diff --git a/chat/services/prompt.py b/chat/services/prompt.py
index 1df074b..48a0a7f 100644
--- a/chat/services/prompt.py
+++ b/chat/services/prompt.py
@@ -37,6 +37,7 @@ import tiktoken
 from chat.llm.client import Message
 from chat.state.edges import get_edge, list_edges_for
 from chat.state.entities import get_bot, get_you
+from chat.state.group_node import get_group_node
 from chat.state.memory import search_memories
 from chat.state.world import (
     active_scene,
@@ -206,6 +207,26 @@ def _build_previous_scene_block(pov_summary: str | None) -> str | None:
     return "PREVIOUS SCENE SUMMARY:\n" + pov_summary
 
 
+def _build_group_node_block(group_node: dict | None) -> str | None:
+    """Render the group-node summary + dynamic as a SHOULD-tier block.
+
+    Used only in 3-entity scenes (you + host + guest). Returns None when
+    the row is missing or both summary and dynamic are empty.
+    """
+    if not group_node:
+        return None
+    summary = (group_node.get("summary") or "").strip()
+    dynamic = (group_node.get("dynamic") or "").strip()
+    if not summary and not dynamic:
+        return None
+    lines = ["Group dynamic:"]
+    if summary:
+        lines.append(f"- Summary: {summary}")
+    if dynamic:
+        lines.append(f"- Dynamic: {dynamic}")
+    return "\n".join(lines)
+
+
 def _closing_instruction(speaker_name: str, addressee_name: str) -> str:
     return (
         f"Continue the scene as {speaker_name}, in their voice, responding "
@@ -287,6 +308,7 @@ def assemble_narrative_prompt(
     budget_soft: int = 6000,
     budget_hard: int = 8000,
     encoding_name: str = "cl100k_base",
+    guest_id: str | None = None,
 ) -> list[Message]:
     """Assemble the narrative prompt for ``speaker_bot_id`` to respond.
 
@@ -313,6 +335,15 @@ def assemble_narrative_prompt(
     if chat is None:
         raise ValueError(f"chat_id {chat_id!r} not found")
 
+    # Auto-detect guest from chat state when caller didn't pass one.
+    # Phase 1 chats have ``guest_bot_id is None``; the auto-detect is a
+    # no-op there and the function behaves exactly as before.
+    if guest_id is None:
+        guest_id = chat.get("guest_bot_id")
+    # A speaker addressing themself as guest doesn't add a third party.
+    if guest_id is not None and guest_id == speaker_bot_id:
+        guest_id = None
+
     you = get_you(conn)
     addressee_id, addressee_name = _resolve_addressee(conn, addressee, you)
 
@@ -325,9 +356,10 @@ def assemble_narrative_prompt(
         addressee_name,
     )
 
-    # Activity for present entities. Phase 1: you + speaker bot. (When a
-    # guest is added in Phase 1+, callers that know about it can pass
-    # extra activities via a future hook; for now we keep it strict.)
+    # Activity for present entities. Core (MUST): you + speaker bot.
+    # Phase 2 (SHOULD-tier): when a third party (guest) is present in
+    # the chat, append their activity in a separate block so it can be
+    # trimmed independently under tight budget.
     activities: list[dict] = []
     you_act = get_activity(conn, "you")
     if you_act is not None:
@@ -341,6 +373,34 @@ def assemble_narrative_prompt(
         activities.append(bot_act)
     activity_block = _build_activity_block(activities)
 
