dap-protocol-guide

DAP initialization establishes a session between debugger and adapter through a handshake that exchanges capabilities and configuration. Start with an initialize request, receive capabilities response, then send initialized notification before breakpoints or execution commands. This sequence ensures both sides agree on supported features before debugging begins.

DAP is a standardized messaging protocol that decouples debuggers from language-specific debug adapters, enabling one debugger UI to debug multiple languages. It defines request/response/event types for breakpoint management, stack inspection, and variable evaluation, eliminating the need to rebuild debugging tools for each language.

Send a setBreakpoints request with breakpoint objects that include line number and a condition expression. The adapter evaluates the condition at runtime and halts execution only when the condition is true. Include source file path and receive confirmation of breakpoint locations in the response.

Each language adapter implements DAP's core message schema but varies in initialization parameters, variable scoping rules, and stack frame representation. Python adapters handle coroutines differently than Java's thread model; JavaScript adapts for async/await. Adapters report their capabilities during initialization so debuggers adjust behavior accordingly.

Violations include skipping initialization, sending requests out of sequence, mismatched thread IDs in stack inspection, and ignoring adapter capability flags. Protocol-level errors occur when breakpoint requests reference unsupported condition syntax or when responses omit required fields. Always validate initialization completion before issuing breakpoint or execution commands.

Yes. After hitting a breakpoint, use variables request with a frame reference to retrieve local, global, and watch variables. Send evaluate request to compute expressions in the current scope. DAP returns variable names, types, and values; complex objects return reference handles for nested inspection.