diff --git a/internal/commands/cards.go b/internal/commands/cards.go
index c15a58cf..4877b6e7 100644
--- a/internal/commands/cards.go
+++ b/internal/commands/cards.go
@@ -7,6 +7,7 @@ import (
 	"math"
 	"strconv"
 	"strings"
+	"time"
 
 	"github.com/spf13/cobra"
 
@@ -1108,10 +1109,26 @@ You can pass either a card ID or a Basecamp URL:
 			}
 
 			// Attachment paths are readable or not regardless of the body, so
-			// check them before the pipe is drained.
+			// check them before the pipe is drained. The due date too:
+			// dateparse.Parse returns unrecognized input unchanged, so a bad
+			// value fails only at the server, after the producer is spent.
+			// todos update already rejects it locally; match that.
 			if err := validateAttachPaths(attachFiles); err != nil {
 				return err
 			}
+			// Every non-empty value is parsed, not just non-blank ones: the
+			// no-change guard above tests due == "", so a whitespace-only
+			// --due passes it, and dateparse.Parse trims that to an empty
+			// date. Parsing it here answers "Invalid due date" instead of
+			// sending an update with nothing in it. Surrounding whitespace on
+			// a real date is already handled by the parser.
+			var parsedDue string
+			if due != "" {
+				parsedDue = dateparse.Parse(due)
+				if _, err := time.Parse("2006-01-02", parsedDue); err != nil {
+					return output.ErrUsage(fmt.Sprintf("Invalid due date: %q", due))
+				}
+			}
 
 			// Syntactic checks first, then "-", then account and network: a
 			// malformed ID is answered without waiting on the producer, and a
@@ -1160,9 +1177,8 @@ You can pass either a card ID or a Basecamp URL:
 			if html != "" {
 				req.Content = &html
 			}
-			if due != "" {
-				dueOn := dateparse.Parse(due)
-				req.DueOn = &dueOn
+			if parsedDue != "" {
+				req.DueOn = &parsedDue
 			}
 			if cmd.Flags().Changed("assignee") {
 				assigneeID, err := resolveAssigneeID(cmd.Context(), app, assignee)
diff --git a/internal/commands/comment.go b/internal/commands/comment.go
index 16348cb5..bbc51ac8 100644
--- a/internal/commands/comment.go
+++ b/internal/commands/comment.go
@@ -1167,10 +1167,16 @@ busybox-ash) it posts a literal leading $ and keeps \n as backslash-n:
 			}
 
 			// Attachment paths are readable or not regardless of the body, so
-			// check them before the pipe is drained.
+			// check them before the pipe is drained. So is whether any target
+			// is even a number: the loop below tolerates individual bad IDs so
+			// a mixed batch still posts, but when none can parse the invocation
+			// creates nothing, and that is knowable from the argument alone.
 			if err := validateAttachPaths(attachFiles); err != nil {
 				return err
 			}
+			if err := requireOneParseableTarget(recordingArg); err != nil {
+				return err
+			}
 
 			var content string
 			if len(args) > 1 {
diff --git a/internal/commands/files.go b/internal/commands/files.go
index b00a9f41..4b117bb4 100644
--- a/internal/commands/files.go
+++ b/internal/commands/files.go
@@ -1272,7 +1272,7 @@ Use - as the content argument to read the document body from stdin:
 
 			// Resolve "-" before any account or network work, so a bad stdin
 			// gets the stdin error rather than "--account is required".
-			if err := rejectSubscribeConflict(cmd.Flags().Changed("subscribe"), noSubscribe); err != nil {
+			if err := rejectSubscribeConflict(cmd.Flags().Changed("subscribe"), noSubscribe, subscribe); err != nil {
 				return err
 			}
 			if err := requireNumericID(*vaultID, "folder ID"); err != nil {
diff --git a/internal/commands/helpers.go b/internal/commands/helpers.go
index d4164d4e..2854d102 100644
--- a/internal/commands/helpers.go
+++ b/internal/commands/helpers.go
@@ -443,6 +443,32 @@ func extractIDs(args []string) []string {
 	return urlarg.ExtractIDs(args)
 }
 
+// requireOneParseableTarget rejects a recording argument whose every
+// comma-separated token fails to parse. Callers tolerate individual bad IDs so
+// a mixed batch still posts what it can, but an all-invalid argument creates
+// nothing — and extractIDs is pure, so that is decidable from the argument
+// alone, before a "-" drains the producer.
+func requireOneParseableTarget(arg string) error {
+	for _, id := range extractIDs([]string{arg}) {
+		if _, err := strconv.ParseInt(id, 10, 64); err == nil {
+			return nil
+		}
+	}
+	return output.ErrUsage(fmt.Sprintf("no valid recording ID in %q", arg))
+}
+
+// hasPersonToken reports whether input holds at least one token resolvePersonIDs
+// would attempt to resolve. It splits the same way, so the pre-read guard and
+// the resolver cannot disagree about what counts as empty.
+func hasPersonToken(input string) bool {
+	for token := range strings.SplitSeq(input, ",") {
+		if strings.TrimSpace(token) != "" {
+			return true
+		}
+	}
+	return false
+}
+
 // resolvePersonIDs splits a comma-separated input string and resolves each
 // token (name, email, ID, or "me") to a person ID via the name resolver.
 func resolvePersonIDs(ctx context.Context, resolver *names.Resolver, input string) ([]int64, error) {
@@ -480,15 +506,21 @@ func resolvePersonIDs(ctx context.Context, resolver *names.Resolver, input strin
 // settle it first: draining a pipe for an invocation this rejects makes the
 // caller wait on a producer whose output is discarded, and lets a blank pipe
 // answer "stdin is empty" instead of naming the conflict.
-func rejectSubscribeConflict(subscribeChanged, noSubscribe bool) error {
+func rejectSubscribeConflict(subscribeChanged, noSubscribe bool, subscribe string) error {
 	if subscribeChanged && noSubscribe {
 		return output.ErrUsage("--subscribe and --no-subscribe are mutually exclusive")
 	}
+	// resolvePersonIDs skips blank tokens, so a value with no resolvable token
+	// can never name anyone — ",,," reaches the same error as "". Deciding it
+	// here rather than after the lookup keeps it ahead of any stdin read.
+	if subscribeChanged && !hasPersonToken(subscribe) {
+		return output.ErrUsage("--subscribe requires at least one person")
+	}
 	return nil
 }
 
