diff --git a/.claude/rules/architecture-map.md b/.claude/rules/architecture-map.md
index e252ef26..2c2677a7 100644
--- a/.claude/rules/architecture-map.md
+++ b/.claude/rules/architecture-map.md
@@ -101,7 +101,9 @@ shell (or, in parallel, in per-test `.result` files aggregated at the end).
 | `coverage/html_index.sh` / `html_file.sh` | HTML page emitters; the only two files exempt from `max_line_length` |
 | `rerun.sh` | `.bashunit/last-failed` cache for `--rerun-failed` |
 | `reports.sh` | JUnit/HTML/TAP/JSON writers |
-| `check_os.sh` / `dependencies.sh` | one-fork OS detect; `command -v` probes (builtins, not forks) |
+| `system/index.sh` | aggregator only — sources the `src/system/` module below |
+| `system/check_os.sh` / `system/dependencies.sh` | one-fork OS detect; `command -v` probes (builtins, not forks) |
+| `system/io.sh` | small I/O helpers |
 | `cli/index.sh` | aggregator only — sources the `src/cli/` module below |
 | `cli/{upgrade,watch,doc,init}.sh` | the non-`test` subcommand implementations; `main.sh` is their only caller |
 | `learn/index.sh` `benchmark.sh` | the remaining non-`test` subcommands; benchmark is shared with `runner/bench.sh` |
diff --git a/bashunit b/bashunit
index 029f1934..55e269fa 100755
--- a/bashunit
+++ b/bashunit
@@ -65,11 +65,9 @@ for arg in "$@"; do
 done
 
 source "$BASHUNIT_ROOT_DIR/src/dev/debug.sh"
-source "$BASHUNIT_ROOT_DIR/src/check_os.sh"
+source "$BASHUNIT_ROOT_DIR/src/system/index.sh"
 source "$BASHUNIT_ROOT_DIR/src/str.sh"
 source "$BASHUNIT_ROOT_DIR/src/globals.sh"
-source "$BASHUNIT_ROOT_DIR/src/dependencies.sh"
-source "$BASHUNIT_ROOT_DIR/src/io.sh"
 source "$BASHUNIT_ROOT_DIR/src/math.sh"
 source "$BASHUNIT_ROOT_DIR/src/parallel.sh"
 source "$BASHUNIT_ROOT_DIR/src/env.sh"
diff --git a/build.sh b/build.sh
index 185f1a9e..aaa56792 100755
--- a/build.sh
+++ b/build.sh
@@ -1,7 +1,7 @@
 #!/usr/bin/env bash
 set -euo pipefail
 
-source src/check_os.sh
+source src/system/check_os.sh
 bashunit::check_os::init
 
 BASHUNIT_ROOT_DIR="$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)"
diff --git a/src/check_os.sh b/src/system/check_os.sh
similarity index 100%
rename from src/check_os.sh
rename to src/system/check_os.sh
diff --git a/src/dependencies.sh b/src/system/dependencies.sh
similarity index 100%
rename from src/dependencies.sh
rename to src/system/dependencies.sh
diff --git a/src/system/index.sh b/src/system/index.sh
new file mode 100644
index 00000000..a1c78417
--- /dev/null
+++ b/src/system/index.sh
@@ -0,0 +1,12 @@
+#!/usr/bin/env bash
+
+# Entry point for the src/system/ module: only `source` lines and comments belong
+# here. build.sh emits a file's body before recursing into its `source` lines, so
+# any statement here would run before its dependencies in the built binary
+# (adrs/adr-010-src-module-directories.md).
+#
+# Capability probing: what this machine is and what it has. The bottom layer --
+# nothing here touches config, test state, console or the runner.
+source "$BASHUNIT_ROOT_DIR/src/system/check_os.sh"
+source "$BASHUNIT_ROOT_DIR/src/system/dependencies.sh"
+source "$BASHUNIT_ROOT_DIR/src/system/io.sh"
diff --git a/src/io.sh b/src/system/io.sh
similarity index 100%
rename from src/io.sh
rename to src/system/io.sh
diff --git a/tests/unit/check_os_test.sh b/tests/unit/check_os_test.sh
index 8cd9c5c3..84ea08f4 100644
--- a/tests/unit/check_os_test.sh
+++ b/tests/unit/check_os_test.sh
@@ -85,7 +85,7 @@ function test_module_load_detects_os_with_a_single_uname_call() {
   # under `build.sh --verify` the built bashunit lives in a folder
   # without src/, and sourcing through $BASHUNIT_ROOT_DIR crashed
   # the whole verification run (silently, until #834 gated it).
-  source "$(dirname "${BASH_SOURCE[0]}")/../../src/check_os.sh"
+  source "$(dirname "${BASH_SOURCE[0]}")/../../src/system/check_os.sh"
 
   assert_have_been_called_times 1 uname
 }