diff --git a/lib/roast/cogs/agent/providers/claude.rb b/lib/roast/cogs/agent/providers/claude.rb
index 2db0c619..4a0a8f90 100644
--- a/lib/roast/cogs/agent/providers/claude.rb
+++ b/lib/roast/cogs/agent/providers/claude.rb
@@ -20,7 +20,13 @@ def initialize(invocation_result)
           def invoke(input)
             invocations = [] #: Array[ClaudeInvocation]
             input.prompts.each do |prompt|
-              invocation = ClaudeInvocation.new(@config, prompt, invocations.last&.result&.session || input.session)
+              previous_session = invocations.last&.result&.session
+              invocation = ClaudeInvocation.new(
+                @config,
+                prompt,
+                previous_session || input.session,
+                fork_session: previous_session.nil?,
+              )
               invocation.run!
               invocations << invocation
               break unless invocation.result.success
diff --git a/lib/roast/cogs/agent/providers/claude/claude_invocation.rb b/lib/roast/cogs/agent/providers/claude/claude_invocation.rb
index f2555877..1c6ad3d7 100644
--- a/lib/roast/cogs/agent/providers/claude/claude_invocation.rb
+++ b/lib/roast/cogs/agent/providers/claude/claude_invocation.rb
@@ -53,8 +53,8 @@ def initialize
               end
             end
 
-            #: (Agent::Config, String, String?) -> void
-            def initialize(config, prompt, session)
+            #: (Agent::Config, String, String?, ?fork_session: bool) -> void
+            def initialize(config, prompt, session, fork_session: true)
               @base_command = config.valid_command #: (String | Array[String])?
               @model = config.valid_model #: String?
               @append_system_prompt = config.valid_append_system_prompt #: String?
@@ -69,6 +69,7 @@ def initialize(config, prompt, session)
               @show_response = config.show_response? #: bool
               @prompt = prompt
               @session = session
+              @fork_session = fork_session #: bool
             end
 
             #: () -> void
@@ -184,7 +185,10 @@ def command_line
               command.push("--model", @model) if @model
               command.push("--system-prompt", @replace_system_prompt) if @replace_system_prompt
               command.push("--append-system-prompt", @append_system_prompt) if @append_system_prompt
-              command.push("--fork-session", "--resume", @session) if @session.present?
+              if @session.present?
+                command.push("--fork-session") if @fork_session
+                command.push("--resume", @session)
+              end
               command << "--dangerously-skip-permissions" unless @apply_permissions
               command
             end
diff --git a/test/roast/cogs/agent/providers/claude/claude_invocation_test.rb b/test/roast/cogs/agent/providers/claude/claude_invocation_test.rb
index 2314ede2..bbb2ef6c 100644
--- a/test/roast/cogs/agent/providers/claude/claude_invocation_test.rb
+++ b/test/roast/cogs/agent/providers/claude/claude_invocation_test.rb
@@ -233,7 +233,7 @@ def failure_status
               assert_equal "Additional instructions", command[prompt_index + 1]
             end
 
-            test "command_line includes session flags when session is set" do
+            test "command_line includes fork-session and resume when session is set" do
               invocation = ClaudeInvocation.new(@config, "Test prompt", "session_123")
 
               command = invocation.send(:command_line)
@@ -244,6 +244,26 @@ def failure_status
               assert_equal "session_123", command[resume_index + 1]
             end
 
+            test "command_line includes resume without fork-session when fork_session is false" do
+              invocation = ClaudeInvocation.new(@config, "Test prompt", "session_123", fork_session: false)
+
+              command = invocation.send(:command_line)
+
+              refute_includes command, "--fork-session"
+              assert_includes command, "--resume"
+              resume_index = command.index("--resume")
+              assert_equal "session_123", command[resume_index + 1]
+            end
+
+            test "command_line omits fork-session when no session is given even if fork_session is true" do
+              invocation = ClaudeInvocation.new(@config, "Test prompt", nil, fork_session: true)
+
+              command = invocation.send(:command_line)
+
+              refute_includes command, "--fork-session"
+              refute_includes command, "--resume"
+            end
+
             test "command_line includes dangerously-skip-permissions when permissions skipped" do
               @config.skip_permissions!
               command = @invocation.send(:command_line)
diff --git a/test/roast/cogs/agent/providers/claude_test.rb b/test/roast/cogs/agent/providers/claude_test.rb
index 9237bece..9a441dd3 100644
--- a/test/roast/cogs/agent/providers/claude_test.rb
+++ b/test/roast/cogs/agent/providers/claude_test.rb
@@ -106,6 +106,43 @@ def mock_status(success:)
             assert_equal ["session_from_first"], sessions_seen
           end
 
+          test "invoke does not fork session for subsequent invocations" do
+            input = Agent::Input.new
+            input.prompts = ["Main task", "Finalizer"]
+
+            fork_flags = []
+            call_count = 0
+            CommandRunner.stubs(:execute).with do |args, **kwargs|
+              call_count += 1
+              fork_flags << args.include?("--fork-session")
+              result_json = { type: "result", subtype: "success", result: "done", session_id: "session_1" }.to_json
+              kwargs[:stdout_handler]&.call(result_json)
+              true
+            end.returns(["", "", mock_status(success: true)])
+
+            @provider.invoke(input)
+
+            assert_equal [false, false], fork_flags
+          end
+
+          test "invoke forks session for first invocation when input has session" do
+            input = Agent::Input.new
+            input.prompts = ["Main task", "Finalizer"]
+            input.session = "external_session"
+
+            fork_flags = []
+            CommandRunner.stubs(:execute).with do |args, **kwargs|
+              fork_flags << args.include?("--fork-session")
+              result_json = { type: "result", subtype: "success", result: "done", session_id: "new_session" }.to_json
+              kwargs[:stdout_handler]&.call(result_json)
+              true
+            end.returns(["", "", mock_status(success: true)])
+
+            @provider.invoke(input)
+
+            assert_equal [true, false], fork_flags
+          end
+
           test "invoke stops on first failed invocation" do
             input = Agent::Input.new
             input.prompts = ["Main task", "Finalizer 1", "Finalizer 2"]