fix(plan): sandbox path resolution in Plan Mode to prevent hallucinations (#22737)

Co-authored-by: gemini-code-assist[bot] <176961590+gemini-code-assist[bot]@users.noreply.github.com>

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

@@ -165,10 +165,19 @@ class WriteFileToolInvocation extends BaseToolInvocation<
       true,
       () => this.config.getApprovalMode(),
     );
-    this.resolvedPath = path.resolve(
-      this.config.getTargetDir(),
-      this.params.file_path,
-    );
+
+    if (this.config.isPlanMode()) {
+      const safeFilename = path.basename(this.params.file_path);
+      this.resolvedPath = path.join(
+        this.config.storage.getPlansDir(),
+        safeFilename,
+      );
+    } else {
+      this.resolvedPath = path.resolve(
+        this.config.getTargetDir(),
+        this.params.file_path,
+      );
+    }
   }
 
   override toolLocations(): ToolLocation[] {