diff --git a/packages/travsr-lsif-py/src/security.ts b/packages/travsr-lsif-py/src/security.ts
index a79b76f3..d5ce9cb2 100644
--- a/packages/travsr-lsif-py/src/security.ts
+++ b/packages/travsr-lsif-py/src/security.ts
@@ -46,11 +46,23 @@ export function assertPathsContained(filePaths: string[], repoRoot: string): voi
  * Pure in-memory containment check — no I/O, safe to call in hot loops.
  * Does NOT follow symlinks; use assertPathsContained for the authoritative check.
  * Used as a belt-and-suspenders filter before emitting each file's ranges.
+ *
+ * Both operands are resolved before comparing (#806). Resolving only filePath
+ * put the two sides in different namespaces on Windows, where resolve()
+ * prepends the current drive letter and normalize() does not, so a genuinely
+ * contained file compared "C:\repo\src\a.ts" against "\repo" and was dropped.
+ *
+ * pathImpl exists so the tests can pin this function's Windows and POSIX
+ * behaviour from any host; production callers never pass it.
  */
-export function isUnderRoot(filePath: string, repoRoot: string): boolean {
-  const normalized = path.normalize(path.resolve(filePath));
-  const root = path.normalize(repoRoot);
-  return normalized.startsWith(root + path.sep) || normalized === root;
+export function isUnderRoot(
+  filePath: string,
+  repoRoot: string,
+  pathImpl: typeof path = path
+): boolean {
+  const normalized = pathImpl.normalize(pathImpl.resolve(filePath));
+  const root = pathImpl.normalize(pathImpl.resolve(repoRoot));
+  return normalized.startsWith(root + pathImpl.sep) || normalized === root;
 }
 
 // ── Internal helpers ───────────────────────────────────────────────────────────
diff --git a/packages/travsr-lsif-py/src/test/security.test.ts b/packages/travsr-lsif-py/src/test/security.test.ts
index 631ea51c..ce2e8319 100644
--- a/packages/travsr-lsif-py/src/test/security.test.ts
+++ b/packages/travsr-lsif-py/src/test/security.test.ts
@@ -45,6 +45,52 @@ test('isUnderRoot: correctly rejects escaped paths', () => {
   assert.ok(!isUnderRoot('/etc/passwd', root));
   assert.ok(!isUnderRoot('/', root));
 });
+// #806: isUnderRoot resolved filePath but not repoRoot. On Windows resolve()
+// prepends the current drive letter and normalize() does not, so the two sides
+// compared "C:\...\foo.py" against "\home\user\project" and every
+// contained file was rejected. The platform-native test above only catches that
+// when run ON Windows; these two pin both platforms' semantics from any host by
+// injecting the flavour explicitly. Inputs are fully qualified — a
+// drive-relative path would pick up the host's cwd and stop being
+// deterministic.
+test('isUnderRoot: Windows semantics, verified from any host (#806)', () => {
+  const root = 'C:\\home\\user\\project';
+  assert.ok(isUnderRoot('C:\\home\\user\\project\\src\\foo.py', root, path.win32));
+  assert.ok(isUnderRoot('C:\\home\\user\\project', root, path.win32));
+  assert.ok(
+    isUnderRoot('C:/home/user/project/src/foo.py', root, path.win32),
+    'forward slashes normalize to backslashes'
+  );
+  assert.ok(!isUnderRoot('C:\\home\\user\\other\\foo.py', root, path.win32));
+  assert.ok(
+    !isUnderRoot('C:\\home\\user\\project-evil\\foo.py', root, path.win32),
+    'prefix match is not enough'
+  );
+  assert.ok(
+    !isUnderRoot('D:\\home\\user\\project\\src\\foo.py', root, path.win32),
+    'same path on another drive is outside'
+  );
+  assert.ok(
+    !isUnderRoot('C:\\home\\user\\project\\src\\..\\..\\..\\etc\\passwd', root, path.win32),
+    'traversal collapses before comparing'
+  );
+});
+
+test('isUnderRoot: POSIX semantics, verified from any host (#806)', () => {
+  const root = '/home/user/project';
+  assert.ok(isUnderRoot('/home/user/project/src/foo.py', root, path.posix));
+  assert.ok(isUnderRoot('/home/user/project', root, path.posix));
+  assert.ok(!isUnderRoot('/home/user/other/foo.py', root, path.posix));
+  assert.ok(
+    !isUnderRoot('/home/user/project-evil/foo.py', root, path.posix),
+    'prefix match is not enough'
+  );
+  assert.ok(
+    !isUnderRoot('/home/user/project/../project-evil/foo.py', root, path.posix),
+    'traversal collapses before comparing'
+  );
+});
+
 
