diff --git a/src/skillspector/nodes/build_context.py b/src/skillspector/nodes/build_context.py
index 694a048..225c26d 100644
--- a/src/skillspector/nodes/build_context.py
+++ b/src/skillspector/nodes/build_context.py
@@ -87,7 +87,10 @@ def _walk_skill_files(skill_dir: Path) -> list[str]:
             continue
         try:
             rel = item.relative_to(skill_dir)
-            paths.append(str(rel))
+            # Use forward slashes on every OS: these relative paths are dict keys
+            # and SARIF/URI locations, so they must be portable (not OS-specific
+            # backslashes on Windows).
+            paths.append(rel.as_posix())
         except ValueError:
             logger.debug("Skipping path (not under skill_dir): %s", item)
             continue
diff --git a/tests/nodes/analyzers/test_semantic_developer_intent.py b/tests/nodes/analyzers/test_semantic_developer_intent.py
index 0ddc704..408daa9 100644
--- a/tests/nodes/analyzers/test_semantic_developer_intent.py
+++ b/tests/nodes/analyzers/test_semantic_developer_intent.py
@@ -342,7 +342,7 @@ def _build_file_cache(skill_dir: Path) -> dict[str, str]:
     for item in sorted(skill_dir.rglob("*")):
         if not item.is_file():
             continue
-        rel = str(item.relative_to(skill_dir))
+        rel = item.relative_to(skill_dir).as_posix()  # forward slashes on every OS
         try:
             cache[rel] = item.read_text(encoding="utf-8", errors="replace")
         except OSError:
diff --git a/tests/nodes/analyzers/test_semantic_security_discovery.py b/tests/nodes/analyzers/test_semantic_security_discovery.py
index 85209f6..5b0c53b 100644
--- a/tests/nodes/analyzers/test_semantic_security_discovery.py
+++ b/tests/nodes/analyzers/test_semantic_security_discovery.py
@@ -317,7 +317,7 @@ def _build_file_cache(skill_dir: Path) -> dict[str, str]:
     for item in sorted(skill_dir.rglob("*")):
         if not item.is_file():
             continue
-        rel = str(item.relative_to(skill_dir))
+        rel = item.relative_to(skill_dir).as_posix()  # forward slashes on every OS
         try:
             cache[rel] = item.read_text(encoding="utf-8", errors="replace")
         except OSError:
diff --git a/tests/nodes/test_semantic_quality_policy.py b/tests/nodes/test_semantic_quality_policy.py
index f80960b..9d52d33 100644
--- a/tests/nodes/test_semantic_quality_policy.py
+++ b/tests/nodes/test_semantic_quality_policy.py
@@ -289,7 +289,7 @@ def _build_file_cache(skill_dir: Path) -> dict[str, str]:
     for item in sorted(skill_dir.rglob("*")):
         if not item.is_file():
             continue
-        rel = str(item.relative_to(skill_dir))
+        rel = item.relative_to(skill_dir).as_posix()  # forward slashes on every OS
         try:
             cache[rel] = item.read_text(encoding="utf-8", errors="replace")
         except OSError:
diff --git a/tests/test_mcp_least_privilege.py b/tests/test_mcp_least_privilege.py
index 9e7852e..8195986 100644
--- a/tests/test_mcp_least_privilege.py
+++ b/tests/test_mcp_least_privilege.py
@@ -98,7 +98,7 @@ def _make_state(fixture_name: str) -> dict:
             continue
         if item.name.startswith(".") and not item.name.startswith(".claude"):
             continue
-        rel = str(item.relative_to(fixture_dir))
+        rel = item.relative_to(fixture_dir).as_posix()  # forward slashes on every OS
         components.append(rel)
     components.sort()
 
diff --git a/tests/test_mcp_tool_poisoning.py b/tests/test_mcp_tool_poisoning.py
index 7d5b524..428897c 100644
--- a/tests/test_mcp_tool_poisoning.py
+++ b/tests/test_mcp_tool_poisoning.py
@@ -121,7 +121,7 @@ def _make_state(
             continue
         if item.name.startswith(".") and not item.name.startswith(".claude"):
             continue
-        rel = str(item.relative_to(fixture_dir))
+        rel = item.relative_to(fixture_dir).as_posix()  # forward slashes on every OS
         components.append(rel)
     components.sort()