fix(fp_mcp): redirect Fortran WRITE(6,...) to stderr to protect JSON-RPC channel

CHANGELOG.md

@@ -136,6 +136,15 @@ The format is loosely based on [Keep a Changelog](https://keepachangelog.com/).
   (`python/mcp-servers/fd_isolation_cases.py`), and
   `python/tests/test_fd_isolation.py` checks that all six servers run every
   one of them. `fp_mcp` follows in #232.
+- **MCP server `fp_mcp`: the same fd isolation** (#232). `fp_mcp` had the
+  most of it: `FPWRTPRF` writes the radial profile table to unit 6 once at
+  preparation and then every `NTG2STEP` steps (every step by default), so
+  the lines that are not JSON-RPC grew with `NTMAX`: in one stdio session
+  (`initialize`, `init`, `run`, library defaults) 63 of the 66 stdout lines
+  at `NTMAX=1`, 83 of 86 at `NTMAX=2`. `main()` now calls `_fd_isolation.install()`
+  immediately before the stdio server is built, as the other six servers
+  do; its `tests/test_fd_isolation.py` runs the shared checks, and
+  `python/tests/test_fd_isolation.py` checks all seven servers.
 
 ## TR Phase L — library-ization (2026-04-18)
 

python/mcp-servers/fp_mcp/README.md

@@ -354,6 +354,8 @@ FP の Fortran 側は FPCOMM 単一状態なので、1 プロセス = 1 イン
 
 MCP サーバの標準エラー出力が LLM クライアントに渡ります。Claude Desktop なら「開発者ツール」相当のログビューア、Claude Code なら実行ターミナルで確認できます。
 
+Fortran 側の `WRITE(6,...)`（unit 6 = fd 1）も同じ標準エラー出力に流れます。stdio サーバでは fd 1 が JSON-RPC の送信パイプなので、`main()` がサーバ構築の直前に共有モジュール `python/_fd_isolation.py` で fd 1 を stderr に付け替えています（#227 / #232 / #257）。FP は半径プロファイルの表を unit 6 に書きます（準備時に 1 回、その後は `NTG2STEP` ステップごと、既定では毎ステップ）。この付け替えが無いと、その出力がステップ数とともに増え、パイプを壊します。端末で手動起動したときや `2>&1` で stdout と stderr が同じファイルになっているときは、unit 6 の出力は捨てられ、その旨の警告が stderr に 1 行出ます（同一ファイルの判定は `fstat` の inode によるので、パイプやコンソールに inode が無い Windows では判定されず、fd 1 は通常どおり stderr に向きます）。ライブラリの出力を残したい場合は stderr を別に渡してください（例: `python -m fp_mcp.server 2>server.log`）。
+
 ## 10. 参考資料
 
 - MCP spec: <https://modelcontextprotocol.io/>

python/mcp-servers/fp_mcp/server.py

@@ -30,6 +30,26 @@
 :mod:`fplib`. This file mirrors ``tr_mcp/server.py``; adding the
 remaining modules (ti / wr / wrx / tot) should be a matter of copying
 this file and swapping the backing library.
+
+fd-isolation (Fortran WRITE(6) vs MCP JSON-RPC)
+-----------------------------------------------
+Fortran WRITE(6,...) targets OS fd 1, which is also the JSON-RPC write
+pipe to the MCP client parent.  Any Fortran diagnostic line corrupts the
+pipe: the client logs "Failed to parse JSONRPC message from server" for
+each junk line, and in the worst case the session drops with
+"Connection closed".
+
+FP is a heavy offender: ``FPWRTPRF`` prints the whole radial profile
+table to unit 6 once at preparation and then every NTG2STEP steps (every
+step by default), so the damage grows with NTMAX (measured client-side on
+the fp_iter01 fixture: 150 parse errors at NTMAX=1, 211 at NTMAX=2).
+
+main() therefore calls ``_fd_isolation.install()`` (the module shared by
+the stdio servers, ``python/_fd_isolation.py``) immediately before the
+stdio server is built -- NOT at import time, see #227 item 1.
+It moves the pipe to a descriptor of its own, rebuilds ``sys.stdout`` on
+it and gives fd 1 to stderr, so that Fortran WRITE(6,...) lands in the
+server's log.
 """
 from __future__ import annotations
 
@@ -85,6 +105,10 @@
     FplibNotImplementedError,
 )
 
+# The fd isolation shared by the stdio servers; it lives beside the wrapper,
+# in ``<repo>/python/``.
+import _fd_isolation  # noqa: E402
+
 
 # =====================================================================
 # Parameter & state schema metadata.
@@ -644,6 +668,11 @@ def main(argv: Optional[List[str]] = None) -> int:
         )
         return 2
 
+    # Install the fd isolation now -- NOT at import time (#227 item 1).
+    # Everything above this point (--help, --print-tools, the MCP-missing
+    # error path) returns before we touch the host process's fds.
+    _fd_isolation.install()
+
     server = build_server()
     # FastMCP >=0.9 exposes .run() for stdio transport by default.
     server.run()

python/mcp-servers/fp_mcp/tests/test_fd_isolation.py

@@ -0,0 +1,43 @@
+"""fp_mcp's share of the fd-isolation checks (fd_isolation_cases): its
+main() installs the shared isolation, and nothing else of it does."""
+import sys
+from pathlib import Path
+
+import pytest
+
+MCP_ROOT = Path(__file__).resolve().parents[2]             # <repo>/python/mcp-servers
+if str(MCP_ROOT) not in sys.path:
+    sys.path.insert(0, str(MCP_ROOT))
+
+pytest.register_assert_rewrite("fd_isolation_cases")
+import fd_isolation_cases as cases  # noqa: E402
+
+SERVER = "fp_mcp"
+
+
+def test_import_leaves_fd1_and_sys_stdout_alone():
+    cases.import_leaves_fd1_and_sys_stdout_alone(SERVER)
+
+
+def test_main_installs_the_isolation_before_the_stdio_server_is_built():
+    cases.main_installs_the_isolation_before_the_stdio_server_is_built(SERVER)
+
+
+@pytest.mark.parametrize("flag", ["--help", "--print-tools"])
+def test_help_and_print_tools_install_nothing(flag):
+    cases.flag_installs_nothing(SERVER, flag)
+
+
+def test_missing_sdk_installs_nothing():
+    cases.missing_sdk_installs_nothing(SERVER)
+
+
+@pytest.mark.skipif(sys.platform == "win32",
+                    reason="fstat gives a pipe no identity there: a merged stream is not recognised")
+def test_stderr_merged_into_stdout_keeps_fortran_output_off_the_stream():
+    cases.stderr_merged_into_stdout_keeps_fortran_output_off_the_stream(SERVER)
+
+
+@pytest.mark.skipif(sys.platform == "win32", reason="preexec_fn is POSIX only")
+def test_closed_fd2_keeps_everything_but_json_rpc_off_the_pipe():
+    cases.closed_fd2_keeps_everything_but_json_rpc_off_the_pipe(SERVER)

python/tests/test_fd_isolation.py

@@ -25,7 +25,7 @@
 import fd_isolation_cases as cases  # noqa: E402
 from fd_isolation_cases import run  # noqa: E402
 
-SERVERS = ("eq_mcp", "tr_mcp", "tot_mcp", "ti_mcp", "wr_mcp", "wrx_mcp")
+SERVERS = ("eq_mcp", "tr_mcp", "tot_mcp", "ti_mcp", "wr_mcp", "wrx_mcp", "fp_mcp")
 
 
 # ----- every server runs every shared check -----