diff --git a/src/claude_session.py b/src/claude_session.py
index 9983d41..b319b23 100644
--- a/src/claude_session.py
+++ b/src/claude_session.py
@@ -37,6 +37,42 @@ DEFAULT_TIMEOUT = 300  # seconds
 
 CLAUDE_BIN = os.environ.get("CLAUDE_BIN", "claude")
 
+# ---------------------------------------------------------------------------
+# Per-channel mutex for send_message
+# ---------------------------------------------------------------------------
+#
+# Two paths can hit `send_message(channel_id, ...)` concurrently for the same
+# channel: a text adapter (Discord/Telegram/WhatsApp) and the voice adapter
+# (`adapter_name="discord-voice"`). The underlying Claude CLI subprocess is
+# blocking (`subprocess.Popen` with stream-json read loop) and stateful via
+# `--resume <session_id>` — interleaving two concurrent invocations on the
+# same channel would corrupt the conversation order.
+#
+# We use `threading.Lock` (NOT `asyncio.Lock`) because `send_message` is sync
+# code typically run from `asyncio.to_thread` in async adapters. asyncio.Lock
+# only serializes coroutines, not threads — it would NOT protect this path.
+#
+# Each channel gets its own lock so DIFFERENT channels still run in parallel.
+# Locks are created lazily on first use; the dict itself is guarded by a
+# small bootstrap lock so two concurrent first-uses don't race on creation.
+_session_locks: dict[str, threading.Lock] = {}
+_session_locks_bootstrap = threading.Lock()
+
+
+def _get_session_lock(channel_id: str) -> threading.Lock:
+    """Return the channel's mutex, creating it on first access.
+
+    Two threads racing to create the same channel's lock would otherwise
+    end up with different lock objects (setdefault is not atomic across
+    the read-modify-write under all interpreter conditions — defensive).
+    """
+    lock = _session_locks.get(channel_id)
+    if lock is not None:
+        return lock
+    with _session_locks_bootstrap:
+        return _session_locks.setdefault(channel_id, threading.Lock())
+
+
 PERSONALITY_FILES = [
     "IDENTITY.md",
     "SOUL.md",
@@ -543,19 +579,28 @@ def send_message(
     timeout: int = DEFAULT_TIMEOUT,
     on_text: Callable[[str], None] | None = None,
 ) -> str:
-    """High-level convenience: auto start or resume based on channel state."""
-    session = get_active_session(channel_id)
-    # Only resume if session has a valid session_id (not a pre-set model placeholder)
-    if session is not None and session.get("session_id"):
-        return resume_session(session["session_id"], message, timeout, on_text=on_text)
-    # Use model from pre-set session if available, otherwise use provided model
-    effective_model = model
-    if session is not None and session.get("model"):
-        effective_model = session["model"]
-    response_text, _session_id = start_session(
-        channel_id, message, effective_model, timeout, on_text=on_text
-    )
-    return response_text
+    """High-level convenience: auto start or resume based on channel state.
+
+    Concurrency: a per-`channel_id` `threading.Lock` serializes invocations
+    that hit the same channel (e.g. text adapter + voice adapter racing on
+    the same Discord guild text channel). Different channels run in
+    parallel — each holds its own lock. Lock is acquired blocking; we rely
+    on `timeout` (default 5 minutes) to bound the worst case rather than
+    a non-blocking acquire (loss of fairness vs adapter-side queueing).
+    """
+    with _get_session_lock(channel_id):
+        session = get_active_session(channel_id)
+        # Only resume if session has a valid session_id (not a pre-set model placeholder)
+        if session is not None and session.get("session_id"):
+            return resume_session(session["session_id"], message, timeout, on_text=on_text)
+        # Use model from pre-set session if available, otherwise use provided model
+        effective_model = model
+        if session is not None and session.get("model"):
+            effective_model = session["model"]
+        response_text, _session_id = start_session(
+            channel_id, message, effective_model, timeout, on_text=on_text
+        )
+        return response_text
 
 
 def clear_session(channel_id: str) -> bool:
diff --git a/src/router.py b/src/router.py
index e733845..c6a0344 100644
--- a/src/router.py
+++ b/src/router.py
@@ -154,8 +154,17 @@ def route_message(
             channel_cfg = _get_channel_config(channel_id)
             model = (channel_cfg or {}).get("default_model") or _get_config().get("bot.default_model", "sonnet")
 
+    # Voice-mode augment: prepend speaker prefix so Claude knows who spoke
+    # in a voice channel. Cheap now, future-proof for multi-speaker later.
