A Study Investigating BGB-26808 Alone or in Combination With Tislelizumab in Participants With Advanced Solid Tumors

Inclusion Criteria: 1. Able to provide a signed and dated written informed consent prior to any study-specific procedures, sampling, or data collection. 2. Eastern Cooperative Oncology Group (ECOG) Performance Status ≤ 1. 3. Phase 1a: Participants with histologically or cytologically confirmed advanced, metastatic, and unresectable solid tumors that are immune-sensitive who have previously received standard systemic therapy, or for whom treatment is not available or not tolerated, or for whom treatment is determined not appropriate based on investigator's judgment and who have not received prior therapy targeting hematopoietic progenitor kinase 1 (HPK1). 4. Phase 1b: Participants with histologically confirmed locally advanced unresectable or metastatic tumor types and who have not had prior systemic treatment. Participants who received prior systemic therapy in a neo-adjuvant or adjuvant setting with curative intent for nonmetastatic disease must have experienced a disease-free interval of ≥ 6 months from the last dose of systemic therapy prior to the first dose of study treatments. 5. ≥ 1 measurable lesion per RECIST v1.1. 6. Able to provide an archived tumor tissue sample. 7. Adequate organ function. 8. Females of childbearing potential must be willing to use a highly effective method of birth control for the duration of the study, and for ≥ 90 days after the last dose of BGB-26808, or for ≥ 120 days after the last dose of tislelizumab, or for ≥ 180 days after the last dose of chemotherapy. 9. Nonsterile males must be willing to use a highly effective method of birth control for the duration of the study treatment period and for ≥ 90 days after the last dose of BGB-26808, or for ≥ 120 days after the last dose of tislelizumab, or for ≥ 180 days after the last dose of chemotherapy. Exclusion Criteria: 1. Prior treatment with anti-PD-1, anti-PD-L1, anti-PD-L2, anti-TIGIT, anti-CTLA4, or any other antibody or drug specifically targeting T-cell costimulation or checkpoint pathways. 2. Uncontrollable pleural effusion, pericardial effusion, or ascites requiring frequent drainage or medical intervention. 3. Clinically significant bleeding from the gastrointestinal tract within 28 days before the first dose of study treatment(s). 4. Active leptomeningeal disease or uncontrolled, untreated brain metastasis. 5. Active autoimmune diseases or history of autoimmune diseases that may relapse 6. Any malignancy ≤ 3 years before the first dose of study treatment(s) except for the specific cancer under investigation in this study and any locally recurring cancer that has been treated with curative intent (eg, resected basal or squamous cell skin cancer, superficial bladder cancer, or carcinoma in situ of the cervix or breast). 7. Any condition that required systemic treatment with either corticosteroids (\> 10 mg daily of prednisone or equivalent) or other immunosuppressive medication ≤ 14 days before the first dose of study treatment(s). 8. History of interstitial lung disease, noninfectious pneumonitis, or uncontrolled lung diseases including pulmonary fibrosis, acute lung diseases. 9. Uncontrolled diabetes. 10. Infection (including tuberculosis infection) requiring systemic (oral or intravenous) antibacterial, antifungal, or antiviral therapy ≤ 14 days before the first dose of study treatment(s). Note: Other protocol defined Inclusion/Exclusion criteria may apply.