[cli] DRY up --prebuilt-dir helpers

Two small simplifications surfaced by re-reading the diff with a 'is
this DRY?' lens:

- Extract _missing_prebuilt_flash_tool(): the
  _ensure_platform_packages_for_prebuilt_upload helper had two
  near-identical 'if platform check; if finder() is not None return 0;
  tool_label = ...' blocks. Lift the platform->(tool_name) selection
  into a tiny helper so the early-return is one line and the install
  body is no longer interleaved with the platform dispatch.

- Collapse the two near-duplicate 'firmware not found' return points in
  upload_using_picotool. Both branches just selected an error message
  and returned 1; reorder so the message selection happens first and
  there's a single 'return 1'.

No behavior change; pure refactor. 587 unit tests pass.

Issue: esphome/device-builder#572

esphome/__main__.py

@@ -69,6 +69,7 @@ from esphome.util import (
     PICOTOOL_PACKAGE,
     FlashImage,
     detect_rp2040_bootsel,
+    get_ltchiptool_path,
     get_picotool_path,
     get_serial_ports,
     is_picotool_usb_permission_error,
@@ -853,9 +854,15 @@ def upload_using_esptool(
     elif CORE.using_toolchain_esp_idf:
         from esphome.espidf import toolchain
 
-        flash_images = [
-            FlashImage(path=toolchain.get_factory_firmware_path(), offset="0x0")
-        ]
+        # For ESP-IDF the upload is a single factory image at 0x0 (it bundles
+        # bootloader + partitions + app). The prebuilt-dir form ships that
+        # single file at the canonical name so the dashboard can flash it
+        # without re-deriving the ESP-IDF build path.
+        factory_path = (
+            CORE.prebuilt_artifact_path("firmware.factory.bin")
+            or toolchain.get_factory_firmware_path()
+        )
+        flash_images = [FlashImage(path=factory_path, offset="0x0")]
     else:
         from esphome.platformio import toolchain
 
@@ -931,9 +938,56 @@ def upload_using_esptool(
     return run_esptool(115200)
 
 
+def upload_using_ltchiptool(_config: ConfigType, port: str) -> int:
+    """Upload a libretiny ``.uf2`` directly via ``ltchiptool flash write``.
+
+    Bypasses PlatformIO so the dashboard's transparent install can flash a
+    libretiny device from just a prebuilt ``.uf2`` and a serial port; no
+    local build tree, ``platformio.ini`` or libretiny package install on the
+    machine running ``esphome upload`` is required (ltchiptool itself is
+    still required, but it ships with the libretiny PlatformIO platform or
+    can be ``pip install``-ed independently).
+
+    Chip family is auto-detected from the UF2 header so we don't need to
+    plumb through ``CORE.data[KEY_LIBRETINY][KEY_FAMILY]``.
+
+    The ``_config`` parameter is unused but kept for signature parity with
+    ``upload_using_platformio`` / ``upload_using_picotool`` so the dispatch
+    in ``upload_program`` can stay symmetric.
+    """
+    firmware = CORE.firmware_bin
+    if not firmware.is_file():
+        _LOGGER.error(
+            "LibreTiny firmware file not found at %s. "
+            "Make sure the project has been compiled first, or that "
+            "--prebuilt-dir points at a directory containing firmware.uf2 "
+            "(or firmware.bin -- libretiny emits the same UF2 content under "
+            "both names).",
+            firmware,
+        )
+        return 1
+
+    ltchiptool = get_ltchiptool_path()
+    if ltchiptool is None:
+        _LOGGER.error(
+            "ltchiptool not found. Install the libretiny PlatformIO platform "
+            "(esphome compile <yaml> with a libretiny board pulls it in) or "
+            "run `pip install ltchiptool` in this environment."
+        )
+        return 1
+
+    _LOGGER.info("Uploading firmware to LibreTiny device via ltchiptool...")
+    cmd = [str(ltchiptool), "flash", "write", "-d", port, str(firmware)]
+    return run_external_process(*cmd)
+
+
 def upload_using_platformio(config: ConfigType, port: str) -> int:
     from esphome.platformio import toolchain
 
+    # `upload_program` routes around this helper when --prebuilt-dir is set
+    # (libretiny→ltchiptool, RP2040→1200bps-touch+picotool), so PlatformIO
+    # is only invoked when a local build tree is available.
+
     # RP2040 platform-raspberrypi build recipe expects firmware.bin.signed for
     # the upload target, but 'nobuild' skips the build phase that creates it.
     # Create it here so the upload doesn't fail.
@@ -972,14 +1026,31 @@ def upload_using_picotool(config: ConfigType) -> int:
     from esphome.platformio import toolchain
 
     idedata = toolchain.get_idedata(config)
-    firmware_elf = Path(idedata.firmware_elf_path)
 
-    if not firmware_elf.is_file():
-        _LOGGER.error(
-            "Firmware ELF file not found at %s. "
-            "Make sure the project has been compiled first.",
-            firmware_elf,
-        )
+    # --prebuilt-dir ships canonical artifacts at the root of the directory,
+    # not the full PlatformIO build tree, so the ELF may not be present.
+    # picotool's "load" target accepts .uf2 / .bin / .elf, so fall back to
+    # CORE.firmware_bin (which resolves to the prebuilt firmware.uf2 when set)
+    # when no ELF is available.
+    elf_path = Path(idedata.firmware_elf_path)
+    if elf_path.is_file():
+        firmware_file: Path = elf_path
+    elif CORE.prebuilt_dir is not None and CORE.firmware_bin.is_file():
+        firmware_file = CORE.firmware_bin
+    else:
+        if CORE.prebuilt_dir is not None:
+            _LOGGER.error(
+                "No firmware found for picotool: neither %s (from idedata) "
+                "nor %s (prebuilt) exists.",
+                elf_path,
+                CORE.firmware_bin,
+            )
+        else:
+            _LOGGER.error(
+                "Firmware ELF file not found at %s. "
+                "Make sure the project has been compiled first.",
+                elf_path,
+            )
         return 1
 
     picotool = get_picotool_path(idedata.cc_path)
@@ -997,7 +1068,7 @@ def upload_using_picotool(config: ConfigType) -> int:
         # so progress bars display in real-time with \r updates.
         # Capture stderr only so we can detect permission errors.
         result = subprocess.run(
-            [str(picotool), "load", "-v", "-x", str(firmware_elf)],
+            [str(picotool), "load", "-v", "-x", str(firmware_file)],
             stderr=subprocess.PIPE,
             timeout=60,
             check=False,
@@ -1026,6 +1097,61 @@ def upload_using_picotool(config: ConfigType) -> int:
     return 0
 
 
+def _rp2040_serial_reset_to_bootsel(port: str, timeout: float = 10.0) -> bool:
+    """Reboot an arduino-pico RP2040 from running firmware into BOOTSEL mode.
+
+    arduino-pico's USB CDC handler treats a 1200bps "touch" (open the port at
+    1200 baud, then close it) as a request to reboot into the rp2040
+    bootloader, exposing the device as a picotool-loadable BOOTSEL endpoint.
+    This is the same mechanism the arduino-pico PlatformIO recipe uses for
+    serial uploads, lifted out so --prebuilt-dir uploads don't need to
+    re-invoke PlatformIO.
+
+    Returns True once a BOOTSEL device shows up on the USB bus.
+    """
+    import serial
+
+    # Look picotool up *before* triggering the reset. If it's missing, the
+    # touch would leave the device stranded in BOOTSEL with nothing able to
+    # flash it; bail out early so the user can recover (the device is still
+    # running the old firmware and re-enumerates as the same serial port).
+    picotool = _find_picotool()
+    if picotool is None:
+        _LOGGER.error(
+            "picotool not found; cannot flash RP2040 after BOOTSEL reset. "
+            "Ensure the RP2040 PlatformIO platform is installed (%s).",
+            PICOTOOL_PACKAGE,
+        )
+        return False
+
+    _LOGGER.info("Rebooting %s into BOOTSEL via 1200bps touch...", port)
+    try:
+        ser = serial.Serial()
+        ser.baudrate = 1200
+        ser.port = port
+        ser.open()
+        # Small wait so the firmware sees the open before we close it; on a
+        # fast host the open+close can otherwise happen inside a single USB
+        # frame and the touch is missed.
+        time.sleep(0.1)
+        ser.close()
+    except (OSError, serial.SerialException) as err:
+        _LOGGER.error("Failed to open %s at 1200 baud for BOOTSEL reset: %s", port, err)
+        return False
+
+    start = time.monotonic()
+    while time.monotonic() - start < timeout:
+        if detect_rp2040_bootsel(picotool).device_count > 0:
+            return True
+        time.sleep(0.2)
+    _LOGGER.error(
+        "RP2040 did not enter BOOTSEL within %.0fs after 1200bps touch on %s.",
+        timeout,
+        port,
+    )
+    return False
+
+
 def _wait_for_serial_port(
     port: str | None = None,
     timeout: float = 30.0,
@@ -1083,10 +1209,74 @@ def check_permissions(port: str):
             )
 
 
+def _missing_prebuilt_flash_tool() -> str | None:
+    """Return the name of the platform-specific flash tool that's expected
+    but not yet on disk, or None if no install step is needed.
+
+    ESP* paths use bundled esptool / smpclient. Libretiny + RP2040 paths
+    use ltchiptool / picotool, which ship inside the PlatformIO platform
+    package and aren't pip-installable on their own.
+    """
+    if CORE.is_libretiny and get_ltchiptool_path() is None:
+        return "ltchiptool"
+    if CORE.target_platform == PLATFORM_RP2040 and _find_picotool() is None:
+        return "picotool"
+    return None
+
+
+def _ensure_platform_packages_for_prebuilt_upload(config: ConfigType) -> int:
+    """Make sure the per-platform flash tool is available before a
+    ``--prebuilt-dir`` upload dispatches.
+
+    On a host that has never compiled the target platform locally (the
+    dashboard's transparent-install scenario), the flash tool isn't on
+    disk yet. Mirror what ``esphome compile`` would do up to the
+    platform-install step (codegen + write platformio.ini), then run
+    ``pio run -t idedata`` so the platform is downloaded without paying
+    for an actual compile.
+
+    Returns 0 if no install was needed or the install succeeded; non-zero
+    on failure so the upload bails out with a clear status.
+    """
+    tool_name = _missing_prebuilt_flash_tool()
+    if tool_name is None:
+        return 0
+
+    _LOGGER.info(
+        "%s not found on this host; configuring the PlatformIO %s "
+        "platform so the upload can flash the prebuilt firmware...",
+        tool_name,
+        CORE.target_platform,
+    )
+    rc = write_cpp(config)
+    if rc != 0:
+        return rc
+
+    from esphome.platformio import toolchain
+
+    return toolchain.prepare_platform_for_upload(config, CORE.verbose)
+
+
 def upload_program(
     config: ConfigType, args: ArgsProtocol, devices: list[str]
 ) -> tuple[int, str | None]:
     host = devices[0]
+
+    # --prebuilt-dir routes every per-platform upload helper at a directory of
+    # prebuilt artifacts instead of the local build tree. Validate once here
+    # so failures surface before we dispatch into platform-specific code that
+    # would otherwise produce confusing "file not found" errors deep in the
+    # esptool / picotool / PlatformIO call stacks.
+    prebuilt_dir = getattr(args, "prebuilt_dir", None)
+    if prebuilt_dir is not None:
+        prebuilt_path = Path(prebuilt_dir).expanduser()
+        if not prebuilt_path.is_dir():
+            raise EsphomeError(f"--prebuilt-dir {prebuilt_dir} is not a directory")
+        CORE.prebuilt_dir = prebuilt_path
+        rc = _ensure_platform_packages_for_prebuilt_upload(config)
+        if rc != 0:
+            return rc, None
+
     try:
         module = importlib.import_module("esphome.components." + CORE.target_platform)
         if getattr(module, "upload_program")(config, args, host):
@@ -1125,6 +1315,27 @@ def upload_program(
         if CORE.target_platform in (PLATFORM_ESP32, PLATFORM_ESP8266):
             file = getattr(args, "file", None)
             exit_code = upload_using_esptool(config, host, file, args.upload_speed)
+        elif CORE.is_libretiny and CORE.prebuilt_dir is not None:
+            # Dashboard transparent-install case: flash a prebuilt .uf2 with
+            # ltchiptool directly so the build tree isn't required. Without
+            # --prebuilt-dir, the normal PlatformIO path is still used so this
+            # is purely additive and existing libretiny serial flows are
+            # unchanged.
+            exit_code = upload_using_ltchiptool(config, host)
+        elif CORE.target_platform == PLATFORM_RP2040 and CORE.prebuilt_dir is not None:
+            # RP2040 serial + --prebuilt-dir: 1200bps-touch reboot into BOOTSEL,
+            # then flash with picotool. Avoids the PlatformIO build-tree
+            # requirement that upload_using_platformio imposes, mirroring the
+            # libretiny→ltchiptool path. Without --prebuilt-dir the original
+            # PlatformIO path is preserved for back-compat.
+            if _rp2040_serial_reset_to_bootsel(host):
+                exit_code = upload_using_picotool(config)
+            else:
+                # Touch reset failed: the helper already logged a specific
+                # error (port open failure, picotool missing, or BOOTSEL
+                # never enumerated). Be explicit about the failed exit code
+                # rather than relying on the function-level default of 1.
+                exit_code = 1
         elif CORE.target_platform == PLATFORM_RP2040 or CORE.is_libretiny:
             exit_code = upload_using_platformio(config, host)
         # else: Unknown target platform, exit_code remains 1
@@ -2092,6 +2303,16 @@ def parse_args(argv):
         "--file",
         help="Manually specify the binary file to upload.",
     )
+    parser_upload.add_argument(
+        "--prebuilt-dir",
+        help=(
+            "Advanced: directory of prebuilt artifacts to flash instead of "
+            "re-deriving paths from the local build tree. Intended for the "
+            "ESPHome dashboard's transparent install flow on hosts that "
+            "don't compile firmware themselves. End users should not need "
+            "this flag."
+        ),
+    )
     parser_upload.add_argument(
         "--ota-platform",
         choices=[CONF_ESPHOME, CONF_WEB_SERVER],

esphome/components/nrf52/__init__.py

@@ -500,9 +500,17 @@ def upload_program(config: ConfigType, args, host: str) -> bool:
         mcumgr_device = host
 
     if mcumgr_device:
-        firmware = Path(
-            CORE.relative_pioenvs_path(CORE.name, "zephyr", "app_update.bin")
-        ).resolve()
+        # `esphome upload --prebuilt-dir <path>` ships the MCUboot-signed
+        # update image at the root of the prebuilt directory; prefer that so
+        # the dashboard's transparent BLE install on a Bluetooth proxy works
+        # without a local Zephyr build tree.
+        prebuilt = CORE.prebuilt_artifact_path("app_update.bin")
+        if prebuilt is not None:
+            firmware = prebuilt.resolve()
+        else:
+            firmware = Path(
+                CORE.relative_pioenvs_path(CORE.name, "zephyr", "app_update.bin")
+            ).resolve()
         asyncio.run(smpmgr_upload(mcumgr_device, firmware))
         return True  # Handled: mcumgr OTA upload
 

esphome/core/__init__.py

@@ -567,6 +567,13 @@ class EsphomeCore:
         self.config_path: Path | None = None
         # The relative path to where all build files are stored
         self.build_path: Path | None = None
+        # Directory of prebuilt artifacts for `esphome upload --prebuilt-dir`.
+        # When set, firmware/partition/bootloader resolution and the idedata
+        # cache prefer files under this directory over the local build tree.
+        # User-facing docs live in esphome/esphome-docs#6600
+        # (`guides/cli.mdx`); see esphome/device-builder#572 for the
+        # cross-platform layout contract that motivates the flag.
+        self.prebuilt_dir: Path | None = None
         # The validated configuration, this is None until the config has been validated
         self.config: ConfigType | None = None
         # The pending tasks in the task queue (mostly for C++ generation)
@@ -633,6 +640,7 @@ class EsphomeCore:
         self.data = {}
         self.config_path = None
         self.build_path = None
+        self.prebuilt_dir = None
         self.config = None
         self.event_loop = _FakeEventLoop()
         self.task_counter = 0
@@ -775,8 +783,33 @@ class EsphomeCore:
     def relative_piolibdeps_path(self, *path: str | Path) -> Path:
         return self.relative_build_path(".piolibdeps", *path)
 
