WebAssembly Debugger

## Step through a Wasm module, forward and backward Open a `.wasm` binary or edit WebAssembly Text and inspect the module without installing a toolchain. The debugger decodes binary sections, function signatures, imports, exports, globals, tables, data segments, instruction offsets, control-flow structure and custom names. WAT is compiled with the WebAssembly Binary Toolkit and can be downloaded again as text or binary. ## Reversible execution Choose an exported function, provide typed arguments, and step into, over, or out of calls. Every instruction records the operand stack, locals, globals, call frames, table state, host-import calls, memory reads, and byte-level memory writes. Because writes are recorded as reversible deltas, Step Back restores the previous state rather than replaying from the beginning. Drag the trace cursor to jump anywhere you have already been. Continuing from an earlier point discards the old future and starts a new timeline, which is how you test a different argument without losing the context you built up getting there. Breakpoints pause before a selected instruction. Read, write and read/write watchpoints pause when an address range is touched, and the memory viewer highlights the bytes the current instruction read or wrote. "Last read" and "last write" jump to the most recent access of an address before the cursor, which is the fastest way to answer "what put this value here". State comparison shows what changed between any two points in the trace. On a loop that is misbehaving, comparing iteration three to iteration four is usually more informative than watching either one. ## Modules are modelled, not instantiated Uploaded code is decoded and interpreted by a purpose-built interpreter. It is not handed to the browser's WebAssembly engine and instantiated. No filesystem or network capability is ever supplied. Common WASI calls are modelled with deterministic, capability-free stubs: `fd_write`, `proc_exit`, `random_get` and `clock_time_get`. Standard output is captured in the UI. This is not a full WASI runtime, and a module that needs real files, sockets or processes receives safe zero-value stubs rather than working I/O. ## Metadata the module carries Custom sections often say more about provenance than the code does. The debugger summarises `producers`, `target_features`, `linking`, `reloc.*`, `dylink.0`, build ID and `sourceMappingURL`, and attributes size per section and per function. The size view answers "why is this bundle 4 MB" faster than any bundler report, because it points at the specific functions responsible. Component Model envelopes are detected and reported with an explicit unsupported boundary. WIT interface inspection and component execution are scoped as later work rather than partially claimed here. ## The security view The security panel highlights powerful host imports, indirect calls, unbounded or unusually large memories, memory growth, shared memory, and explicit trap paths. These are review leads, not a verdict. A clean report does not certify a module as safe, and a flagged import is often entirely legitimate. Use the panel to decide where to point the debugger, not to decide whether to trust a dependency. ## Limits Execution is capped at 50,000 instructions and 32 MiB of linear memory. The interpreter covers core numeric, control-flow, variable, table, memory, call, conversion, sign-extension, saturating-conversion and many bulk-memory instructions, plus commonly used reference-types operations. Modules using SIMD, threads, exception handling, GC or memory64 can still be inspected where decoding succeeds, but execution stops with a clear diagnostic at an unsupported instruction rather than guessing. ## Typical uses Diagnose a divide-by-zero, unreachable, invalid indirect-call or out-of-bounds trap by stepping to it and reading the state that produced it. Review a third-party module's imports and exported surface before integration. Trace compiler-generated control flow and inspect function-level dynamic coverage. Learn stack-machine execution with the editable factorial, memory, nested-call, WASI output and trap examples. Export a portable JSON trace for a bug report.