fix: treat files with only whitespace as empty in read_file tool

- Added trim() check to detect files containing only whitespace characters
- Files with only spaces, tabs, or newlines now show "File is empty" notice
- Original file content is preserved without modification
- Added test case to verify whitespace-only files are treated as empty

Fixes #5789

src/core/tools/__tests__/readFileTool.spec.ts

@@ -481,6 +481,21 @@ describe("read_file tool XML output structure", () => {
 				`<files>\n<file><path>${testFilePath}</path>\n<content/><notice>File is empty</notice>\n</file>\n</files>`,
 			)
 		})
+
+		it("should treat files with only whitespace as empty", async () => {
+			// Setup - file has lines but only whitespace content
+			mockedCountFileLines.mockResolvedValue(3) // File has 3 lines
+			mockedExtractTextFromFile.mockResolvedValue("   \n\t\n  ") // Only whitespace
+			mockProvider.getState.mockResolvedValue({ maxReadFileLine: -1 })
+
+			// Execute
+			const result = await executeReadFileTool({}, { totalLines: 3 })
+
+			// Verify - should show empty file notice even though totalLines > 0
+			expect(result).toBe(
+				`<files>\n<file><path>${testFilePath}</path>\n<content/><notice>File is empty</notice>\n</file>\n</files>`,
+			)
+		})
 	})
 
 	describe("Error Handling Tests", () => {

src/core/tools/readFileTool.ts

@@ -519,9 +519,13 @@ export async function readFileTool(
 				// Handle normal file read
 				const content = await extractTextFromFile(fullPath)
 				const lineRangeAttr = ` lines="1-${totalLines}"`
-				let xmlInfo = totalLines > 0 ? `<content${lineRangeAttr}>\n${content}</content>\n` : `<content/>`
 
-				if (totalLines === 0) {
+				// Check if file is effectively empty (no content or only whitespace)
+				const isEffectivelyEmpty = totalLines === 0 || content.trim() === ""
+
+				let xmlInfo = !isEffectivelyEmpty ? `<content${lineRangeAttr}>\n${content}</content>\n` : `<content/>`
+
+				if (isEffectivelyEmpty) {
 					xmlInfo += `<notice>File is empty</notice>\n`
 				}