+    def prebuilt_artifact_path(self, *names: str) -> Path | None:
+        # Return the first existing prebuilt artifact among ``names``, or None
+        # when no prebuilt dir is configured or none of the candidates exist.
+        # Callers pass canonical basenames (e.g. "firmware.bin", "firmware.uf2")
+        # and the upload helpers fall back to the local build tree when this
+        # returns None.
+        if self.prebuilt_dir is None:
+            return None
+        for name in names:
+            candidate = self.prebuilt_dir / name
+            if candidate.is_file():
+                return candidate
+        return None
+
     @property
     def firmware_bin(self) -> Path:
+        # Prebuilt override priority is platform-aware: on RP2040 and libretiny
+        # the canonical flash artifact is the UF2 (raw firmware.bin won't have
+        # the address/family header picotool / ltchiptool need); on ESP* it's
+        # firmware.bin. When the dashboard ships only the canonical name, the
+        # other entry is a harmless no-op.
+        if self.is_rp2040 or self.is_libretiny:
+            prebuilt_names = ("firmware.uf2", "firmware.bin")
+        else:
+            prebuilt_names = ("firmware.bin", "firmware.uf2")
+        if (prebuilt := self.prebuilt_artifact_path(*prebuilt_names)) is not None:
+            return prebuilt
         # Check if using ESP-IDF toolchain
         if self.using_toolchain_esp_idf:
             return self.relative_build_path("build", f"{self.name}.bin")
@@ -792,6 +825,12 @@ class EsphomeCore:
         # Native ESP-IDF (--toolchain esp-idf): the partition table image is emitted under
         # build/partition_table/partition-table.bin alongside firmware.bin. PlatformIO writes the
         # equivalent file as partitions.bin in the env-specific .pioenvs directory.
+        if (
+            prebuilt := self.prebuilt_artifact_path(
+                "partitions.bin", "partition-table.bin"
+            )
+        ) is not None:
+            return prebuilt
         if self.using_toolchain_esp_idf:
             return self.relative_build_path(
                 "build", "partition_table", "partition-table.bin"
@@ -800,6 +839,8 @@ class EsphomeCore:
 
     @property
     def bootloader_bin(self) -> Path:
+        if (prebuilt := self.prebuilt_artifact_path("bootloader.bin")) is not None:
+            return prebuilt
         if self.using_toolchain_esp_idf:
             return self.relative_build_path("build", "bootloader", "bootloader.bin")
         return self.relative_pioenvs_path(self.name, "bootloader.bin")

esphome/platformio/toolchain.py

@@ -84,6 +84,36 @@ def run_compile(config, verbose):
     return run_platformio_cli_run(config, verbose, *args)
 
 
+def prepare_platform_for_upload(config, verbose) -> int:
+    """Configure the PlatformIO build environment for ``CORE.name`` without
+    compiling, so platform-specific flashing tools end up on disk.
+
+    Used by ``esphome upload --prebuilt-dir`` on hosts that have never
+    compiled the target platform locally. Runs ``pio run -t idedata``: the
+    target fires SConscript (which on libretiny triggers
+    ``ConfigurePythonVenv`` and pip-installs ``ltchiptool`` into the
+    platform's virtualenv, and on RP2040 installs the picotool tool
+    package) but skips the actual compile, so this is much cheaper than a
+    full build.
+
+    The idedata JSON gets emitted to stdout as a side effect of the
+    target; we don't filter it out because the install runs once per cold
+    host and the trailing JSON blob is harmless noise.
+
+    Returns the subprocess exit code; non-zero means the platform install
+    failed and the upload caller should abort.
+    """
+    # ``capture_stdout`` is intentionally False so run_platformio_cli_run
+    # returns the int exit code (str only when capture_stdout=True).
+    result = run_platformio_cli_run(config, verbose, "-t", "idedata")
+    # Belt-and-suspenders: assert we got an int rather than silently
+    # treating a captured stdout string as success.
+    assert isinstance(result, int), (
+        f"prepare_platform_for_upload expected int, got {type(result).__name__}"
+    )
+    return result
+
+
 def _run_idedata(config):
     args = ["-t", "idedata"]
     stdout = run_platformio_cli_run(config, False, *args, capture_stdout=True)
@@ -101,7 +131,60 @@ def _run_idedata(config):
         raise
 
 
+def _resolve_prebuilt_idedata_paths(data: dict, prebuilt_dir: Path) -> None:
+    """Resolve relative paths in a prebuilt idedata.json against prebuilt_dir.
+
+    The dashboard's transparent-install pipeline rewrites ``prog_path`` and
+    ``extra.flash_images[*].path`` to bare basenames before shipping the
+    tarball (the receiver's build-host absolute paths don't resolve on the
+    offloader). Accept both shapes: absolute paths pass through unchanged
+    so a hand-built ``--prebuilt-dir`` still works, and basenames / other
+    relative paths resolve to ``prebuilt_dir / p``. Mutates ``data`` in
+    place.
+
+    ``cc_path`` is left alone because it points at a PlatformIO toolchain
+    binary (``~/.platformio/packages/...``) that lives outside the prebuilt
+    dir; the offloader's local PIO install provides the matching binary.
+
+    Absoluteness follows ``pathlib.Path.is_absolute`` semantics: on Windows
+    a leading slash without a drive letter (e.g. ``/foo/bar``) is rooted
+    but **not** absolute, and will be resolved against ``prebuilt_dir``.
+    Hand-staged directories should use OS-appropriate absolute paths if
+    they want passthrough.
+    """
+    prog = data.get("prog_path")
+    if prog is not None and not Path(prog).is_absolute():
+        data["prog_path"] = str(prebuilt_dir / prog)
+
+    extra = data.get("extra")
+    if isinstance(extra, dict):
+        for image in extra.get("flash_images", []) or []:
+            path = image.get("path") if isinstance(image, dict) else None
+            if path is not None and not Path(path).is_absolute():
+                image["path"] = str(prebuilt_dir / path)
+
+
 def _load_idedata(config):
+    # --prebuilt-dir override: read the shipped idedata.json, resolve any
+    # relative paths inside it against the prebuilt dir, return it directly.
+    # Malformed JSON -> EsphomeError (clean one-line diagnostic, not a raw
+    # JSONDecodeError trace). No schema or path-existence validation; any
+    # missing artifact surfaces later as a "file not found" from the
+    # downstream flasher.
+    if CORE.prebuilt_dir is not None:
+        prebuilt_idedata = CORE.prebuilt_dir / "idedata.json"
+        if prebuilt_idedata.is_file():
+            try:
+                data = json.loads(prebuilt_idedata.read_text(encoding="utf-8"))
+            except json.JSONDecodeError as err:
+                raise EsphomeError(
+                    f"Failed to parse {prebuilt_idedata}: {err}. The dashboard "
+                    "must stage a syntactically valid idedata.json under "
+                    "--prebuilt-dir; the upload cannot proceed without it."
+                ) from err
+            _resolve_prebuilt_idedata_paths(data, CORE.prebuilt_dir)
+            return data
+
     platformio_ini = CORE.relative_build_path("platformio.ini")
     temp_idedata = CORE.relative_internal_path("idedata", f"{CORE.name}.json")
 

esphome/util.py

@@ -5,6 +5,7 @@ import io
 import logging
 from pathlib import Path
 import re
+import shutil
 import subprocess
 import sys
 from typing import TYPE_CHECKING, Any
@@ -401,6 +402,46 @@ def get_serial_ports() -> list[SerialPort]:
 
 PICOTOOL_PACKAGE = "tool-picotool-rp2040-earlephilhower"
 
+# PlatformIO ships ltchiptool inside a libretiny-specific virtualenv under
+# `~/.platformio/penv/.libretiny/`, separate from the toolchain packages
+# layout that picotool uses, so it needs its own lookup helper.
+LTCHIPTOOL_PIO_PENV_NAME = ".libretiny"
+
+
+def get_ltchiptool_path() -> Path | None:
+    """Find the ltchiptool executable.
+
+    Order:
+      1. ``ltchiptool`` on PATH (pip-installed system-wide or in a virtualenv).
+      2. PlatformIO's libretiny penv (where platform-libretiny installs it
+         during its package init). The script subdirectory follows the
+         CPython venv convention: ``Scripts/`` on Windows, ``bin/``
+         elsewhere.
+
+    Returns None if neither is available; callers should surface an
+    actionable error pointing the user at one of those install paths.
+    """
+    on_path = shutil.which("ltchiptool")
+    if on_path is not None:
+        return Path(on_path)
+    if sys.platform == "win32":
+        bin_subdir = "Scripts"
+        binary_name = "ltchiptool.exe"
+    else:
+        bin_subdir = "bin"
+        binary_name = "ltchiptool"
+    pio_penv = (
+        Path.home()
+        / ".platformio"
+        / "penv"
+        / LTCHIPTOOL_PIO_PENV_NAME
+        / bin_subdir
+        / binary_name
+    )
+    if pio_penv.is_file():
+        return pio_penv
+    return None
+
 
 def get_picotool_path(cc_path: str) -> Path | None:
     """Derive the picotool binary path from the PlatformIO toolchain cc_path.

