diff --git a/mcp/streamable_headers.go b/mcp/streamable_headers.go
index 7133e26c9..b9c516018 100644
--- a/mcp/streamable_headers.go
+++ b/mcp/streamable_headers.go
@@ -63,17 +63,24 @@ type headerSchemaProperty struct {
 	Properties map[string]headerSchemaProperty `json:"properties,omitempty"`
 }
 
-// UnmarshalJSON treats a boolean schema as a schema without annotations.
-// A field can be encoded as "region": {"type": "string", "x-mcp-header": "Region"},
-// but also simply as "extra": true.
+// UnmarshalJSON decodes schema properties while ignoring boolean subschemas
+// and preserving non-string type values for primitive-type validation.
 func (p *headerSchemaProperty) UnmarshalJSON(data []byte) error {
 	switch string(bytes.TrimSpace(data)) {
 	case "true", "false":
 		*p = headerSchemaProperty{}
 		return nil
 	}
-	type plain headerSchemaProperty
-	return internaljson.Unmarshal(data, (*plain)(p))
+	var obj struct {
+		Type       json.RawMessage                 `json:"type,omitempty"`
+		XMCPHeader json.RawMessage                 `json:"x-mcp-header,omitempty"`
+		Properties map[string]headerSchemaProperty `json:"properties,omitempty"`
+	}
+	if err := json.Unmarshal(data, &obj); err != nil {
+		return err
+	}
+	*p = headerSchemaProperty{Type: obj.Type, XMCPHeader: obj.XMCPHeader, Properties: obj.Properties}
+	return nil
 }
 
 func (p headerSchemaProperty) primitiveType() bool {
diff --git a/mcp/streamable_headers_test.go b/mcp/streamable_headers_test.go
index d80c781f1..f6a31a775 100644
--- a/mcp/streamable_headers_test.go
+++ b/mcp/streamable_headers_test.go
@@ -442,6 +442,50 @@ func TestValidateToolParamHeaders(t *testing.T) {
 		wantErr    bool
 		wantErrSub string
 	}{
+		{
+			name: "array type on an unannotated sibling does not hide an invalid annotation",
+			tool: &Tool{
+				Name: "test",
+				InputSchema: map[string]any{
+					"type": "object",
+					"properties": map[string]any{
+						"count": map[string]any{"type": "number", "x-mcp-header": "Count"},
+						"note":  map[string]any{"type": []any{"null", "string"}},
+					},
+				},
+			},
+			wantErr:    true,
+			wantErrSub: "primitive types",
+		},
+		{
+			name: "nullable annotated property is rejected as non-primitive",
+			tool: &Tool{
+				Name: "test",
+				InputSchema: map[string]any{
+					"type": "object",
+					"properties": map[string]any{
+						"note": map[string]any{"type": []any{"null", "string"}, "x-mcp-header": "Note"},
+					},
+				},
+			},
+			wantErr:    true,
+			wantErrSub: "primitive types",
+		},
+		{
+			name: "boolean subschema does not hide an invalid annotation",
+			tool: &Tool{
+				Name: "test",
+				InputSchema: map[string]any{
+					"type": "object",
+					"properties": map[string]any{
+						"count": map[string]any{"type": "number", "x-mcp-header": "Count"},
+						"extra": true,
+					},
+				},
+			},
+			wantErr:    true,
+			wantErrSub: "primitive types",
+		},
 		{
 			name: "valid tool with x-mcp-header",
 			tool: &Tool{
@@ -1317,6 +1361,34 @@ func TestExtractToolParamHeaders(t *testing.T) {
 			},
 			want: map[string]string{"user.id": "UserId"},
 		},
+		{
+			name: "array type on an unannotated sibling does not hide annotations",
+			tool: &Tool{
+				Name: "test",
+				InputSchema: map[string]any{
+					"type": "object",
+					"properties": map[string]any{
+						"region": map[string]any{"type": "string", "x-mcp-header": "Region"},
+						"note":   map[string]any{"type": []any{"null", "string"}},
+					},
+				},
+			},
+			want: map[string]string{"region": "Region"},
+		},
+		{
+			name: "boolean subschema does not hide annotations",
+			tool: &Tool{
+				Name: "test",
+				InputSchema: map[string]any{
+					"type": "object",
+					"properties": map[string]any{
+						"region": map[string]any{"type": "string", "x-mcp-header": "Region"},
+						"extra":  true,
+					},
+				},
+			},
+			want: map[string]string{"region": "Region"},
+		},
 	}
 
 	for _, tt := range tests {