diff --git a/CHANGELOG.md b/CHANGELOG.md
index 69142dca..9f5aa18e 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -6,6 +6,7 @@
 - Named snapshot assertions support multiple snapshots per test; mismatches show the resolved path and `--snapshot-update` hint (#986)
 
 ### Changed
+- `assert_true` / `assert_false` name the problem instead of printing a bare number: exit code 127 now reports `unknown command: <arg>` and 126 `not executable: <arg>`. The most common cause is passing a test expression like `[ -d /tmp ]`, which is run as a command word
 - Core comparison assertions report missing required arguments as usage errors instead of comparing against empty values (#983)
 - Performance: Literal snapshots bypass placeholder regex processing unless they contain a placeholder (about 13x faster) (#985)
 - Performance: `assert_within_delta` uses fixed-point arithmetic for common values, with a `bc`/`awk` fallback for unsupported inputs (about 6.6x faster) (#979)
@@ -14,6 +15,7 @@
 - Internal: Split `src/runner.sh` and `src/coverage.sh` into focused modules with no behavior change; see [ADR-010](adrs/adr-010-src-module-directories.md) (#924, #925)
 
 ### Fixed
+- `assert_false` no longer passes when the command does not exist. Exit code 127 is non-zero, so a typo in the command name satisfied the assertion while testing nothing; 127 and 126 now fail both `assert_true` and `assert_false`, because they mean the command never ran
 - `assert_within_delta` accepts a leading `+` on any operand (#979)
 - Invalid `BASHUNIT_SHARD_INDEX` / `BASHUNIT_SHARD_TOTAL` values now fail with a clear error instead of reaching raw arithmetic or reporting no tests
 - Date assertions reject unparseable values instead of crashing or treating them as epoch 0
diff --git a/docs/assertions.md b/docs/assertions.md
index 6bdfba93..3591065d 100644
--- a/docs/assertions.md
+++ b/docs/assertions.md
@@ -36,6 +36,12 @@ to narrow it (`bashunit doc json`).
 ## assert_true
 > `assert_true bool|function|command`
 
+Takes a **command**, not a test expression. A bracketed condition of the form
+used inside `if` is run as a command word, so the shell reports exit code 127 and
+the assertion fails with `unknown command`. Use the `test` builtin instead —
+`assert_true "test -d /tmp"` — or a purpose-built assertion such as
+`assert_directory_exists`.
+
 Reports an error if the argument result in a truthy value: `true` or `0`.
 
 - [assert_false](#assert-false) is similar but different.
diff --git a/src/assert/core.sh b/src/assert/core.sh
index ecc9d569..59beb74c 100755
--- a/src/assert/core.sh
+++ b/src/assert/core.sh
@@ -114,6 +114,30 @@ function bashunit::fail() {
   bashunit::console_results::print_failure_message "${label}" "$message"
 }
 
+_BASHUNIT_ASSERT_EXIT_DESC_OUT=""
+
+##
+# Describes a failing exit code for assert_true / assert_false. 127 and 126 are
+# the two codes the shell reserves for "I could not run this at all", and a bare
+# number tells the reader nothing: the most natural shell idiom,
+# `assert_true "[ -d /tmp ]"`, hits 127 because the argument is run as a command
+# word rather than evaluated, and the failure used to point nowhere near the
+# cause.
+#
+# The wording deliberately avoids the literal phrase "command not found".
+# runner/diagnostics.sh classifies a test as a runtime error by scanning its
+# output for that exact string, so using it here made every such failure report
+# as both Failed and Error for the one cause.
+# Arguments: $1 - exit code, $2 - the command as written
+##
+function bashunit::assert::_describe_exit_code() {
+  case "$1" in
+  127) _BASHUNIT_ASSERT_EXIT_DESC_OUT="unknown command: $2" ;;
+  126) _BASHUNIT_ASSERT_EXIT_DESC_OUT="not executable: $2" ;;
+  *) _BASHUNIT_ASSERT_EXIT_DESC_OUT="exit code: $1" ;;
+  esac
+}
+
 function assert_true() {
   bashunit::assert::should_skip && return 0
 
@@ -140,7 +164,9 @@ function assert_true() {
   local exit_code=$?
 
   if [ "$exit_code" -ne 0 ]; then
-    bashunit::handle_bool_assertion_failure "command or function with zero exit code" "exit code: $exit_code"
+    bashunit::assert::_describe_exit_code "$exit_code" "$actual"
+    bashunit::handle_bool_assertion_failure \
+      "command or function with zero exit code" "$_BASHUNIT_ASSERT_EXIT_DESC_OUT"
   else
     bashunit::state::add_assertions_passed
   fi
@@ -171,11 +197,17 @@ function assert_false() {
   bashunit::run_command_or_eval "$actual"
   local exit_code=$?
 
