diff --git a/packages/daemon/src/session/session-manager.ts b/packages/daemon/src/session/session-manager.ts
index c2664eaca..55cf20019 100644
--- a/packages/daemon/src/session/session-manager.ts
+++ b/packages/daemon/src/session/session-manager.ts
@@ -858,12 +858,26 @@ export class SessionManager {
     // guidance rather than in the delivered text (which the model may summarize away). Deliberately
     // scoped to a terminal report: nothing here asks for progress narration, which would turn every
     // delegated task into channel chatter.
+    //
+    // The full-name/built-in warning and the this-call-is-complete sentence live HERE, not in the
+    // standing collaboration guidance, on purpose (issue #800): on the Claude Code runtime the
+    // session also carries the runtime's own built-in `SendMessage` — a literal name match for a
+    // report-back instruction — and a child that picks it loses its parent report silently; children
+    // also tended to shotgun a redundant direct `toAgent` wake around the correct parent reply.
+    // Both hazards exist exactly (and only) in needsParentReply sessions, so the directive is the
+    // niche injection point: in-thread conversations never see this text, which is what makes the
+    // #801 regression class ("plain replies reach nobody" taught session-wide, reverted by #861)
+    // structurally impossible here.
     const parentReplyAppend = needsReplyToParent
       ? `# Reporting back to your parent session\n` +
         `Another session delegated this work to you and is waiting on the outcome. When you finish — or when you ` +
         `cannot finish — reply to it with ` +
         `\`sendMessage\` \`{"sessionId":"${effectiveOriginSessionId}","message":"..."}\`, saying whether you ` +
-        `succeeded or failed and what the result was (on failure, what went wrong). Send it exactly once, at the ` +
+        `succeeded or failed and what the result was (on failure, what went wrong). Use exactly this tool — ` +
+        `\`mcp__agentconnect__sendMessage\` — by its full name: your runtime may offer a similarly-named ` +
+        `built-in (a bare \`SendMessage\`) that does NOT reach AgentConnect, and anything sent through it is ` +
+        `lost. The parent session IS the agent that delegated this to you, and this one call is the complete ` +
+        `delivery — do not also message that agent directly (no \`toAgent\` wake, no DM). Send it exactly once, at the ` +
         `end; do not report progress along the way, and do not skip it because the task was small or unsuccessful. ` +
         `Your ordinary assistant response in this child session is not delivered to the parent. Do not write the ` +
         `result before or after the tool call; after the tool reports successful delivery, end your turn immediately ` +
diff --git a/packages/daemon/test/session-manager.test.ts b/packages/daemon/test/session-manager.test.ts
index 13d1652d8..46c406cf2 100644
--- a/packages/daemon/test/session-manager.test.ts
+++ b/packages/daemon/test/session-manager.test.ts
@@ -737,6 +737,16 @@ describe('SessionManager', () => {
       expect(metaArg).toContain('{"sessionId":"origin-sess-9","message":"..."}')
       expect(metaArg).toContain('after the tool reports successful delivery, end your turn immediately')
       expect(metaArg).toContain('without repeating the message')
+      // #800 collision warning, scoped to THIS directive (niche: only needsParentReply
+      // sessions carry it — never the session-wide guidance, per the #801/#861 lesson):
+      // the full tool name, and the runtime's similarly-named built-in reaches nothing.
+      expect(metaArg).toContain('`mcp__agentconnect__sendMessage` — by its full name')
+      expect(metaArg).toContain('a bare `SendMessage`')
+      expect(metaArg).toContain('anything sent through it is lost')
+      // Dual-identity clarification: the parent session IS the delegating agent, and the
+      // one sessionId call is the complete delivery — no redundant direct wake or DM.
+      expect(metaArg).toContain('this one call is the complete delivery')
+      expect(metaArg).toContain('do not also message that agent directly')
       // Persisted, so later turns and resumes keep it.
       expect(store.getSession(sessionKey('slack', 'C1', '100.1', 'bot-a'))?.needsParentReply).toBe(1)
       store.close()