diff --git a/internal/config/cache_max_bytes_internal_test.go b/internal/config/cache_max_bytes_internal_test.go
index ae55913..50933c2 100644
--- a/internal/config/cache_max_bytes_internal_test.go
+++ b/internal/config/cache_max_bytes_internal_test.go
@@ -9,6 +9,12 @@ import (
 	"testing"
 )
 
+// Static errors returned by the stub free-space probes below.
+var (
+	errTestStatfsFailed     = errors.New("statfs failed")
+	errTestProbeNotExpected = errors.New("probe must not be called")
+)
+
 // discardLogger returns a logger that swallows all output, for tests
 // that exercise code paths which log.
 func discardLogger() *slog.Logger {
@@ -205,7 +211,7 @@ func TestComputeDefaultCacheMaxBytesAppliesFloorToComputedDefault(t *testing.T)
 func TestComputeDefaultCacheMaxBytesPropagatesProbeError(t *testing.T) {
 	t.Parallel()
 
-	probe := func(string) (uint64, error) { return 0, errors.New("statfs failed") }
+	probe := func(string) (uint64, error) { return 0, errTestStatfsFailed }
 
 	_, err := ComputeDefaultCacheMaxBytes(t.TempDir(), probe)
 	if err == nil {
@@ -284,7 +290,7 @@ func TestResolveCacheMaxBytesDoesNotOverrideExplicitValue(t *testing.T) {
 	probe := func(string) (uint64, error) {
 		t.Error("free-space probe must not be consulted for explicit values")
 
-		return 0, errors.New("probe must not be called")
+		return 0, errTestProbeNotExpected
 	}
 
 	err = c.resolveCacheMaxBytes(discardLogger(), probe)
diff --git a/internal/config/cachesize.go b/internal/config/cachesize.go
index 854d729..5937cfe 100644
--- a/internal/config/cachesize.go
+++ b/internal/config/cachesize.go
@@ -43,7 +43,7 @@ func defaultFreeSpaceProbe(path string) (uint64, error) {
 	}
 
 	if stat.Bsize < 0 {
-		return 0, fmt.Errorf("statfs reported negative block size %d for %q", stat.Bsize, path)
+		return 0, fmt.Errorf("%w %d for %q", errNegativeBlockSize, stat.Bsize, path)
 	}
 
 	blockSize := uint64(stat.Bsize)
diff --git a/internal/config/config.go b/internal/config/config.go
index 6f96908..fb6e02d 100644
--- a/internal/config/config.go
+++ b/internal/config/config.go
@@ -62,7 +62,11 @@ var (
 	errTooFewConnections = errors.New("must be at least 1")
 	errValueTooShort     = errors.New("value too short")
 	errMustBeSetTogether = errors.New("must be set together")
-	errValueNull         = errors.New(
+	errMustNotBeNegative = errors.New("must not be negative")
+	errOverflowsInt64    = errors.New("overflows a 64-bit integer")
+	errNegativeBlockSize = errors.New(
+		"statfs reported negative block size")
+	errValueNull = errors.New(
 		"value is null; omit the key entirely to use the default")
 	errValuesNull = errors.New(
 		"value is null; omit a key entirely to use its default")
@@ -371,8 +375,8 @@ func (c *Config) validate() error {
 	// Zero is valid (it disables the disk cache); only negative
 	// values are rejected. No floor applies to explicit values.
 	if c.CacheMaxBytes < 0 {
-		return fmt.Errorf("config key %q: value %d must not be negative",
-			"cache_max_bytes", c.CacheMaxBytes)
+		return fmt.Errorf("config key %q: value %d %w",
+			keyCacheMaxBytes, c.CacheMaxBytes, errMustNotBeNegative)
 	}
 
 	for _, host := range c.AllowlistHosts {
@@ -619,27 +623,29 @@ func getInt64(sc *smartconfig.Config, key string, defaultVal int64) (int64, erro
 		return val, nil
 	case uint64:
 		if val > math.MaxInt64 {
-			return 0, fmt.Errorf("config key %q: value %d overflows a 64-bit integer",
-				key, val)
+			return 0, fmt.Errorf("config key %q: value %d %w",
+				key, val, errOverflowsInt64)
 		}
 
 		return int64(val), nil
 	case float64:
 		if val != math.Trunc(val) {
-			return 0, fmt.Errorf("config key %q: value %v is not an integer", key, val)
+			return 0, fmt.Errorf("config key %q: value %v is %w",
+				key, val, errNotAnInteger)
 		}
 
 		return int64(val), nil
 	case string:
 		parsed, err := strconv.ParseInt(strings.TrimSpace(val), 10, 64)
 		if err != nil {
-			return 0, fmt.Errorf("config key %q: value %q is not an integer", key, val)
+			return 0, fmt.Errorf("config key %q: value %q is %w",
+				key, val, errNotAnInteger)
 		}
 
 		return parsed, nil
 	default:
-		return 0, fmt.Errorf("config key %q: value %v (%T) is not an integer",
-			key, raw, raw)
+		return 0, fmt.Errorf("config key %q: value %v (%T) is %w",
+			key, raw, raw, errNotAnInteger)
 	}
 }