diff --git a/backend/Dockerfile b/backend/Dockerfile
index 72918976..0725ed5a 100644
--- a/backend/Dockerfile
+++ b/backend/Dockerfile
@@ -22,15 +22,19 @@ ENV WASI_SDK=/opt/wasi-sdk
 
 # Initialize arduino-cli and install cores.
 # - arduino:avr  → Uno / Nano / Mega / etc. (built-in core, always needed)
-# - ATTinyCore:avr → ATtiny85 (Spence Konde's third-party core, drazzy.com index)
-#   Without this the FQBN ATTinyCore:avr:attinyx5:chip=85,clock=internal16mhz
-#   fails with "Platform 'ATTinyCore:avr' not found: platform not installed".
+# - ATTinyCore:avr@1.4.1 → ATtiny85 (Spence Konde's third-party core,
+#   drazzy.com index). Pinned to 1.4.1 because >=1.5.0 depends on
+#   micronucleus hosted at azduino.com (frequently unreachable). 1.4.1's
+#   micronucleus tool comes from github.com/digistump and still supports
+#   the FQBN clock options we ship (clock=16pll, etc.). We never invoke
+#   micronucleus — it is only required for USB upload, not compilation —
+#   but arduino-cli will not install a core whose tool deps cannot fetch.
 RUN arduino-cli config init \
     && arduino-cli config add board_manager.additional_urls \
         http://drazzy.com/package_drazzy.com_index.json \
     && arduino-cli core update-index \
     && arduino-cli core install arduino:avr \
-    && arduino-cli core install ATTinyCore:avr
+    && arduino-cli core install ATTinyCore:avr@1.4.1
 
 WORKDIR /app
 
diff --git a/backend/app/services/arduino_cli.py b/backend/app/services/arduino_cli.py
index 9446cda5..b2de06a5 100644
--- a/backend/app/services/arduino_cli.py
+++ b/backend/app/services/arduino_cli.py
@@ -30,6 +30,17 @@ class ArduinoCLIService:
         "esp32": "esp32:esp32",
     }
 
+    # Version pins for `arduino-cli core install`.  Keyed by core ID; if a core
+    # is in this map we pass `<core>@<version>` instead of just `<core>`.
+    # ATTinyCore: >=1.5.0 depends on micronucleus hosted at azduino.com, which
+    # has been unreachable for extended periods.  1.4.1 is the last release
+    # whose micronucleus tool is on github.com (digistump release) AND still
+    # supports the FQBN options we use (clock=16pll, etc.). Compile-only —
+    # micronucleus itself is never invoked here.
+    CORE_INSTALL_VERSIONS: dict[str, str] = {
+        "ATTinyCore:avr": "1.4.1",
+    }
+
     def __init__(self, cli_path: str = "arduino-cli"):
         self.cli_path = cli_path
         self._ensure_board_urls()
@@ -147,12 +158,14 @@ class ArduinoCLIService:
         if self._is_core_installed(core_id):
             return {"needed": False, "installed": True, "core_id": core_id, "log": ""}
 
-        # Install the core
-        print(f"[arduino-cli] Auto-installing core {core_id} for board {fqbn}...")
+        # Install the core (optionally pinned to a specific version)
+        version = self.CORE_INSTALL_VERSIONS.get(core_id)
+        install_spec = f"{core_id}@{version}" if version else core_id
+        print(f"[arduino-cli] Auto-installing core {install_spec} for board {fqbn}...")
 
         def _install():
             return subprocess.run(
-                [self.cli_path, "core", "install", core_id],
+                [self.cli_path, "core", "install", install_spec],
                 capture_output=True, text=True
             )
 
diff --git a/docker/entrypoint.sh b/docker/entrypoint.sh
index cecac073..bc9b2690 100644
--- a/docker/entrypoint.sh
+++ b/docker/entrypoint.sh
@@ -35,7 +35,7 @@ fi
 arduino-cli core update-index 2>/dev/null || true
 arduino-cli core install arduino:avr 2>/dev/null || true
 arduino-cli core install rp2040:rp2040 2>/dev/null || true
-arduino-cli core install ATTinyCore:avr 2>/dev/null || true
+arduino-cli core install ATTinyCore:avr@1.4.1 2>/dev/null || true
 
 # ESP32 compilation now uses ESP-IDF instead of arduino-cli.
 # arduino-cli ESP32 core is no longer needed for QEMU-compatible builds.