diff --git a/integration-tests/file-system.test.ts b/integration-tests/file-system.test.ts
index 6b150be53b..cd880d66aa 100644
--- a/integration-tests/file-system.test.ts
+++ b/integration-tests/file-system.test.ts
@@ -5,6 +5,8 @@
  */
 
 import { describe, it, expect } from 'vitest';
+import { existsSync } from 'node:fs';
+import * as path from 'node:path';
 import { TestRig, printDebugInfo, validateModelOutput } from './test-helper.js';
 
 describe('file-system', () => {
@@ -90,4 +92,170 @@ describe('file-system', () => {
       console.log('File written successfully with hello message.');
     }
   });
+
+  it('should correctly handle file paths with spaces', async () => {
+    const rig = new TestRig();
+    await rig.setup('should correctly handle file paths with spaces');
+    const fileName = 'my test file.txt';
+
+    const result = await rig.run(`write "hello" to "${fileName}"`);
+
+    const foundToolCall = await rig.waitForToolCall('write_file');
+    if (!foundToolCall) {
+      printDebugInfo(rig, result);
+    }
+    expect(
+      foundToolCall,
+      'Expected to find a write_file tool call',
+    ).toBeTruthy();
+
+    const newFileContent = rig.readFile(fileName);
+    expect(newFileContent).toBe('hello');
+  });
+
+  it('should perform a read-then-write sequence', async () => {
+    const rig = new TestRig();
+    await rig.setup('should perform a read-then-write sequence');
+    const fileName = 'version.txt';
+    rig.createFile(fileName, '1.0.0');
+
+    const prompt = `Read the version from ${fileName} and write the next version 1.0.1 back to the file.`;
+    const result = await rig.run(prompt);
+
+    await rig.waitForTelemetryReady();
+    const toolLogs = rig.readToolLogs();
+
+    const readCall = toolLogs.find(
+      (log) => log.toolRequest.name === 'read_file',
+    );
+    const writeCall = toolLogs.find(
+      (log) =>
+        log.toolRequest.name === 'write_file' ||
+        log.toolRequest.name === 'replace',
+    );
+
+    if (!readCall || !writeCall) {
+      printDebugInfo(rig, result, { readCall, writeCall });
+    }
+
+    expect(readCall, 'Expected to find a read_file tool call').toBeDefined();
+    expect(
+      writeCall,
+      'Expected to find a write_file or replace tool call',
+    ).toBeDefined();
+
+    const newFileContent = rig.readFile(fileName);
+    expect(newFileContent).toBe('1.0.1');
+  });
+
+  it('should replace multiple instances of a string', async () => {
+    const rig = new TestRig();
+    await rig.setup('should replace multiple instances of a string');
+    const fileName = 'ambiguous.txt';
+    const fileContent = 'Hey there, \ntest line\ntest line';
+    const expectedContent = 'Hey there, \nnew line\nnew line';
+    rig.createFile(fileName, fileContent);
+
+    const result = await rig.run(
+      `replace "test line" with "new line" in ${fileName}`,
+    );
+
+    const foundToolCall = await rig.waitForAnyToolCall([
+      'replace',
+      'write_file',
+    ]);
+    if (!foundToolCall) {
+      printDebugInfo(rig, result);
+    }
+    expect(
+      foundToolCall,
+      'Expected to find a replace or write_file tool call',
+    ).toBeTruthy();
+
+    const toolLogs = rig.readToolLogs();
+    const successfulEdit = toolLogs.some(
+      (log) =>
+        (log.toolRequest.name === 'replace' ||
+          log.toolRequest.name === 'write_file') &&
+        log.toolRequest.success,
+    );
+    if (!successfulEdit) {
+      console.error(
+        'Expected a successful edit tool call, but none was found.',
+      );
+      printDebugInfo(rig, result);
+    }
+    expect(successfulEdit, 'Expected a successful edit tool call').toBeTruthy();
+
+    const newFileContent = rig.readFile(fileName);
+    expect(newFileContent).toBe(expectedContent);
+  });
+
+  it('should fail safely when trying to edit a non-existent file', async () => {
+    const rig = new TestRig();
+    await rig.setup(
+      'should fail safely when trying to edit a non-existent file',
+    );
+    const fileName = 'non_existent.txt';
+
+    const result = await rig.run(`In ${fileName}, replace "a" with "b"`);
+
+    await rig.waitForTelemetryReady();
+    const toolLogs = rig.readToolLogs();
+
+    const readAttempt = toolLogs.find(
+      (log) => log.toolRequest.name === 'read_file',
+    );
+    const writeAttempt = toolLogs.find(
+      (log) => log.toolRequest.name === 'write_file',
+    );
+    const successfulReplace = toolLogs.find(
+      (log) => log.toolRequest.name === 'replace' && log.toolRequest.success,
+    );
+
+    // The model can either investigate (and fail) or do nothing.
+    // If it chose to investigate by reading, that read must have failed.
