diff --git a/app/models/assistant/function/upload_account_statement.rb b/app/models/assistant/function/upload_account_statement.rb
index 2faf230af..f557e0818 100644
--- a/app/models/assistant/function/upload_account_statement.rb
+++ b/app/models/assistant/function/upload_account_statement.rb
@@ -118,10 +118,12 @@ class Assistant::Function::UploadAccountStatement < Assistant::Function
     error("validation_failed", e.record.errors.full_messages.join("; "))
   rescue => e
     # The shared upload path can raise from content sniffing, storage or a
-    # validation hook. Those would otherwise surface to the agent as a raw
-    # exception string; give it something it can act on instead.
+    # validation hook. The agent gets a fixed message, never the exception text:
+    # a storage failure can carry bucket names, object keys, paths or request
+    # details, and this response crosses out to an external client. Diagnostics
+    # stay in the server log.
     Rails.logger.error("[UploadAccountStatement] #{e.class}: #{e.message}")
-    error("upload_failed", "The statement could not be stored: #{e.message.truncate(200)}")
+    error("upload_failed", "The statement could not be stored due to an unexpected error. It has been logged for the administrator.")
   end
 
   private
diff --git a/test/models/assistant/function/upload_account_statement_test.rb b/test/models/assistant/function/upload_account_statement_test.rb
index 6a115fd5b..3fcf55734 100644
--- a/test/models/assistant/function/upload_account_statement_test.rb
+++ b/test/models/assistant/function/upload_account_statement_test.rb
@@ -98,29 +98,40 @@ class Assistant::Function::UploadAccountStatementTest < ActiveSupport::TestCase
   end
 
   test "accepts urlsafe base64 without padding" do
-    encoded = Base64.urlsafe_encode64(@content, padding: false)
+    # Chosen so the encoding actually uses the URL-safe alphabet; the usual
+    # fixture encodes to plain base64, which would exercise the padding branch
+    # while leaving the "-_" translation untested.
+    content = "a,b\n1,2\n~~~???\n"
+    encoded = Base64.urlsafe_encode64(content, padding: false)
+    assert_match(/-/, encoded, "fixture must exercise the urlsafe alphabet")
+    assert_match(/_/, encoded, "fixture must exercise the urlsafe alphabet")
+    assert_no_match(/=/, encoded, "fixture must be unpadded")
 
     result = @function.call("filename" => "statement.csv", "content_base64" => encoded)
 
     assert result[:success]
-    assert_equal Digest::SHA256.hexdigest(@content), result[:statement][:content_sha256]
+    assert_equal Digest::SHA256.hexdigest(content), result[:statement][:content_sha256]
   end
 
-  test "rejects content that decodes to zero bytes" do
+  test "rejects blank content" do
+    # Base64.strict_encode64("") is "", which is blank, so this never reaches
+    # the decoder — hence invalid_content rather than empty_file.
     result = @function.call("filename" => "statement.csv", "content_base64" => Base64.strict_encode64(""))
 
     assert_not result[:success]
     assert_equal "invalid_content", result[:error]
   end
 
-  test "reports an unexpected storage failure as a tool error" do
-    AccountStatement.stubs(:create_from_prepared_upload!).raises(StandardError, "storage exploded")
+  test "reports an unexpected storage failure without leaking the exception" do
+    AccountStatement.stubs(:create_from_prepared_upload!).raises(StandardError, "s3://bucket/secret-key exploded")
 
     result = @function.call(params(filename: "statement.csv"))
 
     assert_not result[:success]
     assert_equal "upload_failed", result[:error]
-    assert_match(/storage exploded/, result[:message])
+    # The response crosses out to an external agent, so it must carry none of
+    # the exception's detail.
+    assert_no_match(/s3:|bucket|secret-key|exploded/, result[:message])
   end
 
   test "rejects an unknown account_id rather than silently uploading unlinked" do