+    # SHOULD-tier guest activity extension (Phase 2 / Task 43).
+    guest_activity_block: str | None = None
+    if guest_id is not None:
+        guest_act = get_activity(conn, guest_id)
+        if guest_act is not None:
+            guest_act = dict(guest_act)
+            guest_bot = get_bot(conn, guest_id)
+            guest_act["_display_name"] = (
+                guest_bot["name"] if guest_bot else guest_id
+            )
+            guest_activity_block = _build_activity_block([guest_act])
+
+    # SHOULD-tier group-node block (Phase 2 / Task 43): rendered only
+    # when the group_node row is present AND it covers all three of
+    # you + host + guest (per the Task 43 spec).
+    group_node_block: str | None = None
+    if guest_id is not None:
+        gn = get_group_node(conn, chat_id)
+        if gn is not None:
+            members = set(gn.get("members") or [])
+            host_id = chat.get("host_bot_id")
+            required = {"you"}
+            if host_id is not None:
+                required.add(host_id)
+            required.add(guest_id)
+            if required.issubset(members):
+                group_node_block = _build_group_node_block(gn)
+
     container = None
     if chat.get("active_scene_id"):
         scene = get_scene(conn, chat["active_scene_id"])
@@ -421,6 +481,8 @@ def assemble_narrative_prompt(
         include_previous_scene: bool,
         include_memories_top_k: int,
         dialogue_keep: int,
+        include_guest_activity: bool = True,
+        include_group_node: bool = True,
     ) -> tuple[str, int, list[dict]]:
         # dialogue: keep the last `dialogue_keep` turns verbatim; older
         # turns become an "earlier:" placeholder line.
@@ -447,6 +509,8 @@ def assemble_narrative_prompt(
             other_edges_block if include_other_edges else None,
             scene_block,
             activity_block,
+            guest_activity_block if include_guest_activity else None,
+            group_node_block if include_group_node else None,
             prev_block,
             memories_block,
             dialogue_block,
@@ -463,12 +527,25 @@ def assemble_narrative_prompt(
     nice_memories_k = min(4, len(memory_summaries))
     include_prev = previous_scene_summary is not None
     include_other = other_edges_block is not None
+    include_guest_activity = guest_activity_block is not None
+    include_group_node = group_node_block is not None
 
-    body, total, _ = assemble(
-        include_other_edges=include_other,
-        include_previous_scene=include_prev,
-        include_memories_top_k=nice_memories_k,
-        dialogue_keep=nice_dialogue_keep,
+    def _build(*, prev: bool, mem_k: int, dlg: int, other: bool,
+               guest_act: bool, group: bool) -> tuple[str, int]:
+        body, total, _ = assemble(
+            include_other_edges=other,
+            include_previous_scene=prev,
+            include_memories_top_k=mem_k,
+            dialogue_keep=dlg,
+            include_guest_activity=guest_act,
+            include_group_node=group,
+        )
+        return body, total
+
+    body, total = _build(
+        prev=include_prev, mem_k=nice_memories_k, dlg=nice_dialogue_keep,
+        other=include_other, guest_act=include_guest_activity,
+        group=include_group_node,
     )
 
     # If under soft, we're done.
@@ -478,34 +555,31 @@ def assemble_narrative_prompt(
     # Drop NICE in order: previous scene → memories beyond top-2 →
     # older dialogue turns (collapse to 4).
     if include_prev:
-        body, total, _ = assemble(
-            include_other_edges=include_other,
-            include_previous_scene=False,
-            include_memories_top_k=nice_memories_k,
-            dialogue_keep=nice_dialogue_keep,
-        )
         include_prev = False
+        body, total = _build(
+            prev=include_prev, mem_k=nice_memories_k, dlg=nice_dialogue_keep,
+            other=include_other, guest_act=include_guest_activity,
+            group=include_group_node,
+        )
         if total <= budget_soft:
             return _emit(body, user_turn_prose)
 
     if nice_memories_k > 2:
         nice_memories_k = 2
-        body, total, _ = assemble(
-            include_other_edges=include_other,
-            include_previous_scene=False,
-            include_memories_top_k=nice_memories_k,
-            dialogue_keep=nice_dialogue_keep,
+        body, total = _build(
+            prev=include_prev, mem_k=nice_memories_k, dlg=nice_dialogue_keep,
+            other=include_other, guest_act=include_guest_activity,
+            group=include_group_node,
         )
         if total <= budget_soft:
             return _emit(body, user_turn_prose)
 
