diff --git a/internal/settingscontract/contract_test.go b/internal/settingscontract/contract_test.go
index 4cf2bfd5..ee2d0f50 100644
--- a/internal/settingscontract/contract_test.go
+++ b/internal/settingscontract/contract_test.go
@@ -1210,6 +1210,40 @@ func TestQuotedNumbersAreRejected(t *testing.T) {
 	}
 }
 
+// TestRawInvalidUTF8IsRejected covers the other path to the substitution
+// TestLoneSurrogatesAreRejectedForEveryType guards. A raw 0xff byte inside a
+// quoted string — what an HTTP body carries when a client encodes text in the
+// wrong charset — is not an escape, so the surrogate scan never sees it, but
+// encoding/json still turns it into U+FFFD and reports success. The value is
+// then stored verbatim, which SQLite's json_valid accepts and Postgres jsonb
+// refuses.
+func TestRawInvalidUTF8IsRejected(t *testing.T) {
+	for name, schema := range map[string]*ValueSchema{
+		"string": {Type: TypeString},
+		"object": {Type: TypeObject, SchemaRef: "subtitle-appearance.json"},
+	} {
+		t.Run(name, func(t *testing.T) {
+			raw := json.RawMessage([]byte{'"', 0xff, '"'})
+			if name == "object" {
+				raw = json.RawMessage(append(append([]byte(`{"fontSize":"`), 0xff), `"}`...))
+			}
+			if err := schema.ValidateValue(raw, objSchemas); err == nil {
+				t.Fatal("a value with a raw invalid UTF-8 byte was accepted")
+			}
+			if _, err := schema.NormalizeValue(raw, objSchemas); err == nil {
+				t.Fatal("a value with a raw invalid UTF-8 byte was stored")
+			}
+		})
+	}
+
+	// Valid multi-byte UTF-8 must still pass; the check is on well-formedness,
+	// not on being ASCII.
+	if err := (&ValueSchema{Type: TypeString}).ValidateValue(
+		json.RawMessage(`"日本語 — café 😀"`), nil); err != nil {
+		t.Errorf("valid multi-byte UTF-8 was rejected: %v", err)
+	}
+}
+
 // TestLoneSurrogatesAreRejectedForEveryType pins the check to the whole
 // validation path rather than the object branch it started in. A lone surrogate
 // decodes to U+FFFD on SQLite and is refused outright by Postgres jsonb, so a
diff --git a/internal/settingscontract/validate.go b/internal/settingscontract/validate.go
index e6bc76be..a80994c2 100644
--- a/internal/settingscontract/validate.go
+++ b/internal/settingscontract/validate.go
@@ -10,6 +10,7 @@ import (
 	"regexp"
 	"strconv"
 	"strings"
+	"unicode/utf8"
 
 	"github.com/santhosh-tekuri/jsonschema/v6"
 )
@@ -389,10 +390,19 @@ func (v *ValueSchema) ValidateValue(raw json.RawMessage, objectSchemas map[strin
 		return errors.New("null is not allowed for this setting")
 	}
 
-	// Every type that can carry a string, not just the object branch: a lone
-	// surrogate in a plain string setting (ui.custom_css) decodes to U+FFFD on
-	// SQLite and is rejected outright by Postgres jsonb, so leaving this to the
-	// object case alone let the two backends disagree about the same value.
+	// Both of these run for every type, not just the object branch: a value the
+	// decoder silently rewrites is stored verbatim, and the two backends do not
+	// agree on what they will store. Postgres jsonb rejects U+FFFD-producing
+	// input outright while SQLite's json_valid accepts it, so the same request
+	// succeeds on one deployment and fails on the other.
+	//
+	// Raw bytes and escapes are separate paths to the same substitution: an
+	// invalid UTF-8 byte arrives in the body as itself, a lone surrogate as a
+	// \u escape, and encoding/json turns both into U+FFFD while reporting
+	// success.
+	if !utf8.Valid(trimmed) {
+		return errors.New("value is not valid UTF-8")
+	}
 	if err := rejectLoneSurrogates(trimmed); err != nil {
 		return err
 	}