,
+        version: "old",
+        canReplace: () => false,
+        command: [process.execPath, fixture, contender, "record-start"],
+      }),
+    ),
+  ).rejects.toThrow("Client version old cannot replace server version test")
+
+  expect(await Bun.file(contender + ".started").exists()).toBe(false)
+  expect(existing.exitCode).toBe(null)
+})
+
 test("a legacy health response is still replaced", async () => {
   const directory = await temp()
   const registration = join(directory, "service.json")