mcp-esp32

An MCP server that puts an ESP32 / MicroPython board behind tools an agent can call: enumerate ports, flash firmware, run code over the raw REPL, move files, capture serial output.

The problem this repository is actually about: flashing a board takes tens of seconds, boards reset unexpectedly, the serial link drops bytes, and an agent needs to be able to cancel a flash it started. None of that fits the request/response shape most MCP servers use. This repo is a demonstration of the parts of MCP built for exactly that -- progress notifications, cancellation, and structured (not thrown-and-forgotten) error handling -- applied to a device that is genuinely slow, stateful, and failure-prone.

Result: eight tools, a from-scratch raw-REPL client, a flashing orchestrator that reports progress at least every 5% and can be cancelled mid-write with no orphaned process, and a from-scratch fault-injecting simulator so all of that is verifiable without a board attached. 37 tests, all green, no hardware required. See Verification status below for exactly what that does and does not prove.

Why a long-running hardware operation is a harder MCP shape than a REST wrapper

A REST wrapper around esptool is POST /flash and a 200 once it's done (or a client-side timeout if it isn't). That throws away everything that actually matters when the thing on the other end of a serial cable takes 30 seconds to flash and can silently vanish partway through:

• Progress. A flash with no feedback for 30 seconds is indistinguishable from a hang, to both a human and an agent deciding whether to keep waiting. flash_firmware reports progress via MCP's notifications/progress at least every 5%, not just at the end.

• Cancellation. An agent that started a flash against the wrong port needs to be able to stop it -- not just stop waiting for it, but actually stop the write and not leave a subprocess or a half-open serial port behind. flash_firmware handles notifications/cancelled by signalling the in-flight write to stop, waiting (inside a shielded scope, so the cleanup itself can't be cancelled) for it to actually stop, and only then letting the cancellation propagate.

• Timeouts vs. failure. A REST call that times out tells you nothing about whether the device is busy, dead, or reset. Every failure mode here is a distinct, structured exception (DeviceResetError, FlashTimeoutError, ReplDesyncError, ...) with a kind field an agent can branch on, not a generic timeout.

• Recovery. A dropped byte on a REPL exchange shouldn't corrupt the next one. repl_exec detects a desynchronised raw REPL and resynchronises before the next call, instead of returning garbage or hanging.

• Statefulness under concurrency. A board has one UART. Two tool calls racing against the same port would interleave bytes and corrupt both, so every tool call is serialised per-port (an asyncio.Lock per port name) -- calls against different ports still run fully in parallel.

Related MCP server: esp-mcp

Tools

Architecture

server.py        MCP tool registration; adapts Context.report_progress and
                  cancellation for flash_firmware. Everything else is a
                  direct pass-through to toolkit.py.
toolkit.py        Backend-agnostic async tool implementations. No mcp.Context
                  here -- every reliability property is tested by calling
                  these functions directly.
backends/base.py  Resolves a port string to a BoardHandle: transport
                  factory, flash-session factory, identify(). Ports named
                  "SIM*" route to the simulator; anything else routes to
                  the serial backend.
raw_repl.py       MicroPython raw-REPL client (transport-agnostic).
fsops.py          ls/get/put built on raw_repl.exec(), chunked, size-capped.
flasher.py        Flash orchestration: progress throttling, cancellation,
                  SimulatorFlashSession (talks to the simulator) and
                  SubprocessFlashSession (drives the real esptool CLI).
sim_flash_protocol.py
                  Wire format for the simulator's flash handshake.
backends/simulator.py
                  A pty-backed fake device: real raw-REPL protocol, real
                  code execution against an in-memory filesystem, and a
                  configurable fault injector.
backends/serial_backend.py
                  Real hardware via pyserial + the esptool CLI. Not
                  exercised by anything in this repository -- see below.

The simulator, and exactly what it reproduces

The simulator (backends/simulator.py) is not a mock of the raw-REPL client -- it's a second, independent implementation of the device side of that protocol, running in a background thread on the other end of a real pty. Code sent to it is genuinely executed (via a sandboxed exec, with fake os/machine/sys/ubinascii modules backing an in-memory filesystem), so raw_repl.py cannot tell it apart from a real board at the protocol level.

Its flash handshake is a separate, deliberately simple framed protocol (sim_flash_protocol.py) -- sync, begin, N acknowledged data blocks, end -- not a reimplementation of esptool's real SLIP/ROM-bootloader wire format. Reproducing that byte-for-byte (chip-specific stub loaders, ROM quirks, SLIP escaping) is a separate project from what this repository demonstrates. What matters for exercising the MCP long-running-operation surface is the shape of a flash: a handshake, many acknowledged writes, an end -- with the same opportunities for a reset, a wedged link, or a cancellation. See Design notes for why this was the simpler option.

