This is an open-label, phase 1/2 study evaluating the safety, tolerability, pharmacokinetic (what the body does to the drug), pharmacodynamic (what the drug does to the body), and antitumor activity of CGT4859 in adult participants with intrahepatic cholangiocarcinoma (iCCA) or other advanced solid tumors with FGFR2 and/or FGFR3 genetic alternations.

Key Inclusion Criteria:

1. Histologically confirmed locally advanced, metastatic, and/or unresectable iCCA or other solid tumor with documented FGFR2/3 alteration in blood and/or tumor.
2. Previously treated with, not appropriate for, or declined standard-of-care first-line treatment.
3. Have measurable disease per RECIST v1.1.
4. Eastern Cooperative Oncology Group (ECOG) performance status of 0-1.
5. Have clinically acceptable local laboratory screening results (clinical chemistry and hematology) within certain limits.
6. Resolution of toxicities from prior therapy to ≤Grade 1 (or baseline), including resolution of clinically significant laboratory abnormalities, before the first dose of study drug. Exceptions are alopecia, hypothyroidism, or type 1 diabetes mellitus controlled with medical intervention, and paronychia controlled with local intervention.

Key Exclusion Criteria:

1. Received chemotherapy or anticancer therapies or radiotherapy within certain timeframes before first dose of study drug.
2. Major surgeries (eg, abdominal laparotomy) within 4 weeks of the first dose of study drug.
3. Clinically significant corneal or retinal disorders or current evidence of retinal detachment.
4. Received more than 2 prior FGFRi therapies
5. Active, symptomatic, or untreated brain metastases unless the participant is clinically stable and off corticosteroids for ≥2 months.