diff --git a/src/gn/commands_unittest.cc b/src/gn/commands_unittest.cc
index 8c7ce5f..6fbe9ab 100644
--- a/src/gn/commands_unittest.cc
+++ b/src/gn/commands_unittest.cc
@@ -8,6 +8,7 @@
 #include "base/values.h"
 #include "gn/commands.h"
 #include "gn/label_pattern.h"
+#include "gn/standard_out.h"
 #include "gn/target.h"
 #include "gn/test_with_scope.h"
 #include "util/test/test.h"
@@ -65,6 +66,10 @@
     created_targets.push_back(std::move(target));
   }
 
+  // Prevent Err()  messages sent to stderr from polluting
+  // the tests' own output.
+  ScopedBufferedOutput buffered_output;
+
   for (const auto& test_case : cases) {
     std::vector<const Target*> targets_to_filter = all_targets;
 
diff --git a/src/gn/err.cc b/src/gn/err.cc
index ca61e7c..91a4ac6 100644
--- a/src/gn/err.cc
+++ b/src/gn/err.cc
@@ -204,7 +204,7 @@
 }
 
 bool Err::PrintToStdout() const {
-  FlushBufferedOutput();
+  FlushBufferedLogOutput();
   return InternalPrintToStdout(false, true);
 }
 
diff --git a/src/gn/gn_main.cc b/src/gn/gn_main.cc
index 894f8e7..75441a6 100644
--- a/src/gn/gn_main.cc
+++ b/src/gn/gn_main.cc
@@ -92,7 +92,7 @@
 
   if (retval != 0) {
     // Failed, print out log info we buffered.
-    FlushBufferedOutput();
+    FlushBufferedLogOutput();
   }
 
   exit(retval);  // Don't free memory, it can be really slow!
diff --git a/src/gn/standard_out.cc b/src/gn/standard_out.cc
index ecc5ab8..abd21c9 100644
--- a/src/gn/standard_out.cc
+++ b/src/gn/standard_out.cc
@@ -220,6 +220,8 @@
 // Collects buffered log output for quiet mode.
 class QuietModeBuffer {
  public:
+  using BufferedOutput = ScopedBufferedOutput::Item;
+
   void Append(std::string_view output,
               TextDecoration decoration,
               HtmlEscaping escaping) {
@@ -251,13 +253,13 @@
     output_buffer_.clear();
   }
 
- private:
-  struct BufferedOutput {
-    std::string output;
-    TextDecoration decoration;
-    HtmlEscaping escaping;
-  };
+  // Take a snapshot of the current buffers output items.
+  std::vector<BufferedOutput> GetItems() {
+    std::lock_guard<std::mutex> lock(lock_);
+    return output_buffer_;
+  }
 
+ private:
   std::mutex lock_;
 
   // Set when we're in quiet mode but then flush the output. This means that
@@ -272,6 +274,9 @@
 // Non-null while buffering standard output. Deliberately leaked on shutdown.
 QuietModeBuffer* quiet_mode_buffer = nullptr;
 
+// Non-null while ScopedBufferedOutput singleton exists.
+QuietModeBuffer* scoped_output_buffer = nullptr;
+
 }  // namespace
 
 bool IsColorEnabled() {
@@ -282,12 +287,20 @@
 void OutputString(std::string_view output,
                   TextDecoration dec,
                   HtmlEscaping escaping) {
+  if (scoped_output_buffer) {
+    scoped_output_buffer->Append(output, dec, escaping);
+    return;
+  }
   WriteOutputString(output, dec, escaping);
 }
 
 void OutputLogString(std::string_view output,
                      TextDecoration dec,
                      HtmlEscaping escaping) {
+  if (scoped_output_buffer) {
+    scoped_output_buffer->Append(output, dec, escaping);
+    return;
+  }
   if (quiet_mode_buffer) {
     quiet_mode_buffer->Append(output, dec, escaping);
     return;
@@ -303,12 +316,29 @@
   quiet_mode_buffer = new QuietModeBuffer();
 }
 
-void FlushBufferedOutput() {
+void FlushBufferedLogOutput() {
   if (quiet_mode_buffer) {
     quiet_mode_buffer->Flush();
   }
 }
 
+ScopedBufferedOutput::ScopedBufferedOutput() {
+  DCHECK(!scoped_output_buffer)
+      << "Only one ScopedBufferedOutput instance can exist";
+  scoped_output_buffer = new QuietModeBuffer();
+}
+
+ScopedBufferedOutput::~ScopedBufferedOutput() {
+  DCHECK(scoped_output_buffer);
+  delete scoped_output_buffer;
+  scoped_output_buffer = nullptr;
+}
+
+std::vector<ScopedBufferedOutput::Item> ScopedBufferedOutput::GetItems() {
+  DCHECK(scoped_output_buffer);
+  return scoped_output_buffer->GetItems();
+}
+
 void PrintSectionHelp(const std::string& line,
                       const std::string& topic,
                       const std::string& tag) {
diff --git a/src/gn/standard_out.h b/src/gn/standard_out.h
index 4355c23..9bf1637 100644
--- a/src/gn/standard_out.h
+++ b/src/gn/standard_out.h
@@ -6,6 +6,7 @@
 #define TOOLS_GN_STANDARD_OUT_H_
 
 #include <string>
+#include <vector>
 
 enum TextDecoration {
   DECORATION_NONE = 0,
@@ -42,7 +43,7 @@
 // BufferLogOutput() is not threadsafe and is expected to be called from early
 // process init before we've created any worker thread.
 void BufferLogOutput();
-void FlushBufferedOutput();
+void FlushBufferedLogOutput();
 
 // If printing markdown, this generates table-of-contents entries with
 // links to the actual help; otherwise, prints a one-line description.
@@ -76,4 +77,32 @@
 
 bool IsColorEnabled();
 
+// Helper class used to redirect OutputString() and OutputLogString() calls
+// to an internal buffer. Useful for tests.
+//
+// Only one instance per process of that class can be used though. Usage is
+//
+//  1) Create instance, this automatically sets up the redirection.
+//  2) Call GetItems() to get a snapshot of the currently buffered items.
+//  3) On destruction, buffered output is dropped.
+//
+struct ScopedBufferedOutput {
+  // Constructor sets up the redirection, only one instance per process can
+  // exist at a given time, since this manages global state.
+  ScopedBufferedOutput();
+
+  // Destructor drops all buffered output items.
+  ~ScopedBufferedOutput();
+
+  // The input of a single buffered OutputString() or OutputLogString() call.
+  struct Item {
+    std::string output;
+    TextDecoration decoration;
+    HtmlEscaping escaping;
+  };
+
+  // Retrieve a snapshot (copy) of buffered items so far.
+  std::vector<Item> GetItems();
+};
+
 #endif  // TOOLS_GN_STANDARD_OUT_H_