@@ -27,6 +27,7 @@ accurate and fully typed.
     - [API Key](#api-key)
     - [Personal Access Token](#personal-access-token)
   - [Action Attempts](#action-attempts)
+  - [Setting a value to null](#setting-a-value-to-null)
   - [Pagination](#pagination)
     - [Manually fetch pages with the next_page_cursor](#manually-fetch-pages-with-the-next_page_cursor)
     - [Resume pagination](#resume-pagination)
@@ -43,6 +44,7 @@ accurate and fully typed.
     - [Configuring the Faraday Client](#configuring-the-faraday-client)
     - [Using the Faraday Client](#using-the-faraday-client)
     - [Overriding the Client](#overriding-the-client)
+    - [Serializing URL search params](#serializing-url-search-params)
 - [Development and Testing](#development-and-testing)
   - [Quickstart](#quickstart)
   - [Source code](#source-code)
@@ -226,6 +228,35 @@ rescue Seam::ActionAttemptTimeoutError
 end
 ```
 
+### Setting a value to null
+
+The Seam API distinguishes three states for an updatable parameter:
+omitted (leave the stored value unchanged), null (unset the stored value),
+and a value (set it).
+
+Ruby's `nil` means omitted.
+The SDK removes `nil` parameters from the request entirely,
+so passing `nil` never unsets a value.
+To unset a value, pass the `Seam::NULL` sentinel,
+which the SDK sends as JSON `null` in request bodies
+and as an empty value in query strings:
+
+```ruby
+require "seam"
+
+seam = Seam.new
+
+# Leaves ends_at unchanged.
+seam.access_grants.update(access_grant_id: access_grant_id, ends_at: nil)
+
+# Unsets ends_at so the grant no longer expires.
+seam.access_grants.update(access_grant_id: access_grant_id, ends_at: Seam::NULL)
+```
+
+Only pass `Seam::NULL` for parameters the API documents as nullable.
+Generated methods document nullable parameters
+with `Seam::Null` in their `@param` types, e.g. `[String, Seam::Null, nil]`.
+
 ### Pagination
 
 Some Seam API endpoints that return lists of resources support pagination.
@@ -489,6 +520,61 @@ devices = seam.client.get("/devices/list").body["devices"]
 A Faraday compatible client may be provided to create a `Seam` instance.
 This API is used internally and is not directly supported.
 
+#### Serializing URL search params
+
+The Seam API parses URL search params as complex types.
+If you call it with your own HTTP client,
+`Seam.serialize_url_search_params` is exported for that purpose.
+The `_strict=true` parameter is added to any non-empty query
+so the Seam API uses strict, schema-aware parsing.
+A query with no serializable params remains empty.
+
+```ruby
+require "net/http"
+require "seam"
+
+uri = URI("https://connect.getseam.com/devices/list")
+uri.query = Seam.serialize_url_search_params({device_ids: ["device1", "device2"]})
+
+Net::HTTP.get(uri, {"Authorization" => "Bearer your-api-key"})
+```
+
+The serialization defines the name and value of each search param,
+where every value is a string.
+`Seam::UrlSearchParams` holds those pairs and renders the query string,
+as [URLSearchParams] does for the [reference implementation]:
+
+```ruby
+require "seam"
+
+search_params = Seam::UrlSearchParams.new
+
+Seam.update_url_search_params(search_params, {device_ids: ["device1", "device2"]})
+
+search_params.to_a
+# => [["device_ids", "device1"], ["device_ids", "device2"], ["_strict", "true"]]
+
+search_params.to_s
+# => "device_ids=device1&device_ids=device2&_strict=true"
+```
+
+Pass either the query string or the pairs to your HTTP client.
+A client may percent-encode a few characters differently
+than `URLSearchParams` does,
+which the Seam API reads as the same params either way.
+
+A param set to `nil` is omitted,
+while a param set to `Seam::NULL` is serialized to an empty value,
+which the Seam API reads as null,
+as described in [Setting a value to null](#setting-a-value-to-null).
+A param that cannot be represented raises a `Seam::UnserializableParamError`.
+
+The Seam API parses these params with the corresponding [parser].
