diff --git a/internal/commands/assign.go b/internal/commands/assign.go
index 072b4db1..dc069b47 100644
--- a/internal/commands/assign.go
+++ b/internal/commands/assign.go
@@ -644,7 +644,10 @@ func doAssignStep(cmd *cobra.Command, app *appctx.App, stepIDStr, assigneeID str
 	}
 	assigneeIDs = append(assigneeIDs, assigneeIDInt)
 
+	// The API rejects step updates without a title, so carry over the
+	// current one.
 	updated, err := app.Account().CardSteps().Update(cmd.Context(), stepID, &basecamp.UpdateStepRequest{
+		Title:       step.Title,
 		AssigneeIDs: assigneeIDs,
 	})
 	if err != nil {
@@ -746,7 +749,10 @@ func doUnassignStep(cmd *cobra.Command, app *appctx.App, stepIDStr string, assig
 
 	assigneeIDs := removeID(existingAssigneeIDs(step.Assignees), assigneeIDInt)
 
+	// The API rejects step updates without a title, so carry over the
+	// current one.
 	updated, err := app.Account().CardSteps().Update(cmd.Context(), stepID, &basecamp.UpdateStepRequest{
+		Title:       step.Title,
 		AssigneeIDs: assigneeIDs,
 	})
 	if err != nil {
diff --git a/internal/commands/assign_test.go b/internal/commands/assign_test.go
index 181c7b71..1c455d88 100644
--- a/internal/commands/assign_test.go
+++ b/internal/commands/assign_test.go
@@ -3,6 +3,7 @@ package commands
 import (
 	"bytes"
 	"context"
+	"encoding/json"
 	"errors"
 	"fmt"
 	"io"
@@ -706,3 +707,82 @@ func TestAssignBatchAllFailNeverResolvesAssignee(t *testing.T) {
 			"person resolution should not be attempted when all items fail validation")
 	}
 }
+
+// mockStepAssignTransport serves the current step on GET and captures the
+// PUT body so the assign/unassign step paths can be checked for the
+// carried-over title the API requires.
+type mockStepAssignTransport struct {
+	capturedPut []byte
+}
+
+func (m *mockStepAssignTransport) RoundTrip(req *http.Request) (*http.Response, error) {
+	header := make(http.Header)
+	header.Set("Content-Type", "application/json")
+
+	stepJSON := `{"id": 456, "title": "Existing step", "completed": false, "assignees": [{"id": 11, "name": "Existing Person"}]}`
+
+	switch req.Method {
+	case "GET":
+		var body string
+		switch {
+		case strings.Contains(req.URL.Path, "/card_tables/steps/"):
+			body = stepJSON
+		case strings.Contains(req.URL.Path, "/projects.json"):
+			body = `[{"id": 123, "name": "Test Project"}]`
+		case strings.Contains(req.URL.Path, "/projects/"):
+			body = `{"id": 123, "dock": [{"name": "kanban_board", "id": 789, "title": "Card Table"}]}`
+		case strings.Contains(req.URL.Path, "/people.json"):
+			body = `[{"id": 11, "name": "Existing Person"}, {"id": 99, "name": "New Person"}]`
+		default:
+			return nil, fmt.Errorf("unexpected GET path: %s", req.URL.Path)
+		}
+		return &http.Response{StatusCode: 200, Body: io.NopCloser(strings.NewReader(body)), Header: header}, nil
+	case "PUT":
+		if !strings.HasSuffix(req.URL.Path, "/card_tables/steps/456") {
+			return nil, fmt.Errorf("unexpected PUT path: %s", req.URL.Path)
+		}
+		body, err := io.ReadAll(req.Body)
+		if err != nil {
+			return nil, err
+		}
+		m.capturedPut = body
+		if err := req.Body.Close(); err != nil {
+			return nil, err
+		}
+		return &http.Response{StatusCode: 200, Body: io.NopCloser(strings.NewReader(stepJSON)), Header: header}, nil
+	default:
+		return nil, fmt.Errorf("unexpected request method: %s", req.Method)
+	}
+}
+
+// TestAssignStepCarriesTitle verifies that assigning a person to a step sends
+// the current title in the update, which the API requires.
+func TestAssignStepCarriesTitle(t *testing.T) {
+	transport := &mockStepAssignTransport{}
+	app := setupCardsMockApp(t, transport)
+
+	cmd := NewAssignCmd()
+	err := executeAssignCommand(cmd, app, "456", "--step", "--to", "99")
+	require.NoError(t, err)
+
+	var body map[string]any
+	require.NoError(t, json.Unmarshal(transport.capturedPut, &body))
+	assert.Equal(t, "Existing step", body["title"])
+	assert.Equal(t, []any{float64(11), float64(99)}, body["assignee_ids"])
+}
+
+// TestUnassignStepCarriesTitle verifies that removing a person from a step
+// also sends the current title in the update.
+func TestUnassignStepCarriesTitle(t *testing.T) {
+	transport := &mockStepAssignTransport{}
+	app := setupCardsMockApp(t, transport)
+
+	cmd := NewUnassignCmd()
+	err := executeAssignCommand(cmd, app, "456", "--step", "--from", "11")
+	require.NoError(t, err)
+
+	var body map[string]any
+	require.NoError(t, json.Unmarshal(transport.capturedPut, &body))
+	assert.Equal(t, "Existing step", body["title"])
+	assert.Equal(t, []any{}, body["assignee_ids"])
+}
diff --git a/internal/commands/cards.go b/internal/commands/cards.go
index 49f52e4a..f64361ef 100644
--- a/internal/commands/cards.go
+++ b/internal/commands/cards.go
@@ -2009,6 +2009,14 @@ You can pass either a step ID or a Basecamp URL:
 			req := &basecamp.UpdateStepRequest{}
 			if title != "" {
 				req.Title = title
+			} else {
+				// The API rejects step updates without a title, so carry
+				// over the current one when only other fields change.
+				current, err := app.Account().CardSteps().Get(cmd.Context(), stepID)
+				if err != nil {
+					return convertSDKError(err)
+				}
+				req.Title = current.Title
 			}
 			if dueOn != "" {
 				req.DueOn = dateparse.Parse(dueOn)
diff --git a/internal/commands/cards_test.go b/internal/commands/cards_test.go
index e6ba6884..473de00d 100644
--- a/internal/commands/cards_test.go
+++ b/internal/commands/cards_test.go
@@ -189,6 +189,93 @@ func TestCardsStepUpdateRequiresFields(t *testing.T) {
 	assert.NoError(t, err, "expected help output, not error")
 }
 
