diff --git a/tests/tools/test_mcp_tool_session_expired.py b/tests/tools/test_mcp_tool_session_expired.py
new file mode 100644
index 000000000..67e6e5874
--- /dev/null
+++ b/tests/tools/test_mcp_tool_session_expired.py
@@ -0,0 +1,359 @@
+"""Tests for MCP tool-handler transport-session auto-reconnect.
+
+When a Streamable HTTP MCP server garbage-collects its server-side
+session (idle TTL, server restart, pod rotation, …) it rejects
+subsequent requests with a JSON-RPC error containing phrases like
+``"Invalid or expired session"``.  The OAuth token remains valid —
+only the transport session state needs rebuilding.
+
+Before the #13383 fix, this class of failure fell through as a plain
+tool error with no recovery path, so every subsequent call on the
+affected MCP server failed until the gateway was manually restarted.
+"""
+import json
+import threading
+import time
+from unittest.mock import AsyncMock, MagicMock
+
+import pytest
+
+
+# ---------------------------------------------------------------------------
+# _is_session_expired_error — unit coverage
+# ---------------------------------------------------------------------------
+
+
+def test_is_session_expired_detects_invalid_or_expired_session():
+    """Reporter's exact wpcom-mcp error message (#13383)."""
+    from tools.mcp_tool import _is_session_expired_error
+    exc = RuntimeError("Invalid params: Invalid or expired session")
+    assert _is_session_expired_error(exc) is True
+
+
+def test_is_session_expired_detects_expired_session_variant():
+    """Generic ``session expired`` / ``expired session`` phrasings used
+    by other SDK servers."""
+    from tools.mcp_tool import _is_session_expired_error
+    assert _is_session_expired_error(RuntimeError("Session expired")) is True
+    assert _is_session_expired_error(RuntimeError("expired session: abc")) is True
+
+
+def test_is_session_expired_detects_session_not_found():
+    """Server-side GC produces ``session not found`` / ``unknown session``
+    on some implementations."""
+    from tools.mcp_tool import _is_session_expired_error
+    assert _is_session_expired_error(RuntimeError("session not found")) is True
+    assert _is_session_expired_error(RuntimeError("Unknown session: abc123")) is True
+
+
+def test_is_session_expired_is_case_insensitive():
+    """Match uses lower-cased comparison so servers that emit the
+    message in different cases (SDK formatter quirks) still trigger."""
+    from tools.mcp_tool import _is_session_expired_error
+    assert _is_session_expired_error(RuntimeError("INVALID OR EXPIRED SESSION")) is True
+    assert _is_session_expired_error(RuntimeError("Session Expired")) is True
+
+
+def test_is_session_expired_rejects_unrelated_errors():
+    """Narrow scope: only the specific session-expired markers trigger.
+    A regular RuntimeError / ValueError does not."""
+    from tools.mcp_tool import _is_session_expired_error
+    assert _is_session_expired_error(RuntimeError("Tool failed to execute")) is False
+    assert _is_session_expired_error(ValueError("Missing parameter")) is False
+    assert _is_session_expired_error(Exception("Connection refused")) is False
+    # 401 is handled by the sibling _is_auth_error path, not here.
+    assert _is_session_expired_error(RuntimeError("401 Unauthorized")) is False
+
+
+def test_is_session_expired_rejects_interrupted_error():
+    """InterruptedError is the user-cancel signal — must never route
+    through the session-reconnect path."""
+    from tools.mcp_tool import _is_session_expired_error
+    assert _is_session_expired_error(InterruptedError()) is False
+    assert _is_session_expired_error(InterruptedError("Invalid or expired session")) is False
+
+
+def test_is_session_expired_rejects_empty_message():
+    """Bare exceptions with no message shouldn't match."""
+    from tools.mcp_tool import _is_session_expired_error
+    assert _is_session_expired_error(RuntimeError("")) is False
+    assert _is_session_expired_error(Exception()) is False
+
+
+# ---------------------------------------------------------------------------
+# Handler integration — verify the recovery plumbing wires end-to-end
+# ---------------------------------------------------------------------------
+
+
+def _install_stub_server(name: str = "wpcom"):
+    """Register a minimal server stub that _handle_session_expired_and_retry
+    can signal via _reconnect_event, and that reports ready+session after
+    the event fires."""
+    from tools import mcp_tool
+
+    mcp_tool._ensure_mcp_loop()
+
+    server = MagicMock()
+    server.name = name
+    # _reconnect_event is called via loop.call_soon_threadsafe(…set); use
+    # a threading-safe substitute.