 func applySubscribeFlags(ctx context.Context, resolver *names.Resolver, subscribe string, subscribeChanged, noSubscribe bool) (*[]int64, error) {
-	if err := rejectSubscribeConflict(subscribeChanged, noSubscribe); err != nil {
+	if err := rejectSubscribeConflict(subscribeChanged, noSubscribe, subscribe); err != nil {
 		return nil, err
 	}
 	if noSubscribe {
diff --git a/internal/commands/messages.go b/internal/commands/messages.go
index 3e6c1b94..f221d251 100644
--- a/internal/commands/messages.go
+++ b/internal/commands/messages.go
@@ -461,7 +461,7 @@ Use - as the body argument to read the body from stdin:
 			if edit && body != "" {
 				return output.ErrUsage("cannot combine --edit and body argument")
 			}
-			if err := rejectSubscribeConflict(cmd.Flags().Changed("subscribe"), noSubscribe); err != nil {
+			if err := rejectSubscribeConflict(cmd.Flags().Changed("subscribe"), noSubscribe, subscribe); err != nil {
 				return err
 			}
 			if err := requireNumericID(*messageBoard, "message board ID"); err != nil {
diff --git a/internal/commands/schedule.go b/internal/commands/schedule.go
index 31f7a5e9..b0787de8 100644
--- a/internal/commands/schedule.go
+++ b/internal/commands/schedule.go
@@ -441,7 +441,7 @@ func newScheduleCreateCmd(project, scheduleID *string) *cobra.Command {
 				return err
 			}
 
-			if err := rejectSubscribeConflict(cmd.Flags().Changed("subscribe"), noSubscribe); err != nil {
+			if err := rejectSubscribeConflict(cmd.Flags().Changed("subscribe"), noSubscribe, subscribe); err != nil {
 				return err
 			}
 			if err := requireNumericID(*scheduleID, "schedule ID"); err != nil {
diff --git a/internal/commands/stdin_integration_test.go b/internal/commands/stdin_integration_test.go
index 2332e422..3477cf88 100644
--- a/internal/commands/stdin_integration_test.go
+++ b/internal/commands/stdin_integration_test.go
@@ -387,6 +387,24 @@ func TestDeterministicFailuresRejectedBeforeReadingStdin(t *testing.T) {
 			[]string{"post", "-", "--content-type", "bogus"}, "unsupported --content-type"},
 		{"todos update bad due date", NewTodosCmd,
 			[]string{"update", "1", "--due", "not-a-date", "--description", "-"}, "Invalid due date"},
+		{"chat update bad room", NewChatCmd,
+			[]string{"update", "5", "-", "--room", "nope"}, "Invalid chat room ID"},
+		{"cards create bad card-table id", NewCardsCmd,
+			[]string{"create", "Title", "-", "--column", "Backlog", "--card-table", "nope"}, "Invalid card table ID"},
+		{"cards update bad due date", NewCardsCmd,
+			[]string{"update", "1", "--due", "not-a-date", "--body", "-"}, "Invalid due date"},
+		{"cards update whitespace-only due date", NewCardsCmd,
+			[]string{"update", "1", "--due", " ", "--body", "-"}, "Invalid due date"},
+		{"docs create delimiter-only subscribe", NewDocsCmd,
+			[]string{"documents", "create", "Title", "-", "--subscribe", ",,,"}, "requires at least one person"},
+		{"schedule create delimiter-only subscribe", NewScheduleCmd,
+			[]string{"create", "Title", "--starts-at", "2026-01-01T10:00:00Z", "--ends-at", "2026-01-01T11:00:00Z", "--subscribe", ", ,", "--description", "-"}, "requires at least one person"},
+		{"comments create all-invalid targets", NewCommentsCmd,
+			[]string{"create", "nope,alsonope", "-"}, "no valid recording ID"},
+		{"docs create blank subscribe", NewDocsCmd,
+			[]string{"documents", "create", "Title", "-", "--subscribe", ""}, "requires at least one person"},
+		{"messages create blank subscribe", NewMessagesCmd,
+			[]string{"create", "Title", "-", "--subscribe", ""}, "requires at least one person"},
 		{"chat update bad content-type", NewChatCmd,
 			[]string{"update", "1", "-", "--content-type", "bogus"}, "unsupported --content-type"},
 		{"boost bad id", NewBoostsCmd,