diff --git a/backend/client_config.py b/backend/client_config.py
index c9772b4..c3291b2 100644
--- a/backend/client_config.py
+++ b/backend/client_config.py
@@ -269,14 +269,34 @@ def mcp_entry(platform: str) -> dict:
 
 
 def _hooks_command_claude() -> str:
-    """Claude/WorkBuddy: curl the hook with $CLAUDE_TOOL_USE_INPUT."""
+    """Claude/WorkBuddy: curl the hook, forwarding stdin as the POST body.
+
+    Claude Code writes the PreToolUse JSON payload to the hook command's
+    **stdin** — there is no ``$CLAUDE_TOOL_USE_INPUT`` env var (that was a
+    mistaken assumption from an earlier version; confirmed against Claude
+    Code's hooks docs). A single-quoted ``'$CLAUDE_TOOL_USE_INPUT'`` is never
+    shell-expanded either way, so the old command always POSTed that literal
+    string — invalid JSON, so the backend 422'd on *every* call and curl's
+    non-2xx exit code (0) meant Claude Code silently treated it as "allow".
+    ``-d @-`` tells curl to read the POST body from stdin instead.
+    """
     return (
         f"curl -s -X POST {HOOK_ENDPOINT} "
         "-H 'Content-Type: application/json' "
-        "-d '$CLAUDE_TOOL_USE_INPUT'"
+        "-d @-"
     )
 
 
+# Old, broken Claude command (see _hooks_command_claude docstring) — kept only
+# so _matcher_is_mine still recognises an install written before this fix and
+# upgrades it in place instead of appending a second, duplicate matcher.
+_LEGACY_CLAUDE_HOOK_COMMANDS = (
+    f"curl -s -X POST {HOOK_ENDPOINT} "
+    "-H 'Content-Type: application/json' "
+    "-d '$CLAUDE_TOOL_USE_INPUT'",
+)
+
+
 def _hooks_command_codebuddy() -> str:
     """CodeBuddy: forward stdin JSON via the ``hooks_forward`` helper.
 
@@ -369,21 +389,24 @@ def hooks_matcher(platform: str) -> dict:
 def _matcher_is_mine(matcher: dict, cmd: str) -> bool:
     """True if a PreToolUse matcher is the MyKnowledge hook (by command signature).
 
-    Recognises our hook by its command (claude/curl-$CLAUDE_TOOL_USE_INPUT,
-    codebuddy/hooks_forward.py, or cursor's direct-entry form) so we never
-    mistake a user's own hook for ours.  Handles both the Claude/CodeBuddy
-    nested ``hooks[0].command`` shape and Cursor's direct ``command`` entry.
+    Recognises our hook by its command (claude/curl-@-, codebuddy/hooks_forward.py,
+    or cursor's direct-entry form) so we never mistake a user's own hook for
+    ours.  Handles both the Claude/CodeBuddy nested ``hooks[0].command`` shape
+    and Cursor's direct ``command`` entry.  Also matches
+    ``_LEGACY_CLAUDE_HOOK_COMMANDS`` so an install written before the
+    stdin/``-d @-`` fix is upgraded in place rather than duplicated.
     """
     if not isinstance(matcher, dict):
         return False
+    recognised = (cmd, *_LEGACY_CLAUDE_HOOK_COMMANDS)
     # Cursor entries carry the command directly on the entry.
-    if matcher.get("command") == cmd:
+    if matcher.get("command") in recognised:
         return True
     hooks = matcher.get("hooks")
     if not isinstance(hooks, list) or not hooks:
         return False
     first = hooks[0]
-    return isinstance(first, dict) and first.get("command") == cmd
+    return isinstance(first, dict) and first.get("command") in recognised
 
 
 def _merge_hook_entry(matchers: list, entry: dict, cmd: str) -> list:
@@ -391,16 +414,16 @@ def _merge_hook_entry(matchers: list, entry: dict, cmd: str) -> list:
 