     if nice_dialogue_keep > baseline_keep:
         nice_dialogue_keep = baseline_keep
-        body, total, _ = assemble(
-            include_other_edges=include_other,
-            include_previous_scene=False,
-            include_memories_top_k=nice_memories_k,
-            dialogue_keep=nice_dialogue_keep,
+        body, total = _build(
+            prev=include_prev, mem_k=nice_memories_k, dlg=nice_dialogue_keep,
+            other=include_other, guest_act=include_guest_activity,
+            group=include_group_node,
         )
         if total <= budget_soft:
             return _emit(body, user_turn_prose)
@@ -513,21 +587,37 @@ def assemble_narrative_prompt(
     # Drop more NICE until we're under hard: memories all the way to 0.
     while nice_memories_k > 0 and total > budget_hard:
         nice_memories_k = max(0, nice_memories_k - 1)
-        body, total, _ = assemble(
-            include_other_edges=include_other,
-            include_previous_scene=False,
-            include_memories_top_k=nice_memories_k,
-            dialogue_keep=nice_dialogue_keep,
+        body, total = _build(
+            prev=include_prev, mem_k=nice_memories_k, dlg=nice_dialogue_keep,
+            other=include_other, guest_act=include_guest_activity,
+            group=include_group_node,
+        )
+
+    # Drop SHOULD-tier blocks in order: guest activity → group node →
+    # other edges. (Guest activity goes first per Task 43 spec — it's
+    # the most expendable additive context.)
+    if include_guest_activity and total > budget_hard:
+        include_guest_activity = False
+        body, total = _build(
+            prev=include_prev, mem_k=nice_memories_k, dlg=nice_dialogue_keep,
+            other=include_other, guest_act=include_guest_activity,
+            group=include_group_node,
+        )
+
+    if include_group_node and total > budget_hard:
+        include_group_node = False
+        body, total = _build(
+            prev=include_prev, mem_k=nice_memories_k, dlg=nice_dialogue_keep,
+            other=include_other, guest_act=include_guest_activity,
+            group=include_group_node,
         )
 
-    # Drop SHOULD: other edges.
     if include_other and total > budget_hard:
         include_other = False
-        body, total, _ = assemble(
-            include_other_edges=False,
-            include_previous_scene=False,
-            include_memories_top_k=nice_memories_k,
-            dialogue_keep=nice_dialogue_keep,
+        body, total = _build(
+            prev=include_prev, mem_k=nice_memories_k, dlg=nice_dialogue_keep,
+            other=include_other, guest_act=include_guest_activity,
+            group=include_group_node,
         )
 