+    # (Engineering decision #14 in the plan.) Only the discord-voice adapter
+    # triggers it — text adapters keep the message verbatim.
+    claude_text = text
+    if adapter_name == "discord-voice":
+        user_name = _get_config().get("voice.user_name", "user") or "user"
+        claude_text = f"[speaker:{user_name}] {text}"
+
     try:
-        response = send_message(channel_id, text, model=model, on_text=on_text)
+        response = send_message(channel_id, claude_text, model=model, on_text=on_text)
         _set_last_response(channel_id, response)
         return response, False
     except Exception as e:
diff --git a/tests/test_claude_session_mutex.py b/tests/test_claude_session_mutex.py
new file mode 100644
index 0000000..9a6b2f1
--- /dev/null
+++ b/tests/test_claude_session_mutex.py
@@ -0,0 +1,307 @@
+"""Regression-critical tests for per-channel mutex in src/claude_session.py.
+
+Three scenarios from the eng-review test plan (2026-05-27):
+
+  1. Concurrent `send_message` calls on the SAME channel_id serialize —
+     the second waits for the first to finish before its subprocess runs.
+  2. Concurrent `send_message` calls on DIFFERENT channel_ids run in parallel
+     — independent channels never block each other.
+  3. Acquisition contract is documented and consistent: the lock is acquired
+     blocking (no acquire timeout), which means a hung subprocess on
+     channel X delays subsequent X messages but never X' (X != X'). This
+     test pins that behavior so future refactors must preserve it.
+
+The mutex is `threading.Lock`, NOT `asyncio.Lock`, because `send_message`
+is a sync function typically dispatched via `asyncio.to_thread` from
+async adapters. asyncio.Lock would serialize coroutines only — not the
+subprocess invocation. See plan section "Engineering decisions" #2.
+"""
+
+import json
+import threading
+import time
+from concurrent.futures import ThreadPoolExecutor, as_completed
+from unittest.mock import patch
+
+import pytest
+
+from src import claude_session
+from src.claude_session import (
+    _get_session_lock,
+    _session_locks,
+    send_message,
+)
+
+
+# ---------------------------------------------------------------------------
+# Helpers
+# ---------------------------------------------------------------------------
+
+
+@pytest.fixture(autouse=True)
+def _clear_session_locks():
+    """Each test starts with a fresh lock map so we don't share state."""
+    _session_locks.clear()
+    yield
+    _session_locks.clear()
+
+
+@pytest.fixture
+def temp_sessions(tmp_path, monkeypatch):
+    """Isolated active.json per test — keeps real session state untouched."""
+    sessions_dir = tmp_path / "sessions"
+    sessions_dir.mkdir()
+    sf = sessions_dir / "active.json"
+    sf.write_text("{}")
+    monkeypatch.setattr(claude_session, "SESSIONS_DIR", sessions_dir)
+    monkeypatch.setattr(claude_session, "_SESSIONS_FILE", sf)
+    return sf
+
+
+def _slow_run_claude(sleep_seconds: float, in_critical: threading.Event,
+                     concurrent_seen: threading.Event):
+    """Build a fake `_run_claude` that signals when inside the critical section.
+
+    The fake holds the simulated subprocess for `sleep_seconds`. Any other
+    invocation that overlaps will set `concurrent_seen` — the mutex test
+    asserts this NEVER happens for the same channel_id.
+    """
+    state = {"active": 0, "lock": threading.Lock()}
+
+    def fake(cmd, timeout, on_text=None, cwd=None):
+        with state["lock"]:
+            state["active"] += 1
+            if state["active"] > 1:
+                concurrent_seen.set()
+        in_critical.set()
+        time.sleep(sleep_seconds)
+        with state["lock"]:
+            state["active"] -= 1
+        return {
+            "result": "Hello from Claude!",
+            "session_id": "sess-abc-123",
+            "usage": {"input_tokens": 10, "output_tokens": 5},
+            "total_cost_usd": 0.001,
+            "cost_usd": 0.001,
+            "duration_ms": int(sleep_seconds * 1000),
+            "num_turns": 1,
+            "intermediate_count": 0,
+            "subtype": "success",
+            "is_error": False,
+        }
+
+    return fake
+
+
+# ---------------------------------------------------------------------------
+# Scenario 1 — same channel serializes
+# ---------------------------------------------------------------------------
+
+
+class TestSameChannelSerializes:
+    def test_two_concurrent_calls_same_channel_run_one_at_a_time(
+        self, temp_sessions
+    ):
+        """Two parallel send_message on the SAME channel_id never overlap.