tests/unit_tests/components/test_nrf52.py

@@ -0,0 +1,52 @@
+"""Tests for the nrf52 component's custom upload_program."""
+
+from __future__ import annotations
+
+from pathlib import Path
+from unittest.mock import AsyncMock, MagicMock, patch
+
+from esphome.core import CORE
+
+
+def test_nrf52_upload_program_prebuilt_dir_for_ble_ota(tmp_path: Path) -> None:
+    """`esphome upload --prebuilt-dir <path>` for an nRF52 Bluetooth proxy
+    ships the MCUboot-signed update at <prebuilt-dir>/app_update.bin so the
+    dashboard's transparent BLE install doesn't need a local Zephyr build
+    tree. Verify the nrf52 upload_program picks up that path instead of
+    `CORE.relative_pioenvs_path(..., "zephyr", "app_update.bin")`.
+    """
+    prebuilt = tmp_path / "prebuilt"
+    prebuilt.mkdir()
+    expected_firmware = prebuilt / "app_update.bin"
+    expected_firmware.write_bytes(b"signed-mcuboot-image")
+    CORE.prebuilt_dir = prebuilt
+    CORE.name = "bt-proxy"
+
+    captured_firmware: list[Path] = []
+
+    async def fake_upload(device: str, firmware: Path) -> None:
+        captured_firmware.append(firmware)
+
+    # Patch the heavy nrf52 surface so we exercise only the firmware-path
+    # resolution: BLE scan returns a fake device, smpmgr_upload records the
+    # firmware path it would have flashed.
+    with (
+        patch(
+            "esphome.components.nrf52.ota.smpmgr_scan",
+            new=AsyncMock(return_value="AA:BB:CC:DD:EE:FF"),
+        ),
+        patch(
+            "esphome.components.nrf52.ota.smpmgr_upload",
+            new=fake_upload,
+        ),
+        patch(
+            "esphome.components.nrf52.ble_logger.is_mac_address",
+            return_value=False,
+        ),
+    ):
+        from esphome.components.nrf52 import upload_program
+
+        handled = upload_program({}, MagicMock(), "BLE")
+
+    assert handled is True
+    assert captured_firmware == [expected_firmware.resolve()]

tests/unit_tests/test_core.py

@@ -894,6 +894,119 @@ class TestEsphomeCore:
             "foo/build/.pioenvs/test-device/bootloader.bin"
         )
 
+    def test_prebuilt_artifact_path__none_when_unset(self, target):
+        """prebuilt_artifact_path is the gate on every prebuilt-dir override.
+        When --prebuilt-dir is not set, every consumer must fall back to the
+        local build tree."""
+        assert target.prebuilt_artifact_path("firmware.bin") is None
+
+    def test_prebuilt_artifact_path__returns_first_existing(self, target, tmp_path):
+        """firmware.bin (ESP) and firmware.uf2 (RP2040/libretiny) are both
+        canonical names, so callers pass them in priority order and get the
+        first one that actually exists."""
+        target.prebuilt_dir = tmp_path
+        (tmp_path / "firmware.uf2").write_bytes(b"uf2")
+
+        assert (
+            target.prebuilt_artifact_path("firmware.bin", "firmware.uf2")
+            == tmp_path / "firmware.uf2"
+        )
+
+    def test_prebuilt_artifact_path__none_when_no_candidate_exists(
+        self, target, tmp_path
+    ):
+        """If --prebuilt-dir is set but the directory is empty for the asked
+        names, return None so the caller falls through to the local build
+        tree rather than asserting."""
+        target.prebuilt_dir = tmp_path
+
+        assert target.prebuilt_artifact_path("firmware.bin") is None
+
+    def test_firmware_bin__prebuilt_override(self, target, tmp_path):
+        """CORE.firmware_bin is the single resolution point every OTA path
+        reads. With --prebuilt-dir set and firmware.bin present, it must
+        return the prebuilt path instead of the (non-existent) local build
+        path."""
+        target.name = "test-device"
+        target.toolchain = const.Toolchain.PLATFORMIO
+        target.data[const.KEY_CORE] = {const.KEY_TARGET_PLATFORM: "esp32"}
+        target.prebuilt_dir = tmp_path
+        (tmp_path / "firmware.bin").write_bytes(b"fw")
+
+        assert target.firmware_bin == tmp_path / "firmware.bin"
+
+    def test_firmware_bin__prebuilt_override_uf2(self, target, tmp_path):
+        """RP2040 / libretiny ship firmware.uf2; firmware_bin returns it when
+        firmware.bin is absent so the dashboard only needs to ship one file."""
+        target.name = "test-device"
+        target.toolchain = const.Toolchain.PLATFORMIO
+        target.data[const.KEY_CORE] = {const.KEY_TARGET_PLATFORM: "bk72xx"}
+        target.prebuilt_dir = tmp_path
+        (tmp_path / "firmware.uf2").write_bytes(b"uf2")
+
+        assert target.firmware_bin == tmp_path / "firmware.uf2"
+
+    def test_firmware_bin__prebuilt_prefers_uf2_on_rp2040(self, target, tmp_path):
+        """Critical for RP2040: when both firmware.bin and firmware.uf2 are
+        staged, return the .uf2. picotool's load command needs the UF2
+        header (address + family); a raw .bin would flash to the wrong
+        offset (or refuse to flash). The dashboard's wire format ships only
+        the canonical name, but a hand-staged dir might carry both."""
+        target.name = "test-device"
+        target.toolchain = const.Toolchain.PLATFORMIO
+        target.data[const.KEY_CORE] = {const.KEY_TARGET_PLATFORM: const.PLATFORM_RP2040}
+        target.prebuilt_dir = tmp_path
+        (tmp_path / "firmware.bin").write_bytes(b"raw")
+        (tmp_path / "firmware.uf2").write_bytes(b"uf2-wrapped")
+
+        assert target.firmware_bin == tmp_path / "firmware.uf2"
+
+    def test_firmware_bin__prebuilt_prefers_bin_on_esp32(self, target, tmp_path):
+        """Mirror of the above for ESP32: the canonical artifact is .bin
+        (esptool flashes raw images at fixed offsets); if both are shipped
+        the .uf2 is the irrelevant one."""
+        target.name = "test-device"
+        target.toolchain = const.Toolchain.PLATFORMIO
+        target.data[const.KEY_CORE] = {const.KEY_TARGET_PLATFORM: "esp32"}
+        target.prebuilt_dir = tmp_path
+        (tmp_path / "firmware.bin").write_bytes(b"raw")
+        (tmp_path / "firmware.uf2").write_bytes(b"uf2-wrapped")
+
+        assert target.firmware_bin == tmp_path / "firmware.bin"
+
+    def test_partition_table_bin__prebuilt_override(self, target, tmp_path):
+        target.name = "test-device"
+        target.toolchain = const.Toolchain.PLATFORMIO
+        target.prebuilt_dir = tmp_path
+        (tmp_path / "partitions.bin").write_bytes(b"pt")
+
+        assert target.partition_table_bin == tmp_path / "partitions.bin"
+
+    def test_bootloader_bin__prebuilt_override(self, target, tmp_path):
+        target.name = "test-device"
+        target.toolchain = const.Toolchain.PLATFORMIO
+        target.prebuilt_dir = tmp_path
+        (tmp_path / "bootloader.bin").write_bytes(b"bl")
+
+        assert target.bootloader_bin == tmp_path / "bootloader.bin"
+
+    def test_firmware_bin__prebuilt_dir_set_but_file_missing_falls_through(
+        self, target, tmp_path
+    ):
+        """Setting --prebuilt-dir alone must not break devices that don't ship
+        every artifact (e.g. OTA-only uploads with no bootloader.bin). When
+        the canonical file isn't in the directory, fall back to the local
+        build path so the caller's existing error messages still apply."""
+        target.name = "test-device"
+        target.toolchain = const.Toolchain.PLATFORMIO
+        target.data[const.KEY_CORE] = {const.KEY_TARGET_PLATFORM: "esp32"}
+        target.prebuilt_dir = tmp_path
+        # No firmware.bin in tmp_path on purpose.
+
+        assert target.firmware_bin == Path(
+            "foo/build/.pioenvs/test-device/firmware.bin"
+        )
+
     def test_add_library__extracts_short_name_from_path(self, target):
         """Test add_library extracts short name from library paths like owner/lib."""
         target.data[const.KEY_CORE] = {

tests/unit_tests/test_main.py

@@ -50,6 +50,7 @@ from esphome.__main__ import (
     show_logs,
     upload_program,
     upload_using_esptool,
+    upload_using_ltchiptool,
     upload_using_picotool,
     upload_using_platformio,
 )
@@ -1135,6 +1136,7 @@ class MockArgs:
     ota_platform: str | None = None
     partition_table: bool = False
     bootloader: bool = False
+    prebuilt_dir: str | None = None
 
 
 def test_upload_program_serial_esp32(
@@ -1341,6 +1343,41 @@ def test_upload_using_esptool_with_file_path(
     assert firmware_path.endswith("custom_firmware.bin")
 
