diff --git a/cli/src/lib/harnesses.ts b/cli/src/lib/harnesses.ts
index 1026802..0eeca3a 100644
--- a/cli/src/lib/harnesses.ts
+++ b/cli/src/lib/harnesses.ts
@@ -1,5 +1,6 @@
 import { spawn } from 'node:child_process'
-import { access, constants } from 'node:fs/promises'
+import { access, constants, readFile, stat } from 'node:fs/promises'
+import { homedir } from 'node:os'
 import { dirname, join } from 'node:path'
 import which from 'which'
 
@@ -114,7 +115,32 @@ export interface DiscoveredHarness {
   version: string | null
 }
 
+// Same cache contract as src/main/installer and src/opencode-plugin.
+const DISCOVERY_CACHE_TTL_MS = 5 * 60 * 1000
+
+async function cachedHarness(spec: HarnessSpec): Promise<DiscoveredHarness | null> {
+  try {
+    const file = join(homedir(), '.cache', 'hoist', 'discovery.json')
+    const [text, info] = await Promise.all([readFile(file, 'utf8'), stat(file)])
+    if (Date.now() - info.mtimeMs > DISCOVERY_CACHE_TTL_MS) return null
+    const raw: unknown = JSON.parse(text)
+    if (typeof raw !== 'object' || raw === null || Array.isArray(raw) || !Object.hasOwn(raw, spec.id)) return null
+    const entry: unknown = (raw as Record<string, unknown>)[spec.id]
+    if (typeof entry !== 'object' || entry === null || Array.isArray(entry)) return null
+    const { path, version } = entry as Record<string, unknown>
+    if (typeof path !== 'string' || path.length === 0) return null
+    await access(path, constants.X_OK)
+    return { spec, installed: true, path, version: typeof version === 'string' && version.length > 0 ? version : null }
+  } catch {
+    return null
+  }
+}
+
 export async function discoverHarness(spec: HarnessSpec): Promise<DiscoveredHarness> {
+  return (await cachedHarness(spec)) ?? discoverLiveHarness(spec)
+}
+
+async function discoverLiveHarness(spec: HarnessSpec): Promise<DiscoveredHarness> {
   for (const method of spec.installMethods) {
     const path = await resolveBinary(binaryFor(method, spec.id))
     if (path) {
@@ -149,7 +175,7 @@ export async function installHarness(spec: HarnessSpec, opts: InstallOptions = {
     if (result.code !== 0) {
       throw new Error(`npm install -g ${pkg} failed:\n${result.stderr || result.stdout}`)
     }
-    return discoverHarness(spec)
+    return discoverLiveHarness(spec)
   }
 
   if (method.type === 'brew') {
@@ -171,7 +197,7 @@ export async function installHarness(spec: HarnessSpec, opts: InstallOptions = {
     if (result.code !== 0) {
       throw new Error(`brew ${brewArgs.join(' ')} failed:\n${result.stderr || result.stdout}`)
     }
-    return discoverHarness(spec)
+    return discoverLiveHarness(spec)
   }
 
