diff --git a/packages/cli/src/acp-integration/session/rewrite/LlmRewriter.test.ts b/packages/cli/src/acp-integration/session/rewrite/LlmRewriter.test.ts
index 604293f9d51..7c9ea4769d9 100644
--- a/packages/cli/src/acp-integration/session/rewrite/LlmRewriter.test.ts
+++ b/packages/cli/src/acp-integration/session/rewrite/LlmRewriter.test.ts
@@ -4,7 +4,10 @@
  * SPDX-License-Identifier: Apache-2.0
  */
 
-import { describe, it, expect, vi, beforeEach } from 'vitest';
+import { describe, it, expect, vi, beforeEach, afterEach } from 'vitest';
+import { mkdtempSync, rmSync, writeFileSync } from 'node:fs';
+import { tmpdir } from 'node:os';
+import { join } from 'node:path';
 import type { Config } from '@qwen-code/qwen-code-core';
 import type { TurnContent, MessageRewriteConfig } from './types.js';
 
@@ -278,4 +281,65 @@ describe('LlmRewriter', () => {
       expect(input).not.toContain('上一轮改写结果');
     });
   });
+
+  describe('promptFile', () => {
+    let tempDir: string;
+
+    beforeEach(() => {
+      tempDir = mkdtempSync(join(tmpdir(), 'llm-rewriter-promptfile-'));
+    });
+
+    afterEach(() => {
+      rmSync(tempDir, { recursive: true, force: true });
+    });
+
+    function promptOf(rewriter: unknown): string {
+      return (rewriter as { prompt: string }).prompt;
+    }
+
+    it('loads a custom prompt from a readable file', () => {
+      const filePath = join(tempDir, 'prompt.md');
+      writeFileSync(filePath, '  custom rewrite prompt  ');
+
+      const rewriter = new LlmRewriter(makeConfig(), {
+        enabled: true,
+        target: 'all',
+        promptFile: filePath,
+      } as MessageRewriteConfig);
+
+      expect(promptOf(rewriter)).toBe('custom rewrite prompt');
+    });
+
+    it('falls back to the default prompt when the file is missing', () => {
+      const rewriter = new LlmRewriter(makeConfig(), {
+        enabled: true,
+        target: 'all',
+        promptFile: join(tempDir, 'does-not-exist.md'),
+      } as MessageRewriteConfig);
+
+      expect(promptOf(rewriter)).toContain('rewrites raw coding-agent output');
+    });
+
+    // Regression for #9752: promptFile pointing at a path that exists but
+    // cannot be read as a file (a directory) used to throw EISDIR from the
+    // constructor, crashing ACP session startup.
+    it('falls back to the default prompt when promptFile is a directory', () => {
+      expect(
+        () =>
+          new LlmRewriter(makeConfig(), {
+            enabled: true,
+            target: 'all',
+            promptFile: tempDir,
+          } as MessageRewriteConfig),
+      ).not.toThrow();
+
+      const rewriter = new LlmRewriter(makeConfig(), {
+        enabled: true,
+        target: 'all',
+        promptFile: tempDir,
+      } as MessageRewriteConfig);
+
+      expect(promptOf(rewriter)).toContain('rewrites raw coding-agent output');
+    });
+  });
 });
diff --git a/packages/cli/src/acp-integration/session/rewrite/LlmRewriter.ts b/packages/cli/src/acp-integration/session/rewrite/LlmRewriter.ts
index 5783497f091..6228e466651 100644
--- a/packages/cli/src/acp-integration/session/rewrite/LlmRewriter.ts
+++ b/packages/cli/src/acp-integration/session/rewrite/LlmRewriter.ts
@@ -57,16 +57,29 @@ export class LlmRewriter {
     // promptFile takes precedence over inline prompt
     if (rewriteConfig.promptFile) {
       const filePath = resolve(rewriteConfig.promptFile);
-      if (existsSync(filePath)) {
-        this.prompt = readFileSync(filePath, 'utf-8').trim();
-        debugLogger.info(
-          `Loaded rewrite prompt from file: ${filePath} (${this.prompt.length} chars)`,
-        );
-      } else {
+      if (!existsSync(filePath)) {
         debugLogger.warn(
           `Rewrite prompt file not found: ${filePath}, using default`,
         );
         this.prompt = DEFAULT_REWRITE_PROMPT;
+      } else {
+        // existsSync passes for directories and says nothing about
+        // readability, so the read itself can still fail (EISDIR, EACCES,
+        // ...). Degrade like the missing-file case instead of throwing,
+        // which would crash ACP session startup (#9752).
+        try {
+          this.prompt = readFileSync(filePath, 'utf-8').trim();
+          debugLogger.info(
+            `Loaded rewrite prompt from file: ${filePath} (${this.prompt.length} chars)`,
+          );
+        } catch (error) {
+          debugLogger.warn(
+            `Rewrite prompt file could not be read: ${filePath} (${
+              error instanceof Error ? error.message : String(error)
+            }), using default`,
+          );
+          this.prompt = DEFAULT_REWRITE_PROMPT;
+        }
       }
     } else {
       this.prompt = rewriteConfig.prompt || DEFAULT_REWRITE_PROMPT;
diff --git a/packages/cli/src/acp-integration/session/rewrite/README.md b/packages/cli/src/acp-integration/session/rewrite/README.md
index ad40314be12..e327daa097d 100644
--- a/packages/cli/src/acp-integration/session/rewrite/README.md
+++ b/packages/cli/src/acp-integration/session/rewrite/README.md
@@ -33,3 +33,6 @@ Add to `settings.json`:
 ```
 
 `timeoutMs` sets the per-rewrite LLM call timeout in milliseconds. Defaults to 30000.
+If `promptFile` is missing or cannot be read, rewriting falls back to the
+built-in default prompt. Set `QWEN_DEBUG_LOG_FILE` to capture the fallback
+warning.