diff --git a/quic/core/crypto/proof_verifier.h b/quic/core/crypto/proof_verifier.h
index 7576620..ab09811 100644
--- a/quic/core/crypto/proof_verifier.h
+++ b/quic/core/crypto/proof_verifier.h
@@ -97,6 +97,11 @@
   // for some implementations) that provides useful information for the
   // verifier, e.g. logging handles.
   //
+  // If certificate verification fails, a TLS alert will be sent when closing
+  // the connection. This alert defaults to certificate_unknown. By setting
+  // |*out_alert|, a different alert can be sent to provide a more specific
+  // reason why verification failed.
+  //
   // This function may also return QUIC_PENDING, in which case the ProofVerifier
   // will call back, on the original thread, via |callback| when complete.
   // In this case, the ProofVerifier will take ownership of |callback|.
@@ -109,6 +114,7 @@
       const ProofVerifyContext* context,
       std::string* error_details,
       std::unique_ptr<ProofVerifyDetails>* details,
+      uint8_t* out_alert,
       std::unique_ptr<ProofVerifierCallback> callback) = 0;
 
   // Returns a ProofVerifyContext instance which can be use for subsequent
diff --git a/quic/core/quic_crypto_client_handshaker_test.cc b/quic/core/quic_crypto_client_handshaker_test.cc
index cfae506..2d6db7f 100644
--- a/quic/core/quic_crypto_client_handshaker_test.cc
+++ b/quic/core/quic_crypto_client_handshaker_test.cc
@@ -54,6 +54,7 @@
       const ProofVerifyContext* /*context*/,
       std::string* /*error_details*/,
       std::unique_ptr<ProofVerifyDetails>* /*details*/,
+      uint8_t* /*out_alert*/,
       std::unique_ptr<ProofVerifierCallback> /*callback*/) override {
     return QUIC_SUCCESS;
   }
diff --git a/quic/core/tls_client_handshaker.cc b/quic/core/tls_client_handshaker.cc
index fc69ad6..6ea06b8 100644
--- a/quic/core/tls_client_handshaker.cc
+++ b/quic/core/tls_client_handshaker.cc
@@ -376,6 +376,7 @@
     const std::vector<std::string>& certs,
     std::string* error_details,
     std::unique_ptr<ProofVerifyDetails>* details,
+    uint8_t* out_alert,
     std::unique_ptr<ProofVerifierCallback> callback) {
   const uint8_t* ocsp_response_raw;
   size_t ocsp_response_len;
@@ -390,7 +391,8 @@
 
   return proof_verifier_->VerifyCertChain(
       server_id_.host(), server_id_.port(), certs, ocsp_response, sct_list,
-      verify_context_.get(), error_details, details, std::move(callback));
+      verify_context_.get(), error_details, details, out_alert,
+      std::move(callback));
 }
 
 void TlsClientHandshaker::OnProofVerifyDetailsAvailable(
diff --git a/quic/core/tls_client_handshaker.h b/quic/core/tls_client_handshaker.h
index c2707c7..aadfa0b 100644
--- a/quic/core/tls_client_handshaker.h
+++ b/quic/core/tls_client_handshaker.h
@@ -92,6 +92,7 @@
       const std::vector<std::string>& certs,
       std::string* error_details,
       std::unique_ptr<ProofVerifyDetails>* details,
+      uint8_t* out_alert,
       std::unique_ptr<ProofVerifierCallback> callback) override;
   void OnProofVerifyDetailsAvailable(
       const ProofVerifyDetails& verify_details) override;
diff --git a/quic/core/tls_client_handshaker_test.cc b/quic/core/tls_client_handshaker_test.cc
index 7cd7d45..2488426 100644
--- a/quic/core/tls_client_handshaker_test.cc
+++ b/quic/core/tls_client_handshaker_test.cc
@@ -74,15 +74,16 @@
       const ProofVerifyContext* context,
       std::string* error_details,
       std::unique_ptr<ProofVerifyDetails>* details,
+      uint8_t* out_alert,
       std::unique_ptr<ProofVerifierCallback> callback) override {
     if (!active_) {
-      return verifier_->VerifyCertChain(hostname, port, certs, ocsp_response,
-                                        cert_sct, context, error_details,
-                                        details, std::move(callback));
+      return verifier_->VerifyCertChain(
+          hostname, port, certs, ocsp_response, cert_sct, context,
+          error_details, details, out_alert, std::move(callback));
     }
     pending_ops_.push_back(std::make_unique<VerifyChainPendingOp>(
         hostname, port, certs, ocsp_response, cert_sct, context, error_details,
-        details, std::move(callback), verifier_.get()));
+        details, out_alert, std::move(callback), verifier_.get()));
     return QUIC_PENDING;
   }
 
@@ -123,6 +124,7 @@
                          const ProofVerifyContext* context,
                          std::string* error_details,
                          std::unique_ptr<ProofVerifyDetails>* details,
+                         uint8_t* out_alert,
                          std::unique_ptr<ProofVerifierCallback> callback,
                          ProofVerifier* delegate)
         : hostname_(hostname),
@@ -133,6 +135,7 @@
           context_(context),
           error_details_(error_details),
           details_(details),
+          out_alert_(out_alert),
           callback_(std::move(callback)),
           delegate_(delegate) {}
 