 test('assertPathsContained: accepts files inside root', () => {
   const root = resolveRoot('/tmp');
diff --git a/packages/travsr-lsif-ts/src/security.ts b/packages/travsr-lsif-ts/src/security.ts
index 7eed7eac..e4769db3 100644
--- a/packages/travsr-lsif-ts/src/security.ts
+++ b/packages/travsr-lsif-ts/src/security.ts
@@ -134,11 +134,23 @@ export function assertFilesContained(
  * Pure in-memory containment check — no I/O, safe to call in hot filter loops.
  * Does NOT follow symlinks; use assertFilesContained for the authoritative check.
  * Used as a belt-and-suspenders filter on program.getSourceFiles().
+ *
+ * Both operands are resolved before comparing (#806). Resolving only filePath
+ * put the two sides in different namespaces on Windows, where resolve()
+ * prepends the current drive letter and normalize() does not, so a genuinely
+ * contained file compared "C:\repo\src\a.ts" against "\repo" and was dropped.
+ *
+ * pathImpl exists so the tests can pin this function's Windows and POSIX
+ * behaviour from any host; production callers never pass it.
  */
-export function isUnderRoot(filePath: string, repoRoot: string): boolean {
-  const normalized = path.normalize(path.resolve(filePath));
-  const root = path.normalize(repoRoot);
-  return normalized.startsWith(root + path.sep) || normalized === root;
+export function isUnderRoot(
+  filePath: string,
+  repoRoot: string,
+  pathImpl: typeof path = path
+): boolean {
+  const normalized = pathImpl.normalize(pathImpl.resolve(filePath));
+  const root = pathImpl.normalize(pathImpl.resolve(repoRoot));
+  return normalized.startsWith(root + pathImpl.sep) || normalized === root;
 }
 
 // ── Internal helpers ──────────────────────────────────────────────────────────
diff --git a/packages/travsr-lsif-ts/src/test/security.test.ts b/packages/travsr-lsif-ts/src/test/security.test.ts
index d2d890d7..0328872c 100644
--- a/packages/travsr-lsif-ts/src/test/security.test.ts
+++ b/packages/travsr-lsif-ts/src/test/security.test.ts
@@ -115,6 +115,52 @@ test('isUnderRoot: correctly identifies contained and escaped paths', () => {
   assert.ok(!isUnderRoot('/home/user/other/foo.ts', root));
   assert.ok(!isUnderRoot('/home/user/project-evil/foo.ts', root), 'prefix match is not enough');
 });
+// #806: isUnderRoot resolved filePath but not repoRoot. On Windows resolve()
+// prepends the current drive letter and normalize() does not, so the two sides
+// compared "C:\...\foo.ts" against "\home\user\project" and every
+// contained file was rejected. The platform-native test above only catches that
+// when run ON Windows; these two pin both platforms' semantics from any host by
+// injecting the flavour explicitly. Inputs are fully qualified — a
+// drive-relative path would pick up the host's cwd and stop being
+// deterministic.
+test('isUnderRoot: Windows semantics, verified from any host (#806)', () => {
+  const root = 'C:\\home\\user\\project';
+  assert.ok(isUnderRoot('C:\\home\\user\\project\\src\\foo.ts', root, path.win32));
+  assert.ok(isUnderRoot('C:\\home\\user\\project', root, path.win32));
+  assert.ok(
+    isUnderRoot('C:/home/user/project/src/foo.ts', root, path.win32),
+    'forward slashes normalize to backslashes'
+  );
+  assert.ok(!isUnderRoot('C:\\home\\user\\other\\foo.ts', root, path.win32));
+  assert.ok(
+    !isUnderRoot('C:\\home\\user\\project-evil\\foo.ts', root, path.win32),
+    'prefix match is not enough'
+  );
+  assert.ok(
+    !isUnderRoot('D:\\home\\user\\project\\src\\foo.ts', root, path.win32),
+    'same path on another drive is outside'
+  );
+  assert.ok(
+    !isUnderRoot('C:\\home\\user\\project\\src\\..\\..\\..\\etc\\passwd', root, path.win32),
+    'traversal collapses before comparing'
+  );
+});
+
+test('isUnderRoot: POSIX semantics, verified from any host (#806)', () => {
+  const root = '/home/user/project';
+  assert.ok(isUnderRoot('/home/user/project/src/foo.ts', root, path.posix));
+  assert.ok(isUnderRoot('/home/user/project', root, path.posix));
+  assert.ok(!isUnderRoot('/home/user/other/foo.ts', root, path.posix));
+  assert.ok(
+    !isUnderRoot('/home/user/project-evil/foo.ts', root, path.posix),
+    'prefix match is not enough'
+  );
+  assert.ok(
+    !isUnderRoot('/home/user/project/../project-evil/foo.ts', root, path.posix),
+    'traversal collapses before comparing'
+  );
+});
+
 
 // ── Integration tests against malicious tsconfig fixtures ─────────────────────