+    reconnect_flag = threading.Event()
+
+    class _EventAdapter:
+        def set(self):
+            reconnect_flag.set()
+
+    server._reconnect_event = _EventAdapter()
+
+    # Immediately "ready" — simulates a fast reconnect (_ready.is_set()
+    # is polled by _handle_session_expired_and_retry until the timeout).
+    ready_flag = threading.Event()
+    ready_flag.set()
+    server._ready = MagicMock()
+    server._ready.is_set = ready_flag.is_set
+
+    # session attr must be truthy for the handler's initial check
+    # (``if not server or not server.session``) and for the post-
+    # reconnect readiness probe (``srv.session is not None``).
+    server.session = MagicMock()
+    return server, reconnect_flag
+
+
+def test_call_tool_handler_reconnects_on_session_expired(monkeypatch, tmp_path):
+    """Reporter's exact repro: call_tool raises "Invalid or expired
+    session", handler triggers reconnect, retries once, and returns
+    the retry's successful JSON (not the generic error)."""
+    monkeypatch.setenv("HERMES_HOME", str(tmp_path))
+
+    from tools import mcp_tool
+    from tools.mcp_tool import _make_tool_handler
+
+    server, reconnect_flag = _install_stub_server("wpcom")
+    mcp_tool._servers["wpcom"] = server
+    mcp_tool._server_error_counts.pop("wpcom", None)
+
+    # First call raises session-expired; second call (post-reconnect)
+    # returns a proper MCP tool result.
+    call_count = {"n": 0}
+
+    async def _call_sequence(*a, **kw):
+        call_count["n"] += 1
+        if call_count["n"] == 1:
+            raise RuntimeError("Invalid params: Invalid or expired session")
+        # Second call: mimic the MCP SDK's structured success response.
+        result = MagicMock()
+        result.isError = False
+        result.content = [MagicMock(type="text", text="tool completed")]
+        result.structuredContent = None
+        return result
+
+    server.session.call_tool = _call_sequence
+
+    try:
+        handler = _make_tool_handler("wpcom", "wpcom-mcp-content-authoring", 10.0)
+        out = handler({"slug": "hello"})
+        parsed = json.loads(out)
+        # Retry succeeded — no error surfaced to caller.
+        assert "error" not in parsed, (
+            f"Expected retry to succeed after reconnect; got: {parsed}"
+        )
+        # _reconnect_event was signalled exactly once.
+        assert reconnect_flag.is_set(), (
+            "Handler did not trigger transport reconnect on session-expired "
+            "error — the reconnect flow is the whole point of this fix."
+        )
+        # Exactly 2 call attempts (original + one retry).
+        assert call_count["n"] == 2, (
+            f"Expected 1 original + 1 retry = 2 calls; got {call_count['n']}"
+        )
+    finally:
+        mcp_tool._servers.pop("wpcom", None)
+        mcp_tool._server_error_counts.pop("wpcom", None)
+
+
+def test_call_tool_handler_non_session_expired_error_falls_through(
+    monkeypatch, tmp_path
+):
+    """Preserved-behaviour canary: a non-session-expired exception must
+    NOT trigger reconnect — it must fall through to the generic error
+    path so the caller sees the real failure."""
+    monkeypatch.setenv("HERMES_HOME", str(tmp_path))
+
+    from tools import mcp_tool
+    from tools.mcp_tool import _make_tool_handler
+
+    server, reconnect_flag = _install_stub_server("srv")
+    mcp_tool._servers["srv"] = server
+    mcp_tool._server_error_counts.pop("srv", None)
+
+    async def _raises(*a, **kw):
+        raise RuntimeError("Tool execution failed — unrelated error")
+
+    server.session.call_tool = _raises
+
+    try:
+        handler = _make_tool_handler("srv", "mytool", 10.0)
+        out = handler({"arg": "v"})
+        parsed = json.loads(out)
+        # Generic error path surfaced the failure.
+        assert "MCP call failed" in parsed.get("error", "")
+        # Reconnect was NOT triggered for this unrelated failure.
+        assert not reconnect_flag.is_set(), (
+            "Reconnect must not fire for non-session-expired errors — "
+            "this would cause spurious transport churn on every tool "
+            "failure."