@@ -143,7 +146,7 @@
       // synchronously.
       QuicAsyncStatus status = delegate_->VerifyCertChain(
           hostname_, port_, certs_, ocsp_response_, cert_sct_, context_,
-          error_details_, details_,
+          error_details_, details_, out_alert_,
           std::make_unique<FailingProofVerifierCallback>());
       ASSERT_NE(status, QUIC_PENDING);
       callback_->Run(status == QUIC_SUCCESS, *error_details_, details_);
@@ -158,6 +161,7 @@
     const ProofVerifyContext* context_;
     std::string* error_details_;
     std::unique_ptr<ProofVerifyDetails>* details_;
+    uint8_t* out_alert_;
     std::unique_ptr<ProofVerifierCallback> callback_;
     ProofVerifier* delegate_;
   };
diff --git a/quic/core/tls_handshaker.cc b/quic/core/tls_handshaker.cc
index 495312d..bfe3545 100644
--- a/quic/core/tls_handshaker.cc
+++ b/quic/core/tls_handshaker.cc
@@ -146,6 +146,7 @@
       expected_ssl_error() == SSL_ERROR_WANT_CERTIFICATE_VERIFY) {
     enum ssl_verify_result_t result = verify_result_;
     verify_result_ = ssl_verify_retry;
+    *out_alert = cert_verify_tls_alert_;
     return result;
   }
   const STACK_OF(CRYPTO_BUFFER)* cert_chain = SSL_get0_peer_certificates(ssl());
@@ -164,8 +165,10 @@
   ProofVerifierCallbackImpl* proof_verify_callback =
       new ProofVerifierCallbackImpl(this);
 
+  cert_verify_tls_alert_ = *out_alert;
   QuicAsyncStatus verify_result = VerifyCertChain(
       certs, &cert_verify_error_details_, &verify_details_,
+      &cert_verify_tls_alert_,
       std::unique_ptr<ProofVerifierCallback>(proof_verify_callback));
   switch (verify_result) {
     case QUIC_SUCCESS:
@@ -179,6 +182,7 @@
       return ssl_verify_retry;
     case QUIC_FAILURE:
     default:
+      *out_alert = cert_verify_tls_alert_;
       QUIC_LOG(INFO) << "Cert chain verification failed: "
                      << cert_verify_error_details_;
       return ssl_verify_invalid;
diff --git a/quic/core/tls_handshaker.h b/quic/core/tls_handshaker.h
index 657c9c7..12b27f1 100644
--- a/quic/core/tls_handshaker.h
+++ b/quic/core/tls_handshaker.h
@@ -90,10 +90,16 @@
   // non-owning pointer to |callback|; the callback must live until this
   // function returns QUIC_SUCCESS or QUIC_FAILURE, or until the callback is
   // run.
+  //
+  // If certificate verification fails, |*out_alert| may be set to a TLS alert
+  // that will be sent when closing the connection; it defaults to
+  // certificate_unknown. Implementations of VerifyCertChain may retain the
+  // |out_alert| pointer while performing an async operation.
   virtual QuicAsyncStatus VerifyCertChain(
       const std::vector<std::string>& certs,
       std::string* error_details,
       std::unique_ptr<ProofVerifyDetails>* details,
+      uint8_t* out_alert,
       std::unique_ptr<ProofVerifierCallback> callback) = 0;
   // Called when certificate verification is completed.
   virtual void OnProofVerifyDetailsAvailable(
@@ -173,6 +179,7 @@
   ProofVerifierCallbackImpl* proof_verify_callback_ = nullptr;
   std::unique_ptr<ProofVerifyDetails> verify_details_;
   enum ssl_verify_result_t verify_result_ = ssl_verify_retry;
+  uint8_t cert_verify_tls_alert_ = SSL_AD_CERTIFICATE_UNKNOWN;
   std::string cert_verify_error_details_;
 