     if total > budget_hard:
diff --git a/tests/test_prompt.py b/tests/test_prompt.py
index bef8dc0..2ef4641 100644
--- a/tests/test_prompt.py
+++ b/tests/test_prompt.py
@@ -253,3 +253,244 @@ def test_must_exceeds_budget_hard_raises_value_error(tmp_path):
                 budget_soft=5,
                 budget_hard=10,
             )
+
+
+# ---------------------------------------------------------------------------
+# Task 43: multi-entity prompt assembly (guest_id support)
+# ---------------------------------------------------------------------------
+
+
+def _seed_with_guest(conn) -> None:
+    """Seed a 3-entity scene: you (Sam) + host (Aria, bot_a) + guest (Iris, bot_b).
+
+    Group node row is initialized with summary + dynamic, edges in all
+    relevant directions are seeded, and activities are recorded for all
+    three entities.
+    """
+    append_event(conn, kind="bot_authored", payload={
+        "id": "bot_a",
+        "name": "Aria",
+        "persona": "reserved coworker who notices things",
+        "voice_samples": ["I — sorry, I didn't mean to.", "Right. Of course."],
+        "traits": ["introverted", "observant"],
+        "backstory": "An archivist who joined the firm last spring.",
+        "initial_relationship_to_you": "coworker; mild crush; never voiced",
+        "kickoff_prose": "you stay late at the office",
+    })
+    append_event(conn, kind="bot_authored", payload={
+        "id": "bot_b",
+        "name": "Iris",
+        "persona": "wry transplant from the Boston office",
+        "voice_samples": ["Oh, please.", "Don't make me say it twice."],
+        "traits": ["sardonic", "loyal"],
+        "backstory": "Met Aria at a conference two years back.",
+        "initial_relationship_to_you": "stranger; curious",
+        "kickoff_prose": "",
+    })
+    append_event(conn, kind="you_authored", payload={
+        "name": "Sam",
+        "pronouns": "they/them",
+        "persona": "tired analyst",
+    })
+    append_event(conn, kind="chat_created", payload={
+        "id": "chat_bot_a",
+        "host_bot_id": "bot_a",
+        "guest_bot_id": "bot_b",
+        "initial_time": "2026-04-26T20:00:00+00:00",
+        "narrative_anchor": "Day 1 evening",
+        "weather": "clear",
+    })
+    append_event(conn, kind="container_created", payload={
+        "chat_id": "chat_bot_a",
+        "name": "office bullpen",
+        "type": "workplace",
+        "properties": {"public": False, "moving": False, "audible_range": "room"},
+    })
+    # Edges: host -> you, guest -> you, host -> guest, guest -> host.
+    append_event(conn, kind="edge_update", payload={
+        "source_id": "bot_a",
+        "target_id": "you",
+        "affinity_delta": 12,
+        "trust_delta": 5,
+        "knowledge_facts": ["they work on the same floor"],
+    })
+    append_event(conn, kind="edge_update", payload={
+        "source_id": "bot_a",
+        "target_id": "bot_b",
+        "affinity_delta": 20,
+        "trust_delta": 15,
+        "knowledge_facts": ["studied physics together"],
+    })
+    append_event(conn, kind="edge_update", payload={
+        "source_id": "bot_b",
+        "target_id": "you",
+        "affinity_delta": 4,
+        "trust_delta": 0,
+        "knowledge_facts": ["Aria's coworker"],
+    })
+    append_event(conn, kind="edge_update", payload={
+        "source_id": "bot_b",
+        "target_id": "bot_a",
+        "affinity_delta": 18,
+        "trust_delta": 12,
+        "knowledge_facts": ["former roommate"],
+    })
+    # Activity for all three entities — note distinct verbs so we can
+    # check whose activity got dropped under tight budget.
+    append_event(conn, kind="activity_change", payload={
+        "entity_id": "you",
+        "container_id": 1,
+        "posture": "sitting at your desk",
+        "action": {"verb": "finishing emails"},
+        "attention": "the screen",
+        "holding": ["coffee mug"],
+    })
+    append_event(conn, kind="activity_change", payload={
+        "entity_id": "bot_a",
+        "container_id": 1,
+        "posture": "sitting at her desk",
+        "action": {"verb": "pretending to work"},
+        "attention": "you, in glances",
+    })
+    append_event(conn, kind="activity_change", payload={
+        "entity_id": "bot_b",
+        "container_id": 1,
+        "posture": "leaning against the doorframe",
+        "action": {"verb": "smirking-distinctively"},
+        "attention": "Aria",
+    })
+    append_event(conn, kind="scene_opened", payload={
+        "chat_id": "chat_bot_a",
+        "container_id": 1,
+        "started_at": "2026-04-26T20:00:00+00:00",
+        "participants": ["you", "bot_a", "bot_b"],
+    })
+    append_event(conn, kind="group_node_initialized", payload={
+        "chat_id": "chat_bot_a",
+        "members": ["you", "bot_a", "bot_b"],