 
+def test_upload_using_esptool_idf_prebuilt_factory_bin(
+    tmp_path: Path,
+    mock_run_external_command_main: Mock,
+) -> None:
+    """ESP-IDF builds flash a single ``firmware.factory.bin`` (bootloader +
+    partitions + app bundled) at offset 0x0. With --prebuilt-dir set the
+    esptool helper must pick up ``<prebuilt-dir>/firmware.factory.bin``
+    instead of ``CORE.relative_build_path("build", "firmware.factory.bin")``
+    so the dashboard can flash an ESP-IDF device without a local build."""
+    setup_core(platform=PLATFORM_ESP32, tmp_path=tmp_path, name="test")
+    CORE.toolchain = Toolchain.ESP_IDF
+    CORE.data[KEY_ESP32] = {KEY_VARIANT: VARIANT_ESP32}
+
+    prebuilt = tmp_path / "prebuilt"
+    prebuilt.mkdir()
+    factory_bin = prebuilt / "firmware.factory.bin"
+    factory_bin.write_bytes(b"\x00" * 1024)
+    CORE.prebuilt_dir = prebuilt
+
+    config = {CONF_ESPHOME: {"platformio_options": {}}}
+
+    result = upload_using_esptool(config, "/dev/ttyUSB0", None, None)
+
+    assert result == 0
+    cmd_list = list(mock_run_external_command_main.call_args[0][1:])
+
+    # ESP-IDF prebuilt: single image at offset 0x0 pointing at the prebuilt
+    # factory file, not the local build tree's <build>/firmware.factory.bin.
+    write_flash_idx = cmd_list.index("write-flash")
+    offset_idx = write_flash_idx + 4
+    assert cmd_list[offset_idx] == "0x0"
+    path_arg = cmd_list[offset_idx + 1]
+    assert path_arg == str(factory_bin)
+
+
 @pytest.mark.parametrize(
     "platform,device",
     [
@@ -1372,6 +1409,127 @@ def test_upload_program_serial_platformio_platforms(
     mock_upload_using_platformio.assert_called_once_with(config, device)
 
 
+def test_upload_program_libretiny_serial_with_prebuilt_dir_uses_ltchiptool(
+    mock_get_port_type: Mock,
+    mock_check_permissions: Mock,
+    tmp_path: Path,
+) -> None:
+    """Verify LibreTiny serial + --prebuilt-dir bypasses upload_using_platformio
+    entirely so the dashboard doesn't have to ship a PlatformIO build tree;
+    ltchiptool can flash the prebuilt .uf2 directly. Verify the dispatch."""
+    setup_core(platform=PLATFORM_BK72XX)
+    mock_get_port_type.return_value = "SERIAL"
+
+    prebuilt = tmp_path / "prebuilt"
+    prebuilt.mkdir()
+    (prebuilt / "firmware.uf2").write_bytes(b"uf2")
+
+    args = MockArgs(prebuilt_dir=str(prebuilt))
+    devices = ["/dev/ttyUSB0"]
+
+    with (
+        # Pretend ltchiptool is already installed so we skip the pkg-install
+        # prep branch and exercise the dispatch we actually care about here.
+        patch(
+            "esphome.__main__.get_ltchiptool_path", return_value=tmp_path / "fake-lt"
+        ),
+        patch("esphome.__main__.upload_using_ltchiptool", return_value=0) as mock_lt,
+        patch("esphome.__main__.upload_using_platformio") as mock_pio,
+    ):
+        exit_code, host = upload_program({}, args, devices)
+
+    assert exit_code == 0
+    assert host == "/dev/ttyUSB0"
+    mock_lt.assert_called_once_with({}, "/dev/ttyUSB0")
+    mock_pio.assert_not_called()
+
+
+def test_upload_program_libretiny_serial_without_prebuilt_dir_uses_platformio(
+    mock_upload_using_platformio: Mock,
+    mock_get_port_type: Mock,
+    mock_check_permissions: Mock,
+) -> None:
+    """Regression guard: without --prebuilt-dir, libretiny serial still goes
+    through upload_using_platformio. The ltchiptool path is purely additive."""
+    setup_core(platform=PLATFORM_BK72XX)
+    mock_get_port_type.return_value = "SERIAL"
+    mock_upload_using_platformio.return_value = 0
+
+    args = MockArgs()
+    devices = ["/dev/ttyUSB0"]
+
+    with patch("esphome.__main__.upload_using_ltchiptool") as mock_lt:
+        exit_code, _ = upload_program({}, args, devices)
+
+    assert exit_code == 0
+    mock_upload_using_platformio.assert_called_once_with({}, "/dev/ttyUSB0")
+    mock_lt.assert_not_called()
+
+
+def test_upload_using_ltchiptool_success(tmp_path: Path) -> None:
+    """Verify the ltchiptool helper invokes the binary with the firmware uf2 path."""
+    setup_core(platform=PLATFORM_BK72XX, tmp_path=tmp_path)
+
+    prebuilt = tmp_path / "prebuilt"
+    prebuilt.mkdir()
+    firmware_uf2 = prebuilt / "firmware.uf2"
+    firmware_uf2.write_bytes(b"uf2-data")
+    CORE.prebuilt_dir = prebuilt
+
+    fake_ltchiptool = tmp_path / "ltchiptool"
+    fake_ltchiptool.touch()
+
+    with (
+        patch("esphome.__main__.get_ltchiptool_path", return_value=fake_ltchiptool),
+        patch("esphome.__main__.run_external_process", return_value=0) as mock_run,
+    ):
+        exit_code = upload_using_ltchiptool({}, "/dev/ttyUSB0")
+
+    assert exit_code == 0
+    mock_run.assert_called_once_with(
+        str(fake_ltchiptool),
+        "flash",
+        "write",
+        "-d",
+        "/dev/ttyUSB0",
+        str(firmware_uf2),
+    )
+
+
+def test_upload_using_ltchiptool_missing_firmware(tmp_path: Path) -> None:
+    """Surface a clear error when the resolved firmware path doesn't exist,
+    rather than letting ltchiptool fail with an obscure 'no such file'."""
+    setup_core(platform=PLATFORM_BK72XX, tmp_path=tmp_path)
+
+    prebuilt = tmp_path / "prebuilt"
+    prebuilt.mkdir()
+    # No firmware.uf2 written.
+    CORE.prebuilt_dir = prebuilt
+
+    with patch("esphome.__main__.get_ltchiptool_path") as mock_find:
+        exit_code = upload_using_ltchiptool({}, "/dev/ttyUSB0")
+
+    assert exit_code == 1
+    # get_ltchiptool_path is short-circuited; firmware-missing fails first.
+    mock_find.assert_not_called()
+
+
+def test_upload_using_ltchiptool_not_found(tmp_path: Path) -> None:
+    """When ltchiptool isn't installed anywhere we know how to find, surface
+    an actionable install hint instead of an ENOENT from subprocess."""
+    setup_core(platform=PLATFORM_BK72XX, tmp_path=tmp_path)
+
+    prebuilt = tmp_path / "prebuilt"
+    prebuilt.mkdir()
+    (prebuilt / "firmware.uf2").write_bytes(b"uf2")
+    CORE.prebuilt_dir = prebuilt
+
+    with patch("esphome.__main__.get_ltchiptool_path", return_value=None):
+        exit_code = upload_using_ltchiptool({}, "/dev/ttyUSB0")
+
+    assert exit_code == 1
+
+
 def test_upload_using_platformio_creates_signed_bin_for_rp2040(
     tmp_path: Path,
 ) -> None:
@@ -1412,6 +1570,226 @@ def test_upload_using_platformio_skips_signed_bin_for_non_rp2040(
     assert result == 0
 
