diff --git a/src/gn/err.cc b/src/gn/err.cc
index e72d86a..ca61e7c 100644
--- a/src/gn/err.cc
+++ b/src/gn/err.cc
@@ -92,12 +92,37 @@
   }
 }
 
+}  // namespace
+
+struct ErrOutput {
+  virtual ~ErrOutput() = default;
+  virtual void Write(std::string_view text,
+                     TextDecoration dec = DECORATION_NONE) = 0;
+};
+
+namespace {
+
+struct StdoutErrOutput : public ErrOutput {
+  StdoutErrOutput(bool is_fatal) : is_fatal(is_fatal) {}
+  void Write(std::string_view text, TextDecoration dec) override {
+    OutputErrString(is_fatal, text, dec);
+  }
+  bool is_fatal;
+};
+
+struct StringErrOutput : public ErrOutput {
+  void Write(std::string_view text, TextDecoration dec) override {
+    out.append(text);
+  }
+  std::string out;
+};
+
 // The line length is used to clip the maximum length of the markers we'll
 // make if the error spans more than one line (like unterminated literals).
-void OutputHighlighedPosition(const Location& location,
-                              const Err::RangeList& ranges,
-                              size_t line_length,
-                              bool is_fatal) {
+void OutputHighlightedPosition(const Location& location,
+                               const Err::RangeList& ranges,
+                               size_t line_length,
+                               ErrOutput& output) {
   // Make a buffer of the line in spaces.
   std::string highlight;
   highlight.resize(line_length);
@@ -119,7 +144,7 @@
     highlight.resize(highlight.size() - 1);
 
   highlight += "\n";
-  OutputErrString(is_fatal, highlight, DECORATION_BLUE);
+  output.Write(highlight, DECORATION_BLUE);
 }
 
 }  // namespace
@@ -209,11 +234,23 @@
         return false;
       }
     }
+  }
 
+  StdoutErrOutput output(is_fatal);
+  InternalFormat(output, is_sub_err, is_fatal);
+  return true;
+}
+
+void Err::InternalFormat(ErrOutput& output,
+                         bool is_sub_err,
+                         bool is_fatal) const {
+  DCHECK(info_);
+
+  if (!is_sub_err) {
     if (is_fatal)
-      OutputString("ERROR ", DECORATION_RED);
+      output.Write("ERROR ", DECORATION_RED);
     else
-      OutputLogString("WARNING ", DECORATION_MAGENTA);
+      output.Write("WARNING ", DECORATION_MAGENTA);
   }
 
   // File name and location.
@@ -235,27 +272,32 @@
   std::string colon;
   if (!loc_str.empty() || !toolchain_str.empty())
     colon = ": ";
-  OutputErrString(is_fatal,
-                  loc_str + toolchain_str + colon + info_->message + "\n");
+  output.Write(loc_str + toolchain_str + colon + info_->message + "\n");
 
   // Quoted line.
   if (input_file) {
     std::string line =
         GetNthLine(input_file->contents(), info_->location.line_number());
     if (!base::ContainsOnlyChars(line, base::kWhitespaceASCII)) {
-      OutputErrString(is_fatal, line + "\n", DECORATION_DIM);
-      OutputHighlighedPosition(info_->location, info_->ranges, line.size(),
-                               is_fatal);
+      output.Write(line + "\n", DECORATION_DIM);
+      OutputHighlightedPosition(info_->location, info_->ranges, line.size(),
+                                output);
     }
   }
 
   // Optional help text.
   if (!info_->help_text.empty())
-    OutputErrString(is_fatal, info_->help_text + "\n");
+    output.Write(info_->help_text + "\n");
 
   // Sub errors.
   for (const auto& sub_err : info_->sub_errs)
-    sub_err.InternalPrintToStdout(true, is_fatal);
+    sub_err.InternalFormat(output, true, is_fatal);
+}
 
-  return true;
+std::string Err::to_string() const {
+  if (!has_error())
+    return std::string();
+  StringErrOutput output;
+  InternalFormat(output, false, true);
+  return std::move(output.out);
 }
diff --git a/src/gn/err.h b/src/gn/err.h
index c67da5d..098ea95 100644
--- a/src/gn/err.h
+++ b/src/gn/err.h
@@ -15,6 +15,7 @@
 
 class ParseNode;
 class Value;
+struct ErrOutput;
 
