diff --git a/extensions/copilot/src/extension/tools/node/getErrorsTool.tsx b/extensions/copilot/src/extension/tools/node/getErrorsTool.tsx
index ab4f8bac1b2ab..55dfc9e7e71c3 100644
--- a/extensions/copilot/src/extension/tools/node/getErrorsTool.tsx
+++ b/extensions/copilot/src/extension/tools/node/getErrorsTool.tsx
@@ -18,7 +18,8 @@ import { coalesce } from '../../../util/vs/base/common/arrays';
 import { CancellationToken } from '../../../util/vs/base/common/cancellation';
 import { Disposable } from '../../../util/vs/base/common/lifecycle';
 import { ResourceSet } from '../../../util/vs/base/common/map';
-import { isEqualOrParent } from '../../../util/vs/base/common/resources';
+import { isWindows } from '../../../util/vs/base/common/platform';
+import { isEqual, isEqualOrParent } from '../../../util/vs/base/common/resources';
 import { URI } from '../../../util/vs/base/common/uri';
 import { IInstantiationService } from '../../../util/vs/platform/instantiation/common/instantiation';
 import { DiagnosticSeverity, ExtendedLanguageModelToolResult, LanguageModelPromptTsxPart, MarkdownString, Range } from '../../../vscodeTypes';
@@ -95,13 +96,15 @@ export class GetErrorsTool extends Disposable implements ICopilotTool<IGetErrors
 			let inputUri: URI | undefined;
 			let matchedExactPath = false;
 
+			const normalizedResource = this.normalizeDriveLetter(resource);
 			for (const path of nonNotebookPaths) {
 				// we support file or folder paths
-				if (isEqualOrParent(resource, path.uri)) {
+				const normalizedPath = this.normalizeDriveLetter(path.uri);
+				if (isEqualOrParent(normalizedResource, normalizedPath)) {
 					foundMatch = true;
 
 					// Track the input URI that matched - prefer exact matches, otherwise use the folder
-					const isExactMatch = resource.toString() === path.uri.toString();
+					const isExactMatch = isEqual(normalizedResource, normalizedPath);
 					if (isExactMatch) {
 						// Exact match - this is the file itself, no input folder
 						inputUri = undefined;
@@ -146,6 +149,48 @@ export class GetErrorsTool extends Disposable implements ICopilotTool<IGetErrors
 		return results;
 	}
 
+	/**
+	 * Normalize the casing of a Windows drive letter so that diagnostics can be
+	 * matched regardless of the drive letter casing produced by the model or the
+	 * diagnostics provider.
+	 *
+	 * The model (and some language servers) may emit a path with a lower-case drive
+	 * letter (e.g. `c:\foo`) while the diagnostics service reports the same file with
+	 * an upper-case drive letter (e.g. `C:\foo`), or vice versa - the casing is not
+	 * stable across machines (see microsoft/vscode#319858). On Windows the drive
+	 * letter is case-insensitive, so we lower-case *only* the drive letter to make
+	 * the two URIs compare equal.
+	 *
+	 * We intentionally do not lower-case the rest of the path: file systems may be
+	 * case-sensitive past the drive, so `Foo.ts` and `foo.ts` must still be treated
+	 * as different files. This is a no-op on non-Windows platforms and on paths that
+	 * have no drive letter.
+	 *
+	 * The function operates solely on `uri.path`, which is the path component after
+	 * the scheme and authority are stripped. For a standard Windows local file URI
+	 * (`file:///C:/foo/bar`) the authority is empty and `uri.path` is `/C:/foo/bar`,
+	 * so the drive letter is found at `path[1]`. UNC paths (`file://server/share`)
+	 * store the host in the authority and produce a path without a drive letter, so
+	 * they are returned unchanged. The malformed form `file://C:/foo` (drive letter
+	 * in the authority) is not produced by `URI.file()` and is not handled.
+	 *
+	 * Note - This is made public for testing purposes only.
+	 */
+	public normalizeDriveLetter(uri: URI): URI {
+		if (!isWindows) {
+			return uri;
+		}
+		// `uri.path` is of the form `/c:/folder/file` for file URIs with a drive letter.
+		const path = uri.path;
+		if (path.length >= 3 && path[0] === '/' && path[2] === ':') {
+			const lowerDrive = path[1].toLowerCase();
+			if (lowerDrive !== path[1]) {
+				return uri.with({ path: `/${lowerDrive}${path.slice(2)}` });
+			}
+		}
+		return uri;
+	}
+
 	async invoke(options: vscode.LanguageModelToolInvocationOptions<IGetErrorsParams>, token: CancellationToken) {
 		const getAll = () => this.languageDiagnosticsService.getAllDiagnostics()
 			.map(d => ({ uri: d[0], diagnostics: d[1].filter(e => e.severity <= DiagnosticSeverity.Warning), inputUri: undefined }))
@@ -356,4 +401,4 @@ export class DiagnosticToolOutput extends PromptElement<IDiagnosticToolOutputPro
 			)}
 		</>;
 	}
-}
\ No newline at end of file
+}
diff --git a/extensions/copilot/src/extension/tools/node/test/getErrorsTool.spec.tsx b/extensions/copilot/src/extension/tools/node/test/getErrorsTool.spec.tsx
index 96630579f59b6..a5846ffac16c6 100644
--- a/extensions/copilot/src/extension/tools/node/test/getErrorsTool.spec.tsx
+++ b/extensions/copilot/src/extension/tools/node/test/getErrorsTool.spec.tsx
@@ -14,6 +14,7 @@ import { TestWorkspaceService } from '../../../../platform/test/node/testWorkspa
 import { IWorkspaceService } from '../../../../platform/workspace/common/workspaceService';
 import { createTextDocumentData } from '../../../../util/common/test/shims/textDocument';
 import { CancellationToken } from '../../../../util/vs/base/common/cancellation';
+import { isWindows } from '../../../../util/vs/base/common/platform';
 import { URI } from '../../../../util/vs/base/common/uri';
 import { SyncDescriptor } from '../../../../util/vs/platform/instantiation/common/descriptors';
 import { IInstantiationService } from '../../../../util/vs/platform/instantiation/common/instantiation';
@@ -38,6 +39,8 @@ suite('GetErrorsTool - Tool Invocation', () => {
 	const noErrorFile = URI.file('/test/workspace/src/noErrorFile.ts');
 	const eslintErrorFile = URI.file('/test/workspace/eslint/eslint_unexpected_constant_condition_1.ts');
 	const emptyLineErrorFile = URI.file('/test/workspace/emptyLineError.ts');
+	const upperCaseDriveFile = URI.file('C:/test/workspace/src/windowsDriveFile.ts');
+	const lowerCaseDriveFile = URI.file('c:/test/workspace/src/windowsDriveFile.ts');
 
