diff --git a/lib/prism/static_cache.rb b/lib/prism/static_cache.rb
new file mode 100644
index 0000000..81f63fd
--- /dev/null
+++ b/lib/prism/static_cache.rb
@@ -0,0 +1,102 @@
+module Prism
+  # Conditional-GET (ETag/Last-Modified/304) and If-Range validation for
+  # Middleware::Static/Assets. Kept separate from Prism::ConditionalGet
+  # (which is mixed into Controller and reads/writes headers through
+  # `request`/`headers`/`halt`) because the static-file middleware works
+  # directly on a Rack env and a plain headers Hash, with no controller
+  # instance to hang state off of.
+  module StaticCache
+    # Cheap validators for an arbitrary file: a weak ETag derived from size
+    # and mtime (no content read - the whole point for large files) plus a
+    # Last-Modified date. Weak because two files can share size+mtime
+    # without being byte-identical, so this can only ever be used for
+    # If-None-Match/If-Modified-Since (which allow weak comparison), never
+    # for If-Range (which requires a strong validator - see range_applicable?).
+    def self.weak_validators(size, mtime)
+      etag = %(W/"#{size}-#{mtime.to_i}")
+      [etag, mtime.utc.httpdate]
+    end
+
+    # A strong ETag for a fingerprinted asset: the content hash is already
+    # in the filename (Assets::Compiler put it there), so unlike an
+    # arbitrary static file we get a real content-derived validator for
+    # free - no need to hash the file again just to answer If-Range.
+    def self.strong_etag(fingerprint_hash)
+      %("#{fingerprint_hash}")
+    end
+
+    # True when the client's cached copy is still fresh per
+    # If-None-Match/If-Modified-Since - caller should respond 304.
+    def self.not_modified?(env, etag, last_modified)
+      none_match = env['HTTP_IF_NONE_MATCH']
+      modified_since = env['HTTP_IF_MODIFIED_SINCE']
+      return false if none_match.nil? && modified_since.nil?
+
+      etag_fresh = none_match.nil? || etag_matches?(none_match, etag)
+      time_fresh = modified_since.nil? || not_newer_than?(modified_since, last_modified)
+      etag_fresh && time_fresh
+    end
+
+    # RFC 7233 SS3.2: If-Range names exactly one validator (an entity-tag or
+    # an HTTP-date, never both/neither/a list). When present, a Range
+    # request is only honored if that validator strong-matches the
+    # resource's *current* state; otherwise the Range header is ignored and
+    # the full body is sent - this is what lets a client resume a download
+    # safely (it re-requests the whole file the moment the file changes
+    # instead of stitching a byte range from the old version onto one from
+    # the new). Absent If-Range, Range is always honored (existing behavior).
+    def self.range_applicable?(env, etag, last_modified)
+      if_range = env['HTTP_IF_RANGE']
+      return true if if_range.nil?
+
+      if entity_tag?(if_range)
+        strong_etag_match?(if_range, etag)
+      else
+        exact_date_match?(if_range, last_modified)
+      end
+    end
+
+    def self.entity_tag?(value)
+      value.start_with?('"') || value.start_with?('W/"')
+    end
+    private_class_method :entity_tag?
+
+    # Strong comparison (RFC 7232 SS2.3.2): both sides must be non-weak and
+    # byte-identical. Our own weak validators (plain static files) can
+    # therefore never satisfy an If-Range by entity-tag - only a fingerprinted
+    # asset's strong etag can, and only when the client's tag is not itself
+    # marked weak.
+    def self.strong_etag_match?(header_value, current_etag)
+      return false if current_etag.nil? || current_etag.start_with?('W/')
+      return false if header_value.start_with?('W/')
+
+      header_value == current_etag
+    end
+    private_class_method :strong_etag_match?
+
+    def self.exact_date_match?(header_value, last_modified_header)
+      return false if last_modified_header.nil?
+
+      Time.httpdate(header_value) == Time.httpdate(last_modified_header)
+    rescue ArgumentError
+      false
+    end
+    private_class_method :exact_date_match?
+
+    def self.etag_matches?(header_value, current_etag)
+      return false if current_etag.nil?
+
+      header_value.split(',').map(&:strip).any? { |candidate| candidate == '*' || candidate == current_etag }
+    end
+    private_class_method :etag_matches?
+
+    def self.not_newer_than?(header_value, current_last_modified)
+      return false if current_last_modified.nil?
+
+      Time.httpdate(header_value) >= Time.httpdate(current_last_modified)
+    rescue ArgumentError
+      false
+    end
+    private_class_method :not_newer_than?
+  end
+end
diff --git a/lib/prism/streaming_body.rb b/lib/prism/streaming_body.rb
new file mode 100644
index 0000000..ed2add1
--- /dev/null
+++ b/lib/prism/streaming_body.rb
@@ -0,0 +1,43 @@
+module Prism
+  # ActionController::Live-lite: a response body whose chunks are produced
+  # by the action itself, one at a time, instead of built up into a single
+  # String before headers are ever sent. Response#stream wraps the given
+  # block in one of these instead of the ordinary Array body, and
+  # Server#write_response recognizes the type (see there) to switch the
+  # wire format to HTTP/1.1 chunked Transfer-Encoding.
+  #
+  # Deliberately synchronous, not threaded: the block runs on the same
+  # thread that iterates it (#each below), in the order it calls
+  # `stream.write`. That is enough to let a large export write straight to
+  # the socket as it is generated - never holding the whole payload in
+  # memory at once, and letting the client start receiving bytes before the
+  # last row even exists - without the complexity (and crash-on-abandoned-
+  # connection risk) of ActionController::Live's separate writer thread.
+  class StreamingBody
+    def initialize(&producer)
+      @producer = producer
+    end
+
+    # Rack's body contract: `each` is called exactly once by whatever is
+    # sending the response, and yields every chunk in order. Handing the
+    # producer block a Writer (instead of yielding to it directly) is what
+    # lets action code say `stream.write(row)` repeatedly rather than
+    # returning an Enumerable of precomputed chunks.
+    def each(&sink)
+      @producer.call(Writer.new(sink))
+      nil
+    end
+
+    class Writer
+      def initialize(sink)
+        @sink = sink
+      end
+
+      def write(chunk)
+        @sink.call(chunk.to_s)
+        self
+      end
+      alias_method :<<, :write
+    end
+  end
+end
diff --git a/lib/prism/template.rb b/lib/prism/template.rb
index 3fcd041..02c9197 100644
--- a/lib/prism/template.rb
+++ b/lib/prism/template.rb
@@ -2,33 +2,40 @@ module Prism
   module Template
     module_function
 
