diff --git a/lib/librevox/listener/base.rb b/lib/librevox/listener/base.rb
index 4b96631..e990ba6 100644
--- a/lib/librevox/listener/base.rb
+++ b/lib/librevox/listener/base.rb
@@ -44,7 +44,18 @@ def send_message(msg)
         # while awaiting a reply would deadlock every other sender.
         @write_lock.acquire do
           @reply_promises << promise
-          @connection.send_data(msg)
+          begin
+            @connection.send_data(msg)
+          rescue Exception => error
+            # A promise may only stay queued if its command fully reached the
+            # wire — otherwise no reply will ever arrive for it, and every
+            # later reply would resolve the wrong sender's promise. Must catch
+            # Exception: a sender cancelled mid-send raises Async::Stop, which
+            # is not a StandardError.
+            @reply_promises.delete(promise)
+            promise.reject(error)
+            raise
+          end
         end
 
         reply = promise.wait
diff --git a/test/functional/librevox/listener/cancellation_poisoning_test.rb b/test/functional/librevox/listener/cancellation_poisoning_test.rb
new file mode 100644
index 0000000..a3fd3ff
--- /dev/null
+++ b/test/functional/librevox/listener/cancellation_poisoning_test.rb
@@ -0,0 +1,252 @@
+# frozen_string_literal: true
+
+require_relative '../../../test_helper'
+
+require 'librevox/listener/base'
+
+# A connection whose #send_data can be told, per-message, to park forever
+# (yielding repeatedly, like a real Async::Stream flush blocked on TCP
+# backpressure) until the parked fiber is stopped. This
+# lets a test deterministically catch a sender exactly *inside* send_data —
+# after Base#send_message has already pushed its promise onto the shared
+# @reply_promises FIFO, but before the bytes are recorded as "on the wire".
+#
+# Unlike InterleaveDetectingConnection (concurrency_test.rb), which only
+# yields once and always completes, this connection can be used to simulate
+# a sender that never finishes writing because its controller fiber gets
+# cancelled (Async::Stop) mid-send — the scenario the production framework
+# triggers constantly by killing a call's controller task on hangup.
+class BlockableConnection
+  attr_reader :data
+
+  def initialize
+    @data = []
+    @blocked = Hash.new(false)
+    @failing = Hash.new(false)
+    @entered = Hash.new(0)
+  end
+
+  # Mark +msg+ so that send_data(msg) raises IOError, simulating a write
+  # error on a socket that is otherwise still readable.
+  def fail!(msg)
+    @failing[msg] = true
+  end
+
+  # Mark +msg+ so that send_data(msg) parks indefinitely instead of
+  # completing, until the parked fiber is stopped.
+  def block!(msg)
+    @blocked[msg] = true
+  end
+
+  def entered?(msg)
+    @entered[msg] > 0
+  end
+
+  def send_data(msg)
+    @entered[msg] += 1
+
+    raise IOError, "simulated write failure" if @failing[msg]
+
+    if @blocked[msg]
+      Async::Task.current.yield while @blocked[msg]
+    else
+      Async::Task.current.yield # simulate the ordinary single flush yield
+    end
+
+    @data << msg
+  end
+
+  def close; end
+  def close_write; end
+end
+
+# Regression tests for cancellation-safety of the send path. Base#send_message
+# pushes the reply promise onto @reply_promises *before* @connection.send_data
+# returns; because replies are matched purely positionally
+# (@reply_promises.shift in receive_message), a promise left queued by a sender
+# that was cancelled mid-send would permanently shift reply correlation by one
+# for every later sender on the same shared connection. send_message must
+# therefore remove (and reject) the promise when send_data is interrupted.
+class TestCancellationPoisoning < Minitest::Test
+  prepend Librevox::Test::AsyncTest
+
+  def setup
+    @conn = BlockableConnection.new
+    @listener = Librevox::Listener::Base.new(@conn)
+  end
+
+  # Invariant under test: a sender's own send_message call resolves with its
+  # own reply, even after an unrelated concurrent sender was cancelled
+  # mid-send.
+  def test_stopping_a_sender_mid_send_data_does_not_eat_a_later_senders_reply
+    a_result = nil
+    c_result = nil
+
+    @conn.block!("B")
+
+    a = Async { a_result = @listener.send_message("A") }
+    b = Async { @listener.send_message("B") }
+
+    assert bounded_yield { @conn.entered?("B") },
+      "B never reached send_data — cannot set up the mid-send cancellation window"
+
+    # By now A must have completed (write-lock limit is 1, so B could only
+    # reach send_data after A released it) and B is parked forever inside
+    # send_data, holding a pushed promise with no corresponding bytes ever
+    # recorded on the wire.
+    assert_equal ["A"], @conn.data,
+      "setup invariant broken: A should already be on the wire before B could start"
+
+    # The framework cancels B's controller fiber mid-send (e.g. the caller
+    # hung up while B's command was still being flushed).
+    b.stop
+
+    c = Async { c_result = @listener.send_message("C") }
+
+    assert bounded_yield { @conn.data.size == 2 },
+      "C never reached the wire after B's cancellation released the write-lock"
+    assert_equal ["A", "C"], @conn.data,
+      "B's bytes must never appear on the wire — it was cancelled mid-send"
+
+    # FreeSWITCH replies once per command that actually arrived: A and C.