+
+        We instrument `_run_claude` to signal whenever more than one
+        invocation is concurrently inside it. The mutex MUST prevent that.
+        """
+        in_critical = threading.Event()
+        concurrent_seen = threading.Event()
+        slow = _slow_run_claude(0.25, in_critical, concurrent_seen)
+
+        with patch.object(claude_session, "_run_claude", side_effect=slow):
+            start = time.monotonic()
+            with ThreadPoolExecutor(max_workers=2) as pool:
+                futures = [
+                    pool.submit(send_message, "ch-same", f"msg-{i}")
+                    for i in range(2)
+                ]
+                results = [f.result(timeout=10) for f in futures]
+            elapsed = time.monotonic() - start
+
+        assert not concurrent_seen.is_set(), (
+            "Two send_message calls on the same channel ran concurrently — "
+            "mutex did not serialize them."
+        )
+        assert all(r == "Hello from Claude!" for r in results)
+        # Two serial 0.25s subprocesses must take at least ~0.5s total
+        # (we allow a generous floor — schedulers can be slow).
+        assert elapsed >= 0.45, f"Expected serialized ~0.5s, got {elapsed:.3f}s"
+
+    def test_lock_is_reentrant_per_channel_dict(self, temp_sessions):
+        """`_get_session_lock` returns the SAME lock object for the same channel."""
+        lock_a1 = _get_session_lock("channel-A")
+        lock_a2 = _get_session_lock("channel-A")
+        lock_b = _get_session_lock("channel-B")
+        assert lock_a1 is lock_a2
+        assert lock_a1 is not lock_b
+
+
+# ---------------------------------------------------------------------------
+# Scenario 2 — different channels parallel
+# ---------------------------------------------------------------------------
+
+
+class TestDifferentChannelsParallel:
+    def test_two_concurrent_calls_different_channels_run_in_parallel(
+        self, temp_sessions
+    ):
+        """Different channels MUST NOT block each other.
+
+        We measure elapsed wall-clock: two 0.4s subprocesses on different
+        channels should finish in ~0.4s (parallel), NOT ~0.8s (serialized).
+        """
+        in_critical = threading.Event()
+        # `concurrent_seen` is OK to fire here — we WANT them to overlap.
+        concurrent_seen = threading.Event()
+        slow = _slow_run_claude(0.4, in_critical, concurrent_seen)
+
+        with patch.object(claude_session, "_run_claude", side_effect=slow):
+            start = time.monotonic()
+            with ThreadPoolExecutor(max_workers=2) as pool:
+                f1 = pool.submit(send_message, "ch-A", "msg-A")
+                f2 = pool.submit(send_message, "ch-B", "msg-B")
+                results = [f1.result(timeout=10), f2.result(timeout=10)]
+            elapsed = time.monotonic() - start
+
+        assert all(r == "Hello from Claude!" for r in results)
+        # Parallel execution: total time should be close to 0.4s, well under
+        # 0.7s (would mean serialization). 0.65s ceiling allows for GIL +
+        # scheduler jitter on a busy test box.
+        assert elapsed < 0.65, (
+            f"Different channels appear serialized: elapsed {elapsed:.3f}s "
+            f"(expected ~0.4s parallel, <0.65s ceiling)"
+        )
+        assert concurrent_seen.is_set(), (
+            "Different channels did not overlap — mutex is too coarse "
+            "(should be per-channel, not global)."
+        )
+
+    def test_three_channels_all_overlap(self, temp_sessions):
+        """Stress: three concurrent channels all run in parallel."""
+        in_critical = threading.Event()
+        concurrent_seen = threading.Event()
+        slow = _slow_run_claude(0.3, in_critical, concurrent_seen)
+
+        with patch.object(claude_session, "_run_claude", side_effect=slow):
+            start = time.monotonic()
+            with ThreadPoolExecutor(max_workers=3) as pool:
+                futures = [
+                    pool.submit(send_message, f"ch-{i}", f"msg-{i}")
+                    for i in range(3)
+                ]
+                for f in as_completed(futures, timeout=10):
+                    assert f.result() == "Hello from Claude!"
