diff --git a/contract-tests/service.rb b/contract-tests/service.rb
index 0a1d303..30220bf 100644
--- a/contract-tests/service.rb
+++ b/contract-tests/service.rb
@@ -22,7 +22,9 @@
     capabilities: [
       'headers',
       'last-event-id',
+      'post',
       'read-timeout',
+      'report',
     ],
   }.to_json
 end
@@ -54,6 +56,8 @@
   entity = nil
   sse = SSE::Client.new(
     streamUrl,
+    method: opts[:method] || "GET",
+    payload: opts[:body] || nil,
     headers: opts[:headers] || {},
     last_event_id: opts[:lastEventId],
     read_timeout: opts[:readTimeoutMs].nil? ? nil : (opts[:readTimeoutMs].to_f / 1000),
diff --git a/lib/ld-eventsource/client.rb b/lib/ld-eventsource/client.rb
index c018151..8c80aee 100644
--- a/lib/ld-eventsource/client.rb
+++ b/lib/ld-eventsource/client.rb
@@ -84,6 +84,11 @@ class Client
     # @param socket_factory [#open] (nil)  an optional factory object for creating sockets,
     #   if you want to use something other than the default `TCPSocket`; it must implement
     #   `open(uri, timeout)` to return a connected `Socket`
+    # @param method [String] ("GET")  the HTTP method to use for requests
+    # @param payload [String, Hash, Array, #call] (nil)  optional request payload. If payload is a Hash or
+    #   an Array, it will be converted to JSON and sent as the request body. A string will be sent as a non-JSON
+    #   request body. If payload responds to #call, it will be invoked on each
+    #   request to generate the payload dynamically.
     # @yieldparam [Client] client  the new client instance, before opening the connection
     #
     def initialize(uri,
@@ -95,13 +100,17 @@ def initialize(uri,
           last_event_id: nil,
           proxy: nil,
           logger: nil,
-          socket_factory: nil)
+          socket_factory: nil,
+          method: "GET",
+          payload: nil)
       @uri = URI(uri)
       @stopped = Concurrent::AtomicBoolean.new(false)
 
       @headers = headers.clone
       @connect_timeout = connect_timeout
       @read_timeout = read_timeout
+      @method = method.to_s.upcase
+      @payload = payload
       @logger = logger || default_logger
       http_client_options = {}
       if socket_factory
@@ -262,9 +271,7 @@ def connect
         cxn = nil
         begin
           @logger.info { "Connecting to event stream at #{@uri}" }
-          cxn = @http_client.request("GET", @uri, {
-            headers: build_headers,
-          })
+          cxn = @http_client.request(@method, @uri, build_opts)
           if cxn.status.code == 200
             content_type = cxn.content_type.mime_type
             if content_type && content_type.start_with?("text/event-stream")
@@ -358,5 +365,18 @@ def build_headers
       h['Last-Event-Id'] = @last_id if !@last_id.nil? && @last_id != ""
       h.merge(@headers)
     end
+
+    def build_opts
+      return {headers: build_headers} if @payload.nil?
