This study examines code-mixing between SP and AY in WhatsApp chats. Muysken’s (2000) theory of code-mixing serves as the main framework, supported by Holmes’ (2013) theory of social factors. A qualitative method is employed, with simple frequency counts. The findings reveal five similarities and four differences in SP and AY’s code-mixing practices. Similarities include the use of all three types of code-mixing: insertion, alternation, and congruent lexicalization, with insertion being the most frequent. Constituency and element-switched indicators are consistently found across all three types by both SP and AY. All four indicators also appear in the alternation type used by both. Differences include the least-used types: SP uses alternation the least, while AY uses congruent lexicalization the least. Additionally, SP uses switch sites in insertion; AY does not. SP does not use properties in insertion, while AY does. Conversely, SP uses properties in congruent lexicalization, while AY does not. The study also discusses how participant roles and communicative function influence their use of code-mixing. Future research is encouraged to explore code-mixing through researchers’ firsthand experiences and to analyze speakers with different social relationships to provide broader insights into code-mixing in digital communication.