diff --git a/internal/profiles/resolver.go b/internal/profiles/resolver.go
index 0ae5561..ca69a59 100644
--- a/internal/profiles/resolver.go
+++ b/internal/profiles/resolver.go
@@ -310,9 +310,17 @@ type cliIntent struct {
 }
 
 var judgePathPlaceholderPattern = regexp.MustCompile(`\{\{\s*(prompt|output_schema):([^}]+)\}\}`)
-var scannerIDPattern = regexp.MustCompile(`^[A-Za-z0-9][A-Za-z0-9_-]*$`)
+
+// scannerIDPattern restricts user-defined scanner IDs to lowercase. IDs are
+// lowercased when used as evidence file names, so allowing uppercase would let
+// two case-distinct IDs (Foo and foo) collide on the same output file.
+var scannerIDPattern = regexp.MustCompile(`^[a-z0-9][a-z0-9_-]*$`)
 var scannerTargetPlaceholderPattern = regexp.MustCompile(`\{\{\s*target\s*\}\}`)
 
+// maxScannerIDLength bounds user-defined scanner IDs so <id>.json evidence file
+// names stay within filesystem limits.
+const maxScannerIDLength = 64
+
 type ResolvedRunSet struct {
 	Options     []runner.Options
 	OutputPath  string
@@ -952,7 +960,10 @@ func validateProfile(name string, profile Profile) error {
 			return fmt.Errorf("User-defined scanner in profile %s must include a non-empty id", name)
 		}
 		if scanner.custom && !scannerIDPattern.MatchString(scanner.ID) {
-			return fmt.Errorf("User-defined scanner %s in profile %s has invalid id; use letters, digits, underscores, and hyphens, starting with a letter or digit", scanner.ID, name)
+			return fmt.Errorf("User-defined scanner %s in profile %s has invalid id; use lowercase letters, digits, underscores, and hyphens, starting with a letter or digit", scanner.ID, name)
+		}
+		if scanner.custom && len(scanner.ID) > maxScannerIDLength {
+			return fmt.Errorf("User-defined scanner id in profile %s is %d characters; scanner IDs are used as file names and must be at most %d characters", name, len(scanner.ID), maxScannerIDLength)
 		}
 		if scanner.custom && strings.TrimSpace(scanner.Command) == "" {
 			return fmt.Errorf("User-defined scanner %s in profile %s must include a non-empty command", scanner.ID, name)
diff --git a/internal/profiles/resolver_test.go b/internal/profiles/resolver_test.go
index accb291..4c8ca2d 100644
--- a/internal/profiles/resolver_test.go
+++ b/internal/profiles/resolver_test.go
@@ -1078,7 +1078,42 @@ profiles:
 `)
 
 	_, err := ResolveArgs([]string{"./skill", "--config", config, "--profile", "review"}, dir)
-	if err == nil || err.Error() != "User-defined scanner foo=bar in profile review has invalid id; use letters, digits, underscores, and hyphens, starting with a letter or digit" {
+	if err == nil || err.Error() != "User-defined scanner foo=bar in profile review has invalid id; use lowercase letters, digits, underscores, and hyphens, starting with a letter or digit" {
+		t.Fatalf("err = %v", err)
+	}
+}
+
+func TestResolveArgsRejectsUppercaseUserDefinedScannerID(t *testing.T) {
+	dir := t.TempDir()
+	config := filepath.Join(dir, ".clawscan.yml")
+	writeFile(t, config, `version: 1
+profiles:
+  review:
+    scanners:
+      - id: Foo
+        command: scanner {{target}}
+`)
+
+	_, err := ResolveArgs([]string{"./skill", "--config", config, "--profile", "review"}, dir)
+	if err == nil || err.Error() != "User-defined scanner Foo in profile review has invalid id; use lowercase letters, digits, underscores, and hyphens, starting with a letter or digit" {
+		t.Fatalf("err = %v", err)
+	}
+}
+
+func TestResolveArgsRejectsOversizedUserDefinedScannerID(t *testing.T) {
+	dir := t.TempDir()
+	config := filepath.Join(dir, ".clawscan.yml")
+	longID := strings.Repeat("a", 65)
+	writeFile(t, config, `version: 1
+profiles:
+  review:
+    scanners:
+      - id: `+longID+`
+        command: scanner {{target}}
+`)
+
+	_, err := ResolveArgs([]string{"./skill", "--config", config, "--profile", "review"}, dir)
+	if err == nil || err.Error() != "User-defined scanner id in profile review is 65 characters; scanner IDs are used as file names and must be at most 64 characters" {
 		t.Fatalf("err = %v", err)
 	}
 }