 	beforeEach(() => {
 		collection = createExtensionUnitTestingServices();
@@ -50,8 +53,9 @@ suite('GetErrorsTool - Tool Invocation', () => {
 		const eslintErrorDoc = createTextDocumentData(eslintErrorFile, 'if (true) {\n  console.log("This is a constant condition");\n}', 'ts').document;
 		// File with a trailing empty line where the error is reported
 		const emptyLineErrorDoc = createTextDocumentData(emptyLineErrorFile, 'codeunit 50100 MyCU {\n  procedure Foo() {\n\n', 'ts').document;
+		const upperCaseDriveDoc = createTextDocumentData(upperCaseDriveFile, 'const drive = "C";\n', 'ts').document;
 
-		collection.define(IWorkspaceService, new SyncDescriptor(TestWorkspaceService, [[workspaceFolder], [tsDoc1, tsDoc2, jsDoc, noErrorDoc, eslintErrorDoc, emptyLineErrorDoc]]));
+		collection.define(IWorkspaceService, new SyncDescriptor(TestWorkspaceService, [[workspaceFolder], [tsDoc1, tsDoc2, jsDoc, noErrorDoc, eslintErrorDoc, emptyLineErrorDoc, upperCaseDriveDoc]]));
 
 		// Set up diagnostics service
 		diagnosticsService = new TestLanguageDiagnosticsService();
@@ -138,6 +142,21 @@ suite('GetErrorsTool - Tool Invocation', () => {
 		]);
 	});
 
+	test.skipIf(!isWindows)('getDiagnostics - matches Windows file path with different drive letter casing', () => {
+		const diagnostic = {
+			message: 'Drive letter casing diagnostic',
+			range: new Range(0, 6, 0, 11),
+			severity: DiagnosticSeverity.Error
+		};
+		diagnosticsService.setDiagnostics(upperCaseDriveFile, [diagnostic]);
+
+		const results = tool.getDiagnostics([{ uri: lowerCaseDriveFile, range: undefined }]);
+
+		expect(results).toEqual([
+			{ uri: upperCaseDriveFile, diagnostics: [diagnostic] }
+		]);
+	});
+
 	test('getDiagnostics - filters by folder path', () => {
 		// Test with folder path
 		const srcFolder = URI.file('/test/workspace/src');
@@ -211,6 +230,22 @@ suite('GetErrorsTool - Tool Invocation', () => {
 		expect(msg).toMatchSnapshot();
 	});
 