   throw new Error(`Install method not supported: ${(method as InstallMethod).type}`)
diff --git a/cli/test/discovery-cache.test.ts b/cli/test/discovery-cache.test.ts
new file mode 100644
index 0000000..56f6e05
--- /dev/null
+++ b/cli/test/discovery-cache.test.ts
@@ -0,0 +1,142 @@
+import { afterEach, describe, expect, test } from 'bun:test'
+import { chmod, mkdir, mkdtemp, readFile, rm, utimes, writeFile } from 'node:fs/promises'
+import { tmpdir } from 'node:os'
+import { dirname, join } from 'node:path'
+
+const moduleUrl = new URL('../src/lib/harnesses.ts', import.meta.url).href
+const homes: string[] = []
+const quote = (value: string) => `'${value.replaceAll("'", "'\\''")}'`
+
+afterEach(async () => {
+  await Promise.all(homes.splice(0).map((home) => rm(home, { recursive: true, force: true })))
+})
+
+async function fixture() {
+  const home = await mkdtemp(join(tmpdir(), 'hoist-cli-discovery-'))
+  homes.push(home)
+  const bin = join(home, 'bin')
+  const cacheFile = join(home, '.cache', 'hoist', 'discovery.json')
+  const calls = join(home, 'calls')
+  await mkdir(bin)
+  await mkdir(dirname(cacheFile), { recursive: true })
+  await writeFile(calls, '')
+
+  async function executable(path: string, body: string) {
+    await mkdir(dirname(path), { recursive: true })
+    await writeFile(path, `#!/bin/sh\n${body}\n`, { mode: 0o755 })
+    return path
+  }
+
+  async function tool(path: string, version: string) {
+    return executable(path, `printf '%s\\n' ${quote(path)} >> ${quote(calls)}\nprintf '%s\\n' ${quote(version)}`)
+  }
+
+  async function cache(raw: unknown, ageMs = 0) {
+    await writeFile(cacheFile, JSON.stringify(raw))
+    const time = new Date(Date.now() - ageMs)
+    await utimes(cacheFile, time, time)
+  }
+
+  // An isolated HOME/PATH prevents real harness or npm execution. An unexpected
+  // npm fallback is visible in the calls log, even when it finds nothing.
+  await executable(join(bin, 'npm'), `printf 'npm %s\\n' "$*" >> ${quote(calls)}\nprintf '%s\\n' ${quote(join(home, 'npm', 'node_modules'))}`)
+
+  async function discover(expression = 'await discoverHarness(findHarness("claude-code"))') {
+    const child = Bun.spawn([
+      process.execPath, '-e',
+      `import { discoverHarness, discoverAll, findHarness, installHarness } from ${JSON.stringify(moduleUrl)}; console.log(JSON.stringify(${expression}))`,
+    ], {
+      env: { ...process.env, HOME: home, PATH: bin },
+      stdout: 'pipe',
+      stderr: 'pipe',
+    })
+    const [stdout, stderr, code] = await Promise.all([
+      new Response(child.stdout).text(), new Response(child.stderr).text(), child.exited,
+    ])
+    expect(stderr).toBe('')
+    expect(code).toBe(0)
+    return JSON.parse(stdout)
+  }
+
+  return { home, bin, cacheFile, calls, executable, tool, cache, discover }
+}
+
+describe.skipIf(process.platform === 'win32')('CLI OpenCode discovery cache (isolated fixtures)', () => {
+  test('fresh cache wins over PATH without running the cached binary or npm', async () => {
+    const f = await fixture()
+    const path = await f.tool(join(f.home, 'hidden', 'claude'), 'must not run')
+    await f.tool(join(f.bin, 'claude'), 'PATH version')
+    await f.cache({ 'claude-code': { path, version: 'cached 2.1.204' } })
+    expect(await f.discover()).toMatchObject({ installed: true, path, version: 'cached 2.1.204' })
+    expect(await readFile(f.calls, 'utf8')).toBe('')
+  })
+
+  test('discoverAll reads all plugin harness ids and normalises unknown versions', async () => {
+    const f = await fixture()
+    const raw: Record<string, { path: string; version: unknown }> = {}
+    for (const [id, binary, version] of [
+      ['claude-code', 'claude', 'cached claude'], ['opencode', 'opencode', null], ['codex', 'codex', 42],
+    ] as const) {
+      raw[id] = { path: await f.tool(join(f.home, 'hidden', binary), 'must not run'), version }
+    }
+    await f.cache(raw)
+    const results = await f.discover('await discoverAll()')
+    expect(results.map((h: { installed: boolean }) => h.installed)).toEqual([true, true, true])
+    expect(results.map((h: { version: string | null }) => h.version)).toEqual(['cached claude', null, null])
+    expect(await readFile(f.calls, 'utf8')).toBe('')
+  })
+
+  for (const scenario of ['absent', 'stale', 'malformed JSON', 'array', 'null', 'missing id', 'missing path', 'non-string path', 'deleted binary', 'non-executable binary']) {
+    test(`${scenario} cache falls back to PATH and reads the live version`, async () => {
+      const f = await fixture()
+      const live = await f.tool(join(f.bin, 'claude'), 'live 3.0')
+      const cached = await f.tool(join(f.home, 'hidden', 'claude'), 'must not run')
+      switch (scenario) {
+        case 'stale': await f.cache({ 'claude-code': { path: cached, version: 'old' } }, 5 * 60 * 1000 + 1000); break
+        case 'malformed JSON': await writeFile(f.cacheFile, '{'); break
+        case 'array': await f.cache([]); break
+        case 'null': await f.cache(null); break
+        case 'missing id': await f.cache({ codex: { path: cached } }); break
+        case 'missing path': await f.cache({ 'claude-code': { version: 'old' } }); break
+        case 'non-string path': await f.cache({ 'claude-code': { path: 42 } }); break
+        case 'deleted binary': await f.cache({ 'claude-code': { path: join(f.home, 'gone') } }); break
+        case 'non-executable binary':
+          await chmod(cached, 0o644)
+          await f.cache({ 'claude-code': { path: cached } })
+          break
+      }
+      expect(await f.discover()).toMatchObject({ installed: true, path: live, version: 'live 3.0' })
+      expect(await readFile(f.calls, 'utf8')).toBe(`${live}\n`)
+    })
+  }
+
+  test('absent cache preserves npm-global discovery when PATH misses', async () => {
+    const f = await fixture()
+    const path = await f.tool(join(f.home, 'npm', 'bin', 'claude'), 'npm-global 3.0')
+    expect(await f.discover()).toMatchObject({ installed: true, path, version: 'npm-global 3.0' })
+    expect(await readFile(f.calls, 'utf8')).toBe(`npm root -g\n${path}\n`)
+  })
+
+  test('absent cache and missing binary keep the not-installed result', async () => {
+    const f = await fixture()
+    expect(await f.discover()).toMatchObject({ installed: false, path: null, version: null })
+  })
+
+  for (const prefer of ['npm', 'brew']) {
+    test(`${prefer} install bypasses a fresh cache to report the installed version`, async () => {
+      const f = await fixture()
+      const live = await f.tool(join(f.bin, 'claude'), 'new 4.0')
+      const cached = await f.tool(join(f.home, 'hidden', 'claude'), 'must not run')
+      await f.cache({ 'claude-code': { path: cached, version: 'old 2.0' } })
+      if (prefer === 'brew') {
+        await f.executable(join(f.bin, 'brew'), `printf 'brew %s\\n' "$*" >> ${quote(f.calls)}\nexit 0`)
+      }
+      expect(await f.discover(`await installHarness(findHarness("claude-code"), { prefer: "${prefer}" })`))
+        .toMatchObject({ installed: true, path: live, version: 'new 4.0' })
+      const calls = await readFile(f.calls, 'utf8')
+      expect(calls).toContain(prefer === 'npm' ? 'npm install -g @anthropic-ai/claude-code' : 'brew upgrade claude-code')
+      expect(calls).toContain(live)
+      expect(calls).not.toContain(cached)
+    })
+  }
+})