diff --git a/.changeset/calm-paths-agree.md b/.changeset/calm-paths-agree.md
new file mode 100644
index 00000000..3bf24900
--- /dev/null
+++ b/.changeset/calm-paths-agree.md
@@ -0,0 +1,5 @@
+---
+"leadtype": patch
+---
+
+Fix snippet typechecking on Windows by normalizing virtual file paths before TypeScript compiler host lookups.
diff --git a/packages/leadtype/src/lint/snippet-typecheck.test.ts b/packages/leadtype/src/lint/snippet-typecheck.test.ts
index 789343a0..fb3fde7a 100644
--- a/packages/leadtype/src/lint/snippet-typecheck.test.ts
+++ b/packages/leadtype/src/lint/snippet-typecheck.test.ts
@@ -3,6 +3,7 @@ import { tmpdir } from "node:os";
 import path from "node:path";
 import { afterEach, describe, expect, it } from "vitest";
 import { lintDocs } from "./runner";
+import { toPosixPath } from "./snippet-typecheck";
 
 const tempDirs: string[] = [];
 
@@ -188,4 +189,40 @@ describe("snippet typechecking", () => {
     );
     expect(violations).toEqual([]);
   });
+
+  it("catches typecheck errors when projectRoot contains Windows-style backslashes (Windows regression integration test)", async () => {
+    const { projectRoot, srcDir } = await createTypecheckProject();
+
+    // Create a path containing backslashes to simulate a Windows environment path separator mismatch
+    const backslashedProjectRoot = `${projectRoot}${path.sep}subdir\\subsubdir`;
+
+    const violations = await lintWithTypecheck(
+      backslashedProjectRoot,
+      srcDir,
+      [
+        "```ts",
+        "// @check",
+        'const x: number = "this is a string";',
+        "```",
+        "",
+      ].join("\n")
+    );
+
+    // This should successfully detect the type error
+    expect(violations).toEqual([
+      expect.objectContaining({
+        rule: "snippet:types",
+        severity: "error",
+        message: expect.stringContaining("string"),
+      }),
+    ]);
+  });
+
+  describe("toPosixPath", () => {
+    it("converts Windows backslash paths to POSIX forward slash paths", () => {
+      expect(
+        toPosixPath("C:\\project\\root\\.leadtype-snippet-0\\main.ts")
+      ).toBe("C:/project/root/.leadtype-snippet-0/main.ts");
+    });
+  });
 });
diff --git a/packages/leadtype/src/lint/snippet-typecheck.ts b/packages/leadtype/src/lint/snippet-typecheck.ts
index 6af71669..49268ab3 100644
--- a/packages/leadtype/src/lint/snippet-typecheck.ts
+++ b/packages/leadtype/src/lint/snippet-typecheck.ts
@@ -217,6 +217,10 @@ function isUnresolvedPackageImport(
   return !(specifier.startsWith(".") || specifier.startsWith("/"));
 }
 
+export function toPosixPath(filePath: string): string {
+  return filePath.replace(/\\/g, "/");
+}
+
 export function typecheckSnippets(
   options: TypecheckSnippetsOptions
 ): SnippetTypecheckIssue[] {
@@ -229,7 +233,7 @@ export function typecheckSnippets(
   for (const [index, snippet] of options.snippets.entries()) {
     const virtualDir = `${projectRoot}/.leadtype-snippet-${index}`;
     for (const file of toVirtualFiles(snippet, virtualDir)) {
-      virtualFiles.set(file.path, file);
+      virtualFiles.set(toPosixPath(file.path), file);
     }
   }
 
@@ -245,16 +249,16 @@ export function typecheckSnippets(
   const baseGetSourceFile = host.getSourceFile.bind(host);
   const baseDirectoryExists = host.directoryExists?.bind(host);
   host.fileExists = (fileName) =>
-    virtualFiles.has(fileName) || baseFileExists(fileName);
+    virtualFiles.has(toPosixPath(fileName)) || baseFileExists(fileName);
   host.readFile = (fileName) =>
-    virtualFiles.get(fileName)?.content ?? baseReadFile(fileName);
+    virtualFiles.get(toPosixPath(fileName))?.content ?? baseReadFile(fileName);
   // Module resolution probes the containing directory; virtual dirs never
   // exist on disk, so sibling imports (`./helpers`) need this to resolve.
   host.directoryExists = (directoryName) =>
-    virtualDirs.has(directoryName) ||
+    virtualDirs.has(toPosixPath(directoryName)) ||
     (baseDirectoryExists?.(directoryName) ?? false);
   host.getSourceFile = (fileName, languageVersion, ...rest) => {
-    const virtual = virtualFiles.get(fileName);
+    const virtual = virtualFiles.get(toPosixPath(fileName));
     if (virtual) {
       return ts.createSourceFile(fileName, virtual.content, languageVersion);
     }
@@ -287,7 +291,9 @@ export function typecheckSnippets(
       continue;
     }
     const fileName = diagnostic.file?.fileName;
-    const virtual = fileName ? virtualFiles.get(fileName) : undefined;
+    const virtual = fileName
+      ? virtualFiles.get(toPosixPath(fileName))
+      : undefined;
     if (!virtual) {
       continue; // diagnostics from real project files aren't lint's business
     }