+    reply!("+OK reply-for-A")
+    reply!("+OK reply-for-C")
+
+    assert bounded_yield { a_result },
+      "A never got its reply back"
+    assert_equal "+OK reply-for-A", a_result.headers[:reply_text]
+
+    resolved = bounded_yield { c_result }
+    assert resolved,
+      "C's send_message never returned — its reply promise was starved. " \
+      "B's cancelled-mid-send promise was not removed from @reply_promises, " \
+      "so the reply meant for C (the 2nd real reply on the wire) was shifted " \
+      "onto B's orphaned promise instead, leaving C's promise permanently " \
+      "unresolved."
+
+    assert_equal "+OK reply-for-C", c_result.headers[:reply_text],
+      "C's send_message resolved with the wrong reply — its own reply was " \
+      "consumed by B's orphaned ghost promise"
+  ensure
+    a&.stop
+    b&.stop
+    c&.stop
+  end
+
+  # Same scenario, one sender deeper: without cleanup, one orphaned promise
+  # shifts *every* later sender's reply onto the previous sender's promise,
+  # forever (until connection_closed / reconnect) — not just the one sender
+  # that raced with the cancellation. With cleanup, all remaining senders
+  # keep receiving their own replies.
+  def test_reply_routing_stays_correct_for_all_senders_after_a_cancellation
+    a_result = nil
+    c_result = nil
+    d_result = nil
+
+    @conn.block!("B")
+
+    a = Async { a_result = @listener.send_message("A") }
+    b = Async { @listener.send_message("B") }
+
+    assert bounded_yield { @conn.entered?("B") },
+      "B never reached send_data — cannot set up the mid-send cancellation window"
+
+    b.stop # B cancelled mid-send; its promise is now a permanent orphan
+
+    c = Async { c_result = @listener.send_message("C") }
+    assert bounded_yield { @conn.data.size == 2 }, "C never reached the wire"
+
+    d = Async { d_result = @listener.send_message("D") }
+    assert bounded_yield { @conn.data.size == 3 }, "D never reached the wire"
+
+    assert_equal ["A", "C", "D"], @conn.data,
+      "exactly 3 real commands reached the wire (B never did)"
+
+    # FreeSWITCH sends exactly one reply per command that actually arrived:
+    # 3 replies, for A, C, D in that order.
+    reply!("+OK reply-1") # meant for A
+    reply!("+OK reply-2") # meant for C -- consumed by B's ghost instead
+    reply!("+OK reply-3") # meant for D -- consumed by C's promise instead
+
+    assert bounded_yield { a_result }, "A never got a reply"
+    assert_equal "+OK reply-1", a_result.headers[:reply_text]
+
+    resolved = bounded_yield { c_result }
+    assert resolved, "C's send_message never returned within the bounded wait"
+    assert_equal "+OK reply-2", c_result.headers[:reply_text],
+      "C should resolve with its own reply (reply-2), not a later sender's reply " \
+      "— cross-talk caused by B's orphaned ghost promise shifting the FIFO"
+
+    resolved = bounded_yield { d_result }
+    assert resolved,
+      "D's send_message never returned — once one promise is orphaned, the " \
+      "newest sender is always the one left hanging (the permanent, silent " \
+      "stall seen in production)"
+    assert_equal "+OK reply-3", d_result.headers[:reply_text],
+      "D should resolve with its own reply (reply-3)"
+  ensure
+    a&.stop
+    b&.stop
+    c&.stop
+    d&.stop
+  end
+
+  # A send that fails with an exception (rather than being cancelled) must
+  # take the same cleanup path: the error propagates to the sender, and the
+  # orphaned promise must not stay behind to eat a later sender's reply.
+  def test_a_failing_send_does_not_leave_its_promise_in_the_reply_queue
+    a_result = nil
+    c_result = nil
+    errors = []
+
+    @conn.fail!("B")
+
+    a = Async { a_result = @listener.send_message("A") }
+    b = Async do
+      @listener.send_message("B")
+    rescue IOError => e
+      errors << e
+    end
+    c = Async { c_result = @listener.send_message("C") }
+
+    assert bounded_yield { errors.size == 1 },
+      "the write failure must propagate to B's sender"
+
+    assert bounded_yield { @conn.data.size == 2 }
+    assert_equal ["A", "C"], @conn.data
+
+    reply!("+OK reply-for-A")
+    reply!("+OK reply-for-C")
+
+    assert bounded_yield { a_result && c_result },
+      "A and C must both resolve — B's failed send must not occupy a FIFO slot"
+    assert_equal "+OK reply-for-A", a_result.headers[:reply_text]
+    assert_equal "+OK reply-for-C", c_result.headers[:reply_text]
+  ensure
+    a&.stop
+    b&.stop
+    c&.stop
+  end
+
+  private
+
+  # Yields to other fibers up to +max+ times, checking the block after each
+  # yield, so a broken invariant fails fast with a clear assertion message
+  # instead of hanging the test (and the whole suite) forever.
+  def bounded_yield(max: 10_000)
+    return true if yield
+
+    max.times do
+      Async::Task.current.yield
+      return true if yield
+    end
+
+    false
+  end
+
+  def reply!(text)
+    @listener.receive_message(
+      Librevox::Protocol::Response.new("Content-Type: command/reply\nReply-Text: #{text}", "")
+    )
+  end
+end