+
+      # Resolve payload if it's callable
+      resolved_payload = @payload.respond_to?(:call) ? @payload.call : @payload
+
+      if resolved_payload.is_a?(Hash) || resolved_payload.is_a?(Array)
+        {headers: build_headers, json: resolved_payload}
+      else
+        {headers: build_headers, body: resolved_payload.to_s}
+      end
+    end
   end
 end
diff --git a/spec/client_spec.rb b/spec/client_spec.rb
index ea7b849..636b1dc 100644
--- a/spec/client_spec.rb
+++ b/spec/client_spec.rb
@@ -447,4 +447,294 @@ def send_stream_content(res, content, keep_open:)
       end
     end
   end
+
+  describe "HTTP method parameter" do
+    it "defaults to GET method" do
+      with_server do |server|
+        requests = Queue.new
+        server.setup_response("/") do |req,res|
+          requests << req
+          send_stream_content(res, "", keep_open: true)
+        end
+
+        with_client(subject.new(server.base_uri)) do |client|
+          received_req = requests.pop
+          expect(received_req.request_method).to eq("GET")
+        end
+      end
+    end
+
+    it "uses explicit GET method" do
+      with_server do |server|
+        requests = Queue.new
+        server.setup_response("/") do |req,res|
+          requests << req
+          send_stream_content(res, "", keep_open: true)
+        end
+
+        with_client(subject.new(server.base_uri, method: "GET")) do |client|
+          received_req = requests.pop
+          expect(received_req.request_method).to eq("GET")
+        end
+      end
+    end
+
+    it "uses explicit POST method" do
+      with_server do |server|
+        requests = Queue.new
+        server.setup_response("/") do |req,res|
+          requests << req
+          send_stream_content(res, "", keep_open: true)
+        end
+
+        with_client(subject.new(server.base_uri, method: "POST")) do |client|
+          received_req = requests.pop
+          expect(received_req.request_method).to eq("POST")
+        end
+      end
+    end
+
+    it "normalizes method to uppercase" do
+      with_server do |server|
+        requests = Queue.new
+        server.setup_response("/") do |req,res|
+          requests << req
+          send_stream_content(res, "", keep_open: true)
+        end
+
+        with_client(subject.new(server.base_uri, method: "post")) do |client|
+          received_req = requests.pop
+          expect(received_req.request_method).to eq("POST")
+        end
+      end
+    end
+  end
+
+  describe "payload parameter" do
+    it "sends string payload as body" do
+      with_server do |server|
+        requests = Queue.new
+        server.setup_response("/") do |req,res|
+          requests << req
+          send_stream_content(res, "", keep_open: true)
+        end
+
+        payload = "test-string-payload"
+        with_client(subject.new(server.base_uri, method: "POST", payload: payload)) do |client|
+          received_req = requests.pop
+          expect(received_req.request_method).to eq("POST")
+          expect(received_req.body).to eq(payload)
+        end
+      end
+    end
+
+    it "sends hash payload as JSON" do
+      with_server do |server|
+        requests = Queue.new
+        server.setup_response("/") do |req,res|
+          requests << req
+          send_stream_content(res, "", keep_open: true)
+        end
+
+        payload = {user: "test", id: 123}
+        with_client(subject.new(server.base_uri, method: "POST", payload: payload)) do |client|
+          received_req = requests.pop
+          expect(received_req.request_method).to eq("POST")
+          expect(received_req.header["content-type"].first).to include("application/json")
+          parsed_body = JSON.parse(received_req.body)
+          expect(parsed_body).to eq({"user" => "test", "id" => 123})
+        end
+      end
+    end
+
+    it "sends array payload as JSON" do
+      with_server do |server|
+        requests = Queue.new
+        server.setup_response("/") do |req,res|
+          requests << req
+          send_stream_content(res, "", keep_open: true)
+        end
+
+        payload = ["item1", "item2", "item3"]
+        with_client(subject.new(server.base_uri, method: "POST", payload: payload)) do |client|
+          received_req = requests.pop
+          expect(received_req.request_method).to eq("POST")
+          expect(received_req.header["content-type"].first).to include("application/json")
+          parsed_body = JSON.parse(received_req.body)
+          expect(parsed_body).to eq(["item1", "item2", "item3"])
+        end
+      end
+    end
+
+    it "works with GET method and payload" do
+      with_server do |server|
+        requests = Queue.new
+        server.setup_response("/") do |req,res|
+          requests << req
+          send_stream_content(res, "", keep_open: true)
+        end
+
+        payload = "get-with-payload"
+        with_client(subject.new(server.base_uri, method: "GET", payload: payload)) do |client|
+          received_req = requests.pop
+          expect(received_req.request_method).to eq("GET")
+          expect(received_req.body).to eq(payload)
+        end
+      end
+    end
+  end
+
+  describe "callable payload parameter" do
+    it "invokes lambda payload on each request" do
+      with_server do |server|
+        requests = Queue.new