+// mockStepUpdateTransport serves the current step on GET and captures the
+// update body on PUT. It only answers the single-step endpoint so a stray
+// call to the wrong path fails the test instead of passing on stale data.
+type mockStepUpdateTransport struct {
+	getCount    int
+	capturedPut []byte
+}
+
+func (t *mockStepUpdateTransport) RoundTrip(req *http.Request) (*http.Response, error) {
+	header := make(http.Header)
+	header.Set("Content-Type", "application/json")
+
+	if !strings.HasSuffix(req.URL.Path, "/card_tables/steps/456") {
+		return nil, fmt.Errorf("unexpected request path: %s", req.URL.Path)
+	}
+
+	switch req.Method {
+	case "GET":
+		t.getCount++
+	case "PUT":
+		body, err := io.ReadAll(req.Body)
+		if err != nil {
+			return nil, err
+		}
+		t.capturedPut = body
+		if err := req.Body.Close(); err != nil {
+			return nil, err
+		}
+	default:
+		return nil, fmt.Errorf("unexpected request method: %s", req.Method)
+	}
+
+	// Echo the title back so callers see the effective value, not a constant.
+	title := "Current title"
+	if t.capturedPut != nil {
+		var put struct {
+			Title string `json:"title"`
+		}
+		if err := json.Unmarshal(t.capturedPut, &put); err == nil && put.Title != "" {
+			title = put.Title
+		}
+	}
+	stepJSON := fmt.Sprintf(`{"id": 456, "title": %q, "completed": false, "assignees": []}`, title)
+
+	return &http.Response{
+		StatusCode: 200,
+		Body:       io.NopCloser(strings.NewReader(stepJSON)),
+		Header:     header,
+	}, nil
+}
+
+// TestCardsStepUpdateAssigneesOnlyCarriesTitle verifies that updating only
+// assignees fetches the current step and includes its title in the request —
+// the API rejects step updates without a title.
+func TestCardsStepUpdateAssigneesOnlyCarriesTitle(t *testing.T) {
+	transport := &mockStepUpdateTransport{}
+	app := setupCardsMockApp(t, transport)
+
+	cmd := newCardsStepUpdateCmd()
+	err := executeCommand(cmd, app, "456", "--assignees", "789")
+	require.NoError(t, err)
+
+	assert.Equal(t, 1, transport.getCount)
+
+	var body map[string]any
+	require.NoError(t, json.Unmarshal(transport.capturedPut, &body))
+	assert.Equal(t, "Current title", body["title"])
+	assert.Equal(t, []any{float64(789)}, body["assignee_ids"])
+}
+
+// TestCardsStepUpdateWithTitleSkipsFetch verifies that an explicit title is
+// sent as-is without fetching the current step.
+func TestCardsStepUpdateWithTitleSkipsFetch(t *testing.T) {
+	transport := &mockStepUpdateTransport{}
+	app := setupCardsMockApp(t, transport)
+
+	cmd := newCardsStepUpdateCmd()
+	err := executeCommand(cmd, app, "456", "New title")
+	require.NoError(t, err)
+
+	assert.Equal(t, 0, transport.getCount)
+
+	var body map[string]any
+	require.NoError(t, json.Unmarshal(transport.capturedPut, &body))
+	assert.Equal(t, "New title", body["title"])
+}
+
 // TestCardsStepMoveRequiresCard tests that --card is required for step move.
 func TestCardsStepMoveShowsHelp(t *testing.T) {
 	app, _ := setupTestApp(t)