+        "summary": "Three coworkers catching up after hours UNIQUE-GROUP-SUMMARY.",
+        "dynamic": "warm-but-prickly UNIQUE-GROUP-DYNAMIC",
+    })
+    project(conn)
+
+
+def test_assemble_with_no_guest_matches_phase1(tmp_path):
+    """Regression: 2-entity scenario without guest_id behaves exactly as Phase 1."""
+    db = tmp_path / "t.db"
+    apply_migrations(db)
+    with open_db(db) as conn:
+        _seed_basic(conn)
+        msgs = assemble_narrative_prompt(
+            conn,
+            chat_id="chat_bot_a",
+            speaker_bot_id="bot_a",
+            recent_dialogue=[],
+            retrieved_memory_summaries=[],
+        )
+    body = msgs[0].content
+    # Phase 1 must blocks present.
+    assert "Aria" in body
+    assert "PERSONA" in body
+    assert "Sam" in body
+    assert "ACTIVITIES" in body
+    assert "62/100" in body  # speaker → addressee edge intact
+    # No guest content leaks in.
+    assert "Group dynamic" not in body
+    assert "Iris" not in body
+
+
+def test_assemble_with_guest_includes_group_node_summary(tmp_path):
+    """When guest is present (auto-detected via chat.guest_bot_id) and a
+    group_node row exists, its summary + dynamic are rendered."""
+    db = tmp_path / "t.db"
+    apply_migrations(db)
+    with open_db(db) as conn:
+        _seed_with_guest(conn)
+        msgs = assemble_narrative_prompt(
+            conn,
+            chat_id="chat_bot_a",
+            speaker_bot_id="bot_a",
+            recent_dialogue=[],
+            retrieved_memory_summaries=[],
+        )
+    body = msgs[0].content
+    assert "Group dynamic" in body
+    assert "UNIQUE-GROUP-SUMMARY" in body
+    assert "UNIQUE-GROUP-DYNAMIC" in body
+    # Guest activity also present (SHOULD-tier, fits at default budget).
+    assert "smirking-distinctively" in body
+    # Speaker's other edges include the host -> guest direction.
+    assert "Iris" in body
+
+
+def test_assemble_when_speaker_is_guest_orients_edges_correctly(tmp_path):
+    """When the guest is the speaker, identity is the guest, the
+    addressee edge is guest → you, and other edges include guest → host."""
+    db = tmp_path / "t.db"
+    apply_migrations(db)
+    with open_db(db) as conn:
+        _seed_with_guest(conn)
+        msgs = assemble_narrative_prompt(
+            conn,
+            chat_id="chat_bot_a",
+            speaker_bot_id="bot_b",  # guest as speaker
+            recent_dialogue=[],
+            retrieved_memory_summaries=[],
+        )
+    body = msgs[0].content
+    # Speaker identity is the guest's persona.
+    assert "You are Iris." in body
+    assert "wry transplant from the Boston office" in body
+    # Edge to addressee is guest → you (Sam) with the seeded values
+    # (default 50 + 4 affinity = 54).
+    assert "YOUR EDGE TO Sam" in body
+    assert "54/100" in body
+    # Other edges include guest → host (Aria) with seeded value
+    # (default 50 + 18 = 68).
+    assert "OTHER EDGES" in body
+    assert "Aria" in body
+    assert "68/100" in body
+
+
+def test_assemble_with_tight_budget_drops_guest_activity_first(tmp_path):
+    """Under tight budget MUST blocks survive but SHOULD-tier guest
+    activity is dropped first."""
+    db = tmp_path / "t.db"
+    apply_migrations(db)
+    with open_db(db) as conn:
+        _seed_with_guest(conn)
+        # Short dialogue so MUST core (speaker identity + edge + last 4
+        # turns + closing) sits comfortably under the hard budget while
+        # SHOULD-tier additions (guest activity, group node, other edges)
+        # would push over.
+        dialogue = [
+            {"speaker": "you", "text": "line-16 hi there"},
+            {"speaker": "bot_a", "text": "line-17 hey"},
+            {"speaker": "you", "text": "line-18 quiet night"},
+            {"speaker": "bot_a", "text": "line-19 indeed"},
+        ]
+        msgs = assemble_narrative_prompt(
+            conn,
+            chat_id="chat_bot_a",
+            speaker_bot_id="bot_a",
+            recent_dialogue=dialogue,
+            retrieved_memory_summaries=[],
+            # MUST core ~310 tokens; SHOULD additions (guest activity +
+            # group node + other edges) push it well over 380. budget_hard
+            # is set just above MUST core so SHOULD-tier blocks must be
+            # trimmed away.
+            budget_soft=250,
+            budget_hard=340,
+        )
+    body = msgs[0].content
+    # MUST: speaker identity, edge to addressee, last 4 dialogue turns.
+    assert "Aria" in body
+    assert "YOUR EDGE TO Sam" in body
+    for i in range(16, 20):
+        assert f"line-{i:02d}" in body
+    # Guest activity (SHOULD-tier) must be dropped under tight budget.
+    assert "smirking-distinctively" not in body
+    # Token budget honoured.
+    import tiktoken
+    enc = tiktoken.get_encoding("cl100k_base")
+    assert len(enc.encode(body)) <= 340