Summary

When a message_sending plugin hook cancels outbound delivery (returns { cancel: true }), the message_sent hook is not fired. This means plugins tracking delivery outcomes — audit logging, analytics, delivery confirmation — have no visibility into cancelled sends.

Severity

Medium — Affects plugins relying on message_sent for comprehensive delivery tracking. The send is correctly prevented, but the cancellation is silent to downstream observers.

Steps to Reproduce

1. Register a plugin with both message_sending and message_sent hooks:

   api.on("message_sending", (event) => {
     if (shouldBlock(event.content)) return { cancel: true };
   });
   api.on("message_sent", (event) => {
     console.log("SENT HOOK:", event.success, event.error);
   });

2. Trigger an outbound message through deliverOutboundPayloads (proactive/tool send path).
3. Have the message_sending hook return { cancel: true }.

Expected

message_sent fires with { success: false, error: "canceled by message_sending hook" }.

Actual

message_sent does not fire. The cancellation is invisible to message_sent subscribers.

Root Cause

In src/infra/outbound/deliver.ts (line 488-489), when sendingResult?.cancel is true, the code does continue without calling emitMessageSent:

if (sendingResult?.cancel) {
  continue;  // BUG: skips emitMessageSent entirely
}

Context

• The text-only and sendPayload success paths for message_sent were fixed in commit 1d8bda4a2 (PR fix: complete message_sent hook dispatch coverage (follow-up to #14882) #15104), but the cancel path was overlooked.
• PR feat(hooks): wire outbound message lifecycle hooks #12584 (open, Greptile score 4/5) explicitly handles this case by firing message_sent(false, "canceled by message_sending hook") — the pattern we should follow.
• This is a residual gap from the original hook wiring in PR fix: wire 9 unwired plugin hooks to core code #14882.

Fix

One-line addition — fire emitMessageSent(false, "canceled by message_sending hook") before the continue:

if (sendingResult?.cancel) {
  emitMessageSent(false, "canceled by message_sending hook");
  continue;
}

A fix with 6 new tests (25 total passing) exists on branch fix/message-sent-hook-cancel-path.

Related

• message_sending hook not invoked for message tool sends (only agent responses) #16126 — message_sending hook not invoked for message tool sends
• message_sending and message_sent plugin hooks never fire on common text delivery paths #15389 — message_sending and message_sent hooks never fire on common text delivery paths
• [Bug]: Plugin hooks: 9 of 14 registered hook types are never invoked (silent no-op) #14571 — 9 of 14 registered hook types are never invoked
• PR fix: wire 9 unwired plugin hooks to core code #14882 — Original (incomplete) hook wiring
• PR fix: complete message_sent hook dispatch coverage (follow-up to #14882) #15104 — Partial fix for message_sent on success paths
• PR feat(hooks): wire outbound message lifecycle hooks #12584 — Comprehensive hook wiring (open, handles cancel path correctly)