 
+def test_upload_program_prebuilt_dir_sets_core_attr(
+    mock_upload_using_esptool: Mock,
+    mock_get_port_type: Mock,
+    mock_check_permissions: Mock,
+    tmp_path: Path,
+) -> None:
+    """--prebuilt-dir must stash the validated path on CORE before dispatching
+    so that all per-platform helpers and CORE.firmware_bin / etc. resolve to
+    the prebuilt artifacts."""
+    setup_core(platform=PLATFORM_ESP32)
+    mock_get_port_type.return_value = "SERIAL"
+    mock_upload_using_esptool.return_value = 0
+
+    prebuilt = tmp_path / "artifacts"
+    prebuilt.mkdir()
+
+    config = {}
+    args = MockArgs(prebuilt_dir=str(prebuilt))
+    devices = ["/dev/ttyUSB0"]
+
+    exit_code, _ = upload_program(config, args, devices)
+
+    assert exit_code == 0
+    assert CORE.prebuilt_dir == prebuilt
+
+
+def test_upload_program_prebuilt_dir_installs_libretiny_platform_if_missing(
+    mock_get_port_type: Mock,
+    mock_check_permissions: Mock,
+    tmp_path: Path,
+) -> None:
+    """The dashboard's transparent-install scenario: ltchiptool isn't on
+    disk yet (this host has never compiled a libretiny config locally), so
+    upload_program triggers `pio pkg install` to download the libretiny
+    platform package -- same prep as compile, no actual compile -- before
+    dispatching the upload helper."""
+    setup_core(platform=PLATFORM_BK72XX, tmp_path=tmp_path)
+    mock_get_port_type.return_value = "SERIAL"
+
+    prebuilt = tmp_path / "prebuilt"
+    prebuilt.mkdir()
+    (prebuilt / "firmware.uf2").write_bytes(b"uf2")
+    args = MockArgs(prebuilt_dir=str(prebuilt))
+
+    with (
+        # First call: ltchiptool missing -> install. After install, the
+        # upload_using_ltchiptool dispatch finds it (we mock that helper
+        # to skip the actual flash).
+        patch("esphome.__main__.get_ltchiptool_path", return_value=None) as mock_find,
+        patch("esphome.__main__.write_cpp", return_value=0) as mock_write_cpp,
+        patch(
+            "esphome.platformio.toolchain.prepare_platform_for_upload", return_value=0
+        ) as mock_prep,
+        patch("esphome.__main__.upload_using_ltchiptool", return_value=0),
+    ):
+        exit_code, _ = upload_program({}, args, ["/dev/ttyUSB0"])
+
+    assert exit_code == 0
+    mock_find.assert_called()
+    mock_write_cpp.assert_called_once()
+    mock_prep.assert_called_once()
+
+
+def test_upload_program_prebuilt_dir_skips_install_when_tool_present(
+    mock_get_port_type: Mock,
+    mock_check_permissions: Mock,
+    tmp_path: Path,
+) -> None:
+    """When ltchiptool is already on disk (e.g. HA add-on dashboards with
+    PIO platforms pre-warmed), don't run codegen or pkg install -- that
+    would surprise the user with a heavy side effect on every upload."""
+    setup_core(platform=PLATFORM_BK72XX, tmp_path=tmp_path)
+    mock_get_port_type.return_value = "SERIAL"
+
+    prebuilt = tmp_path / "prebuilt"
+    prebuilt.mkdir()
+    (prebuilt / "firmware.uf2").write_bytes(b"uf2")
+    args = MockArgs(prebuilt_dir=str(prebuilt))
+
+    with (
+        patch(
+            "esphome.__main__.get_ltchiptool_path",
+            return_value=tmp_path / "ltchiptool",
+        ),
+        patch("esphome.__main__.write_cpp") as mock_write_cpp,
+        patch("esphome.platformio.toolchain.prepare_platform_for_upload") as mock_prep,
+        patch("esphome.__main__.upload_using_ltchiptool", return_value=0),
+    ):
+        exit_code, _ = upload_program({}, args, ["/dev/ttyUSB0"])
+
+    assert exit_code == 0
+    mock_write_cpp.assert_not_called()
+    mock_prep.assert_not_called()
+
+
+def test_upload_program_prebuilt_dir_write_cpp_failure_aborts_upload(
+    mock_get_port_type: Mock,
+    mock_check_permissions: Mock,
+    tmp_path: Path,
+) -> None:
+    """Codegen failure (write_cpp returning non-zero) must short-circuit
+    before pkg install runs; pkg install against a broken platformio.ini
+    would just fail less informatively. Symmetric with the pkg-install
+    failure test below."""
+    setup_core(platform=PLATFORM_BK72XX, tmp_path=tmp_path)
+    mock_get_port_type.return_value = "SERIAL"
+
+    prebuilt = tmp_path / "prebuilt"
+    prebuilt.mkdir()
+    args = MockArgs(prebuilt_dir=str(prebuilt))
+
+    with (
+        patch("esphome.__main__.get_ltchiptool_path", return_value=None),
+        patch("esphome.__main__.write_cpp", return_value=3),
+        patch("esphome.platformio.toolchain.prepare_platform_for_upload") as mock_prep,
+        patch("esphome.__main__.upload_using_ltchiptool") as mock_upload,
+    ):
+        exit_code, host = upload_program({}, args, ["/dev/ttyUSB0"])
+
+    assert exit_code == 3
+    assert host is None
+    mock_prep.assert_not_called()
+    mock_upload.assert_not_called()
+
+
+def test_upload_program_prebuilt_dir_pkg_install_failure_aborts_upload(
+    mock_get_port_type: Mock,
+    mock_check_permissions: Mock,
+    tmp_path: Path,
+) -> None:
+    """If `pio pkg install` fails (network error, registry outage, etc.),
+    bail out with that exit code rather than dispatching the upload helper
+    against an environment that's known to be missing the flash tool."""
+    setup_core(platform=PLATFORM_RP2040, tmp_path=tmp_path)
+    mock_get_port_type.return_value = "SERIAL"
+
+    prebuilt = tmp_path / "prebuilt"
+    prebuilt.mkdir()
+    args = MockArgs(prebuilt_dir=str(prebuilt))
+
+    with (
+        patch("esphome.__main__._find_picotool", return_value=None),
+        patch("esphome.__main__.write_cpp", return_value=0),
+        patch(
+            "esphome.platformio.toolchain.prepare_platform_for_upload", return_value=2
+        ),
+        patch("esphome.__main__.upload_using_picotool") as mock_upload,
+        patch("esphome.__main__._rp2040_serial_reset_to_bootsel") as mock_reset,
+    ):
+        exit_code, host = upload_program({}, args, ["/dev/ttyACM0"])
+
+    assert exit_code == 2
+    assert host is None
+    mock_upload.assert_not_called()
+    mock_reset.assert_not_called()
+
+
+def test_upload_program_prebuilt_dir_missing_raises(
+    mock_get_port_type: Mock,
+    mock_check_permissions: Mock,
+    tmp_path: Path,
+) -> None:
+    """Catch a missing --prebuilt-dir before dispatching to a per-platform
+    helper; deferring would surface as a confusing "file not found" deep in
+    esptool / picotool / PlatformIO."""
+    setup_core(platform=PLATFORM_ESP32)
+    mock_get_port_type.return_value = "SERIAL"
+
+    missing = tmp_path / "does-not-exist"
+
+    config = {}
+    args = MockArgs(prebuilt_dir=str(missing))
+    devices = ["/dev/ttyUSB0"]
+
+    with pytest.raises(EsphomeError, match="not a directory"):
+        upload_program(config, args, devices)
+
+
+def test_upload_using_picotool_falls_back_to_firmware_bin_when_elf_missing(
+    tmp_path: Path,
+) -> None:
+    """`--prebuilt-dir` ships a flat firmware.uf2, not the build tree's ELF.
+    picotool load accepts uf2/bin/elf, so when the idedata ELF is missing
+    fall through to CORE.firmware_bin (the prebuilt .uf2) instead of failing.
+    """
+    setup_core(platform=PLATFORM_RP2040, tmp_path=tmp_path)
+
+    prebuilt = tmp_path / "prebuilt"
+    prebuilt.mkdir()
+    firmware_uf2 = prebuilt / "firmware.uf2"
+    firmware_uf2.write_bytes(b"uf2-data")
+    CORE.prebuilt_dir = prebuilt
+
+    # idedata points at an ELF that doesn't exist on disk (typical for
+    # prebuilt-dir flat layouts).
+    mock_idedata = MagicMock()
+    mock_idedata.firmware_elf_path = str(tmp_path / "build" / "firmware.elf")
+    mock_idedata.cc_path = "/fake/path/gcc"
+
+    mock_result = MagicMock()
+    mock_result.returncode = 0
+    mock_result.stderr = b""
+
+    # Stub the picotool lookup to short-circuit the toolchain probe.
+    with (
+        patch("esphome.platformio.toolchain.get_idedata", return_value=mock_idedata),
+        patch(
+            "esphome.__main__.get_picotool_path",
+            return_value=tmp_path / "picotool",
+        ),
+        patch("subprocess.run", return_value=mock_result) as mock_run,
+    ):
+        exit_code = upload_using_picotool({})
+
+    assert exit_code == 0
+    # Verify picotool was handed the prebuilt .uf2, not the missing ELF.
+    cmd = mock_run.call_args[0][0]
+    assert str(firmware_uf2) in cmd
+
+
 def test_upload_program_serial_upload_failed(
     mock_upload_using_esptool: Mock,
     mock_get_port_type: Mock,
@@ -1434,6 +1812,145 @@ def test_upload_program_serial_upload_failed(
     mock_upload_using_esptool.assert_called_once()
 
