diff --git a/admin/claude_export.go b/admin/claude_export.go
index b559bf36..a345c24a 100644
--- a/admin/claude_export.go
+++ b/admin/claude_export.go
@@ -273,24 +273,32 @@ func prepareClaudeTimezoneCredentialUpdateWithHeaders(row *database.AccountRow,
 	if requestedHeaders != nil {
 		baseHeaders = requestedHeaders
 	}
+	// Keys are canonicalized up front: the generated fingerprint says
+	// "X-Stainless-OS" while previously persisted headers come back as
+	// "X-Stainless-Os", and normalizeCustomHeaders treats a case-only clash
+	// with different values as an error. Without this a same-timezone save
+	// failed whenever the freshly rolled OS differed from the stored one.
+	// 先统一成规范大小写：指纹生成的是 X-Stainless-OS，落库后读回是 X-Stainless-Os，
+	// 否则同时区保存时随机到不同 OS 就会触发"大小写重复且值冲突"。
 	merged := make(map[string]string)
 	keepIdentity := requestedHeaders == nil && strings.EqualFold(strings.TrimSpace(row.GetCredential("timezone")), timezone)
 	for name, value := range auth.GenerateClaudeFingerprint(timezone).Headers() {
-		merged[name] = value
+		merged[http.CanonicalHeaderKey(strings.TrimSpace(name))] = value
 	}
 	for name, value := range baseHeaders {
 		lowerName := strings.ToLower(strings.TrimSpace(name))
+		canonicalName := http.CanonicalHeaderKey(strings.TrimSpace(name))
 		if _, isIdentity := identity[lowerName]; isIdentity {
 			// Keep a complete existing fingerprint stable when the operator
 			// saves the same timezone again; a timezone change (or explicit
 			// header patch) intentionally rotates the identity snapshot.
 			if keepIdentity {
-				merged[name] = value
+				merged[canonicalName] = value
 			}
 			continue
 		}
 		if isClaudeSafeOperationalHeader(name) {
-			merged[name] = value
+			merged[canonicalName] = value
 		}
 	}
 	normalized, err := normalizeCustomHeaders(merged)
diff --git a/admin/claude_timezone_headers_test.go b/admin/claude_timezone_headers_test.go
new file mode 100644
index 00000000..f8d7f94e
--- /dev/null
+++ b/admin/claude_timezone_headers_test.go
@@ -0,0 +1,44 @@
+package admin
+
+import (
+	"testing"
+
+	"github.com/codex2api/database"
+)
+
+// A Claude account whose stored identity headers use Go's canonical casing
+// ("X-Stainless-Os") must survive a same-timezone save: the fresh fingerprint
+// is keyed "X-Stainless-OS", and a case-insensitive clash with a different
+// random OS used to fail the whole update with
+// "custom_headers 包含大小写重复且值冲突的请求头: X-Stainless-Os".
+func TestPrepareClaudeTimezoneUpdate_KeepsCanonicalCasedIdentityOnSameTimezone(t *testing.T) {
+	stored := map[string]string{
+		"User-Agent": "claude-cli/2.1.259 (external, cli)", "X-App": "cli",
+		"X-Stainless-Arch": "x64", "X-Stainless-Lang": "js", "X-Stainless-Os": "Windows",
+		"X-Stainless-Package-Version": "0.65.0", "X-Stainless-Runtime": "node", "X-Stainless-Runtime-Version": "v20.18.1",
+	}
+	for i := 0; i < 30; i++ { // the generated OS is random; every save must succeed
+		headersAny := make(map[string]interface{}, len(stored))
+		for k, v := range stored {
+			headersAny[k] = v
+		}
+		row := &database.AccountRow{Platform: "anthropic", Credentials: map[string]interface{}{
+			"upstream_type": "claude", "timezone": "Asia/Shanghai", "custom_headers": headersAny,
+		}}
+		updates := map[string]interface{}{}
+		applied, err := prepareClaudeTimezoneCredentialUpdateWithHeaders(row, "Asia/Shanghai", updates, nil)
+		if err != nil {
+			t.Fatalf("iteration %d: %v", i, err)
+		}
+		if !applied {
+			t.Fatal("update must apply to a Claude row")
+		}
+		headers, _ := updates["custom_headers"].(map[string]string)
+		if headers["X-Stainless-Os"] != "Windows" {
+			t.Fatalf("iteration %d: stored identity must win on same timezone, got %v", i, headers)
+		}
+		if _, dup := headers["X-Stainless-OS"]; dup {
+			t.Fatalf("iteration %d: headers must be canonical-cased only, got %v", i, headers)
+		}
+	}
+}