fix(core): resolve nested plan directory duplication and relative path policies (#25138)

packages/core/src/tools/write-file.ts

@@ -49,6 +49,7 @@ import { debugLogger } from '../utils/debugLogger.js';
 import { WRITE_FILE_DEFINITION } from './definitions/coreTools.js';
 import { resolveToolDeclaration } from './definitions/resolver.js';
 import { detectOmissionPlaceholders } from './omissionPlaceholderDetector.js';
+import { resolveAndValidatePlanPath } from '../utils/planUtils.js';
 import { isGemini3Model } from '../config/models.js';
 import { discoverJitContext, appendJitContext } from './jit-context.js';
 
@@ -168,11 +169,20 @@ class WriteFileToolInvocation extends BaseToolInvocation<
     );
 
     if (this.config.isPlanMode()) {
-      const safeFilename = path.basename(this.params.file_path);
-      this.resolvedPath = path.join(
-        this.config.storage.getPlansDir(),
-        safeFilename,
-      );
+      try {
+        this.resolvedPath = resolveAndValidatePlanPath(
+          this.params.file_path,
+          this.config.storage.getPlansDir(),
+          this.config.getProjectRoot(),
+        );
+      } catch (e) {
+        debugLogger.error(
+          'Failed to resolve plan path during WriteFileTool invocation setup',
+          e,
+        );
+        // Validation fails, set resolvedPath to something that will fail validation downstream or just the raw path.
+        this.resolvedPath = this.params.file_path;
+      }
     } else {
       this.resolvedPath = path.resolve(
         this.config.getTargetDir(),
@@ -499,7 +509,20 @@ export class WriteFileTool
       return `Missing or empty "file_path"`;
     }
 
-    const resolvedPath = path.resolve(this.config.getTargetDir(), filePath);
+    let resolvedPath: string;
+    if (this.config.isPlanMode()) {
+      try {
+        resolvedPath = resolveAndValidatePlanPath(
+          filePath,
+          this.config.storage.getPlansDir(),
+          this.config.getProjectRoot(),
+        );
+      } catch (err) {
+        return err instanceof Error ? err.message : String(err);
+      }
+    } else {
+      resolvedPath = path.resolve(this.config.getTargetDir(), filePath);
+    }
 
     const validationError = this.config.validatePathAccess(resolvedPath);
     if (validationError) {