diff --git a/packages/context/agent-instructions/README.i18n.yaml b/packages/context/agent-instructions/README.i18n.yaml
index 481606e8b5..1323a03a91 100644
--- a/packages/context/agent-instructions/README.i18n.yaml
+++ b/packages/context/agent-instructions/README.i18n.yaml
@@ -2,5 +2,5 @@
 # side as of the last confirmed-consistent state. Both languages carry equal authority;
 # after editing either side, bring the other along and re-record with:
 #   pnpm run verify-translation-pairing --write packages/context/agent-instructions/README.md
-README.md: d69a989a455512177798446e4a3e07b95b5f44d1
-README.zh.md: 96a456bd576cacb06382de4b8e34db4b5976327f
+README.md: 114202cdd7cd4d2fd1d2c82330d4f373a3735b0d
+README.zh.md: 029f87d3b53317b198c011829faf85b41e32c9b3
diff --git a/packages/context/agent-instructions/README.md b/packages/context/agent-instructions/README.md
index d69a989a45..114202cdd7 100644
--- a/packages/context/agent-instructions/README.md
+++ b/packages/context/agent-instructions/README.md
@@ -35,6 +35,8 @@ The first request includes one durable baseline message with the user-global `$D
 
 The defaults suit a typical checkout: `.git` marks the project root, `AGENTS.md` and `CLAUDE.md` are the base candidates, and `AGENTS.local.md` and `CLAUDE.local.md` are additive local overlays. Only `maxBytes` is required — it caps the complete rendered baseline so each deployment chooses its prompt budget explicitly.
 
+Root discovery climbs only when a marker probe confirms that the marker is absent. A permission or I/O failure stops discovery and surfaces the host or filesystem-provider error instead of selecting an ancestor project.
+
 ```yaml
 - name: '@deepseek-ai/dsh-agent-instructions'
   config:
diff --git a/packages/context/agent-instructions/README.zh.md b/packages/context/agent-instructions/README.zh.md
index 96a456bd57..029f87d3b5 100644
--- a/packages/context/agent-instructions/README.zh.md
+++ b/packages/context/agent-instructions/README.zh.md
@@ -35,6 +35,8 @@ kind: "package-reference"
 
 默认设置适合典型检出：`.git` 标记项目根目录，`AGENTS.md` 与 `CLAUDE.md` 是基础候选，`AGENTS.local.md` 与 `CLAUDE.local.md` 是叠加的本地 overlay。只有 `maxBytes` 必填——它限制完整渲染后的基线，让每个部署显式选择自己的提示词预算。
 
+只有确认项目根标记不存在时，项目根发现才会继续上溯。权限或 I/O 失败会停止发现，并返回 Host 或文件系统提供方的错误，而不会选择祖先项目。
+
 ```yaml
 - name: '@deepseek-ai/dsh-agent-instructions'
   config:
diff --git a/packages/context/agent-instructions/src/files.ts b/packages/context/agent-instructions/src/files.ts
index 01492e3e91..32388aeb86 100644
--- a/packages/context/agent-instructions/src/files.ts
+++ b/packages/context/agent-instructions/src/files.ts
@@ -95,6 +95,10 @@ function isMissingPathError(error: unknown): boolean {
   return error instanceof Error && 'code' in error && (error.code === 'ENOENT' || error.code === 'ENOTDIR')
 }
 
+function isMissingProviderPathError(error: unknown): boolean {
+  return error instanceof Error && 'code' in error && error.code === 'FS_NOT_FOUND'
+}
+
 async function nodeStatFile(path: string, signal?: AbortSignal): Promise<StatFileProbe> {
   try {
     signal?.throwIfAborted()
@@ -147,11 +151,10 @@ async function existsAsMarker(path: string, fileSystem?: FileSystem, signal?: Ab
     try {
       const target = await fileSystem.resolve(path, signalOptions(signal))
       return await fileSystem.stat(target, signal) !== undefined
-    } catch {
+    } catch (error: unknown) {
       signal?.throwIfAborted()
-      // TODO(root-marker-unavailable): preserve provider failure separately from
-      // absence and stop discovery; continuing upward can cross into an ancestor project.
-      return false
+      if (isMissingProviderPathError(error)) return false
+      throw error
     }
   }
   try {
@@ -159,9 +162,10 @@ async function existsAsMarker(path: string, fileSystem?: FileSystem, signal?: Ab
     await stat(path)
     signal?.throwIfAborted()
     return true
-  } catch {
+  } catch (error: unknown) {
     signal?.throwIfAborted()
-    return false
+    if (isMissingPathError(error)) return false
+    throw error
   }
 }
 
@@ -172,6 +176,7 @@ async function existsAsMarker(path: string, fileSystem?: FileSystem, signal?: Ab
  * @param fileSystem - optional provider used instead of host filesystem probes.
  * @param signal - cancellation for provider and host probes.
  * @returns the discovered project root, or `cwd` when no marker exists.
+ * @throws the original marker metadata error or cancellation reason when a probe is unavailable.
  */
 export async function findProjectRoot(
   cwd: string,
diff --git a/packages/context/agent-instructions/tests/agent-instructions.spec.ts b/packages/context/agent-instructions/tests/agent-instructions.spec.ts
index 68b1e5897d..c8c89a03b1 100644
--- a/packages/context/agent-instructions/tests/agent-instructions.spec.ts
+++ b/packages/context/agent-instructions/tests/agent-instructions.spec.ts
@@ -77,6 +77,7 @@ async function write(path: string, content: string): Promise<void> {
 