 // Result of doing some operation. Check has_error() to see if an error
 // occurred.
@@ -105,6 +106,10 @@
   // newlines or separators.
   bool PrintToStdout() const;
 
+  // Converts the error to a string, formatted as it would be if calling
+  // PrintToStdout with text decoration disabled.
+  std::string to_string() const;
+
   // Prints to standard out but uses a "WARNING" messaging instead of the
   // normal "ERROR" messaging. This is a property of the printing system rather
   // than of the Err class because there is no expectation that code calling a
@@ -121,6 +126,7 @@
 
  private:
   bool InternalPrintToStdout(bool is_sub_err, bool is_fatal) const;
+  void InternalFormat(ErrOutput& output, bool is_sub_err, bool is_fatal) const;
 
   std::unique_ptr<ErrInfo> info_;  // Non-null indicates error.
 };
diff --git a/src/gn/import_manager.cc b/src/gn/import_manager.cc
index 69f44b4..c422c91 100644
--- a/src/gn/import_manager.cc
+++ b/src/gn/import_manager.cc
@@ -42,7 +42,8 @@
   if (err->has_error()) {
     // If there was an error, append the caller location so the error message
     // displays a why the file was imported (esp. useful for failed asserts).
-    err->AppendSubErr(Err(node_for_err, "whence it was imported."));
+    err->AppendSubErr(
+        Err(node_for_err, "whence " + file.value() + " was imported."));
     return nullptr;
   }
   scope->ClearProcessingImport();
diff --git a/src/gn/template.cc b/src/gn/template.cc
index a01cd8c..5a1c936 100644
--- a/src/gn/template.cc
+++ b/src/gn/template.cc
@@ -106,7 +106,8 @@
   if (err->has_error()) {
     // If there was an error, append the caller location so the error message
     // displays a stack trace of how it got here.
-    err->AppendSubErr(Err(invocation, "whence it was called."));
+    err->AppendSubErr(
+        Err(invocation, "whence " + template_name + " was called."));
     return Value();
   }
 
@@ -124,7 +125,8 @@
     if (!invoker_value->scope_value()->CheckForUnusedVars(err)) {
       // If there was an error, append the caller location so the error message
       // displays a stack trace of how it got here.
-      err->AppendSubErr(Err(invocation, "whence it was called."));
+      err->AppendSubErr(
+          Err(invocation, "whence " + template_name + " was called."));
       return Value();
     }
   }
@@ -133,7 +135,8 @@
   if (!template_scope.CheckForUnusedVars(err)) {
     // If there was an error, append the caller location so the error message
     // displays a stack trace of how it got here.
-    err->AppendSubErr(Err(invocation, "whence it was called."));
+    err->AppendSubErr(
+        Err(invocation, "whence " + template_name + " was called."));
     return Value();
   }
 
diff --git a/src/gn/template_unittest.cc b/src/gn/template_unittest.cc
index 58dbf66..b4fdf1a 100644
--- a/src/gn/template_unittest.cc
+++ b/src/gn/template_unittest.cc
@@ -91,3 +91,26 @@
   input.parsed()->Execute(setup.scope(), &err);
   ASSERT_SUCCESS(input);
 }
+
+TEST(Template, ErrorStackTrace) {
+  TestWithScope setup;
+  TestParseInput input(
+      "template(\"my_template\") {\n"
+      "  print(invoker.undefined_var)\n"
+      "}\n"
+      "my_template(\"lala\") {\n"
+      "}");
+  ASSERT_SUCCESS(input);
+
+  Err err;
+  input.parsed()->Execute(setup.scope(), &err);
+  EXPECT_TRUE(err.has_error());
+  EXPECT_EQ(err.to_string(),
+            "ERROR at //test:2:17: No value named \"undefined_var\" in scope "
+            "\"invoker\"\n"
+            "  print(invoker.undefined_var)\n"
+            "                ^------------\n"
+            "See //test:4:1: whence my_template was called.\n"
+            "my_template(\"lala\") {\n"
+            "^--------------------\n");
+}