+        )
+    finally:
+        mcp_tool._servers.pop("srv", None)
+        mcp_tool._server_error_counts.pop("srv", None)
+
+
+def test_session_expired_handler_returns_none_without_loop(monkeypatch):
+    """Defensive: if the MCP loop isn't running (cold start / shutdown
+    race), the handler must fall through cleanly instead of hanging
+    or raising."""
+    from tools import mcp_tool
+    from tools.mcp_tool import _handle_session_expired_and_retry
+
+    # Install a server stub but make the event loop unavailable.
+    server = MagicMock()
+    server._reconnect_event = MagicMock()
+    server._ready = MagicMock()
+    server._ready.is_set = MagicMock(return_value=True)
+    server.session = MagicMock()
+    mcp_tool._servers["srv-noloop"] = server
+
+    monkeypatch.setattr(mcp_tool, "_mcp_loop", None)
+
+    try:
+        out = _handle_session_expired_and_retry(
+            "srv-noloop",
+            RuntimeError("Invalid or expired session"),
+            lambda: '{"ok": true}',
+            "tools/call",
+        )
+        assert out is None, (
+            "Without an event loop, session-expired handler must fall "
+            "through to caller's generic error path — not hang or raise."
+        )
+    finally:
+        mcp_tool._servers.pop("srv-noloop", None)
+
+
+def test_session_expired_handler_returns_none_without_server_record():
+    """If the server has been torn down / isn't in _servers, fall
+    through cleanly — nothing to reconnect to."""
+    from tools.mcp_tool import _handle_session_expired_and_retry
+    out = _handle_session_expired_and_retry(
+        "does-not-exist",
+        RuntimeError("Invalid or expired session"),
+        lambda: '{"ok": true}',
+        "tools/call",
+    )
+    assert out is None
+
+
+def test_session_expired_handler_returns_none_when_retry_also_fails(
+    monkeypatch, tmp_path
+):
+    """If the retry after reconnect also raises, fall through to the
+    generic error path (don't loop forever, don't mask the second
+    failure)."""
+    monkeypatch.setenv("HERMES_HOME", str(tmp_path))
+
+    from tools import mcp_tool
+    from tools.mcp_tool import _handle_session_expired_and_retry
+
+    server, _ = _install_stub_server("srv-retry-fail")
+    mcp_tool._servers["srv-retry-fail"] = server
+
+    def _retry_raises():
+        raise RuntimeError("retry blew up too")
+
+    try:
+        out = _handle_session_expired_and_retry(
+            "srv-retry-fail",
+            RuntimeError("Invalid or expired session"),
+            _retry_raises,
+            "tools/call",
+        )
+        assert out is None, (
+            "When the retry itself fails, the handler must return None "
+            "so the caller's generic error path runs — no retry loop."
+        )
+    finally:
+        mcp_tool._servers.pop("srv-retry-fail", None)
+
+
+# ---------------------------------------------------------------------------
+# Parallel coverage for resources/list, resources/read, prompts/list,
+# prompts/get — all four handlers share the same exception path.
+# ---------------------------------------------------------------------------
+
+
+@pytest.mark.parametrize(
+    "handler_factory, handler_kwargs, session_method, op_label",
+    [
+        ("_make_list_resources_handler", {"tool_timeout": 10.0}, "list_resources", "list_resources"),
+        ("_make_read_resource_handler", {"tool_timeout": 10.0}, "read_resource", "read_resource"),
+        ("_make_list_prompts_handler", {"tool_timeout": 10.0}, "list_prompts", "list_prompts"),
+        ("_make_get_prompt_handler", {"tool_timeout": 10.0}, "get_prompt", "get_prompt"),
+    ],
+)
+def test_non_tool_handlers_also_reconnect_on_session_expired(
+    monkeypatch, tmp_path, handler_factory, handler_kwargs, session_method, op_label
+):
+    """All four non-``tools/call`` MCP handlers share the recovery
+    pattern and must reconnect the same way on session-expired."""
+    monkeypatch.setenv("HERMES_HOME", str(tmp_path))
+
+    from tools import mcp_tool
+
+    server, reconnect_flag = _install_stub_server(f"srv-{op_label}")
+    mcp_tool._servers[f"srv-{op_label}"] = server
+    mcp_tool._server_error_counts.pop(f"srv-{op_label}", None)
+
+    call_count = {"n": 0}
+
+    async def _sequence(*a, **kw):
+        call_count["n"] += 1
+        if call_count["n"] == 1:
+            raise RuntimeError("Invalid or expired session")
+        # Return something with the shapes each handler expects.