-  if [ "$exit_code" -eq 0 ]; then
-    bashunit::handle_bool_assertion_failure "command or function with non-zero exit code" "exit code: $exit_code"
-  else
-    bashunit::state::add_assertions_passed
-  fi
+  # 127/126 mean the command never ran. Treating "did not run" as "returned
+  # false" let a typo in the command name satisfy this assertion while testing
+  # nothing, so those are failures here as well as in assert_true.
+  case "$exit_code" in
+  0 | 126 | 127)
+    bashunit::assert::_describe_exit_code "$exit_code" "$actual"
+    bashunit::handle_bool_assertion_failure \
+      "command or function with non-zero exit code" "$_BASHUNIT_ASSERT_EXIT_DESC_OUT"
+    ;;
+  *) bashunit::state::add_assertions_passed ;;
+  esac
 }
 
 function bashunit::run_command_or_eval() {
diff --git a/tests/acceptance/snapshots/bashunit_test_sh.test_bashunit_should_display_all_assert_docs.snapshot b/tests/acceptance/snapshots/bashunit_test_sh.test_bashunit_should_display_all_assert_docs.snapshot
index cdcc7766..53f416a7 100644
--- a/tests/acceptance/snapshots/bashunit_test_sh.test_bashunit_should_display_all_assert_docs.snapshot
+++ b/tests/acceptance/snapshots/bashunit_test_sh.test_bashunit_should_display_all_assert_docs.snapshot
@@ -2,6 +2,12 @@
 --------------
 > `assert_true bool|function|command`
 
+Takes a **command**, not a test expression. A bracketed condition of the form
+used inside `if` is run as a command word, so the shell reports exit code 127 and
+the assertion fails with `unknown command`. Use the `test` builtin instead —
+`assert_true "test -d /tmp"` — or a purpose-built assertion such as
+`assert_directory_exists`.
+
 Reports an error if the argument result in a truthy value: `true` or `0`.
 
 - assert_false is similar but different.
diff --git a/tests/unit/assert/basic_test.sh b/tests/unit/assert/basic_test.sh
index 01102c2e..2872cbf8 100644
--- a/tests/unit/assert/basic_test.sh
+++ b/tests/unit/assert/basic_test.sh
@@ -183,3 +183,37 @@ function test_assert_not_same_with_custom_label() {
     "$(bashunit::console_results::print_failed_test "my custom label" "foo" "to not be" "foo")" \
     "$(assert_not_same "foo" "foo" "my custom label")"
 }
+
+# Exit code 127 is the shell's not-found code. Reporting the bare number
+# gives the reader nothing to act on -- and the most natural shell idiom,
+# a `[ ... ]` test expression, produces exactly this because assert_true runs
+# its argument as a command word rather than evaluating it.
+function test_assert_true_reports_a_missing_command_by_name() {
+  assert_same \
+    "$(bashunit::console_results::print_failed_test \
+      "Assert true reports a missing command by name" \
+      "command or function with zero exit code" "but got " \
+      "unknown command: definitely_not_a_command")" \
+    "$(assert_true "definitely_not_a_command")"
+}
+
+function test_assert_true_reports_a_bracket_expression_as_not_found() {
+  assert_same \
+    "$(bashunit::console_results::print_failed_test \
+      "Assert true reports a bracket expression as not found" \
+      "command or function with zero exit code" "but got " \
+      "unknown command: [ -d /tmp ]")" \
+    "$(assert_true "[ -d /tmp ]")"
+}
+
+# assert_false only failed when the exit code was 0, so a command that does not
+# exist satisfied it: 127 is non-zero, therefore "false". A typo in the command
+# name made the assertion pass while testing nothing.
+function test_assert_false_fails_when_the_command_does_not_exist() {
+  assert_same \
+    "$(bashunit::console_results::print_failed_test \
+      "Assert false fails when the command does not exist" \
+      "command or function with non-zero exit code" "but got " \
+      "unknown command: definitely_not_a_command")" \
+    "$(assert_false "definitely_not_a_command")"
+}