diff --git a/internal/core/config.go b/internal/core/config.go
index 8a59fa6..eb126de 100644
--- a/internal/core/config.go
+++ b/internal/core/config.go
@@ -1,6 +1,7 @@
 package core
 
 import (
+	"errors"
 	"fmt"
 
 	"github.com/caarlos0/env/v10"
@@ -137,11 +138,23 @@ type Config struct {
 
 var config *Config
 
+// startupErrors collects configuration failures raised while packages
+// initialise. They cannot be logged where they happen, because logging is not
+// configured until the command runs, and printing them bypasses the
+// configured log format. Validate reports them instead.
+var startupErrors []error
+
+// RecordStartupError stores a configuration failure for Validate to report.
+// Only safe to call during package initialisation, which is single threaded.
+func RecordStartupError(err error) {
+	if err != nil {
+		startupErrors = append(startupErrors, err)
+	}
+}
+
 func init() {
 	config = &Config{}
-	if err := env.Parse(config); err != nil {
-		fmt.Printf("%+v\n", err)
-	}
+	RecordStartupError(env.Parse(config))
 }
 
 func ReadConfig() *Config {
@@ -150,6 +163,10 @@ func ReadConfig() *Config {
 
 // Validate reports settings that would otherwise fail quietly at request time.
 func (c *Config) Validate() error {
+	if len(startupErrors) > 0 {
+		return fmt.Errorf("invalid configuration: %w", errors.Join(startupErrors...))
+	}
+
 	if c.OutputFormat.Default == "" {
 		return nil
 	}
diff --git a/internal/core/config_test.go b/internal/core/config_test.go
new file mode 100644
index 0000000..2c768de
--- /dev/null
+++ b/internal/core/config_test.go
@@ -0,0 +1,56 @@
+package core
+
+import (
+	"errors"
+	"testing"
+
+	"github.com/stretchr/testify/require"
+)
+
+// A malformed setting used to be printed and ignored, so the service started
+// with silent defaults. Validate reports it instead.
+func TestValidateReportsStartupErrors(t *testing.T) {
+	saved := startupErrors
+	t.Cleanup(func() { startupErrors = saved })
+
+	startupErrors = []error{errors.New(`strconv.ParseInt: parsing "abc"`)}
+
+	config := *ReadConfig()
+	err := config.Validate()
+
+	require.Error(t, err)
+	require.Contains(t, err.Error(), "invalid configuration")
+	require.Contains(t, err.Error(), "abc")
+}
+
+func TestValidateAcceptsAWorkingConfiguration(t *testing.T) {
+	saved := startupErrors
+	t.Cleanup(func() { startupErrors = saved })
+	startupErrors = nil
+
+	config := *ReadConfig()
+	config.OutputFormat.Default = ""
+
+	require.NoError(t, config.Validate())
+}
+
+func TestValidateRejectsAnUnknownDefaultFormat(t *testing.T) {
+	saved := startupErrors
+	t.Cleanup(func() { startupErrors = saved })
+	startupErrors = nil
+
+	config := *ReadConfig()
+	config.OutputFormat.Default = "WEBP"
+
+	require.Error(t, config.Validate(), "the map is keyed in lower case")
+}
+
+func TestRecordStartupErrorIgnoresNil(t *testing.T) {
+	saved := startupErrors
+	t.Cleanup(func() { startupErrors = saved })
+	startupErrors = nil
+
+	RecordStartupError(nil)
+
+	require.Empty(t, startupErrors)
+}
diff --git a/internal/core/encryption.go b/internal/core/encryption.go
index bcda20d..6b4cc76 100644
--- a/internal/core/encryption.go
+++ b/internal/core/encryption.go
@@ -13,7 +13,6 @@ import (
 	"fmt"
 	"github.com/caarlos0/env/v10"
 	"io"
-	"log/slog"
 	"strings"
 )
 
@@ -22,13 +21,11 @@ var salt = []byte("go-dims")
 
 func init() {
 	envConfig := Signing{}
-	if err := env.Parse(&envConfig); err != nil {
-		fmt.Printf("%+v\n", err)
-	}
+	RecordStartupError(env.Parse(&envConfig))
 
 	var err error
 	if encryptionKey, err = deriveKey(envConfig.SigningKey); err != nil {
-		slog.Error("failed to derive encryption key", "error", err)
+		RecordStartupError(fmt.Errorf("failed to derive the encryption key: %w", err))
 		return
 	}
 }
diff --git a/internal/core/image.go b/internal/core/image.go
index efff7ab..a3dd2b3 100644
--- a/internal/core/image.go
+++ b/internal/core/image.go
@@ -1,7 +1,6 @@
 package core
 
 import (
-	"fmt"
 	"github.com/beetlebugorg/go-dims/internal/gox/imagex/colorx"
 	"github.com/caarlos0/env/v10"
 	"golang.org/x/exp/slices"
@@ -46,9 +45,7 @@ var defaultSourceBackend SourceBackend
 
 func RegisterImageBackend(sourceBackend SourceBackend) {
 	config := Source{}
-	if err := env.Parse(&config); err != nil {
-		fmt.Printf("%+v\n", err)
-	}
+	RecordStartupError(env.Parse(&config))
 
 	allSourceBackends = append(allSourceBackends, sourceBackend)
 
diff --git a/internal/source/file.go b/internal/source/file.go
index 25edcbd..a52dcef 100644
--- a/internal/source/file.go
+++ b/internal/source/file.go
@@ -2,7 +2,6 @@ package source
 
 import (
 	"context"
-	"fmt"
 	"github.com/beetlebugorg/go-dims/internal/core"
 	"github.com/caarlos0/env/v10"
 	"github.com/davidbyttow/govips/v2/vips"
@@ -19,9 +18,7 @@ type fileSourceBackend struct {
 
 func init() {
 	envConfig := core.FileSource{}
-	if err := env.Parse(&envConfig); err != nil {
-		fmt.Printf("%+v\n", err)
-	}
+	core.RecordStartupError(env.Parse(&envConfig))
 
 	core.RegisterImageBackend(NewFileSourceBackend(envConfig.BaseDir))
 }
diff --git a/internal/source/s3.go b/internal/source/s3.go
index 8fb30aa..d3ff7a3 100644
--- a/internal/source/s3.go
+++ b/internal/source/s3.go
@@ -2,7 +2,6 @@ package source
 
 import (
 	"context"
-	"fmt"
 	"github.com/aws/aws-sdk-go-v2/aws"
 	"github.com/aws/aws-sdk-go-v2/config"
 	"github.com/aws/aws-sdk-go-v2/service/s3"
@@ -37,9 +36,7 @@ var s3Client = sync.OnceValues(func() (*s3.Client, error) {
 
 func init() {
 	envConfig := core.S3{}
-	if err := env.Parse(&envConfig); err != nil {
-		fmt.Printf("%+v\n", err)
-	}
+	core.RecordStartupError(env.Parse(&envConfig))
 
 	core.RegisterImageBackend(NewS3SourceBackend(envConfig))
 }