+            elapsed = time.monotonic() - start
+
+        # 3 × 0.3s in parallel ≈ 0.3s; serial would be ~0.9s.
+        assert elapsed < 0.6, (
+            f"Three channels serialized: {elapsed:.3f}s (expected <0.6s)"
+        )
+
+
+# ---------------------------------------------------------------------------
+# Scenario 3 — acquisition behavior documented and consistent
+# ---------------------------------------------------------------------------
+
+
+class TestAcquisitionBehavior:
+    """Pin the chosen acquisition policy: blocking, no timeout.
+
+    Project style is to bound subprocess execution via `timeout` (default
+    5 min) rather than fail-fast on lock acquire. Reasons:
+
+    - Adapter callers (Discord/Telegram/voice) already serialize work via
+      asyncio.to_thread; queue depth is naturally bounded.
+    - A non-blocking acquire would surface a timing error to the user
+      ("busy, try again") for an entirely transient and self-resolving
+      condition. Blocking gives FIFO-ish ordering with simple semantics.
+    - If a subprocess truly hangs past `timeout`, _run_claude raises
+      TimeoutError → the held lock releases (via `with`) → queued
+      callers proceed.
+
+    This test pins that: a second caller waits and eventually proceeds; it
+    does not raise an exception on contention.
+    """
+
+    def test_contested_acquire_blocks_then_proceeds(self, temp_sessions):
+        in_critical = threading.Event()
+        concurrent_seen = threading.Event()
+        slow = _slow_run_claude(0.3, in_critical, concurrent_seen)
+
+        results: list[str | BaseException] = []
+
+        def run(label: str):
+            try:
+                results.append(send_message("ch-contend", label))
+            except BaseException as e:
+                results.append(e)
+
+        with patch.object(claude_session, "_run_claude", side_effect=slow):
+            t1 = threading.Thread(target=run, args=("first",))
+            t1.start()
+            # Wait until the first call is inside the critical section so
+            # the second is GUARANTEED to contend on the lock.
+            assert in_critical.wait(timeout=2.0), "first call never entered"
+            in_critical.clear()
+            t2 = threading.Thread(target=run, args=("second",))
+            t2.start()
+            t1.join(timeout=5.0)
+            t2.join(timeout=5.0)
+
+        assert len(results) == 2
+        # Both must return the canned response — no exception, no error.
+        assert all(r == "Hello from Claude!" for r in results), (
+            f"Contended acquire surfaced an error instead of blocking: {results}"
+        )
+        # Critical-section overlap check: contended calls MUST serialize.
+        assert not concurrent_seen.is_set(), (
+            "Contended same-channel calls ran concurrently — mutex broken."
+        )
+
+    def test_lock_released_on_subprocess_exception(self, temp_sessions):
+        """If `_run_claude` raises, the lock MUST be released so the next
+        caller can proceed (otherwise a single error deadlocks the channel
+        forever)."""
+
+        call_count = {"n": 0}
+
+        def flaky(cmd, timeout, on_text=None, cwd=None):
+            call_count["n"] += 1
+            if call_count["n"] == 1:
+                raise RuntimeError("simulated subprocess crash")
+            return {
+                "result": "Hello from Claude!",
+                "session_id": "sess-abc-123",
+                "usage": {"input_tokens": 10, "output_tokens": 5},
+                "total_cost_usd": 0.001,
+                "cost_usd": 0.001,
+                "duration_ms": 50,
+                "num_turns": 1,
+                "intermediate_count": 0,
+                "subtype": "success",
+                "is_error": False,
+            }
+
+        with patch.object(claude_session, "_run_claude", side_effect=flaky):
+            with pytest.raises(RuntimeError, match="simulated subprocess crash"):
+                send_message("ch-recover", "first")
+
+            # Second call MUST acquire the lock (proves the first released it).
+            # We use a short timeout via a thread so a deadlock would fail loudly.
+            done = threading.Event()
+            result_box: list[str] = []
+
+            def second():
+                result_box.append(send_message("ch-recover", "second"))
+                done.set()
+
+            t = threading.Thread(target=second)
+            t.start()
+            assert done.wait(timeout=3.0), (
+                "Second call deadlocked — lock was not released on exception."
+            )
+            t.join(timeout=1.0)
+            assert result_box == ["Hello from Claude!"]