+        # Explicitly set primitive attrs — MagicMock's default auto-attr
+        # behaviour surfaces ``MagicMock`` values for optional fields
+        # like ``description``, which break ``json.dumps`` downstream.
+        result = MagicMock()
+        result.resources = []
+        result.prompts = []
+        result.contents = []
+        result.messages = []  # get_prompt
+        result.description = None  # get_prompt optional field
+        return result
+
+    setattr(server.session, session_method, _sequence)
+
+    factory = getattr(mcp_tool, handler_factory)
+    # list_resources / list_prompts take (server_name, timeout).
+    # read_resource / get_prompt take the same signature.
+    try:
+        handler = factory(f"srv-{op_label}", **handler_kwargs)
+        if op_label == "read_resource":
+            out = handler({"uri": "file://foo"})
+        elif op_label == "get_prompt":
+            out = handler({"name": "p1"})
+        else:
+            out = handler({})
+        parsed = json.loads(out)
+        assert "error" not in parsed, (
+            f"{op_label}: expected retry success, got {parsed}"
+        )
+        assert reconnect_flag.is_set(), (
+            f"{op_label}: reconnect should fire for session-expired"
+        )
+        assert call_count["n"] == 2, (
+            f"{op_label}: expected 1 original + 1 retry"
+        )
+    finally:
+        mcp_tool._servers.pop(f"srv-{op_label}", None)
+        mcp_tool._server_error_counts.pop(f"srv-{op_label}", None)
diff --git a/tools/mcp_tool.py b/tools/mcp_tool.py
index 0d5615b0f..565dbfca0 100644
--- a/tools/mcp_tool.py
+++ b/tools/mcp_tool.py
@@ -1582,6 +1582,129 @@ def _handle_auth_error_and_retry(
         "server": server_name,
     }, ensure_ascii=False)
 
+
+# Substrings (lower-cased match) that indicate the MCP server rejected
+# the request because its server-side transport session expired /
+# was garbage-collected.  The caller's OAuth token is still valid —
+# only the transport-layer session state needs rebuilding.  See #13383.
+_SESSION_EXPIRED_MARKERS: tuple = (
+    "invalid or expired session",
+    "expired session",
+    "session expired",
+    "session not found",
+    "unknown session",
+)
+
+
+def _is_session_expired_error(exc: BaseException) -> bool:
+    """Return True if ``exc`` looks like an MCP transport session expiry.
+
+    Streamable HTTP MCP servers may garbage-collect server-side session
+    state while the OAuth token remains valid — idle TTL, server
+    restart, horizontal-scaling pod rotation, etc.  The SDK surfaces
+    this as a JSON-RPC error whose message contains phrases like
+    ``"Invalid or expired session"``.  This class of failure is
+    distinct from :func:`_is_auth_error`: re-running the OAuth refresh
+    flow would be pointless because the access token is fine.  What's
+    needed is a transport reconnect — tear down and rebuild the
+    ``streamablehttp_client`` + ``ClientSession`` pair, which is
+    exactly what ``MCPServerTask._reconnect_event`` triggers.
+    """
+    if isinstance(exc, InterruptedError):
+        return False
+    # Exception messages vary across SDK versions + server
+    # implementations, so match on a small allow-list of stable
+    # substrings rather than exception type.  Kept narrow to avoid
+    # false positives on unrelated server errors.
+    msg = str(exc).lower()
+    if not msg:
+        return False
+    return any(marker in msg for marker in _SESSION_EXPIRED_MARKERS)
+
+
+def _handle_session_expired_and_retry(
+    server_name: str,
+    exc: BaseException,
+    retry_call,
+    op_description: str,
+):
+    """Trigger a transport reconnect and retry once on session expiry.
+
+    Unlike :func:`_handle_auth_error_and_retry`, this does **not** call
+    the OAuth manager's ``handle_401`` — the access token is still
+    valid, only the server-side session state is stale.  Setting
+    ``_reconnect_event`` causes the server task's lifecycle loop to
+    tear down the current ``streamablehttp_client`` + ``ClientSession``
+    and rebuild them, reusing the existing OAuth provider instance.
+    See #13383.
+
+    Args:
+        server_name: Name of the MCP server that raised.
+        exc: The exception from the failed call.
+        retry_call: Zero-arg callable that re-runs the operation,
+            returning the same JSON string format as the handler.
+        op_description: Human-readable name of the operation (logs).
