diff --git a/crates/libfmp/src/error.rs b/crates/libfmp/src/error.rs
index efe7610..3fa1706 100644
--- a/crates/libfmp/src/error.rs
+++ b/crates/libfmp/src/error.rs
@@ -85,7 +85,9 @@ pub enum ConfigurationErrorKind {
     InvalidQueryName,
     /// A credential is empty.
     EmptyCredential,
-    /// Direct FMP access was configured without an API credential.
+    /// No API credential was available: direct FMP access was configured
+    /// without one, or [`crate::ClientBuilder::from_env`] found
+    /// `FMP_API_KEY` unset or blank (for any base URL, proxies included).
     MissingCredential,
     /// Authentication was configured over plaintext HTTP to a non-loopback host.
     InsecureAuthentication,
diff --git a/sdk/go/errors.go b/sdk/go/errors.go
index 11bf27f..9782739 100644
--- a/sdk/go/errors.go
+++ b/sdk/go/errors.go
@@ -77,7 +77,9 @@ const (
 	ConfigurationKindInvalidQueryName
 	// ConfigurationKindEmptyCredential: a credential is empty.
 	ConfigurationKindEmptyCredential
-	// ConfigurationKindMissingCredential: direct FMP access has no credential.
+	// ConfigurationKindMissingCredential: no credential was available, either
+	// for direct FMP access or because NewClientFromEnv found FMP_API_KEY
+	// unset or blank (for any base URL, proxies included).
 	ConfigurationKindMissingCredential
 	// ConfigurationKindInsecureAuthentication: a credential over plain HTTP to a non-loopback host.
 	ConfigurationKindInsecureAuthentication