diff --git a/.changeset/calm-services-start.md b/.changeset/calm-services-start.md
new file mode 100644
index 0000000000..ccdd17b6b0
--- /dev/null
+++ b/.changeset/calm-services-start.md
@@ -0,0 +1,5 @@
+---
+"@opencode-ai/client": patch
+---
+
+Reuse a same-version background service when a repeated health probe succeeds instead of replacing an endpoint another client may already be using.
diff --git a/packages/client/src/effect/service.ts b/packages/client/src/effect/service.ts
index f047d462bf..3c42f0de56 100644
--- a/packages/client/src/effect/service.ts
+++ b/packages/client/src/effect/service.ts
@@ -59,11 +59,11 @@ const discoverLocal = Effect.fnUntraced(function* (options: Options) {
 export const start = Effect.fn("service.start")(function* (options: StartOptions = {}) {
   const compatible = yield* discover(options)
   if (compatible !== undefined) return compatible
-  const mismatched = yield* find(options)
-  yield* Effect.sync(() =>
-    options.onStart?.(mismatched === undefined ? "missing" : "version-mismatch", mismatched?.info),
-  )
-  if (mismatched !== undefined) yield* kill(mismatched.info, options).pipe(Effect.ignore)
+  const existing = yield* find(options)
+  if (existing?.version !== undefined && (options.version === undefined || existing.version === options.version))
+    return existing.endpoint
+  yield* Effect.sync(() => options.onStart?.(existing === undefined ? "missing" : "version-mismatch", existing?.info))
+  if (existing !== undefined) yield* kill(existing.info, options).pipe(Effect.ignore)
 
   const [command, ...args] = options.command ?? ["opencode", "serve", "--service"]
   if (command === undefined) return yield* Effect.fail(new Error("Missing service command"))
@@ -138,6 +138,7 @@ const read = Effect.fnUntraced(function* (file?: string) {
 type LocalService = {
   readonly info: Info
   readonly endpoint: Endpoint
+  readonly version?: string
 }
 
 const probe = Effect.fnUntraced(function* (info: Info, version?: string, allowLegacy = false) {
@@ -161,7 +162,7 @@ const probe = Effect.fnUntraced(function* (info: Info, version?: string, allowLe
     if (health.value.pid !== info.pid) return undefined
     if (info.version !== undefined && health.value.version !== info.version) return undefined
     if (version !== undefined && health.value.version !== version) return undefined
-    return { info, endpoint } satisfies LocalService
+    return { info, endpoint, version: health.value.version } satisfies LocalService
   }
   if (
     !allowLegacy ||
diff --git a/packages/client/test/fixture/service.ts b/packages/client/test/fixture/service.ts
new file mode 100644
index 0000000000..3f77edb419
--- /dev/null
+++ b/packages/client/test/fixture/service.ts
@@ -0,0 +1,39 @@
+import { rename, writeFile } from "node:fs/promises"
+
+const [registration, mode] = process.argv.slice(2)
+if (registration === undefined || mode === undefined) throw new Error("Missing service fixture arguments")
+
+let requests = 0
+const server = Bun.serve({
+  port: 0,
+  async fetch(request) {
+    if (new URL(request.url).pathname !== "/api/health") return new Response(null, { status: 404 })
+    requests += 1
+    if (mode === "modern" && requests === 1) {
+      await writeFile(registration + ".first-request", "")
+      while (!(await Bun.file(registration + ".release").exists())) await Bun.sleep(5)
+      return new Response(null, { status: 503 })
+    }
+    if (mode === "legacy") return Response.json({ healthy: true })
+    return Response.json({ healthy: true, version: "test", pid: process.pid })
+  },
+})
+
+await writeFile(
+  registration + ".tmp",
+  JSON.stringify({
+    id: crypto.randomUUID(),
+    version: mode === "legacy" ? undefined : "test",
+    url: server.url.toString(),
+    pid: process.pid,
+  }),
+  { mode: 0o600 },
+)
+await rename(registration + ".tmp", registration)
+
+const shutdown = () => {
+  server.stop(true)
+  process.exit()
+}
+process.on("SIGTERM", shutdown)
+process.on("SIGINT", shutdown)
diff --git a/packages/client/test/service.test.ts b/packages/client/test/service.test.ts
new file mode 100644
index 0000000000..21428c9166
--- /dev/null
+++ b/packages/client/test/service.test.ts
@@ -0,0 +1,91 @@
+import { NodeFileSystem } from "@effect/platform-node"
+import { afterEach, expect, test } from "bun:test"
+import { Effect } from "effect"
+import { mkdtemp, rm, writeFile } from "node:fs/promises"
+import { tmpdir } from "node:os"
+import { join } from "node:path"
+import { Service } from "../src/effect/index"
+
+const fixture = join(import.meta.dir, "fixture/service.ts")
+const processes: Bun.Subprocess[] = []
+const directories: string[] = []
+
+afterEach(async () => {
+  processes.forEach((process) => process.kill("SIGTERM"))
+  await Promise.all(processes.splice(0).map((process) => process.exited))
+  await Promise.all(directories.splice(0).map((directory) => rm(directory, { recursive: true, force: true })))
+})
+
+test("a concurrent same-version start cannot invalidate a resolved endpoint", async () => {
+  const directory = await temp()
+  const registration = join(directory, "service.json")
+  spawn(registration, "modern")
+  await waitForFile(registration)
+  const original = await Bun.file(registration).json()
+
+  const starts: Service.StartReason[] = []
+  const first = run(
+    Service.start({
+      file: registration,
+      version: "test",
+      command: [],
+      onStart: (reason) => starts.push(reason),
+    }),
+  )
+  await waitForFile(registration + ".first-request")
+
+  const resolved = await run(Service.start({ file: registration, version: "test" }))
+  expect(resolved.url).toBe(original.url)
+
+  await writeFile(registration + ".release", "")
+  await first
+
+  expect(starts).toEqual([])
+  expect(await Bun.file(registration).json()).toEqual(original)
+  expect(await health(resolved.url)).toEqual({ healthy: true, version: "test", pid: original.pid })
+})
+
+test("a legacy health response is still replaced", async () => {
+  const directory = await temp()
+  const registration = join(directory, "service.json")
+  const existing = spawn(registration, "legacy")
+  await waitForFile(registration)
+
+  const starts: Service.StartReason[] = []
+  const result = run(Service.start({ file: registration, command: [], onStart: (reason) => starts.push(reason) }))
+
+  await expect(result).rejects.toThrow("Missing service command")
+  expect(starts).toEqual(["version-mismatch"])
+  await existing.exited
+})
+
+function run<A, E>(effect: Effect.Effect<A, E, never>) {
+  return Effect.runPromise(effect.pipe(Effect.provide(NodeFileSystem.layer)))
+}
+
+function spawn(registration: string, mode: string, ...args: string[]) {
+  const subprocess = Bun.spawn([process.execPath, fixture, registration, mode, ...args], {
+    stdout: "ignore",
+    stderr: "inherit",
+  })
+  processes.push(subprocess)
+  return subprocess
+}
+
+async function temp() {
+  const directory = await mkdtemp(join(tmpdir(), "opencode-client-service-"))
+  directories.push(directory)
+  return directory
+}
+
+async function waitForFile(file: string) {
+  for (let attempt = 0; attempt < 600; attempt++) {
+    if (await Bun.file(file).exists()) return
+    await Bun.sleep(5)
+  }
+  throw new Error(`Timed out waiting for ${file}`)
+}
+
+async function health(url: string) {
+  return fetch(new URL("/api/health", url), { signal: AbortSignal.timeout(1_000) }).then((response) => response.json())
+}