   int expected_ssl_error_ = SSL_ERROR_WANT_READ;
diff --git a/quic/core/tls_server_handshaker.cc b/quic/core/tls_server_handshaker.cc
index aa49597..e0fe84d 100644
--- a/quic/core/tls_server_handshaker.cc
+++ b/quic/core/tls_server_handshaker.cc
@@ -397,6 +397,7 @@
     const std::vector<std::string>& /*certs*/,
     std::string* /*error_details*/,
     std::unique_ptr<ProofVerifyDetails>* /*details*/,
+    uint8_t* /*out_alert*/,
     std::unique_ptr<ProofVerifierCallback> /*callback*/) {
   QUIC_BUG << "Client certificates are not yet supported on the server";
   return QUIC_FAILURE;
diff --git a/quic/core/tls_server_handshaker.h b/quic/core/tls_server_handshaker.h
index 2fba611..4d12bf3 100644
--- a/quic/core/tls_server_handshaker.h
+++ b/quic/core/tls_server_handshaker.h
@@ -97,6 +97,7 @@
       const std::vector<std::string>& certs,
       std::string* error_details,
       std::unique_ptr<ProofVerifyDetails>* details,
+      uint8_t* out_alert,
       std::unique_ptr<ProofVerifierCallback> callback) override;
   void OnProofVerifyDetailsAvailable(
       const ProofVerifyDetails& verify_details) override;
diff --git a/quic/qbone/qbone_session_test.cc b/quic/qbone/qbone_session_test.cc
index 92f8fa7..5059274 100644
--- a/quic/qbone/qbone_session_test.cc
+++ b/quic/qbone/qbone_session_test.cc
@@ -167,13 +167,14 @@
       const ProofVerifyContext* context,
       std::string* error_details,
       std::unique_ptr<ProofVerifyDetails>* details,
+      uint8_t* out_alert,
       std::unique_ptr<ProofVerifierCallback> callback) override {
     if (!proof_verifier_) {
       return QUIC_FAILURE;
     }
     return proof_verifier_->VerifyCertChain(
         hostname, port, certs, ocsp_response, cert_sct, context, error_details,
-        details, std::move(callback));
+        details, out_alert, std::move(callback));
   }
 
   std::unique_ptr<ProofVerifyContext> CreateDefaultContext() override {
diff --git a/quic/quic_transport/web_transport_fingerprint_proof_verifier.cc b/quic/quic_transport/web_transport_fingerprint_proof_verifier.cc
index 2ea1e44..5932b9c 100644
--- a/quic/quic_transport/web_transport_fingerprint_proof_verifier.cc
+++ b/quic/quic_transport/web_transport_fingerprint_proof_verifier.cc
@@ -139,6 +139,7 @@
     const ProofVerifyContext* /*context*/,
     std::string* error_details,
     std::unique_ptr<ProofVerifyDetails>* details,
+    uint8_t* /*out_alert*/,
     std::unique_ptr<ProofVerifierCallback> /*callback*/) {
   if (certs.empty()) {
     *details = std::make_unique<Details>(Status::kInternalError);
diff --git a/quic/quic_transport/web_transport_fingerprint_proof_verifier.h b/quic/quic_transport/web_transport_fingerprint_proof_verifier.h
index fba1f83..ad83788 100644
--- a/quic/quic_transport/web_transport_fingerprint_proof_verifier.h
+++ b/quic/quic_transport/web_transport_fingerprint_proof_verifier.h
@@ -100,6 +100,7 @@
       const ProofVerifyContext* context,
       std::string* error_details,
       std::unique_ptr<ProofVerifyDetails>* details,
+      uint8_t* out_alert,
       std::unique_ptr<ProofVerifierCallback> callback) override;
   std::unique_ptr<ProofVerifyContext> CreateDefaultContext() override;
 
diff --git a/quic/quic_transport/web_transport_fingerprint_proof_verifier_test.cc b/quic/quic_transport/web_transport_fingerprint_proof_verifier_test.cc
index b527ae6..a84a8a5 100644
--- a/quic/quic_transport/web_transport_fingerprint_proof_verifier_test.cc
+++ b/quic/quic_transport/web_transport_fingerprint_proof_verifier_test.cc
@@ -40,12 +40,13 @@
   VerifyResult Verify(absl::string_view certificate) {
     VerifyResult result;
     std::unique_ptr<ProofVerifyDetails> details;
+    uint8_t tls_alert;
     result.status = verifier_->VerifyCertChain(
         /*hostname=*/"", /*port=*/0,
         std::vector<std::string>{std::string(certificate)},
         /*ocsp_response=*/"",
         /*cert_sct=*/"",
-        /*context=*/nullptr, &result.error, &details,
+        /*context=*/nullptr, &result.error, &details, &tls_alert,
         /*callback=*/nullptr);
     result.detailed_status =
         static_cast<WebTransportFingerprintProofVerifier::Details*>(
diff --git a/quic/test_tools/quic_test_client.cc b/quic/test_tools/quic_test_client.cc
index d7f2033..5b93de5 100644
--- a/quic/test_tools/quic_test_client.cc
+++ b/quic/test_tools/quic_test_client.cc
@@ -82,6 +82,7 @@
       const ProofVerifyContext* /*context*/,
       std::string* /*error_details*/,
       std::unique_ptr<ProofVerifyDetails>* /*details*/,
+      uint8_t* /*out_alert*/,
       std::unique_ptr<ProofVerifierCallback> /*callback*/) override {
     return ProcessCerts(certs, cert_sct);
   }
diff --git a/quic/tools/fake_proof_verifier.h b/quic/tools/fake_proof_verifier.h
index 2d7d7f4..8707879 100644
--- a/quic/tools/fake_proof_verifier.h
+++ b/quic/tools/fake_proof_verifier.h
@@ -38,6 +38,7 @@
       const ProofVerifyContext* /*context*/,
       std::string* /*error_details*/,
       std::unique_ptr<ProofVerifyDetails>* /*details*/,
+      uint8_t* /*out_alert*/,
       std::unique_ptr<ProofVerifierCallback> /*callback*/) override {
     return QUIC_SUCCESS;
   }