Organization	12 Months Ended
	Dec. 31, 2025
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Organization	OrganizationLyell Immunopharma, Inc. (the “Company”) was incorporated in Delaware in June 2018. The Company is a late‑stage clinical cell therapy company advancing a pipeline of proprietary next-generation autologous chimeric antigen receptor (“CAR”) T-cell product candidates for patients with hematologic malignancies and solid tumors. The Company’s lead product candidate, rondecabtagene autoleucel (“ronde-cel”, also known as LYL314), is an autologous dual‑targeting CD19/CD20 CAR T‑cell therapy in development for large B-cell lymphoma (“LBCL”). The Company has recently acquired exclusive global rights outside of mainland China, Hong Kong, Macau and Taiwan, to a novel guanylyl cyclase C (“GCC”)-targeted CAR T-cell product candidate in early clinical development for refractory metastatic colorectal cancer (“mCRC”) and other GCC-expressing cancers. The Company’s primary activities since incorporation have been to develop investigational T‑cell therapies, conduct research and development, execute clinical trials, enable and execute manufacturing activities in support of its product candidate development efforts, submit regulatory submissions, acquire technology and product candidates, enter into strategic license and collaboration arrangements, organize and staff the Company, conduct business planning, establish its intellectual property portfolio, raise capital and provide general and administrative support for these activities.