diff --git a/Dockerfile.standalone b/Dockerfile.standalone
index bff895a0..30b215a4 100644
--- a/Dockerfile.standalone
+++ b/Dockerfile.standalone
@@ -280,6 +280,17 @@ ENV IDF_CCACHE_ENABLE=1
 # compiles even though some flags (-I, -fmacro-prefix-map) embed absolute
 # paths into the command line.
 ENV CCACHE_BASEDIR=/var/lib/velxio-build
+# hash_dir=false. ESP-IDF compiles with -ggdb, and for debug builds ccache
+# folds the cwd into every cache key by default; CCACHE_BASEDIR does NOT
+# exempt it (verified with CCACHE_DEBUG on ccache 4.11: "Hashing CWD
+# /var/lib/velxio-build/esp32s3/v_<hash>/project/build"). Since each build
+# variant lives in its own dir, every variant had a private key space and a
+# cold variant of an already-built board recompiled all ~2100 IDF objects
+# from scratch — on a busy box that overran the 600s ninja timeout. With
+# hash_dir off, variants of the same board/sdkconfig share objects. The
+# only difference is DW_AT_comp_dir inside .debug_info of a reused object,
+# which never reaches the flashed .bin.
+ENV CCACHE_NOHASHDIR=1
 # Cache cap + compression set as env vars (rather than via `ccache
 # --set-config` at image-build time) because $CCACHE_DIR is a docker volume:
 # anything written into /var/cache/ccache during the RUN step is masked at
diff --git a/backend/app/services/espidf_compiler.py b/backend/app/services/espidf_compiler.py
index 98f744f7..10166478 100644
--- a/backend/app/services/espidf_compiler.py
+++ b/backend/app/services/espidf_compiler.py
@@ -195,6 +195,17 @@ def _evict_cold_variants(target_dir: Path, keep: int) -> None:
         shutil.rmtree(d, ignore_errors=True)
 
 
+def _ninja_log_step_count(build_dir: 'str | Path') -> int:
+    """Number of completed build steps recorded in ``build_dir/.ninja_log``
+    (0 when absent/unreadable). Diagnostics only — used to say how far a
+    timed-out build got."""
+    try:
+        with open(Path(build_dir) / '.ninja_log', encoding='utf-8', errors='replace') as fh:
+            return max(0, sum(1 for _ in fh) - 1)  # first line is the header
+    except OSError:
+        return 0
+
+
 # ── .ino prototype generation ────────────────────────────────────────────────
 # arduino-cli runs ctags over the sketch and injects forward declarations so
 # the classic Arduino idiom — helpers defined AFTER setup()/loop() that use
@@ -3587,6 +3598,10 @@ class ESPIDFCompiler:
         try:
             cmake_result = await asyncio.to_thread(_run_cmake)
         except subprocess.TimeoutExpired:
+            logger.error(
+                f'[espidf] cmake configure timed out after {cmake_timeout}s '
+                f'in {build_dir}'
+            )
             return {
                 'success': False,
                 'error': f'ESP-IDF cmake configure timed out ({cmake_timeout}s)',
@@ -3626,9 +3641,24 @@ class ESPIDFCompiler:
         try:
             ninja_result = await asyncio.to_thread(_run_ninja)
         except subprocess.TimeoutExpired:
+            # Silent before — a 600s failure left no server-side trace at all,
+            # so a slow-box incident (2026-08-17: cold S3 variant, ccache
+            # missing on every object) had to be reconstructed from the
+            # variant's .ninja_log. Say where it happened and how far it got.
+            done_steps = _ninja_log_step_count(build_dir)
+            logger.error(
+                f'[espidf] ninja build timed out after {NINJA_TIMEOUT_S}s in '
+                f'{build_dir} ({done_steps} steps logged)'
+            )
             return {
                 'success': False,
-                'error': f'ESP-IDF build timed out ({NINJA_TIMEOUT_S}s)',
+                # ninja is incremental and the variant dir persists: the
+                # objects built so far are kept, so retrying picks up where
+                # this attempt stopped instead of starting over.
+                'error': (
+                    f'ESP-IDF build timed out ({NINJA_TIMEOUT_S}s) — the partial '
+                    f'build is kept; compile again to resume where it stopped'
+                ),
                 'stdout': '',
                 'stderr': '',
             }
diff --git a/backend/tests/test_espidf_options.py b/backend/tests/test_espidf_options.py
index 11976feb..ff1a8133 100644
--- a/backend/tests/test_espidf_options.py
+++ b/backend/tests/test_espidf_options.py
@@ -446,3 +446,20 @@ def test_render_sdkconfig_idf5_per_target_drops(compiler: ESPIDFCompiler) -> Non
         _TEMPLATE_DIR, idf_target='esp32s3', use_idf5=True,
     )
     assert 'CONFIG_ESPTOOLPY_FLASHFREQ_26M' not in s3_text  # S3 has no 26M
+
+
+def test_ninja_log_step_count(tmp_path) -> None:
+    """Diagnostic helper for the ninja-timeout log line: counts completed
+    steps in .ninja_log (header excluded), 0 when the log is missing."""
+    from app.services.espidf_compiler import _ninja_log_step_count
+
+    build_dir = tmp_path / 'build'
+    assert _ninja_log_step_count(build_dir) == 0
+    build_dir.mkdir()
+    (build_dir / '.ninja_log').write_text(
+        '# ninja log v6\n'
+        '1\t20\t0\tesp-idf/a.obj\tdeadbeef\n'
+        '2\t30\t0\tesp-idf/b.obj\tcafebabe\n',
+        encoding='utf-8',
+    )
+    assert _ninja_log_step_count(build_dir) == 2