+        server.setup_response("/") do |req,res|
+          requests << req
+          send_stream_content(res, "", keep_open: false)  # Close to trigger reconnect
+        end
+
+        counter = 0
+        callable_payload = -> { counter += 1; "request-#{counter}" }
+
+        with_client(subject.new(server.base_uri, method: "POST", payload: callable_payload, reconnect_time: reconnect_asap)) do |client|
+          # Wait for first request
+          req1 = requests.pop
+          expect(req1.body).to eq("request-1")
+
+          # Wait for reconnect and second request
+          req2 = requests.pop
+          expect(req2.body).to eq("request-2")
+        end
+      end
+    end
+
+    it "invokes proc payload on each request" do
+      with_server do |server|
+        requests = Queue.new
+        server.setup_response("/") do |req,res|
+          requests << req
+          send_stream_content(res, "", keep_open: false)
+        end
+
+        counter = 0
+        callable_payload = proc { counter += 1; {request_id: counter, timestamp: Time.now.to_i} }
+
+        with_client(subject.new(server.base_uri, method: "POST", payload: callable_payload, reconnect_time: reconnect_asap)) do |client|
+          # Wait for first request
+          req1 = requests.pop
+          parsed_body1 = JSON.parse(req1.body)
+          expect(parsed_body1["request_id"]).to eq(1)
+
+          # Wait for reconnect and second request
+          req2 = requests.pop
+          parsed_body2 = JSON.parse(req2.body)
+          expect(parsed_body2["request_id"]).to eq(2)
+          expect(parsed_body2["timestamp"]).to be >= parsed_body1["timestamp"]
+        end
+      end
+    end
+
+    it "invokes custom callable object payload" do
+      with_server do |server|
+        requests = Queue.new
+        server.setup_response("/") do |req,res|
+          requests << req
+          send_stream_content(res, "", keep_open: true)
+        end
+
+        class TestPayloadGenerator
+          def initialize
+            @counter = 0
+          end
+
+          def call
+            @counter += 1
+            {generator: "test", count: @counter}
+          end
+        end
+
+        callable_payload = TestPayloadGenerator.new
+        with_client(subject.new(server.base_uri, method: "POST", payload: callable_payload)) do |client|
+          received_req = requests.pop
+          parsed_body = JSON.parse(received_req.body)
+          expect(parsed_body).to eq({"generator" => "test", "count" => 1})
+        end
+      end
+    end
+
+    it "handles callable returning string" do
+      with_server do |server|
+        requests = Queue.new
+        server.setup_response("/") do |req,res|
+          requests << req
+          send_stream_content(res, "", keep_open: true)
+        end
+
+        callable_payload = -> { "dynamic-string-#{rand(1000)}" }
+        with_client(subject.new(server.base_uri, method: "POST", payload: callable_payload)) do |client|
+          received_req = requests.pop
+          expect(received_req.body).to match(/^dynamic-string-\d+$/)
+        end
+      end
+    end
+
+    it "handles callable returning hash" do
+      with_server do |server|
+        requests = Queue.new
+        server.setup_response("/") do |req,res|
+          requests << req
+          send_stream_content(res, "", keep_open: true)
+        end
+
+        callable_payload = -> { {type: "dynamic", value: rand(1000)} }
+        with_client(subject.new(server.base_uri, method: "POST", payload: callable_payload)) do |client|
+          received_req = requests.pop
+          expect(received_req.header["content-type"].first).to include("application/json")
+          parsed_body = JSON.parse(received_req.body)
+          expect(parsed_body["type"]).to eq("dynamic")
+          expect(parsed_body["value"]).to be_a(Integer)
+        end
+      end
+    end
+
+    it "handles callable returning array" do
+      with_server do |server|
+        requests = Queue.new
+        server.setup_response("/") do |req,res|
+          requests << req
+          send_stream_content(res, "", keep_open: true)
+        end
+
+        callable_payload = -> { ["dynamic", Time.now.to_i] }
+        with_client(subject.new(server.base_uri, method: "POST", payload: callable_payload)) do |client|
+          received_req = requests.pop
+          expect(received_req.header["content-type"].first).to include("application/json")
+          parsed_body = JSON.parse(received_req.body)
+          expect(parsed_body[0]).to eq("dynamic")
+          expect(parsed_body[1]).to be_a(Integer)
+        end
+      end
+    end
+
+    it "handles callable returning other types by converting to string" do
+      with_server do |server|
+        requests = Queue.new
+        server.setup_response("/") do |req,res|
+          requests << req
+          send_stream_content(res, "", keep_open: true)
+        end
+
+        test_object = Object.new
+        def test_object.to_s
+          "custom-object-string"
+        end
+
+        callable_payload = -> { test_object }
+        with_client(subject.new(server.base_uri, method: "POST", payload: callable_payload)) do |client|
+          received_req = requests.pop
+          expect(received_req.body).to eq("custom-object-string")
+        end
+      end
+    end
+  end
 end