diff --git a/chat/services/interjection.py b/chat/services/interjection.py
new file mode 100644
index 0000000..84fac62
--- /dev/null
+++ b/chat/services/interjection.py
@@ -0,0 +1,100 @@
+"""Interjection classifier service (T39).
+
+Per Requirements §6.2, when a guest is present and the addressee bot has
+just spoken, the *non-addressee* bot may follow on with a brief
+interjection beat. This service decides whether that interjection
+fires. Conservative bias: most turns return ``should_interject=False``
+— the addressee has the floor and an interjection is the exception.
+Trigger ``True`` only when the silent witness's character, given their
+persona and edges, would plausibly speak up: jealousy, surprise, strong
+agreement worth voicing, correcting a factual falsehood, urgency.
+
+T44 (turn flow) calls this and, on ``True``, generates the brief
+follow-on response as the silent witness. Classifier failure falls back
+to ``should_interject=False`` with ``reason="fallback"`` so the chat
+keeps moving (§3.3 graceful-degradation rule); callers that care can
+distinguish a real "no" from a degraded "no" by the reason string.
+"""
+
+from __future__ import annotations
+
+from pydantic import BaseModel
+
+from chat.llm.classify import classify
+from chat.llm.client import LLMClient
+
+
+class InterjectionDecision(BaseModel):
+    """Whether the silent witness interjects, plus a short reason.
+
+    Defaults are a deliberate no-op: ``should_interject=False`` with an
+    empty reason. The classifier-failure fallback uses
+    ``reason="fallback"`` so it's distinguishable from a real "no".
+    """
+
+    should_interject: bool = False
+    reason: str = ""
+
+
+_SYSTEM = (
+    "You decide whether a silent witness character interjects after the "
+    "addressee character finishes speaking. STRONGLY default to false — "
+    "the addressee has the floor and most turns should NOT have an "
+    "interjection. Only return true when the silent witness's character, "
+    "given their persona and edges, would plausibly speak up: jealousy, "
+    "surprise, strong agreement worth voicing, correcting a factual "
+    "falsehood, urgency. Output strict JSON matching the schema."
+)
+
+
+async def detect_interjection(
+    client: LLMClient,
+    *,
+    classifier_model: str,
+    addressee_name: str,
+    addressee_just_said: str,
+    silent_witness_name: str,
+    silent_witness_persona: str,
+    silent_witness_edge_to_addressee: dict,  # {affinity, trust, summary}
+    silent_witness_edge_to_you: dict,
+    you_just_said: str,
+    timeout_s: float = 30.0,
+) -> InterjectionDecision:
+    """Decide whether the silent witness bot interjects after the addressee
+    finishes speaking.
+
+    The two ``silent_witness_edge_*`` dicts carry the silent witness's
+    directed edges toward the addressee and toward the user ("you"),
+    each shaped ``{affinity: int, trust: int, summary: str}``. Missing
+    keys fall back to a 50/50 baseline with an empty summary so this
+    function tolerates partially-populated edge state without raising.
+    """
+    user = (
+        f"You said: {you_just_said}\n\n"
+        f"{addressee_name} just said: {addressee_just_said}\n\n"
+        f"Silent witness: {silent_witness_name}\n"
+        f"Persona: {silent_witness_persona}\n"
+        f"Edge {silent_witness_name} -> {addressee_name}: "
+        f"affinity={silent_witness_edge_to_addressee.get('affinity', 50)}, "
+        f"trust={silent_witness_edge_to_addressee.get('trust', 50)}, "
+        f"summary={silent_witness_edge_to_addressee.get('summary', '')}\n"
+        f"Edge {silent_witness_name} -> you: "
+        f"affinity={silent_witness_edge_to_you.get('affinity', 50)}, "
+        f"trust={silent_witness_edge_to_you.get('trust', 50)}, "
+        f"summary={silent_witness_edge_to_you.get('summary', '')}\n\n"
+        f"Should {silent_witness_name} interject?"
+    )
+    return await classify(
+        client,
+        model=classifier_model,
+        system=_SYSTEM,
+        user=user,
+        schema=InterjectionDecision,
+        default=InterjectionDecision(
+            should_interject=False, reason="fallback"
+        ),
+        timeout_s=timeout_s,
+    )
+
+
+__all__ = ["InterjectionDecision", "detect_interjection"]
diff --git a/tests/test_interjection.py b/tests/test_interjection.py
new file mode 100644
index 0000000..8200d11
--- /dev/null
+++ b/tests/test_interjection.py
@@ -0,0 +1,89 @@
+"""Tests for the interjection classifier service (T39).
+
+Per Requirements §6.2, when a guest is present and the addressee bot has
+just spoken, the *non-addressee* bot may interject with a brief follow-on
+beat. The classifier wrapped here decides whether that interjection
+should fire. The default bias is strongly toward False — the addressee
+has the floor — so an interjection only fires when the silent witness's
+character would plausibly speak up.
+
+These tests cover:
+
+* The classifier returning ``should_interject=True`` is honored.
+* The classifier returning ``should_interject=False`` is honored.
+* Repeated invalid JSON exhausts the classifier retries and falls back
+  to ``should_interject=False`` with ``reason="fallback"``.
+"""
+
+from __future__ import annotations
+
+import json
+
+import pytest
+
+from chat.llm.mock import MockLLMClient
+from chat.services.interjection import (
+    InterjectionDecision,
+    detect_interjection,
+)
+
+
+def _kwargs() -> dict:
+    """Reasonable, non-empty kwargs for ``detect_interjection``."""
+    return dict(
+        classifier_model="x",
+        addressee_name="Alice",
+        addressee_just_said="I think we should leave now.",
+        silent_witness_name="Bob",
+        silent_witness_persona="Skeptical engineer, blunt, protective of the user.",
+        silent_witness_edge_to_addressee={
+            "affinity": 40,
+            "trust": 30,
+            "summary": "old rival; mild distrust",
+        },
+        silent_witness_edge_to_you={
+            "affinity": 70,
+            "trust": 80,
+            "summary": "long-time confidant",
+        },
+        you_just_said="Where do you both think we should go?",
+    )
+
+
+@pytest.mark.asyncio
+async def test_interjection_returns_true_when_classifier_decides_yes():
+    canned = json.dumps({"should_interject": True, "reason": "jealousy"})
+    mock = MockLLMClient(canned=[canned])
+    result = await detect_interjection(mock, **_kwargs())
+    assert isinstance(result, InterjectionDecision)
+    assert result.should_interject is True
+    assert result.reason == "jealousy"
+
+
+@pytest.mark.asyncio
+async def test_interjection_returns_false_when_classifier_decides_no():
+    canned = json.dumps(
+        {"should_interject": False, "reason": "addressee has the floor"}
+    )
+    mock = MockLLMClient(canned=[canned])
+    result = await detect_interjection(mock, **_kwargs())
+    assert isinstance(result, InterjectionDecision)
+    assert result.should_interject is False
+    assert result.reason == "addressee has the floor"
+
+
+@pytest.mark.asyncio
+async def test_interjection_falls_back_to_false_on_classifier_failure():
+    """``classify`` retries 3 times; after all fail it returns the default.
+
+    The default carries ``should_interject=False`` and
+    ``reason="fallback"`` so callers can tell a real "no" from a
+    classifier-degraded "no" if they ever care to.
+    """
+    mock = MockLLMClient(
+        canned=["this is not json", "still not json", "still not json"]
+    )
+    result = await detect_interjection(mock, **_kwargs())
+    assert isinstance(result, InterjectionDecision)
+    assert result.should_interject is False
+    assert result.reason == "fallback"