diff --git a/agent_tool/shell/README.mbt.md b/agent_tool/shell/README.mbt.md
index 569d7379a..bf5903e07 100644
--- a/agent_tool/shell/README.mbt.md
+++ b/agent_tool/shell/README.mbt.md
@@ -57,7 +57,7 @@ workspace root, respecting `moon -C <dir>` and explicit cwd changes such as
 `cd ./dir && moon ...`. Bare relative `cd dir` is skipped because `CDPATH` can
 change which directory the shell enters. This covers commands such as
 `moon add`, `moon remove`, `moon update`, `moon fmt`, `moon info`,
-`moon test --update`, and `moon ide rename ... --apply`. Read-only variants
+`moon test -u`/`moon test --update`, and `moon ide rename ... --apply`. Read-only variants
 such as dry-run commands and `moon fmt --check` are left alone, as are Moon
 invocations with per-command environment overrides such as `FOO=bar moon` or
 `env FOO=bar moon`. Follow-up checks are skipped when `timeout_ms` is set and
@@ -96,12 +96,15 @@ source paths are indirect, as in `while read f; do sed -i ... "$f"; done`.
 Too-complex commands with visible MoonBit source creation or tree transfer
 markers are also rejected before execution.
 
-Narrowly recognized Moon commands that are expected to write source or package
-metadata run outside the source-write sandbox: `moon fmt`, `moon info`,
-`moon add`, `moon remove`, `moon update`, `moon test --update`, and
-`moon ide rename ... --apply`. Compounds are conservative; `moon fmt &&
-moon check` is trusted, while a broad script or source rewrite through shell is
-not.
+Recognized Moon commands that can run pre-build hooks or intentionally update
+source and package metadata run outside the source-write sandbox. This includes
+`moon check`, `moon build`, `moon test`, package-based `moon run`, `moon fmt`,
+`moon info`, `moon add`, `moon remove`, `moon update`, `moon test -u`/`moon test
+--update`, and `moon ide rename ... --apply`. Compounds are conservative; `moon
+fmt && moon check` is trusted. A trusted writer may pipe output through the
+read-only `head` limiter, so commands such as `moon test -u 2>&1 | head -40` can
+update snapshots while bounding output. Other pipeline consumers, broad scripts,
+and source rewrites through shell remain sandboxed.
 
 Recognized first-party Git porcelain composes outside the source-write sandbox.
 This lets ordinary workflows such as `git fetch && git rebase`, `git pull
diff --git a/agent_tool/shell/moon_auto_check.mbt b/agent_tool/shell/moon_auto_check.mbt
index 6850f34df..6748e9f69 100644
--- a/agent_tool/shell/moon_auto_check.mbt
+++ b/agent_tool/shell/moon_auto_check.mbt
@@ -517,6 +517,7 @@ test "detect moon commands that should trigger follow-up check" {
     ),
   )
   assert_true(should_auto_check_moon_command("moon test --update"))
+  assert_true(should_auto_check_moon_command("moon test -u"))
   assert_true(should_auto_check_moon_command("command moon fmt"))
   assert_true(should_auto_check_moon_command("command -- moon fmt"))
   assert_true(should_auto_check_moon_command("command -p moon fmt"))
diff --git a/agent_tool/shell/review_guard.mbt b/agent_tool/shell/review_guard.mbt
index 41a788984..16da99c8f 100644
--- a/agent_tool/shell/review_guard.mbt
+++ b/agent_tool/shell/review_guard.mbt
@@ -181,6 +181,13 @@ test "read-only guard blocks mutating moon anywhere; allows read-only commands"
       scratch_dir=None,
     ),
   )
+  assert_true(
+    review_read_only_blocks(
+      @shell_parse.parse_for_policy("moon test -u; true"),
+      base_cwd=".",
+      scratch_dir=None,
+    ),
+  )
   assert_true(
     review_read_only_blocks(
       @shell_parse.parse_for_policy("FOO=bar moon fmt"),
diff --git a/agent_tool/shell/sandbox.mbt b/agent_tool/shell/sandbox.mbt
index 3b2ad8502..7e693b587 100644
--- a/agent_tool/shell/sandbox.mbt
+++ b/agent_tool/shell/sandbox.mbt
@@ -181,10 +181,17 @@ fn commands_are_trusted_source_writes(
 ) -> Bool {
   let mut has_trusted_write = false
   for index, command in commands {
-    if command.separator_before is Some("|" | "||") {
-      return false
+    let command_class = trusted_command_class(command, commands, index)
+    match command.separator_before {
+      Some("||") => return false
+      // A trusted source-writing command may feed a read-only output consumer
+      // such as `head`. Never trust a writer after a pipe: that would make the
+      // whole shell run unsandboxed for a command shape that is not needed for
+      // validation output limiting.
+      Some("|") if !(command_class is TrustedCommandRead) => return false
+      _ => ()
     }
-    match trusted_command_class(command, commands, index) {
+    match command_class {
       TrustedCommandWrite => has_trusted_write = true
       TrustedCommandNeutral | TrustedCommandRead => ()
       UntrustedCommand => return false
@@ -220,12 +227,29 @@ fn trusted_command_class(
           }
         "moon" => trusted_moon_command_class(command, command_index)
         "git" => trusted_git_command_class(command, command_index)
+        "head" => trusted_head_command_class(command, command_index)
         _ => UntrustedCommand
       }
     _ => UntrustedCommand
   }
 }
 