 
+def test_upload_program_rp2040_serial_with_prebuilt_dir_uses_picotool(
+    mock_upload_using_picotool: Mock,
+    mock_get_port_type: Mock,
+    mock_check_permissions: Mock,
+    tmp_path: Path,
+) -> None:
+    """RP2040 serial + --prebuilt-dir reboots into BOOTSEL via 1200bps
+    touch and flashes with picotool, avoiding upload_using_platformio's
+    build-tree requirement."""
+    setup_core(platform=PLATFORM_RP2040)
+    mock_get_port_type.return_value = "SERIAL"
+    mock_upload_using_picotool.return_value = 0
+
+    prebuilt = tmp_path / "prebuilt"
+    prebuilt.mkdir()
+    (prebuilt / "firmware.uf2").write_bytes(b"uf2")
+
+    args = MockArgs(prebuilt_dir=str(prebuilt))
+    devices = ["/dev/ttyACM0"]
+
+    with (
+        # Pretend picotool is already installed so the pkg-install prep
+        # branch short-circuits and we exercise the dispatch we care about.
+        patch(
+            "esphome.__main__._find_picotool", return_value=tmp_path / "fake-picotool"
+        ),
+        patch(
+            "esphome.__main__._rp2040_serial_reset_to_bootsel",
+            return_value=True,
+        ) as mock_reset,
+        patch("esphome.__main__.upload_using_platformio") as mock_pio,
+    ):
+        exit_code, host = upload_program({}, args, devices)
+
+    assert exit_code == 0
+    assert host == "/dev/ttyACM0"
+    mock_reset.assert_called_once_with("/dev/ttyACM0")
+    mock_upload_using_picotool.assert_called_once_with({})
+    mock_pio.assert_not_called()
+
+
+def test_upload_program_rp2040_serial_with_prebuilt_dir_reset_fails(
+    mock_upload_using_picotool: Mock,
+    mock_get_port_type: Mock,
+    mock_check_permissions: Mock,
+    tmp_path: Path,
+) -> None:
+    """If 1200bps touch fails to put the RP2040 into BOOTSEL, the upload
+    must abort (not fall through to a generic exit_code = 1 with no
+    diagnostic)."""
+    setup_core(platform=PLATFORM_RP2040)
+    mock_get_port_type.return_value = "SERIAL"
+
+    prebuilt = tmp_path / "prebuilt"
+    prebuilt.mkdir()
+    args = MockArgs(prebuilt_dir=str(prebuilt))
+
+    with (
+        patch(
+            "esphome.__main__._find_picotool", return_value=tmp_path / "fake-picotool"
+        ),
+        patch("esphome.__main__._rp2040_serial_reset_to_bootsel", return_value=False),
+    ):
+        exit_code, host = upload_program({}, args, ["/dev/ttyACM0"])
+
+    assert exit_code == 1
+    assert host is None
+    mock_upload_using_picotool.assert_not_called()
+
+
+def test_upload_program_rp2040_serial_without_prebuilt_dir_uses_platformio(
+    mock_upload_using_platformio: Mock,
+    mock_get_port_type: Mock,
+    mock_check_permissions: Mock,
+) -> None:
+    """Regression guard: without --prebuilt-dir, RP2040 serial still goes
+    through upload_using_platformio. The 1200bps-touch path is purely
+    additive."""
+    setup_core(platform=PLATFORM_RP2040)
+    mock_get_port_type.return_value = "SERIAL"
+    mock_upload_using_platformio.return_value = 0
+
+    with patch("esphome.__main__._rp2040_serial_reset_to_bootsel") as mock_reset:
+        exit_code, _ = upload_program({}, MockArgs(), ["/dev/ttyACM0"])
+
+    assert exit_code == 0
+    mock_upload_using_platformio.assert_called_once_with({}, "/dev/ttyACM0")
+    mock_reset.assert_not_called()
+
+
+def test_rp2040_serial_reset_to_bootsel_success(tmp_path: Path) -> None:
+    """The 1200bps-touch helper opens then closes the port at 1200 baud
+    (arduino-pico's USB CDC interprets that as a request to enter BOOTSEL),
+    then polls picotool until a BOOTSEL device shows up."""
+    from esphome.__main__ import _rp2040_serial_reset_to_bootsel
+    from esphome.util import BootselResult
+
+    setup_core(platform=PLATFORM_RP2040, tmp_path=tmp_path)
+
+    mock_serial_instance = MagicMock()
+
+    with (
+        patch("serial.Serial", return_value=mock_serial_instance),
+        patch("esphome.__main__._find_picotool", return_value=tmp_path / "picotool"),
+        patch(
+            "esphome.__main__.detect_rp2040_bootsel",
+            return_value=BootselResult(device_count=1),
+        ),
+    ):
+        assert _rp2040_serial_reset_to_bootsel("/dev/ttyACM0") is True
+
+    # The open/close at 1200 baud is what triggers the reset; verify both happened.
+    assert mock_serial_instance.baudrate == 1200
+    assert mock_serial_instance.port == "/dev/ttyACM0"
+    mock_serial_instance.open.assert_called_once()
+    mock_serial_instance.close.assert_called_once()
+
+
+def test_rp2040_serial_reset_to_bootsel_no_bootsel(tmp_path: Path) -> None:
+    """When BOOTSEL never appears (e.g. firmware doesn't implement the
+    1200bps touch handler) return False with a clear timeout log instead
+    of hanging."""
+    from esphome.__main__ import _rp2040_serial_reset_to_bootsel
+    from esphome.util import BootselResult
+
+    setup_core(platform=PLATFORM_RP2040, tmp_path=tmp_path)
+
+    with (
+        patch("serial.Serial", return_value=MagicMock()),
+        patch("esphome.__main__._find_picotool", return_value=tmp_path / "picotool"),
+        patch(
+            "esphome.__main__.detect_rp2040_bootsel",
+            return_value=BootselResult(device_count=0),
+        ),
+    ):
+        # Short timeout so the test doesn't sit on the wall clock.
+        assert _rp2040_serial_reset_to_bootsel("/dev/ttyACM0", timeout=0.1) is False
+
+
 def test_upload_program_bootsel(
     mock_upload_using_picotool: Mock,
     mock_get_port_type: Mock,

tests/unit_tests/test_platformio_toolchain.py

@@ -236,6 +236,189 @@ def test_load_idedata_regenerates_on_corrupted_cache(
     assert result["prog_path"] == "/new/firmware.elf"
 
