diff --git a/fuzzers/tint_ast_fuzzer/fuzzer.cc b/fuzzers/tint_ast_fuzzer/fuzzer.cc
index 421ee4e..a6c3129 100644
--- a/fuzzers/tint_ast_fuzzer/fuzzer.cc
+++ b/fuzzers/tint_ast_fuzzer/fuzzer.cc
@@ -106,9 +106,10 @@
     fuzzer.EnableInspector();
     fuzzer.Run(data, size);
     if (fuzzer.HasErrors()) {
-      std::cout << "Fuzzing " << target.name
-                << " produced an error:" << std::endl
-                << fuzzer.GetErrors() << std::endl;
+      std::cout << "Fuzzing " << target.name << " produced an error"
+                << std::endl;
+      auto printer = tint::diag::Printer::create(stderr, true);
+      tint::diag::Formatter{}.format(fuzzer.Diagnostics(), printer.get());
     }
   }
 
diff --git a/fuzzers/tint_common_fuzzer.cc b/fuzzers/tint_common_fuzzer.cc
index 9e36291..153cf8f 100644
--- a/fuzzers/tint_common_fuzzer.cc
+++ b/fuzzers/tint_common_fuzzer.cc
@@ -217,7 +217,7 @@
   }
 
   if (!program.IsValid()) {
-    errors_ = diag::Formatter().format(program.Diagnostics());
+    diagnostics_ = program.Diagnostics();
     return 0;
   }
 
@@ -234,68 +234,77 @@
 
     auto entry_points = inspector.GetEntryPoints();
     if (inspector.has_error()) {
-      errors_ = inspector.error();
+      diagnostics_.add_error(tint::diag::System::Inspector, inspector.error());
       return 0;
     }
 
     for (auto& ep : entry_points) {
       auto remapped_name = inspector.GetRemappedNameForEntryPoint(ep.name);
       if (inspector.has_error()) {
-        errors_ = inspector.error();
+        diagnostics_.add_error(tint::diag::System::Inspector,
+                               inspector.error());
         return 0;
       }
 
       auto constant_ids = inspector.GetConstantIDs();
       if (inspector.has_error()) {
-        errors_ = inspector.error();
+        diagnostics_.add_error(tint::diag::System::Inspector,
+                               inspector.error());
         return 0;
       }
 
       auto uniform_bindings =
           inspector.GetUniformBufferResourceBindings(ep.name);
       if (inspector.has_error()) {
-        errors_ = inspector.error();
+        diagnostics_.add_error(tint::diag::System::Inspector,
+                               inspector.error());
         return 0;
       }
 
       auto storage_bindings =
           inspector.GetStorageBufferResourceBindings(ep.name);
       if (inspector.has_error()) {
-        errors_ = inspector.error();
+        diagnostics_.add_error(tint::diag::System::Inspector,
+                               inspector.error());
         return 0;
       }
 
       auto readonly_bindings =
           inspector.GetReadOnlyStorageBufferResourceBindings(ep.name);
       if (inspector.has_error()) {
-        errors_ = inspector.error();
+        diagnostics_.add_error(tint::diag::System::Inspector,
+                               inspector.error());
         return 0;
       }
 
       auto sampler_bindings = inspector.GetSamplerResourceBindings(ep.name);
       if (inspector.has_error()) {
-        errors_ = inspector.error();
+        diagnostics_.add_error(tint::diag::System::Inspector,
+                               inspector.error());
         return 0;
       }
 
       auto comparison_sampler_bindings =
           inspector.GetComparisonSamplerResourceBindings(ep.name);
       if (inspector.has_error()) {
-        errors_ = inspector.error();
+        diagnostics_.add_error(tint::diag::System::Inspector,
+                               inspector.error());
         return 0;
       }
 
       auto sampled_texture_bindings =
           inspector.GetSampledTextureResourceBindings(ep.name);
       if (inspector.has_error()) {
-        errors_ = inspector.error();
+        diagnostics_.add_error(tint::diag::System::Inspector,
+                               inspector.error());
         return 0;
       }
 
       auto multisampled_texture_bindings =
           inspector.GetMultisampledTextureResourceBindings(ep.name);
       if (inspector.has_error()) {
-        errors_ = inspector.error();
+        diagnostics_.add_error(tint::diag::System::Inspector,
+                               inspector.error());
         return 0;
       }
     }
diff --git a/fuzzers/tint_common_fuzzer.h b/fuzzers/tint_common_fuzzer.h
index 3bdaf23..32475cc 100644
--- a/fuzzers/tint_common_fuzzer.h
+++ b/fuzzers/tint_common_fuzzer.h
@@ -110,8 +110,6 @@
 
   int Run(const uint8_t* data, size_t size);
 
-  const std::string& GetErrors() const { return errors_; }
-
   const std::vector<uint32_t>& GetGeneratedSpirv() const {
     return generated_spirv_;
   }
@@ -122,7 +120,9 @@
 
   const std::string& GetGeneratedMsl() const { return generated_msl_; }
 
-  bool HasErrors() const { return !errors_.empty(); }
+  const tint::diag::List& Diagnostics() const { return diagnostics_; }
+
+  bool HasErrors() const { return diagnostics_.contains_errors(); }
 
  private:
   InputFormat input_;
@@ -130,7 +130,7 @@
   transform::Manager* transform_manager_;
   transform::DataMap transform_inputs_;
   bool inspector_enabled_;
-  std::string errors_;
+  tint::diag::List diagnostics_;
 
   std::vector<uint32_t> generated_spirv_;
   std::string generated_wgsl_;
diff --git a/src/diagnostic/formatter.cc b/src/diagnostic/formatter.cc
index 4b20879..e162c89 100644
--- a/src/diagnostic/formatter.cc
+++ b/src/diagnostic/formatter.cc
@@ -15,6 +15,7 @@
 #include "src/diagnostic/formatter.h"
 
 #include <algorithm>
+#include <iterator>
 #include <vector>
 
 #include "src/diagnostic/diagnostic.h"
@@ -96,9 +97,7 @@
   /// @param c the character to print `n` times
   /// @param n the number of times to print character `c`
   void repeat(char c, size_t n) {
-    while (n-- > 0) {
-      stream << c;
-    }
+    std::fill_n(std::ostream_iterator<char>(stream), n, c);
   }
 
  private: