diff --git a/src/commands/run.ts b/src/commands/run.ts
index 917f9593f..f089b8c4e 100644
--- a/src/commands/run.ts
+++ b/src/commands/run.ts
@@ -420,7 +420,9 @@ export class RunCommand extends ApifyCommand<typeof RunCommand> {
 	}
 
 	/**
-	 * Ensures the input that the actor will be ran with locally matches the input schema (and prefills default values if missing)
+	 * Validates the input against the input schema and writes to disk only when necessary.
+	 * When the user already has an INPUT.json and no override is provided, it validates in-memory
+	 * without modifying the file (returns null so the finally block is a no-op).
 	 * @param inputOverride Optional input received through command flags
 	 */
 	private async validateAndStoreInput(inputOverride?: { input: Record<string, unknown>; source: string }) {
@@ -546,15 +548,8 @@ export class RunCommand extends ApifyCommand<typeof RunCommand> {
 				);
 			}
 
-			// Step 4: store the input
-			await mkdir(dirname(inputFilePath), { recursive: true });
-			await writeFile(inputFilePath, JSON.stringify(fullInput, null, 2));
-
-			return {
-				existingInput,
-				inputFilePath,
-				writtenAt: Date.now(),
-			};
+			// Don't write to the file — leave the user's INPUT.json untouched
+			return null;
 		}
 
 		return null;
diff --git a/test/local/commands/crawlee/run.test.ts b/test/local/commands/crawlee/run.test.ts
index 6d06d891b..69174d587 100644
--- a/test/local/commands/crawlee/run.test.ts
+++ b/test/local/commands/crawlee/run.test.ts
@@ -68,22 +68,25 @@ describe('apify run', () => {
 		expect(lastErrorMessage()).toMatch(/Field awesome is required/i);
 	});
 
-	it('prefills input with defaults', async () => {
+	it('validates input without modifying file', async () => {
 		await writeFile(inputPath, originalInput);
 
 		await testRunCommand(RunCommand, {});
 
 		const output = JSON.parse(await readFile(outputPath, 'utf8'));
 		expect(output).toStrictEqual({ awesome: true, help: 'this_maze_is_not_meant_for_you' });
+
+		const inputAfterRun = await readFile(inputPath, 'utf8');
+		expect(inputAfterRun).toBe(originalInput);
 	});
 
-	it('should restore the original input file after run', async () => {
+	it('does not modify input file during run', async () => {
 		await writeFile(inputPath, originalInputWithExtraField);
 
 		await testRunCommand(RunCommand, {});
 
-		const input = JSON.parse(await readFile(inputPath, 'utf8'));
-		expect(input).toStrictEqual({ awesome: true, extra: 'field' });
+		const inputAfterRun = await readFile(inputPath, 'utf8');
+		expect(inputAfterRun).toBe(originalInputWithExtraField);
 
 		const output = JSON.parse(await readFile(outputPath, 'utf8'));
 		expect(output).toStrictEqual({ awesome: true, help: 'this_maze_is_not_meant_for_you', extra: 'field' });
diff --git a/test/local/commands/run.test.ts b/test/local/commands/run.test.ts
index dbbf6fc9b..547b0451e 100644
--- a/test/local/commands/run.test.ts
+++ b/test/local/commands/run.test.ts
@@ -333,14 +333,18 @@ writeFileSync(String.raw\`${joinPath('result.txt')}\`, 'hello world');
 			expect(lastErrorMessage()).toMatch(/Field help must be string/i);
 		});
 
-		it('automatically inserts missing defaulted fields', async () => {
-			writeFileSync(inputPath, '{"awesome": true}', { flag: 'w' });
+		it('does not inject defaults into existing input file', async () => {
+			const originalContent = '{"awesome": true}';
+			writeFileSync(inputPath, originalContent, { flag: 'w' });
 			copyFileSync(defaultsInputSchemaPath, inputSchemaPath);
 
 			await testRunCommand(RunCommand, {});
 
 			const output = JSON.parse(readFileSync(outputPath, 'utf8'));
 			expect(output).toStrictEqual({ awesome: true, help: 'this_maze_is_not_meant_for_you' });
+
+			const inputAfterRun = readFileSync(inputPath, 'utf8');
+			expect(inputAfterRun).toBe(originalContent);
 		});
 
 		it('does not insert missing prefilled fields', async () => {