diff --git a/lib/httpapi/claude.go b/lib/httpapi/claude.go
index 641efe47..9ccefdd2 100644
--- a/lib/httpapi/claude.go
+++ b/lib/httpapi/claude.go
@@ -5,6 +5,11 @@ import (
 	st "github.com/coder/agentapi/lib/screentracker"
 )
 
+// formatPaste wraps message in bracketed paste escape sequences.
+// These sequences start with ESC (\x1b), which TUI selection
+// widgets (e.g. Claude Code's numbered-choice prompt) interpret
+// as "cancel". For selection prompts, callers should use
+// MessageTypeRaw to send raw keystrokes directly instead.
 func formatPaste(message string) []st.MessagePart {
 	return []st.MessagePart{
 		// Bracketed paste mode start sequence
diff --git a/lib/screentracker/pty_conversation.go b/lib/screentracker/pty_conversation.go
index cce8c677..80c9b942 100644
--- a/lib/screentracker/pty_conversation.go
+++ b/lib/screentracker/pty_conversation.go
@@ -3,6 +3,7 @@ package screentracker
 import (
 	"context"
 	"encoding/json"
+	"errors"
 	"fmt"
 	"log/slog"
 	"os"
@@ -16,6 +17,26 @@ import (
 	"golang.org/x/xerrors"
 )
 
+const (
+	// writeStabilizeEchoTimeout is the timeout for the echo
+	// detection WaitFor loop in writeStabilize Phase 1. The
+	// effective ceiling may be slightly longer because the 1s
+	// stability check inside the condition runs outside
+	// WaitFor's timeout select. Non-echoing agents (e.g. TUI
+	// agents using bracketed paste) will hit this timeout,
+	// which is non-fatal.
+	//
+	// TODO: move to PTYConversationConfig if agents need
+	// different echo detection windows.
+	writeStabilizeEchoTimeout = 2 * time.Second
+
+	// writeStabilizeProcessTimeout is the maximum time to wait
+	// for the screen to change after sending a carriage return.
+	// This detects whether the agent is actually processing the
+	// input.
+	writeStabilizeProcessTimeout = 15 * time.Second
+)
+
 // A screenSnapshot represents a snapshot of the PTY at a specific time.
 type screenSnapshot struct {
 	timestamp time.Time
@@ -411,7 +432,19 @@ func (c *PTYConversation) sendMessage(ctx context.Context, messageParts ...Messa
 	return nil
 }
 
-// writeStabilize writes messageParts to the screen and waits for the screen to stabilize after the message is written.
+// writeStabilize writes messageParts to the PTY and waits for
+// the agent to process them. It operates in two phases:
+//
+// Phase 1 (echo detection): writes the message text and waits
+// for the screen to change and stabilize. This detects agents
+// that echo typed input. If the screen doesn't change within
+// writeStabilizeEchoTimeout, this is non-fatal — many TUI
+// agents buffer bracketed-paste input without rendering it.
+//
+// Phase 2 (processing detection): writes a carriage return
+// and waits for the screen to change, indicating the agent
+// started processing. This phase is fatal on timeout — if the
+// agent doesn't react to Enter, it's unresponsive.
 func (c *PTYConversation) writeStabilize(ctx context.Context, messageParts ...MessagePart) error {
 	screenBeforeMessage := c.cfg.AgentIO.ReadScreen()
 	for _, part := range messageParts {
@@ -421,7 +454,7 @@ func (c *PTYConversation) writeStabilize(ctx context.Context, messageParts ...Me
 	}
 	// wait for the screen to stabilize after the message is written
 	if err := util.WaitFor(ctx, util.WaitTimeout{
-		Timeout:     15 * time.Second,
+		Timeout:     writeStabilizeEchoTimeout,
 		MinInterval: 50 * time.Millisecond,
 		InitialWait: true,
 		Clock:       c.cfg.Clock,
@@ -441,14 +474,25 @@ func (c *PTYConversation) writeStabilize(ctx context.Context, messageParts ...Me
 		}
 		return false, nil
 	}); err != nil {
-		return xerrors.Errorf("failed to wait for screen to stabilize: %w", err)
+		if !errors.Is(err, util.WaitTimedOut) {
+			// Context cancellation or condition errors are fatal.
+			return xerrors.Errorf("failed to wait for screen to stabilize: %w", err)
+		}
+		// Phase 1 timeout is non-fatal: the agent may not echo
+		// input (e.g. TUI agents buffer bracketed-paste content
+		// internally). Proceed to Phase 2 to send the carriage
+		// return.
+		c.cfg.Logger.Info(
+			"echo detection timed out, sending carriage return",
+			"timeout", writeStabilizeEchoTimeout,
+		)
 	}
 
 	// wait for the screen to change after the carriage return is written
 	screenBeforeCarriageReturn := c.cfg.AgentIO.ReadScreen()
 	lastCarriageReturnTime := time.Time{}
 	if err := util.WaitFor(ctx, util.WaitTimeout{
-		Timeout:     15 * time.Second,
+		Timeout:     writeStabilizeProcessTimeout,
 		MinInterval: 25 * time.Millisecond,
 		Clock:       c.cfg.Clock,
 	}, func() (bool, error) {
diff --git a/lib/screentracker/pty_conversation_test.go b/lib/screentracker/pty_conversation_test.go
index 0e7d9635..8d65e064 100644
--- a/lib/screentracker/pty_conversation_test.go
+++ b/lib/screentracker/pty_conversation_test.go
@@ -8,6 +8,7 @@ import (
 	"log/slog"
 	"os"
 	"sync"
+	"sync/atomic"
 	"testing"
 	"time"
 