+	test.skipIf(!isWindows)('Tool invocation - filePath matches diagnostics when Windows drive letter casing differs', async () => {
+		diagnosticsService.setDiagnostics(upperCaseDriveFile, [
+			{
+				message: 'Drive letter casing diagnostic',
+				range: new Range(0, 6, 0, 11),
+				severity: DiagnosticSeverity.Error
+			}
+		]);
+
+		const pathRep = accessor.get(IPromptPathRepresentationService);
+		const result = await tool.invoke({ input: { filePaths: [lowerCaseDriveFile.fsPath] }, toolInvocationToken: null! }, CancellationToken.None);
+		const msg = await toolResultToString(accessor, result);
+		expect(msg).toContain(`<errors path="${pathRep.getFilePath(upperCaseDriveFile)}">`);
+		expect(msg).toContain('Drive letter casing diagnostic');
+	});
+
 	test('Tool invocation - with folder path includes diagnostics from contained files', async () => {
 		const pathRep = accessor.get(IPromptPathRepresentationService);
 		const srcFolderUri = URI.file('/test/workspace/src');
@@ -260,4 +295,82 @@ suite('GetErrorsTool - Tool Invocation', () => {
 		const msg = await toolResultToString(accessor, result);
 		expect(msg).toMatchSnapshot();
 	});
-});
\ No newline at end of file
+});
+
+suite('GetErrorsTool - normalizeDriveLetter', () => {
+	let accessor: ITestingServicesAccessor;
+	let collection: TestingServiceCollection;
+	let tool: GetErrorsTool;
+
+	beforeEach(() => {
+		collection = createExtensionUnitTestingServices();
+		const workspaceFolder = URI.file('/test/workspace');
+		const tsDoc = createTextDocumentData(URI.file('/test/workspace/file.ts'), '', 'ts').document;
+		collection.define(IWorkspaceService, new SyncDescriptor(TestWorkspaceService, [[workspaceFolder], [tsDoc]]));
+		collection.define(ILanguageDiagnosticsService, new TestLanguageDiagnosticsService());
+		const fileSystemService = new MockFileSystemService();
+		collection.define(IFileSystemService, fileSystemService);
+		accessor = collection.createTestingAccessor();
+		tool = accessor.get(IInstantiationService).createInstance(GetErrorsTool);
+	});
+
+	afterEach(() => {
+		accessor.dispose();
+	});
+
+	test.skipIf(isWindows)('non-Windows: returns the same URI instance for any path shape', () => {
+		const uris = [
+			URI.parse('file:///C:/foo/bar.ts'),
+			URI.parse('file:///c:/foo/bar.ts'),
+			URI.parse('file:///foo/bar.ts'),
+			URI.parse('file:///'),
+		];
+		for (const uri of uris) {
+			expect(tool.normalizeDriveLetter(uri)).toBe(uri);
+		}
+	});
+
+	test.skipIf(!isWindows)('Windows: lowercases an uppercase drive letter', () => {
+		const uri = URI.parse('file:///C:/foo/bar.ts');
+		const result = tool.normalizeDriveLetter(uri);
+		expect(result.path).toBe('/c:/foo/bar.ts');
+	});
+
+	test.skipIf(!isWindows)('Windows: returns same URI instance when drive letter is already lowercase', () => {
+		const uri = URI.parse('file:///c:/foo/bar.ts');
+		expect(tool.normalizeDriveLetter(uri)).toBe(uri);
+	});
+
+	test.skipIf(!isWindows)('Windows: preserves case of path components after the drive letter', () => {
+		const uri = URI.parse('file:///C:/MyProject/SrcFile.ts');
+		const result = tool.normalizeDriveLetter(uri);
+		expect(result.path).toBe('/c:/MyProject/SrcFile.ts');
+	});
+
+	test.skipIf(!isWindows)('Windows: returns URI unchanged when path has no drive letter', () => {
+		const uri = URI.parse('file:///foo/bar.ts');
+		expect(tool.normalizeDriveLetter(uri)).toBe(uri);
+	});
+
+	test.skipIf(!isWindows)('Windows: returns URI unchanged when path is too short to contain a drive letter', () => {
+		const uri = URI.parse('file:///c');
+		expect(tool.normalizeDriveLetter(uri)).toBe(uri);
+	});
+
+	test.skipIf(!isWindows)('Windows: returns URI unchanged for root path', () => {
+		const uri = URI.parse('file:///');
+		expect(tool.normalizeDriveLetter(uri)).toBe(uri);
+	});
+
+	test.skipIf(!isWindows)('Windows: normalizes drive letter when path ends right after the colon', () => {
+		const uri = URI.parse('file:///C:');
+		const result = tool.normalizeDriveLetter(uri);
+		expect(result.path).toBe('/c:');
+	});
+
+	test.skipIf(!isWindows)('Windows: returns URI unchanged when drive position holds a non-letter character', () => {
+		// digit at drive position — `toLowerCase()` is a no-op so the URI is returned as-is
+		const uri = URI.parse('file:///1:/foo/bar.ts');
+		expect(tool.normalizeDriveLetter(uri)).toBe(uri);
+	});
+});