diff --git a/internal/runner/runner_test.go b/internal/runner/runner_test.go
index dcf8ff5..d47003a 100644
--- a/internal/runner/runner_test.go
+++ b/internal/runner/runner_test.go
@@ -901,6 +901,55 @@ func TestUserDefinedScannerUsesIsolatedCwd(t *testing.T) {
 	}
 }
 
+func TestSanitizedDeclaredEnvNamesStripsValues(t *testing.T) {
+	env := []string{"SECRET_KEY=value", "TOKEN", "EMPTY="}
+	names := sanitizedDeclaredEnvNames(env)
+	if len(names) != 3 || names[0] != "SECRET_KEY" || names[1] != "TOKEN" || names[2] != "EMPTY" {
+		t.Fatalf("names = %#v", names)
+	}
+}
+
+func TestRedactDeclaredEnvValuesRemovesSecrets(t *testing.T) {
+	env := map[string]string{"SECRET_KEY": "my-secret-value"}
+	names := []string{"SECRET_KEY"}
+	message := "Error: my-secret-value failed"
+	result := redactDeclaredEnvValues(message, env, names)
+	if result != "Error: [redacted] failed" {
+		t.Fatalf("result = %q", result)
+	}
+}
+
+func TestUserDefinedScannerRedactsDeclaredEnvInError(t *testing.T) {
+	adapter := NewUserDefinedScanner(UserDefinedScannerConfig{
+		ID: "test", Command: "test {{target}}", Targets: []string{"skill"},
+		Env: []string{"DECLARED_SECRET"},
+	})
+	registry, err := NewScannerRegistry(adapter)
+	if err != nil {
+		t.Fatal(err)
+	}
+	commandRunner := &recordingCommandRunner{
+		stderr: "Error: my-declared-secret-value failed",
+		err:    errCommandFailed,
+	}
+	result, err := (ExternalScannerRunner{
+		Registry: registry, CommandRunner: commandRunner, Env: map[string]string{"DECLARED_SECRET": "my-declared-secret-value"},
+		SandboxMode: SandboxModeOff,
+	}).RunScanner("test", t.TempDir()+"/file.txt", "2026-07-21T00:00:00Z")
+	if err != nil {
+		t.Fatal(err)
+	}
+	if result.Status != "failed" {
+		t.Fatalf("status = %q", result.Status)
+	}
+	if !strings.Contains(result.Error, "[redacted]") {
+		t.Fatalf("error = %q, should contain [redacted]", result.Error)
+	}
+	if strings.Contains(result.Error, "my-declared-secret-value") {
+		t.Fatalf("error = %q, should not contain secret value", result.Error)
+	}
+}
+
 func TestUnsafeWindowsShellTargetDetectsInjectionCharacters(t *testing.T) {
 	unsafe := []string{
 		`%PATH%`,
diff --git a/internal/runner/user_defined_scanner.go b/internal/runner/user_defined_scanner.go
index 86318e1..8161eb2 100644
--- a/internal/runner/user_defined_scanner.go
+++ b/internal/runner/user_defined_scanner.go
@@ -63,6 +63,36 @@ func unsafeWindowsShellTarget(target string) bool {
 	return strings.ContainsAny(target, "%\"!")
 }
 
+// sanitizedDeclaredEnvNames returns bare variable names from declared env
+// entries, dropping any accidental =value suffix so an inline value is never
+// mistaken for a name.
+func sanitizedDeclaredEnvNames(env []string) []string {
+	names := make([]string, 0, len(env))
+	for _, entry := range env {
+		name := entry
+		if i := strings.IndexByte(entry, '='); i >= 0 {
+			name = entry[:i]
+		}
+		if name = strings.TrimSpace(name); name != "" {
+			names = append(names, name)
+		}
+	}
+	return names
+}
+
+// redactDeclaredEnvValues scrubs the values of a user-defined scanner's declared
+// env vars from free-text output. A scanner's env: entries exist to hand it
+// secrets, so whatever their spelling their values must never be persisted, even
+// when the name evades the secret-name heuristic.
+func redactDeclaredEnvValues(message string, env map[string]string, names []string) string {
+	for _, name := range names {
+		if value := env[name]; value != "" {
+			message = strings.ReplaceAll(message, value, "[redacted]")
+		}
+	}
+	return message
+}
+
 func (adapter userDefinedScannerAdapter) Run(runner ExternalScannerRunner, target string, startedAt string) (ScannerResult, error) {
 	shell := userDefinedScannerShell(runtime.GOOS, runner.SandboxMode)
 	targetReplacement := shell.quote(target)
@@ -109,6 +139,7 @@ func (adapter userDefinedScannerAdapter) Run(runner ExternalScannerRunner, targe
 	raw := strings.TrimSpace(output.Stdout)
 	if runErr != nil {
 		message := commandError(runErr, output.Stderr, runner.Env)
+		message = redactDeclaredEnvValues(message, runner.Env, sanitizedDeclaredEnvNames(adapter.config.Env))
 		if json.Valid([]byte(raw)) {
 			return ScannerResult{
 				Status: "completed", StartedAt: startedAt, CompletedAt: completedAt, Command: fullCommand,