fix(mcp): ti_mcp, wr_mcp and wrx_mcp keep Fortran output off the JSON-RPC pipe (#262)

CHANGELOG.md

@@ -48,6 +48,16 @@ The format is loosely based on [Keep a Changelog](https://keepachangelog.com/).
   terminal shows that line and no longer the library's output. Where
   `fstat` gives a descriptor no identity (Windows for a pipe or a console)
   the merged layout is not recognised and fd 1 goes to stderr as usual.
+- **MCP servers `ti_mcp`, `wr_mcp`, `wrx_mcp`: the same fd isolation** (#262).
+  The three servers had the layout of the other three and the same exposure:
+  `main()` now calls `_fd_isolation.install()` immediately before the stdio
+  server is built, importing a server module leaves the importing process's
+  fd 1 and `sys.stdout` alone, and `--help` / `--print-tools` / the
+  missing-SDK path return before anything is redirected. Each server's
+  `tests/test_fd_isolation.py` runs the shared checks
+  (`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.
 
 ## TR Phase L — library-ization (2026-04-18)
 

python/mcp-servers/ti_mcp/server.py

@@ -20,6 +20,21 @@
 The server is intentionally small — the real heavy lifting is in
 :mod:`tilib`. This file mirrors ``tr_mcp/server.py`` and swaps the
 backing library, registry, and state schema to match TI.
+
+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".
+
+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
 
@@ -76,6 +91,10 @@
     TilibNotImplementedError,
 )
 
+# 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.
@@ -690,6 +709,11 @@ 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/ti_mcp/tests/test_fd_isolation.py

@@ -0,0 +1,43 @@
+"""ti_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 = "ti_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/mcp-servers/wr_mcp/server.py

@@ -29,6 +29,21 @@
 :mod:`wrlib`. This file mirrors ``tr_mcp/server.py`` with only the
 module-specific names swapped so that future ti / wrx / fp servers
 can follow the same pattern.
+
+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".
+
+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
 
@@ -84,6 +99,10 @@
     WrlibNotImplementedError,
 )
 
+# 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.
@@ -724,6 +743,11 @@ 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/wr_mcp/tests/test_fd_isolation.py

@@ -0,0 +1,43 @@
+"""wr_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 = "wr_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/mcp-servers/wrx_mcp/server.py

@@ -28,6 +28,21 @@
 This mirrors the ``tr_mcp`` reference implementation as closely as
 possible; the main wrx-specific additions are the WRX_RUN_OK gate and
 the different parameter registry / state schema.
+
+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".
+
+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
 
@@ -83,6 +98,10 @@
     WrxlibNotImplementedError,
 )
 
+# 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.
@@ -718,6 +737,11 @@ 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/wrx_mcp/tests/test_fd_isolation.py

@@ -0,0 +1,43 @@
+"""wrx_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 = "wrx_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")
+SERVERS = ("eq_mcp", "tr_mcp", "tot_mcp", "ti_mcp", "wr_mcp", "wrx_mcp")
 
 
 # ----- every server runs every shared check -----