-    def render(source, locals = {}, view = nil)
+    def render(source, locals = {}, view = nil, helpers = nil)
       code = compile(source)
-      scope = Context.new(view).__binding
+      scope = Context.new(view, helpers).__binding
       locals.each { |name, value| scope.local_variable_set(name.to_sym, value) }
       scope.eval(code, '(prism template)')
     end
 
     def compile(source)
-      buffer = +"__out = +''\n"
+      buffer = +"@__out = +''\n"
       source.split(/(<%.*?%>)/m).each do |chunk|
         if chunk.start_with?('<%==')
-          buffer << "__out << (#{chunk[4...-2]}).to_s\n"
+          buffer << "@__out << (#{chunk[4...-2]}).to_s\n"
         elsif chunk.start_with?('<%=')
-          buffer << "__out << escape_html((#{chunk[3...-2]}).to_s)\n"
+          buffer << "@__out << escape_html((#{chunk[3...-2]}).to_s)\n"
         elsif chunk.start_with?('<%')
           buffer << "#{chunk[2...-2]}\n"
         else
-          buffer << "__out << #{chunk.inspect}\n"
+          buffer << "@__out << #{chunk.inspect}\n"
         end
       end
-      buffer << "__out\n"
+      buffer << "@__out\n"
       buffer
     end
 
+    # Rendering context for compiled templates. Block-taking view helpers
+    # (form_for, content_tag with a block, ...) are invoked from a plain
+    # `<% %>` tag, so they append their own markup to @__out directly
+    # instead of returning a string; `capture` lets them do that safely by
+    # temporarily swapping the buffer so nested `<%=`/`<%==` output lands
+    # in the captured fragment rather than the surrounding page.
     class Context
-      def initialize(view = nil)
+      def initialize(view = nil, helpers = nil)
         @view = view
+        @helpers = helpers
       end
 
       def escape_html(text)
@@ -46,6 +53,24 @@ module Prism
         @view.render_collection(name, collection, locals: locals, as: as)
       end
 
+      def capture
+        previous = @__out
+        @__out = +''
+        yield
+        @__out
+      ensure
+        @__out = previous
+      end
+
+      def method_missing(name, *args, **kwargs, &block)
+        return @helpers.public_send(name, *args, **kwargs, &block) if @helpers&.respond_to?(name)
+        super
+      end
+
+      def respond_to_missing?(name, include_private = false)
+        (@helpers && @helpers.respond_to?(name, include_private)) || super
+      end
+
       def __binding
         binding
       end
diff --git a/lib/prism/testing.rb b/lib/prism/testing.rb
new file mode 100644
index 0000000..2bd77d6
--- /dev/null
+++ b/lib/prism/testing.rb
@@ -0,0 +1,3 @@
+require_relative 'testing/test_response'
+require_relative 'testing/integration_session'
+require_relative 'testing/integration_test'