Architecture

probe-web does not reimplement probe-rs. It runs probe-rs itself, either in the browser or as a native server, and talks to it over probe-rs's own RPC protocol. The browser side is a thin client on top.

 your page ─ @probe-web/ui components ─┐
            @probe-web/dap adapter ────┤
                                       ▼
                         @probe-web/client  (TypeScript SDK)
                                       │
                         probe-web-core (wasm): probe-rs RPC client, defmt decoder
                         ┌─────────────┴──────────────┐
                 postMessage                       WebSocket
                         │                            │
     Web Worker: probe-web-local (wasm)        probe-rs serve (native)
     probe-rs + nusb over WebUSB               probe-rs + native USB
                         │                            │
                     debug probe                  debug probe

The pieces

	What it is
@probe-web/client	The TypeScript SDK: Client, Session, Core, Debugger. Wraps the wasm client with typed, promise-based calls.
crates/probe-web-core	probe-rs's unmodified probe-rs-rpc-client, compiled to wasm, plus defmt decoding, ELF symbol lookup and the coredump encoder. It runs on the page's thread (or in Node).
crates/probe-web-local	probe-rs compiled to wasm, running in a dedicated Web Worker and serving the same RPC protocol over postMessage. USB goes through nusb's WebUSB backend.
crates/probe-web-targets	CMSIS-Pack and .FLM parsing, compiled to a separate wasm module that loads only when a user imports a pack.
tools/wire-gen	Generates packages/client/src/wire.ts from probe-rs-rpc's schema.

Because the WebUSB worker and probe-rs serve speak the same protocol, one client serves both transports, and a fix in the SDK applies to both.

The wire contract

probe-rs's RPC messages are Rust types serialized with postcard. The client deserializes them in wasm and hands them to JavaScript through serde-wasm-bindgen with fixed settings:

• enums are externally tagged: a unit variant is a string ("Ok"), others are an object with one key ({ Halted: … });
• 64-bit integers are bigint, so addresses never lose precision;
• None and unit are null;
• Vec<u8> is a plain number[] (the SDK converts to Uint8Array where it matters).

wire.ts describes those values. It is generated from the same postcard-schema model the wire uses, so the TypeScript types cannot drift from probe-rs. CI regenerates it and fails if the committed file differs. Most code uses the SDK's own types; Wire.* types appear where the SDK passes probe-rs values through, such as probe entries, progress events and boot info.

Capability negotiation

The client reads the server's schema report on connect and compares it with its own. A server may implement a subset: the WebUSB worker leaves out disassembly, for example. The client records what is missing instead of refusing to connect, and fails only if an endpoint both sides have disagrees in its types. client.supports(path) exposes the result.

The worker

The worker hosts probe-rs's async core. In the browser it cannot block: every USB transfer is awaited, and timing uses browser timers, not threads. That shapes a few behaviours:

• Crashes are contained. A panic inside the worker posts a fatal: message before the worker dies. The transport then rejects every pending and later call with kind: 'worker-crashed', rather than leaving promises hanging.
• Recovery on attach. A page that goes away mid-command can leave a probe in the middle of a transfer. The worker's attach reopens the probe and retries once, and CMSIS-DAP probes are resynchronized and health-checked when opened.
• RTT scans are targeted. Scanning all of RAM for the RTT control block over WebUSB takes hundreds of milliseconds. The SDK reads the _SEGGER_RTT address from the ELF when it has one, and the worker remembers where it found the block.

The probe-rs forks

probe-rs upstream is synchronous and does not build for wasm. probe-web builds on two branches of beriberikix/probe-rs, which Cargo fetches as git dependencies (Cargo.lock pins the commits):

Branch	Used by	What it changes
webusb/nusb-0.2.7	probe-web-local, probe-web-targets	An async port of probe-rs and probe-rs-debug on nusb, with a WebUSB backend.
wasm-rpc-client	probe-web-core, tools/wire-gen, probe-rs serve	probe-rs-rpc and its client building for wasm32; serve accepting browser WebSocket clients and surviving clients that disconnect mid-operation.

A third fork, of cmsis-pack-manager (wasm/optional-network), makes its .pdsc parser build without networking.

The intent is to follow upstream as the async work lands there. To build against local checkouts of the forks, uncomment the paths override in .cargo/config.toml.