@@ -447,6 +448,130 @@ func TestMessages(t *testing.T) {
 		c, _, _ := newConversation(context.Background(), t)
 		assert.ErrorIs(t, c.Send(st.MessagePartText{Content: ""}), st.ErrMessageValidationEmpty)
 	})
+
+	t.Run("send-message-no-echo-agent-reacts", func(t *testing.T) {
+		ctx, cancel := context.WithTimeout(context.Background(), testTimeout)
+		t.Cleanup(cancel)
+
+		// Given: an agent that doesn't echo typed input but
+		// reacts to carriage return by updating the screen.
+		c, _, mClock := newConversation(ctx, t, func(cfg *st.PTYConversationConfig) {
+			a := &testAgent{screen: "prompt"}
+			a.onWrite = func(data []byte) {
+				if string(data) == "\r" {
+					a.screen = "processing..."
+				}
+			}
+			cfg.AgentIO = a
+		})
+		advanceFor(ctx, t, mClock, interval*threshold)
+
+		// When: a message is sent. Phase 1 times out (no echo),
+		// Phase 2 writes \r and the agent reacts.
+		sendAndAdvance(ctx, t, c, mClock, st.MessagePartText{Content: "hello"})
+
+		// Then: Send succeeds and the user message is recorded.
+		msgs := c.Messages()
+		require.True(t, len(msgs) >= 2)
+		var foundUserMsg bool
+		for _, msg := range msgs {
+			if msg.Role == st.ConversationRoleUser && msg.Message == "hello" {
+				foundUserMsg = true
+				break
+			}
+		}
+		assert.True(t, foundUserMsg, "expected user message 'hello' in conversation")
+	})
+
+	t.Run("send-message-no-echo-no-react", func(t *testing.T) {
+		ctx, cancel := context.WithTimeout(context.Background(), testTimeout)
+		t.Cleanup(cancel)
+
+		// Given: an agent that is completely unresponsive — it
+		// neither echoes input nor reacts to carriage return.
+		c, _, mClock := newConversation(ctx, t, func(cfg *st.PTYConversationConfig) {
+			a := &testAgent{screen: "prompt"}
+			a.onWrite = func(data []byte) {}
+			cfg.AgentIO = a
+		})
+		advanceFor(ctx, t, mClock, interval*threshold)
+
+		// When: a message is sent. Both Phase 1 (echo) and
+		// Phase 2 (processing) time out.
+		// Note: can't use sendAndAdvance here because it calls
+		// require.NoError internally.
+		var sendErr error
+		var sendDone atomic.Bool
+		go func() {
+			sendErr = c.Send(st.MessagePartText{Content: "hello"})
+			sendDone.Store(true)
+		}()
+		advanceUntil(ctx, t, mClock, func() bool { return sendDone.Load() })
+
+		// Then: Send fails with a Phase 2 error (not Phase 1).
+		require.Error(t, sendErr)
+		assert.Contains(t, sendErr.Error(), "failed to wait for processing to start")
+	})
+
+	t.Run("send-message-no-echo-context-cancelled", func(t *testing.T) {
+		// Given: a non-echoing agent and a cancellable context.
+		// The onWrite signals when writeStabilize starts writing
+		// message parts — this is used to synchronize the cancel.
+		sendCtx, sendCancel := context.WithCancel(context.Background())
+		t.Cleanup(sendCancel)
+
+		writeStarted := make(chan struct{}, 1)
+		c, _, mClock := newConversation(sendCtx, t, func(cfg *st.PTYConversationConfig) {
+			a := &testAgent{screen: "prompt"}
+			a.onWrite = func(data []byte) {
+				select {
+				case writeStarted <- struct{}{}:
+				default:
+				}
+			}
+			cfg.AgentIO = a
+		})
+		advanceFor(sendCtx, t, mClock, interval*threshold)
+
+		// When: a message is sent and the context is cancelled
+		// during Phase 1 (after the message is written to the
+		// PTY, before echo detection completes).
+		var sendErr error
+		var sendDone atomic.Bool
+		go func() {
+			sendErr = c.Send(st.MessagePartText{Content: "hello"})
+			sendDone.Store(true)
+		}()
+
+		// Advance tick-by-tick until writeStabilize starts
+		// (onWrite fires). This gives the send loop goroutine
+		// scheduling time between advances.
+		advanceUntil(sendCtx, t, mClock, func() bool {
+			select {
+			case <-writeStarted:
+				return true
+			default:
+				return false
+			}
+		})
+
+		// writeStabilize Phase 1 is now running. Its WaitFor is
+		// blocked on a mock timer sleep select. Cancel: the
+		// select sees ctx.Done() immediately.
+		sendCancel()
+
+		// WaitFor returns ctx.Err(). The errors.Is guard in
+		// Phase 1 propagates it as fatal. Use Eventually since
+		// the goroutine needs scheduling time.
+		require.Eventually(t, sendDone.Load, 5*time.Second, 10*time.Millisecond)
+
+		// Then: the error wraps context.Canceled, not a Phase 2
+		// timeout. This validates the errors.Is(WaitTimedOut)
+		// guard.
+		require.Error(t, sendErr)
+		assert.ErrorIs(t, sendErr, context.Canceled)
+		assert.NotContains(t, sendErr.Error(), "failed to wait for processing to start")
+	})
 }
 
 func TestStatePersistence(t *testing.T) {