+
+    Returns:
+        A JSON string if reconnect + retry was attempted and produced
+        a response, or ``None`` to fall through to the caller's
+        generic error path (not a session-expired error, no server
+        record, reconnect didn't ready in time, or retry also failed).
+    """
+    if not _is_session_expired_error(exc):
+        return None
+
+    with _lock:
+        srv = _servers.get(server_name)
+    if srv is None or not hasattr(srv, "_reconnect_event"):
+        return None
+
+    loop = _mcp_loop
+    if loop is None or not loop.is_running():
+        return None
+
+    logger.info(
+        "MCP server '%s': %s failed with session-expired error (%s); "
+        "signalling transport reconnect and retrying once.",
+        server_name, op_description, exc,
+    )
+
+    # Trigger the same reconnect mechanism the OAuth recovery path
+    # uses, then wait briefly for the new session to come back ready.
+    loop.call_soon_threadsafe(srv._reconnect_event.set)
+    deadline = time.monotonic() + 15
+    ready = False
+    while time.monotonic() < deadline:
+        if srv.session is not None and srv._ready.is_set():
+            ready = True
+            break
+        time.sleep(0.25)
+    if not ready:
+        logger.warning(
+            "MCP server '%s': reconnect did not ready within 15s after "
+            "session-expired error; falling through to error response.",
+            server_name,
+        )
+        return None
+
+    try:
+        result = retry_call()
+        try:
+            parsed = json.loads(result)
+            if "error" not in parsed:
+                _server_error_counts[server_name] = 0
+                return result
+        except (json.JSONDecodeError, TypeError):
+            _server_error_counts[server_name] = 0
+            return result
+    except Exception as retry_exc:
+        logger.warning(
+            "MCP %s/%s retry after session reconnect failed: %s",
+            server_name, op_description, retry_exc,
+        )
+    return None
+
+
 # Dedicated event loop running in a background daemon thread.
 _mcp_loop: Optional[asyncio.AbstractEventLoop] = None
 _mcp_thread: Optional[threading.Thread] = None
@@ -1868,6 +1991,16 @@ def _make_tool_handler(server_name: str, tool_name: str, tool_timeout: float):
             if recovered is not None:
                 return recovered
 
+            # Transport session expiry (#13383): same reconnect flow
+            # but skips OAuth recovery because the access token is
+            # still valid — only the server-side session is stale.
+            recovered = _handle_session_expired_and_retry(
+                server_name, exc, _call_once,
+                f"tools/call {tool_name}",
+            )
+            if recovered is not None:
+                return recovered
+
             _bump_server_error(server_name)
             logger.error(
                 "MCP tool %s/%s call failed: %s",
@@ -1920,6 +2053,11 @@ def _make_list_resources_handler(server_name: str, tool_timeout: float):
             recovered = _handle_auth_error_and_retry(
                 server_name, exc, _call_once, "resources/list",
             )
+            if recovered is not None:
+                return recovered
+            recovered = _handle_session_expired_and_retry(
+                server_name, exc, _call_once, "resources/list",
+            )
             if recovered is not None:
                 return recovered
             logger.error(
@@ -1974,6 +2112,11 @@ def _make_read_resource_handler(server_name: str, tool_timeout: float):
             recovered = _handle_auth_error_and_retry(
                 server_name, exc, _call_once, "resources/read",
             )
+            if recovered is not None:
+                return recovered
+            recovered = _handle_session_expired_and_retry(
+                server_name, exc, _call_once, "resources/read",
+            )
             if recovered is not None:
                 return recovered
             logger.error(
@@ -2031,6 +2174,11 @@ def _make_list_prompts_handler(server_name: str, tool_timeout: float):
             recovered = _handle_auth_error_and_retry(
                 server_name, exc, _call_once, "prompts/list",
             )
+            if recovered is not None:
+                return recovered
+            recovered = _handle_session_expired_and_retry(
+                server_name, exc, _call_once, "prompts/list",
+            )
             if recovered is not None:
                 return recovered
             logger.error(
@@ -2096,6 +2244,11 @@ def _make_get_prompt_handler(server_name: str, tool_timeout: float):
             recovered = _handle_auth_error_and_retry(
                 server_name, exc, _call_once, "prompts/get",
             )
+            if recovered is not None:
+                return recovered
+            recovered = _handle_session_expired_and_retry(
+                server_name, exc, _call_once, "prompts/get",
+            )
             if recovered is not None:
                 return recovered
             logger.error(