+
+[URLSearchParams]: https://developer.mozilla.org/en-US/docs/Web/API/URLSearchParams
+[reference implementation]: https://github.com/seamapi/url-search-params-serializer
+[parser]: https://github.com/seamapi/url-search-params-parser
+
 ## Development and Testing
 
 ### Quickstart
 
@@ -94,10 +94,12 @@ export const rubyParameterType = (parameter: Parameter): string => {
           parameter.format,
           parameter.format === 'number' && parameter.isInt,
         )
-  return nullable(type, {
-    isOptional: !parameter.isRequired,
-    isNullable: parameter.isNullable,
-  })
+  // Only a nullable parameter accepts the Seam::NULL sentinel, and only an
+  // optional parameter accepts nil.
+  const union = [type]
+  if (parameter.isNullable) union.push('Seam::Null')
+  if (!parameter.isRequired) union.push('nil')
+  return union.join(', ')
 }
 
 export const rubyParamDoc = (
 
@@ -1,5 +1,8 @@
 # frozen_string_literal: true
 
+require_relative "seam/null"
+require_relative "seam/url_search_params"
+require_relative "seam/strict_url_search_params_serializer"
 require_relative "seam/http"
 require_relative "seam/http_without_workspace"
 require_relative "seam/webhook"
 
@@ -0,0 +1,23 @@
+# frozen_string_literal: true
+
+require "singleton"
+
+module Seam
+  # The type of the {Seam::NULL} sentinel.
+  class Null
+    include Singleton
+
+    def to_s
+      "NULL"
+    end
+
+    def inspect
+      "NULL"
+    end
+  end
+
+  # Sentinel for an explicit JSON null: +nil+ omits a parameter, +Seam::NULL+
+  # unsets its stored value. Only for parameters the API documents as
+  # nullable.
+  NULL = Null.instance
+end
@@ -5,6 +5,7 @@
 require_relative "defaults"
 require_relative "version"
 require_relative "paginator"
+require_relative "strict_url_search_params_serializer"
 
 module Seam
   module Http
@@ -16,7 +17,11 @@ def self.create_faraday_client(endpoint, auth_headers, faraday_options = {}, far
         default_options = {
           url: endpoint,
           headers: auth_headers.merge(default_headers),
-          request: {timeout: timeout, open_timeout: timeout}
+          request: {
+            timeout: timeout,
+            open_timeout: timeout,
+            params_encoder: UrlSearchParamsEncoder
+          }
         }
 
         options = deep_merge(default_options, faraday_options)
@@ -33,6 +38,7 @@ def self.create_faraday_client(endpoint, auth_headers, faraday_options = {}, far
         faraday_retry_options = default_faraday_retry_options.merge(faraday_retry_options)
 
         Faraday.new(options) do |builder|
+          builder.use ReplaceNullMiddleware
           builder.request :json
           builder.use Seam::PaginationMiddleware
           builder.response :json
@@ -97,6 +103,14 @@ def seam_api_error_response?(env)
         end
       end
 
+      class ReplaceNullMiddleware < Faraday::Middleware
+        def on_request(env)
+          return unless env.body.is_a?(Hash) || env.body.is_a?(Array)
+
+          env.body = Seam.replace_null(env.body)
+        end
+      end
+
       def self.deep_merge(hash1, hash2)
         result = hash1.dup
         hash2.each do |key, value|
@@ -111,5 +125,39 @@ def self.deep_merge(hash1, hash2)
 
       private_class_method :deep_merge
     end
+
+    module UrlSearchParamsEncoder
+      # Pairs decoded from a query string already present in the request
+      # path, passed through {encode} verbatim rather than re-serialized.
+      Decoded = Struct.new(:values)
+
+      def self.encode(params)
+        search_params = Seam::UrlSearchParams.new
+        map_params = {}
+
+        params.each do |name, value|
+          if value.is_a?(Decoded)
+            value.values.each { |element| search_params.append(name, element) }
+          else
+            map_params[name] = value
+          end
+        end
+
+        return search_params.to_s if map_params.empty?
+
+        Seam.update_url_search_params(search_params, map_params)
+        search_params.to_s
+      end
+
+      # Called by Faraday when a request path carries its own query string.
+      def self.decode(query)
+        return {} if query.nil? || query.empty?
+
+        pairs = URI.decode_www_form(query.encode(Encoding::UTF_8))
+        pairs.each_with_object({}) do |(name, value), decoded|
+          (decoded[name] ||= Decoded.new([])).values << value
+        end
+      end
+    end
   end
 end