+///|
+/// `head` is a read-only output limiter used to keep compiler and test output
+/// bounded (`moon test ... 2>&1 | head -30`). Trust only the direct command
+/// without environment wrappers; source-path redirects are rejected before
+/// this classifier runs.
+fn trusted_head_command_class(
+  command : @shell_parse.SimpleCommand,
+  head_index : Int,
+) -> TrustedCommandClass {
+  if head_index == 0 && !command_has_custom_environment(command) {
+    TrustedCommandRead
+  } else {
+    UntrustedCommand
+  }
+}
+
 ///|
 /// Classify a `git` command for the trusted-source-write path. Recognized
 /// first-party porcelain composes by default, while aliases, plumbing, external
diff --git a/agent_tool/shell/sandbox_wbtest.mbt b/agent_tool/shell/sandbox_wbtest.mbt
index 163a9e6a2..84c25afb4 100644
--- a/agent_tool/shell/sandbox_wbtest.mbt
+++ b/agent_tool/shell/sandbox_wbtest.mbt
@@ -16,10 +16,18 @@ test "sandbox trusts only narrow moon source-writing commands" {
   assert_true(mode_for("moon fmt >/dev/null") is TrustedSourceWrite)
   assert_true(mode_for("moon info > /tmp/log") is TrustedSourceWrite)
   assert_true(mode_for("moon test --update 2>/tmp/err") is TrustedSourceWrite)
+  assert_true(mode_for("moon test -u 2>/tmp/err") is TrustedSourceWrite)
   assert_true(mode_for("moon fmt > out.log") is TrustedSourceWrite)
   assert_true(mode_for("moon check") is TrustedSourceWrite)
   assert_true(mode_for("moon build") is TrustedSourceWrite)
   assert_true(mode_for("moon test") is TrustedSourceWrite)
+  assert_true(
+    mode_for("moon test cfront/build -u 2>&1 | head -30") is TrustedSourceWrite,
+  )
+  assert_true(
+    mode_for("moon test cfront/build --update 2>&1 | head -n 40")
+    is TrustedSourceWrite,
+  )
   assert_true(mode_for("moon run cmd/main") is TrustedSourceWrite)
   assert_true(mode_for("moon run cmd/main -- --help") is TrustedSourceWrite)
   assert_true(mode_for("moon run cmd/main -- -u") is TrustedSourceWrite)
@@ -48,6 +56,21 @@ test "sandbox trusts only narrow moon source-writing commands" {
   assert_true(mode_for("sed -i '' 's/a/b/' *.mbt") is SourceReadOnly)
 }
 
+///|
+test "sandbox trusts only read-only consumers after moon output pipes" {
+  assert_true(mode_for("moon test 2>&1 | head -30") is TrustedSourceWrite)
+  assert_true(
+    mode_for("moon test -u 2>&1 | head -40 | head -20") is TrustedSourceWrite,
+  )
+  for
+    cmd in [
+      "moon test -u | tee snapshots.txt", "moon test -u | sh", "moon test -u | moon fmt",
+      "env LIMIT=30 head -30 | moon test -u",
+    ] {
+    assert_true(mode_for(cmd) is SourceReadOnly)
+  }
+}
+
 ///|
 async test "sandbox preblocks source-writing script snippets" {
   let root = "/workspace"