A FaultConfig on the simulated board injects, on demand:

• FlashFault.RESET_MID_FLASH -- the device sends an explicit reset marker after N data blocks and then goes silent, standing in for a brown-out or watchdog reset partway through a write. Tested in test_flasher_simulator.py::test_device_reset_mid_flash_is_a_structured_error_not_a_hang.

• FlashFault.SILENT_TIMEOUT -- the device stops responding with no reset marker at all, standing in for a wedged link. Tested in test_silent_timeout_surfaces_as_flash_timeout_error.

• ReplFault.GARBAGE -- one raw-REPL response is replaced with bytes that don't match any expected marker, standing in for a dropped/corrupted byte. Tested in test_raw_repl.py::test_garbage_on_the_line_is_detected_and_resynced_transparently.

• block_delay_s -- adds latency per flash block, used to make cancellation-mid-flash deterministically testable without a real multi- second flash.

Cancellation itself doesn't need fault injection -- it's tested by cancelling a real (simulated) flash in progress and asserting partial progress was reported and, for the subprocess path, that the child process was actually reaped (test_flasher_subprocess.py::test_cancellation_leaves_no_orphan_process).

Per-port locking is tested by timing two concurrent calls against the same simulated port (they serialise) against two calls on different ports (they don't) -- test_toolkit.py::test_concurrent_calls_on_same_port_are_serialised.

Verification status

Everything in this repository was built and tested against the simulator described above. No physical ESP32 or other hardware was available or used at any point. backends/serial_backend.py (real pyserial + the esptool CLI) is written to the same BoardHandle contract the simulator satisfies and its subprocess-lifecycle logic (progress parsing, cancel-and-reap, reset-string detection) is tested against a stand-in script (tests/fixtures/fake_esptool.py) that mimics esptool's stdout shape -- but the module itself has never been run against a real board or even a real copy of esptool. Treat it as "should work", not "verified".

Installation

python -m venv .venv
.venv/bin/pip install -e ".[dev]"        # simulator only
.venv/bin/pip install -e ".[serial]"     # adds pyserial + esptool for real hardware

Running it

mcp-esp32                 # starts the MCP server on stdio
mcp-esp32 --demo          # narrated walkthrough of every tool against the simulator, no MCP client needed

--demo calls toolkit.py directly (the same functions the tests call) and prints each result, including a deliberate reset-mid-flash fault, so the whole tool surface is visible without wiring up an MCP client.

Testing

pytest

37 tests, no hardware, no network access, no secrets. CI (.github/workflows/ci.yml) runs the suite plus --demo on Python 3.11 and 3.12.

Design notes

Decisions made without a way to check them against real hardware; simpler option chosen in each case.

• Simulated flash protocol instead of real SLIP/ROM-bootloader bytes. Reimplementing esptool's actual wire protocol (chip-specific stub loaders, ROM quirks) would be a second project and wouldn't change what's being demonstrated -- the MCP-level orchestration around a slow, faulty write. The simulator's protocol has the same shape (sync/begin/data×N/end, acknowledged blocks, an explicit reset signal) and is documented as such in sim_flash_protocol.py.

• fs_get/fs_put transfer content as hex-encoded lines over repeated raw-REPL exec() calls, not a dedicated binary protocol. MicroPython's raw REPL keeps globals alive across exec() calls (it's a persistent interpreter, not a fresh one per call), so fs_put opens a file handle in one call and writes to it in several more -- this works identically against real hardware and the simulator, and needed no new wire protocol.

• Base64 in the MCP tool signatures, hex on the wire. fs_get/fs_put take/return content_base64 because raw bytes aren't JSON-safe; internally fsops.py uses ubinascii.hexlify/unhexlify since that's what MicroPython actually has available on-device. MAX_TRANSFER_BYTES (512 KiB) is checked before a transfer starts, not discovered partway through.

• Ports named SIM* always route to the simulator (backends/base.py), auto-created on first use. No environment variable or config flag needed to use the simulator -- call any tool with a port starting with SIM and it exists.

• Errors are structured return values, not raised exceptions, for every expected failure mode (reset, timeout, desync, confirmation-required). A tool call that hits one of these returns {"error": {"kind": ..., ...}} rather than an MCP tool error, so an agent can branch on kind without parsing a message string. Actual bugs still raise and surface as normal tool errors.

• Cancellation cleanup runs inside an anyio.CancelScope(shield=True). Without shielding, the first checkpoint after catching the cancellation would immediately re-raise (cancel scopes are level-triggered), which would skip waiting for the flash thread to actually stop before the request is torn down.

• esptool is shelled out to as a CLI subprocess, not used as a library. Its Python API is not considered stable across versions; the CLI's stdout format ((NN %) progress lines, specific fatal-error strings) is what every existing esptool-wrapping tool already depends on, and it's the surface tests/fixtures/fake_esptool.py can stand in for without needing esptool internals to match.

Requirements

Python 3.11+. mcp for the server itself; pyserial and esptool are optional (only needed for the serial backend against real hardware) and their absence never breaks import -- see tests/test_optional_deps.py.