diff --git a/plugins/kanban/dashboard/plugin_api.py b/plugins/kanban/dashboard/plugin_api.py
index 7cc814dcf..0a49172a8 100644
--- a/plugins/kanban/dashboard/plugin_api.py
+++ b/plugins/kanban/dashboard/plugin_api.py
@@ -38,7 +38,6 @@ from __future__ import annotations
 import asyncio
 import json
 import logging
-import os
 import sqlite3
 import time
 from dataclasses import asdict
@@ -1620,10 +1619,12 @@ def specify_task_endpoint(
     """
     board = _resolve_board(board)
     # Pin the board for the duration of this call so the specifier module
-    # (which calls ``kb.connect()`` with no args) hits the right DB.
-    prev_env = os.environ.get("HERMES_KANBAN_BOARD")
-    try:
-        os.environ["HERMES_KANBAN_BOARD"] = board or kanban_db.DEFAULT_BOARD
+    # (which calls ``kb.connect()`` with no args) hits the right DB. Use a
+    # context-local override rather than mutating the process-global
+    # HERMES_KANBAN_BOARD env var — this endpoint runs in FastAPI's
+    # threadpool, so two concurrent requests for different boards would
+    # otherwise race on the shared env var and cross-write (issue #38323).
+    with kanban_db.scoped_current_board(board or kanban_db.DEFAULT_BOARD):
         # Import lazily so a missing auxiliary client at import time
         # doesn't break plugin load.
         from hermes_cli import kanban_specify  # noqa: WPS433 (intentional)
@@ -1632,11 +1633,6 @@ def specify_task_endpoint(
             task_id,
             author=(payload.author or None),
         )
-    finally:
-        if prev_env is None:
-            os.environ.pop("HERMES_KANBAN_BOARD", None)
-        else:
-            os.environ["HERMES_KANBAN_BOARD"] = prev_env
 
     return {
         "ok": bool(outcome.ok),
@@ -2233,19 +2229,16 @@ def decompose_task_endpoint(
     can take minutes on reasoning models.
     """
     board = _resolve_board(board)
-    prev_env = os.environ.get("HERMES_KANBAN_BOARD")
-    try:
-        os.environ["HERMES_KANBAN_BOARD"] = board or kanban_db.DEFAULT_BOARD
+    # Context-local board pin (see specify endpoint above): this sync
+    # endpoint runs in FastAPI's threadpool, so mutating the process-global
+    # HERMES_KANBAN_BOARD env var would let concurrent requests for
+    # different boards race and cross-write (issue #38323).
+    with kanban_db.scoped_current_board(board or kanban_db.DEFAULT_BOARD):
         from hermes_cli import kanban_decompose  # noqa: WPS433 (intentional)
         outcome = kanban_decompose.decompose_task(
             task_id,
             author=(payload.author or None),
         )
-    finally:
-        if prev_env is None:
-            os.environ.pop("HERMES_KANBAN_BOARD", None)
-        else:
-            os.environ["HERMES_KANBAN_BOARD"] = prev_env
 
     return {
         "ok": bool(outcome.ok),