diff --git a/apps/checker/pkg/assertions/assertions.go b/apps/checker/pkg/assertions/assertions.go
index 82d15976..1ce31ef2 100644
--- a/apps/checker/pkg/assertions/assertions.go
+++ b/apps/checker/pkg/assertions/assertions.go
@@ -73,16 +73,14 @@ func (target HeaderTarget) HeaderEvaluate(s string) bool {
 		return false
 	}
 
-	v, found := headers[target.Key]
-	if !found {
-		return false
+	for key, value := range headers {
+		if strings.EqualFold(key, target.Key) {
+			t := StringTargetType{Comparator: target.Comparator, Target: target.Target}
+			return t.StringEvaluate(fmt.Sprintf("%v", value))
+		}
 	}
 
-	t := StringTargetType{Comparator: target.Comparator, Target: target.Target}
-	// convert all headers to array
-	str := fmt.Sprintf("%v", v)
-
-	return t.StringEvaluate(str)
+	return false
 }
 
 func (target StatusTarget) StatusEvaluate(value int64) bool {
diff --git a/apps/checker/pkg/assertions/assertions_test.go b/apps/checker/pkg/assertions/assertions_test.go
index a876d07b..f0ec18af 100644
--- a/apps/checker/pkg/assertions/assertions_test.go
+++ b/apps/checker/pkg/assertions/assertions_test.go
@@ -103,6 +103,10 @@ func TestHeaderTarget_HeaderEvaluate(t *testing.T) {
 		{name: "Header 1", fields: fields{Comparator: request.StringEmpty, Target: "", Key: "headers1"}, args: args{s: `{"Content-Type":"text/plain;charset=UTF-8","Strict-Transport-Security":"max-age=3153600000","Vary":"Accept-Encoding"}`}, want: false},
 		{name: "Header 2", fields: fields{Comparator: request.StringNotEmpty, Target: "", Key: "headers1"}, args: args{s: `{"Content-Type":"text/plain;charset=UTF-8","Strict-Transport-Security":"max-age=3153600000","headers1":"Accept-Encoding"}`}, want: true},
 		{name: "it should return false if it can not decode the headers", fields: fields{Comparator: request.StringContains, Target: "Accept-Encoding", Key: "Vary"}, args: args{s: `}`}, want: false},
+		{name: "Lowercase assertion key", fields: fields{Comparator: request.StringEquals, Target: "application/json", Key: "content-type"}, args: args{s: `{"Content-Type":"application/json"}`}, want: true},
+		{name: "Mixed-case assertion and lowercase header", fields: fields{Comparator: request.StringEquals, Target: "application/json", Key: "CoNtEnT-TyPe"}, args: args{s: `{"content-type":"application/json"}`}, want: true},
+		{name: "Case-sensitive header value", fields: fields{Comparator: request.StringEquals, Target: "application/json", Key: "content-type"}, args: args{s: `{"Content-Type":"application/JSON"}`}, want: false},
+		{name: "Negative assertion uses actual header value", fields: fields{Comparator: request.StringNotEquals, Target: "application/json", Key: "content-type"}, args: args{s: `{"Content-Type":"application/json"}`}, want: false},
 	}
 	for _, tt := range tests {
 		t.Run(tt.name, func(t *testing.T) {
diff --git a/apps/checker/pkg/job/http_job_test.go b/apps/checker/pkg/job/http_job_test.go
index 58fc1155..2bb41ab7 100644
--- a/apps/checker/pkg/job/http_job_test.go
+++ b/apps/checker/pkg/job/http_job_test.go
@@ -171,7 +171,7 @@ func TestHTTPJob_HeaderAssertions(t *testing.T) {
 			Retry:   1,
 			HeaderAssertions: []*v1.HeaderAssertion{
 				{
-					Key:        "X-Custom",
+					Key:        "x-custom",
 					Comparator: v1.StringComparator_STRING_COMPARATOR_EQUAL,
 					Target:     target,
 				},
diff --git a/packages/assertions/src/v1.ts b/packages/assertions/src/v1.ts
index 9b480988..9ac74cc0 100644
--- a/packages/assertions/src/v1.ts
+++ b/packages/assertions/src/v1.ts
@@ -308,8 +308,14 @@ export class HeaderAssertion implements Assertion {
         message: "Invalid request type for header assertion",
       };
     }
+
+    const key = this.schema.key.toLowerCase();
+    const value = Object.entries(req.header).find(
+      ([name]) => name.toLowerCase() === key,
+    )?.[1];
+
     const { success, message } = evaluateString(
-      req.header[this.schema.key] ?? "",
+      value ?? "",
       this.schema.compare,
       this.schema.target,
     );