diff --git a/packages/daemon/src/session/session-manager.ts b/packages/daemon/src/session/session-manager.ts
index cf957230c..ba6fa15b6 100644
--- a/packages/daemon/src/session/session-manager.ts
+++ b/packages/daemon/src/session/session-manager.ts
@@ -817,8 +817,17 @@ export class SessionManager {
     // reach humans, post at a channel root, or reply into a parent session. It has no
     // visible in-thread form: speaking in the current conversation is an ordinary reply.
     // `toAgent` without a `channel` is the postless, channel-invisible wake.
+    // The precedence bullet leads for a measured reason (issue #800): on the Claude
+    // Code runtime the session also carries the runtime's own built-in `SendMessage`
+    // (agent-teams messaging) — a literal name match for a report-back instruction —
+    // and a child that picks it loses its parent report silently. Costs ~80 standing
+    // tokens per session.
     const collabAppend =
       `# Collaborating with other agents\n` +
+      `- AgentConnect's tools (the \`agentconnect\` MCP server, e.g. \`mcp__agentconnect__sendMessage\`) are the ` +
+      `ONLY channel that reaches other agents and humans here. Your runtime may offer built-in tools with similar ` +
+      `names (e.g. a bare \`SendMessage\`) — those do NOT reach AgentConnect and anything sent through them is ` +
+      `lost. Never use them for messaging, reporting back, or collaboration.\n` +
       `- To reach a specific agent privately, call \`sendMessage\` with ` +
       `\`{"toAgent":"<agent id>","message":"..."}\` — it wakes ONLY that agent, delivered directly to it ` +
       `(nothing is posted to the channel). That bare form is FIRE-AND-FORGET: the peer answers inside its own ` +
diff --git a/packages/daemon/test/session-manager.test.ts b/packages/daemon/test/session-manager.test.ts
index 96dccde66..1b6bba82e 100644
--- a/packages/daemon/test/session-manager.test.ts
+++ b/packages/daemon/test/session-manager.test.ts
@@ -1934,6 +1934,30 @@ describe('SessionManager — collaboration preamble', () => {
     store.close()
   })
 
+  it('leads the guidance with the tool-precedence rule naming the built-in SendMessage hazard (issue #800)', async () => {
+    // Measured (tool-surface A/B, 2026-08-09): a Claude Code child 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. The standing context must say, before anything
+    // else about collaboration, that AgentConnect's MCP tools are the only
+    // channel that reaches anyone and that similarly-named built-ins are lost.
+    const store = newStore()
+    const host = { newSession: vi.fn(async () => 'acp-1') } as any
+    const sm = new SessionManager({ store, hostFor: async () => host, agentById: () => agent, memory })
+    const { blocks } = await sm.handle('bot-a', msg({ ts: '100.1', text: 'hi' }))
+    const first = (blocks[0] as any).text as string
+    expect(first).toContain('ONLY channel that reaches other agents and humans')
+    expect(first).toContain('mcp__agentconnect__sendMessage')
+    expect(first).toContain('a bare `SendMessage`')
+    expect(first).toContain('do NOT reach AgentConnect')
+    expect(first).toContain('Never use them for messaging, reporting back, or collaboration')
+    // It LEADS the collaboration section: the collision fires exactly when the
+    // model is choosing a messaging tool, so the warning must come first.
+    const section = first.slice(first.indexOf('# Collaborating with other agents'))
+    expect(section.indexOf('ONLY channel that reaches')).toBeLessThan(section.indexOf('To reach a specific agent'))
+    store.close()
+  })
+
   it('states the needsReply rule in the standing context, not only in the tool descriptor', async () => {
     // The descriptor is one input among many; this context is ALWAYS present and used to
     // present the bare `toAgent` form as the normal way to reach a peer privately, with no