 
+def test_load_idedata_uses_prebuilt_dir_when_set(
+    setup_core: Path, mock_run_platformio_cli_run: Mock
+) -> None:
+    """`esphome upload --prebuilt-dir <path>` is expected to ship the rendered
+    idedata.json next to the artifacts and bypass PlatformIO entirely. Verify
+    that _load_idedata returns the prebuilt copy verbatim without consulting
+    platformio.ini mtime or invoking ``platformio run -t idedata``."""
+    CORE.build_path = str(setup_core / "build" / "test")
+    CORE.name = "test"
+
+    prebuilt_dir = setup_core / "prebuilt"
+    prebuilt_dir.mkdir()
+    prebuilt_idedata = prebuilt_dir / "idedata.json"
+    prebuilt_idedata.write_text(
+        json.dumps({"prog_path": str(prebuilt_dir / "firmware.elf")})
+    )
+
+    CORE.prebuilt_dir = prebuilt_dir
+
+    result = toolchain._load_idedata({"name": "test"})
+
+    assert result["prog_path"] == str(prebuilt_dir / "firmware.elf")
+    # Never re-runs PlatformIO when prebuilt idedata is supplied: the dashboard
+    # ships these artifacts from a paired build server with no local PIO tree.
+    mock_run_platformio_cli_run.assert_not_called()
+
+
+def test_load_idedata_resolves_basenames_against_prebuilt_dir(
+    setup_core: Path, mock_run_platformio_cli_run: Mock
+) -> None:
+    """The dashboard's wire format ships idedata.json with bare basenames in
+    prog_path and extra.flash_images[*].path (the receiver's build-host
+    absolute paths don't resolve on the offloader). Verify upstream resolves
+    those basenames against CORE.prebuilt_dir so the offloader doesn't have
+    to write a fresh idedata.json on every install. Tracks the request in
+    esphome/device-builder#570."""
+    CORE.build_path = str(setup_core / "build" / "test")
+    CORE.name = "test"
+
+    prebuilt_dir = setup_core / "prebuilt"
+    prebuilt_dir.mkdir()
+    prebuilt_idedata = prebuilt_dir / "idedata.json"
+    prebuilt_idedata.write_text(
+        json.dumps(
+            {
+                "prog_path": "firmware.elf",
+                "extra": {
+                    "flash_images": [
+                        {"path": "bootloader.bin", "offset": "0x1000"},
+                        {"path": "partitions.bin", "offset": "0x8000"},
+                    ]
+                },
+            }
+        )
+    )
+    CORE.prebuilt_dir = prebuilt_dir
+
+    result = toolchain._load_idedata({"name": "test"})
+
+    assert result["prog_path"] == str(prebuilt_dir / "firmware.elf")
+    images = result["extra"]["flash_images"]
+    assert images[0]["path"] == str(prebuilt_dir / "bootloader.bin")
+    assert images[1]["path"] == str(prebuilt_dir / "partitions.bin")
+    mock_run_platformio_cli_run.assert_not_called()
+
+
+def test_load_idedata_absolute_paths_in_prebuilt_pass_through(
+    setup_core: Path, mock_run_platformio_cli_run: Mock
+) -> None:
+    """A hand-built --prebuilt-dir with absolute paths in idedata.json still
+    works: absolute paths pass through unchanged so we don't break the
+    documented-but-rarer shape just to support the dashboard's wire format.
+    """
+    CORE.build_path = str(setup_core / "build" / "test")
+    CORE.name = "test"
+
+    prebuilt_dir = setup_core / "prebuilt"
+    prebuilt_dir.mkdir()
+    # Use a tmp-rooted absolute path so the test passes on Windows too;
+    # Path.is_absolute() requires a drive letter on win32, so a bare
+    # "/somewhere/else/..." string would be classified as relative there
+    # and trigger the wrong code path.
+    abs_bootloader = str(setup_core / "elsewhere" / "bootloader.bin")
+    prebuilt_idedata = prebuilt_dir / "idedata.json"
+    prebuilt_idedata.write_text(
+        json.dumps(
+            {
+                "prog_path": str(prebuilt_dir / "firmware.elf"),
+                "extra": {
+                    "flash_images": [
+                        {"path": abs_bootloader, "offset": "0x1000"},
+                    ]
+                },
+            }
+        )
+    )
+    CORE.prebuilt_dir = prebuilt_dir
+
+    result = toolchain._load_idedata({"name": "test"})
+
+    assert result["prog_path"] == str(prebuilt_dir / "firmware.elf")
+    assert result["extra"]["flash_images"][0]["path"] == abs_bootloader
+
+
+def test_resolve_prebuilt_idedata_paths_missing_prog_path(tmp_path: Path) -> None:
+    """Defensive: a prebuilt idedata.json without prog_path (e.g. when the
+    dashboard only ships extra.flash_images) must not raise -- the resolver
+    skips fields that aren't present so partial-idedata shapes don't crash
+    upload dispatch."""
+    data = {"extra": {"flash_images": []}, "cc_path": "/some/cc"}
+    toolchain._resolve_prebuilt_idedata_paths(data, tmp_path)
+    assert "prog_path" not in data
+    assert data["cc_path"] == "/some/cc"  # left alone
+
+
+def test_resolve_prebuilt_idedata_paths_no_extra_section(tmp_path: Path) -> None:
+    """Defensive: idedata.json without an `extra` section at all (some
+    platforms don't ship flash_images) must not crash."""
+    data = {"prog_path": "firmware.elf"}
+    toolchain._resolve_prebuilt_idedata_paths(data, tmp_path)
+    assert data["prog_path"] == str(tmp_path / "firmware.elf")
+
+
+def test_resolve_prebuilt_idedata_paths_empty_flash_images(tmp_path: Path) -> None:
+    """Defensive: empty extra.flash_images (libretiny / ESP8266 / nRF52 ship
+    this; whole image is one file at offset 0x0). Resolver must not raise."""
+    data = {
+        "prog_path": "firmware.elf",
+        "extra": {"flash_images": []},
+    }
+    toolchain._resolve_prebuilt_idedata_paths(data, tmp_path)
+    assert data["prog_path"] == str(tmp_path / "firmware.elf")
+    assert data["extra"]["flash_images"] == []
+
+
+def test_load_idedata_prebuilt_malformed_json_raises_esphomeerror(
+    setup_core: Path, mock_run_platformio_cli_run: Mock
+) -> None:
+    """A malformed prebuilt idedata.json must surface as a one-line
+    EsphomeError, not an unhandled JSONDecodeError stack trace; the
+    dashboard's transparent-install flow needs a clean diagnostic to
+    bubble back to the operator, not the contents of the broken file."""
+    CORE.build_path = str(setup_core / "build" / "test")
+    CORE.name = "test"
+
+    prebuilt_dir = setup_core / "prebuilt"
+    prebuilt_dir.mkdir()
+    (prebuilt_dir / "idedata.json").write_text("{not valid json")
+    CORE.prebuilt_dir = prebuilt_dir
+
+    with pytest.raises(EsphomeError, match="Failed to parse"):
+        toolchain._load_idedata({"name": "test"})
+    mock_run_platformio_cli_run.assert_not_called()
+
+
+def test_load_idedata_falls_back_when_prebuilt_idedata_missing(
+    setup_core: Path, mock_run_platformio_cli_run: Mock
+) -> None:
+    """If --prebuilt-dir is set but the directory has no idedata.json, the
+    normal local-build-tree path runs. Lets the dashboard skip idedata for
+    OTA-only uploads (where firmware.bin alone is enough) without breaking."""
+    CORE.build_path = str(setup_core / "build" / "test")
+    CORE.name = "test"
+
+    prebuilt_dir = setup_core / "prebuilt"
+    prebuilt_dir.mkdir()
+    # Intentionally no idedata.json in prebuilt_dir.
+    CORE.prebuilt_dir = prebuilt_dir
+
+    platformio_ini = setup_core / "build" / "test" / "platformio.ini"
+    platformio_ini.parent.mkdir(parents=True, exist_ok=True)
+    platformio_ini.write_text("content")
+
+    mock_run_platformio_cli_run.return_value = json.dumps(
+        {"prog_path": "/local/firmware.elf"}
+    )
+
+    result = toolchain._load_idedata({"name": "test"})
+
+    assert result["prog_path"] == "/local/firmware.elf"
+    mock_run_platformio_cli_run.assert_called_once()
+
+
 def test_run_idedata_parses_json_from_output(
     setup_core: Path, mock_run_platformio_cli_run: Mock
 ) -> None:

tests/unit_tests/test_util.py

@@ -582,6 +582,55 @@ def test_run_external_process_line_callbacks() -> None:
     assert any("from subprocess" in r for r in results)
 
 
+def test_get_ltchiptool_path_on_path(tmp_path: Path) -> None:
+    """Verify ltchiptool installed via pip (system or venv) wins over the PIO
+    penv lookup; that's how a dashboard packaging strategy is most likely to
+    install it."""
+    fake_ltchiptool = tmp_path / "ltchiptool"
+    fake_ltchiptool.touch()
+
+    with patch("esphome.util.shutil.which", return_value=str(fake_ltchiptool)):
+        assert util.get_ltchiptool_path() == fake_ltchiptool
+
+
+def test_get_ltchiptool_path_pio_penv(tmp_path: Path) -> None:
+    """Fall back to PlatformIO's libretiny penv when ltchiptool isn't on
+    PATH; this is the install location platform-libretiny uses. The
+    script subdir follows the CPython venv convention -- ``Scripts/``
+    on Windows, ``bin/`` elsewhere -- which matches what PlatformIO
+    creates."""
+    fake_home = tmp_path / "home"
+    if sys.platform == "win32":
+        bin_subdir = "Scripts"
+        binary_name = "ltchiptool.exe"
+    else:
+        bin_subdir = "bin"
+        binary_name = "ltchiptool"
+    pio_bin = fake_home / ".platformio" / "penv" / ".libretiny" / bin_subdir
+    pio_bin.mkdir(parents=True)
+    expected = pio_bin / binary_name
+    expected.touch()
+
+    with (
+        patch("esphome.util.shutil.which", return_value=None),
+        patch("esphome.util.Path.home", return_value=fake_home),
+    ):
+        assert util.get_ltchiptool_path() == expected
+
+
+def test_get_ltchiptool_path_not_found(tmp_path: Path) -> None:
+    """Return None when ltchiptool isn't anywhere we know how to find so
+    callers can surface an install hint instead of guessing a wrong path."""
+    fake_home = tmp_path / "home"
+    fake_home.mkdir()
+
+    with (
+        patch("esphome.util.shutil.which", return_value=None),
+        patch("esphome.util.Path.home", return_value=fake_home),
+    ):
+        assert util.get_ltchiptool_path() is None
+
+
 def test_get_picotool_path_found(tmp_path: Path) -> None:
     """Test picotool path derivation from cc_path."""
     # Create the expected directory structure