     The matcher is identified by **command signature** (``_matcher_is_mine``), so a
     user's own hook sharing a matcher string is never touched.  If our hook already
-    exists but its ``matcher`` string is outdated (e.g. the old Claude ``Bash`` that
-    skipped Write/Edit), we replace it with the current entry instead of leaving the
-    stale one — otherwise re-running ``write_kind`` on an existing install would keep
-    the broken matcher and the fix would never take effect.
+    exists but is stale — an outdated ``matcher`` (e.g. the old Claude ``Bash`` that
+    skipped Write/Edit) or an outdated ``command`` (e.g. the old
+    ``$CLAUDE_TOOL_USE_INPUT`` payload bug) — we overwrite it with the current entry
+    unconditionally. Comparing only ``matcher`` here previously let a stale
+    ``command`` survive re-installs whenever the matcher string already matched,
+    so re-running ``write_kind`` silently kept the broken command forever.
     """
     for i, m in enumerate(matchers):
         if _matcher_is_mine(m, cmd):
-            if isinstance(m, dict) and m.get("matcher") == entry.get("matcher"):
-                return matchers  # already current — nothing to do
-            matchers[i] = entry  # upgrade stale matcher in place
+            matchers[i] = entry  # upgrade stale matcher/command in place
             return matchers
     matchers.append(entry)
     return matchers
diff --git a/tests/test_client_config.py b/tests/test_client_config.py
index fe6b7b6..038177e 100644
--- a/tests/test_client_config.py
+++ b/tests/test_client_config.py
@@ -146,15 +146,33 @@ def test_upgrades_stale_matcher_in_place(self, fake_home: Path) -> None:
         write_kind("ClaudeCode", "hooks")
         data = _read_json(s)
         matchers = data["hooks"]["PreToolUse"]
-        # 我们的钩子 matcher 已升级，且仍是唯一的 MyKnowledge 钩子（不重复追加）。
+        # 我们的钩子已升级到当前命令（-d @-），且仍是唯一的 MyKnowledge 钩子（不重复追加）。
         my = [m for m in matchers
-              if "$CLAUDE_TOOL_USE_INPUT" in m["hooks"][0]["command"]]
+              if "-d @-" in m["hooks"][0]["command"]]
         assert len(my) == 1
         assert my[0]["matcher"] == "Bash|Write|Edit"
+        assert "$CLAUDE_TOOL_USE_INPUT" not in my[0]["hooks"][0]["command"]
         # 用户的钩子（不同 command）不受影响。
         user = [m for m in matchers if m["hooks"][0]["command"] == "user-hook"]
         assert user and user[0]["matcher"] == "Edit|Write"
 
+    def test_upgrades_legacy_command_not_duplicated(self, fake_home: Path) -> None:
+        """Writing twice over a legacy-command install never yields 2+ of our matchers."""
+        s = fake_home / ".claude" / "settings.json"
+        _write_json(s, {"hooks": {
+            "PreToolUse": [
+                {"matcher": "Bash|Write|Edit",
+                 "hooks": [{"type": "command",
+                            "command": "curl -s -X POST http://127.0.0.1:8080/hooks/pre-tool-use "
+                                       "-H 'Content-Type: application/json' -d '$CLAUDE_TOOL_USE_INPUT'"}]},
+            ],
+        }})
+        write_kind("ClaudeCode", "hooks")
+        write_kind("ClaudeCode", "hooks")
+        matchers = _read_json(s)["hooks"]["PreToolUse"]
+        assert len(matchers) == 1
+        assert "-d @-" in matchers[0]["hooks"][0]["command"]
+
 
 class TestAgent:
     def test_creates_agent_file(self, fake_home: Path) -> None:
@@ -414,11 +432,12 @@ def test_bad_kind(self, fake_home: Path) -> None:
 
 class TestHooksMatcher:
     def test_commands_differ_by_platform(self, fake_home: Path) -> None:
-        """ClaudeCode uses curl/$CLAUDE_TOOL_USE_INPUT; CodeBuddyIDE uses helper script."""
+        """ClaudeCode uses curl reading stdin (-d @-); CodeBuddyIDE uses helper script."""
         claude = hooks_matcher("ClaudeCode")
         codebuddy = hooks_matcher("CodeBuddyIDE")
         assert claude["hooks"][0]["command"] != codebuddy["hooks"][0]["command"]
-        assert "$CLAUDE_TOOL_USE_INPUT" in claude["hooks"][0]["command"]
+        assert "-d @-" in claude["hooks"][0]["command"]
+        assert "$CLAUDE_TOOL_USE_INPUT" not in claude["hooks"][0]["command"]
         assert "backend.hooks_forward" in codebuddy["hooks"][0]["command"]
 
     def test_codebuddy_matcher_all(self, fake_home: Path) -> None:
@@ -515,7 +534,7 @@ def test_remove_hooks_keeps_others(self, fake_home: Path) -> None:
         data = _read_json(s)
         cmds = [m["hooks"][0]["command"] for m in data["hooks"]["PreToolUse"]]
         # our exact MyKnowledge command gone, user's own hooks preserved
-        assert "$CLAUDE_TOOL_USE_INPUT" not in "\n".join(cmds)
+        assert _hooks_command_claude() not in cmds
         assert user_bash_cmd in cmds
         assert "fmt" in cmds