+    if (readAttempt && readAttempt.toolRequest.success) {
+      console.error(
+        'A read_file attempt succeeded for a non-existent file when it should have failed.',
+      );
+      printDebugInfo(rig, result);
+    }
+    if (readAttempt) {
+      expect(
+        readAttempt.toolRequest.success,
+        'If model tries to read the file, that attempt must fail',
+      ).toBe(false);
+    }
+
+    // CRITICAL: Verify that no matter what the model did, it never successfully
+    // wrote or replaced anything.
+    if (writeAttempt) {
+      console.error(
+        'A write_file attempt was made when no file should be written.',
+      );
+      printDebugInfo(rig, result);
+    }
+    expect(
+      writeAttempt,
+      'write_file should not have been called',
+    ).toBeUndefined();
+
+    if (successfulReplace) {
+      console.error('A successful replace occurred when it should not have.');
+      printDebugInfo(rig, result);
+    }
+    expect(
+      successfulReplace,
+      'A successful replace should not have occurred',
+    ).toBeUndefined();
+
+    // Final verification: ensure the file was not created.
+    const filePath = path.join(rig.testDir!, fileName);
+    const fileExists = existsSync(filePath);
+
+    expect(fileExists, 'The non-existent file should not be created').toBe(
+      false,
+    );
+  });
 });
diff --git a/integration-tests/replace.test.ts b/integration-tests/replace.test.ts
index 0de23a04c7..6c06f73862 100644
--- a/integration-tests/replace.test.ts
+++ b/integration-tests/replace.test.ts
@@ -91,4 +91,98 @@ describe('replace', () => {
     const newFileContent = rig.readFile(fileName);
     expect(newFileContent).toBe(expectedContent);
   });
+
+  it('should fail safely when old_string is not found', async () => {
+    const rig = new TestRig();
+    await rig.setup('should fail safely when old_string is not found');
+    const fileName = 'no_match.txt';
+    const fileContent = 'hello world';
+    rig.createFile(fileName, fileContent);
+
+    const prompt = `replace "goodbye" with "farewell" in ${fileName}`;
+    await rig.run(prompt);
+
+    await rig.waitForTelemetryReady();
+    const toolLogs = rig.readToolLogs();
+
+    const replaceAttempt = toolLogs.find(
+      (log) => log.toolRequest.name === 'replace',
+    );
+    const readAttempt = toolLogs.find(
+      (log) => log.toolRequest.name === 'read_file',
+    );
+
+    // VERIFY: The model must have at least tried to read the file or perform a replace.
+    expect(
+      readAttempt || replaceAttempt,
+      'Expected model to attempt a read_file or replace',
+    ).toBeDefined();
+
+    // If the model tried to replace, that specific attempt must have failed.
+    if (replaceAttempt) {
+      if (replaceAttempt.toolRequest.success) {
+        console.error(
+          'The replace tool succeeded when it was expected to fail',
+        );
+        console.error('Tool call args:', replaceAttempt.toolRequest.args);
+      }
+      expect(
+        replaceAttempt.toolRequest.success,
+        'If replace is called, it must fail',
+      ).toBe(false);
+    }
+
+    // CRITICAL: The final content of the file must be unchanged.
+    const newFileContent = rig.readFile(fileName);
+    expect(newFileContent).toBe(fileContent);
+  });
+
+  it('should insert a multi-line block of text', async () => {
+    const rig = new TestRig();
+    await rig.setup('should insert a multi-line block of text');
+    const fileName = 'insert_block.js';
+    const originalContent = 'function hello() {\n  // INSERT_CODE_HERE\n}';
+    const newBlock = "console.log('hello');\n  console.log('world');";
+    const expectedContent = `function hello() {\n  ${newBlock}\n}`;
+    rig.createFile(fileName, originalContent);
+
+    const prompt = `In ${fileName}, replace "// INSERT_CODE_HERE" with:\n${newBlock}`;
+    const result = await rig.run(prompt);
+
+    const foundToolCall = await rig.waitForToolCall('replace');
+    if (!foundToolCall) {
+      printDebugInfo(rig, result);
+    }
+    expect(foundToolCall, 'Expected to find a replace tool call').toBeTruthy();
+
+    const newFileContent = rig.readFile(fileName);
+
+    expect(newFileContent.replace(/\r\n/g, '\n')).toBe(
+      expectedContent.replace(/\r\n/g, '\n'),
+    );
+  });
+
+  it('should delete a block of text', async () => {
+    const rig = new TestRig();
+    await rig.setup('should delete a block of text');
+    const fileName = 'delete_block.txt';
+    const blockToDelete =
+      '## DELETE THIS ##\nThis is a block of text to delete.\n## END DELETE ##';
+    const originalContent = `Hello\n${blockToDelete}\nWorld`;
+    const expectedContent = 'Hello\nWorld';
+    rig.createFile(fileName, originalContent);
+
+    const prompt = `In ${fileName}, delete the entire block from "## DELETE THIS ##" to "## END DELETE ##" including the markers.`;
+    const result = await rig.run(prompt);
+
+    const foundToolCall = await rig.waitForToolCall('replace');
+    if (!foundToolCall) {
+      printDebugInfo(rig, result);
+    }
+    expect(foundToolCall, 'Expected to find a replace tool call').toBeTruthy();
+
+    const newFileContent = rig.readFile(fileName);
+
+    expect(newFileContent).toBe(expectedContent);
+  });
 });