We are currently supporting a key client on a strategic initiative and are looking for an experienced Mainframe System Engineer (Medior to Senior level) to join a dedicated mission within a critical z/OS environment. Assignment Overview This is a dedicated freelance assignment (or CDI via portage) with an initial duration of 6 to 12 months, with strong potential for extension depending on client needs and project evolution. The role is focused on mainframe transformation, migration, and upgrade projects, including infrastructure modernization and system enhancements. Location * On-site presence is mandatory Conditions Availability for weekend work (overtime may be required) High-demand environment requiring strong stress management capabilities Key Responsibilities Actively contribute to mainframe LPAR migrations, specifically from z16 to z17 Manage and optimize z/OS environments Work closely with IBM teams on hardware configuration and setup Lead or support migration of mainframe tools, particularly from Broadcom solutions to IBM or BMC software Participate in operating system upgrades and ongoing maintenance activities Contribute to upgrades of third-party software (DB2, CICS, COBOL, IBM MQ, IWS, etc.) Ensure stability, performance, and reliability of critical systems Operate within structured IT processes aligned with ITIL standards Required Skills & Experience Proven experience as a Mainframe System Engineer (Medior or Senior level) Deep expertise in z/OS (mandatory) Demonstrated experience with mainframe LPAR migration (z16 to z17) Strong background in hardware configuration in collaboration with IBM Experience with mainframe tool migration (Broadcom to IBM or BMC environments) Strong involvement in OS and software upgrade projects Solid knowledge of mainframe ecosystems and third-party tools ITIL knowledge is mandatory Fluency in English is mandatory (written and spoken) Soft Skills Excellent communication skills, both written and verbal Ability to work under pressure in highly critical environments Strong sense of ownership and accountability Proactive, autonomous, and solution-oriented mindset Team player with strong collaboration skills Hays plc est l'un des principaux groupes mondiaux de recrutement de professionnels. Hays est l'expert du recrutement de personnes qualifiées, professionnelles et compétentes dans le monde entier. Hays est le leader du marché au Royaume-Uni et en Asie-Pacifique et également l'un des leaders du marché en Europe continentale et en Amérique latine. Hays intervient pour le compte de milliers d’entreprises des secteurs privé et public, sur des postes en CDI, CDD, Travail Temporaire et Contracting. Hays France & Luxembourg compte aujourd’hui plus de 650 salariés répartis au sein de 22 bureaux (Aix-en-Provence, Amiens, Biarritz, Bordeaux, Clermont-Ferrand, Dijon, Grenoble, La Rochelle, Lille, Lyon, Montpellier, Nancy, Nantes, Nice, Paris, Reims, Rennes, Rouen, Strasbourg, Toulouse, Tours et au Luxembourg) couvrant 25 secteurs d’activité.