 class RecordingFileSystem extends FileSystem {
   entries = new Map<string, { type: FsInfo['type']; content?: string; version?: FsVersion }>()
+  missingOnStat = new Set<string>()
   throwOnStat = new Set<string>()
   throwOnRead = new Set<string>()
   omitSizes = new Set<string>()
@@ -105,6 +106,9 @@ class RecordingFileSystem extends FileSystem {
   override async stat(target: FsTarget, signal?: AbortSignal): Promise<FsInfo | undefined> {
     if (signal !== undefined) this.signals.push(signal)
     signal?.throwIfAborted()
+    if (this.missingOnStat.has(target.targetKey)) {
+      throw Object.assign(new Error(`not found: ${target.displayPath}`), { code: 'FS_NOT_FOUND' })
+    }
     if (this.throwOnStat.has(target.targetKey)) throw new Error(`stat failed: ${target.displayPath}`)
     const entry = this.entries.get(target.targetKey)
     if (entry === undefined) return undefined
@@ -2073,6 +2077,30 @@ describe('workspace context request injection', () => {
     }
   })
 
+  it('continues provider root discovery when a marker is confirmed absent', async () => {
+    const root = join(await tempRepo(), 'virtual-repo')
+    const cwd = join(root, 'pkg')
+    const home = join(await tempRepo(), 'virtual-home')
+    const ctx = new Context()
+    try {
+      await ctx.plugin(RecordingFileSystem)
+      const fs = ctx.fs as RecordingFileSystem
+      fs.missingOnStat.add(join(cwd, '.git'))
+      fs.entries.set(join(root, '.git'), { type: 'directory' })
+      fs.entries.set(join(root, 'AGENTS.md'), { type: 'file', content: 'provider parent rule' })
+      await mountWorkspaceContextPlugin(ctx, { dshHome: home, maxBytes: 65536 })
+      const agent = await stubAgent(cwd)
+
+      await composeBaselinePrefix(ctx, agent)
+
+      expect(derivedText(agent)).toContain('provider parent rule')
+    } finally {
+      await ctx.fiber.dispose()
+      await rm(dirname(root), { recursive: true, force: true })
+      await rm(dirname(home), { recursive: true, force: true })
+    }
+  })
+
   it('keeps the direct provider API usable without an operation signal', async () => {
     const root = resolve('/virtual/no-signal-repo')
     const home = resolve('/virtual/no-signal-home')
@@ -2291,23 +2319,24 @@ describe('workspace context request injection', () => {
     }
   })
 
-  it('treats ctx.fs marker lookup failures as absent root markers', async () => {
+  it('surfaces ctx.fs marker lookup failures instead of crossing into an ancestor project', async () => {
     const root = await tempRepo()
+    const cwd = join(root, 'pkg')
     const home = await tempRepo()
     try {
-      await mkdir(join(root, '.git'), { recursive: true })
-      await write(join(root, 'AGENTS.md'), 'repo rule')
       const ctx = new Context()
       await ctx.plugin(RecordingFileSystem)
       const fs = ctx.fs as RecordingFileSystem
-      fs.throwOnStat.add(join(root, '.git'))
-      fs.entries.set(join(root, 'AGENTS.md'), { type: 'file', content: 'repo rule' })
+      fs.throwOnStat.add(join(cwd, '.git'))
+      fs.entries.set(join(root, '.git'), { type: 'directory' })
+      fs.entries.set(join(root, 'AGENTS.md'), { type: 'file', content: 'ancestor rule must not load' })
       await mountWorkspaceContextPlugin(ctx, { dshHome: home, maxBytes: 65536 })
-      const agent = await stubAgent(root)
+      const agent = await stubAgent(cwd)
 
-      await composeBaselinePrefix(ctx, agent)
+      await expect(composeBaselinePrefix(ctx, agent))
+        .rejects.toThrow(`stat failed: ${join(cwd, '.git')}`)
 
-      expect(derivedText(agent)).toContain('repo rule')
+      expectNoDerivedMessages(agent)
     } finally {
       await rm(root, { recursive: true, force: true })
       await rm(home, { recursive: true, force: true })
@@ -2540,6 +2569,42 @@ describe('workspace context request injection', () => {
       await rm(home, { recursive: true, force: true })
     }
   })
+
+  it('surfaces host marker metadata failures instead of crossing into an ancestor project', async () => {
+    const root = await tempRepo()
+    const home = await tempRepo()
+    try {
+      const cwd = join(root, 'pkg')
+      const markerPath = join(cwd, '.git')
+      const failure = Object.assign(new Error(`permission denied: ${markerPath}`), {
+        code: 'EACCES',
+        path: markerPath,
+      })
+      await mkdir(join(root, '.git'), { recursive: true })
+      await write(join(root, 'AGENTS.md'), 'ancestor rule must not load')
+      await mkdir(cwd, { recursive: true })
+      vi.resetModules()
+      vi.doMock('node:fs/promises', async (importOriginal) => {
+        const actual = await importOriginal<typeof import('node:fs/promises')>()
+        return {
+          ...actual,
+          stat: async (path: string) => {
+            if (path === markerPath) throw failure
+            return actual.stat(path)
+          },
+        }
+      })
+      const isolated = await import('@deepseek-ai/dsh-agent-instructions')
+
+      await expect(isolated.loadBaselineInstructions({ cwd, dshHome: home, maxBytes: 65536 }))
+        .rejects.toBe(failure)
+    } finally {
+      vi.doUnmock('node:fs/promises')
+      vi.resetModules()
+      await rm(root, { recursive: true, force: true })
+      await rm(home, { recursive: true, force: true })
+    }
+  })
 })
 
 describe('dynamic nested workspace context injection', () => {