diff --git a/CHANGELOG.md b/CHANGELOG.md
index 90d860edf..aeb86d09d 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -46,6 +46,10 @@ breaking changes may land in a minor release.
 
 ### Changed
 
+- Start bmad-loop's psmux servers with `PSMUX_NO_WARM=1`: their warm pool and hidden
+  `__warm__` standby were never used, and dropping them cuts an idle project from 16 psmux
+  processes and about 935 MiB of summed working sets to 4 and about 175 MiB. The operator's
+  own psmux keeps its pool (#903).
 - Make an un-enabled builtin `[python]` plugin wholly inert: its hooks, workflows and
   seeds no longer run without its module (#496, #497).
 - Run granted project plugins from a verified out-of-tree snapshot: `{scripts}` points
@@ -79,6 +83,9 @@ breaking changes may land in a minor release.
   directory. Under worktree isolation on Windows psmux, the session held the first unit's
   worktree, so its teardown journaled `worktree-teardown-degraded` and left an empty
   `worktrees/1` behind (#902).
+- Refuse psmux option values shaped `-X=value`, `-t…` or `--`: psmux 3.4.0 rewrites the
+  first silently and rejects the others, so the value is now warned about and its key freed
+  (#905).
 - Fire the TUI's stale state-root note on psmux too: the psmux backend now answers which
   value a new pane inherits through psmux 3.4.0's named `show-environment` (#660).
 - Fix the OpenCode V2 SSE-reconnect end-to-end test failing on slow runners (Windows): the
diff --git a/docs/multiplexer-backends.md b/docs/multiplexer-backends.md
index c952d1106..71b317be3 100644
--- a/docs/multiplexer-backends.md
+++ b/docs/multiplexer-backends.md
@@ -101,7 +101,10 @@ session-qualified (`session:@N`).
 
 **Refused option values.** An `@` option value must read back verbatim. A value that is
 empty, contains a double quote or a line break, or has leading or trailing whitespace is
-refused with a stderr warning, and the option reads as unset. Ordinary
+refused with a stderr warning, and the option reads as unset. So are three `-`-leading
+shapes that psmux's client rewrites before parsing: `-X=value` (one
+letter, `=`, then at least one character), any value starting with `-t`, and `--`. Other
+`-`-leading values (`-u`, `-1=x`, `--x=y`) pass. Ordinary
 Windows paths (spaced, UNC, apostrophed, trailing separator) pass. The project ownership
 tag is a hex digest of the project path, so it always passes.
 
diff --git a/src/bmad_loop/adapters/psmux_backend.py b/src/bmad_loop/adapters/psmux_backend.py
index f955c5829..a6bf72d16 100644
--- a/src/bmad_loop/adapters/psmux_backend.py
+++ b/src/bmad_loop/adapters/psmux_backend.py
@@ -644,6 +644,18 @@ def new_session(
             )
         }
         env["PSMUX_ALLOW_NESTING"] = "1"
+        # psmux's warm pool (spare shells per server) and its hidden `__warm__`
+        # standby server serve only default-shell creates: every window this
+        # backend opens carries a command and every session passes `-c`, so
+        # neither is ever claimed, yet together they cost about 0.75 GiB of
+        # summed working sets per idle project (#903). The server reads this at
+        # boot, and this call is the one that boots it; scoped to the spawn, so
+        # the operator's own psmux keeps its pool. Ceilings: the server's panes
+        # inherit it, so a psmux session started from inside one starts cold;
+        # and a psmux config that says `set -g warm on` re-enables the pool at
+        # load, which stays the operator's call (forcing `warm off` at runtime
+        # would also kill the registry's shared standby).
+        env["PSMUX_NO_WARM"] = "1"
         # The initial window is a window launch too: the same PowerShell floor
         # as `_shell_wrap`, checked before psmux creates anything.
         self._require_pwsh_floor()
@@ -947,10 +959,21 @@ def _transportable(value: str) -> bool:
         # the wire now carries them — a value that reads back different is the
         # same silently-unprunable window whichever hop mangled it. An empty
         # value is a silent server-side no-op (`-u` is the verb for that).
+        #
+        # A few `-`-leading shapes still lose the WIRE half, to the client's
+        # argv normalizers that run before `set-option` parses (psmux/psmux#795,
+        # cli.rs at v3.4.0): `normalize_flag_equals` splits `-X=value` (one
+        # ASCII letter, a nonempty suffix) into `-X value` and the write
+        # succeeds silently; `normalize_attached_target_flag` splits `-t…` into
+        # a target flag, and `--` ends the options, so those two fail rc 1 and
+        # would leave a stale value in place. Other `-` values (`-u`, `-1=x`,
+        # `--x=y`, `-x=`) are stored verbatim since #583.
         if not value:
             return False
         if '"' in value or value != value.strip():
             return False
+        if value == "--" or value.startswith("-t") or re.match(r"-[A-Za-z]=.", value):
+            return False
         return value.splitlines() == [value]
 
     def set_session_option(self, name: str, option: str, value: str) -> None:
diff --git a/tests/test_psmux_backend.py b/tests/test_psmux_backend.py
index cc4202e2e..f3fadb902 100644
--- a/tests/test_psmux_backend.py
+++ b/tests/test_psmux_backend.py
@@ -361,6 +361,9 @@ def test_new_session_bypasses_nesting_guard(rec, monkeypatch, tmp_path):
     # the no-op belt: create is verified by a has-session probe afterwards
     assert rec.argv == ["psmux", "has-session", "-t", "=s"]
     assert create_kwargs["env"]["PSMUX_ALLOW_NESTING"] == "1"
+    # the server this call boots keeps no warm pool and spawns no `__warm__`
+    # standby: neither serves a `-c` session or a command-bearing window (#903)
+    assert create_kwargs["env"]["PSMUX_NO_WARM"] == "1"
     # the claude session vars are scrubbed from the create env (the psmux server
     # this call may cold-start would otherwise hand them to every window)
     assert "CLAUDE_CODE_SSE_PORT" not in create_kwargs["env"]
@@ -1681,6 +1684,15 @@ def test_kill_window_unroutable_target_skips_cleanup_but_kills(monkeypatch):
     assert [c[0][1] for c in rec_.calls] == ["kill-window"]
 
 
+# Shapes psmux's client argv normalizers rewrite or refuse before `set-option`
+# parses its value (psmux/psmux#795): `-X=value` reads back as `-X value` with
+# rc 0, and `-t…`/`--` fail rc 1, leaving a prior value in place.
+_PSMUX_ARGV_SHAPES = ["-x=foo", "-a=b c", "-Z=1", "-t", "-tx", "-t=foo", "-target", "--"]
+# Their measured neighbours, which psmux 3.4.0 stores verbatim: the refusal must
+# not widen to them.
+_DASH_SAFE_CONTROLS = ["-1=x", "--x=y", "-x=", "-a="]
+
+
 @pytest.mark.parametrize(
     "value",
     [
@@ -1691,6 +1703,7 @@ def test_kill_window_unroutable_target_skips_cleanup_but_kills(monkeypatch):
         "",  # empty write is a silent server-side no-op; unset exists for this
         " C:\\p x",  # leading space survives the wire but this backend's reads strip
         "C:\\p x ",  # trailing space, same round-trip loss
+        *_PSMUX_ARGV_SHAPES,
     ],
 )
 def test_set_window_option_refuses_untransportable_values(monkeypatch, capsys, value):
@@ -1721,6 +1734,7 @@ def test_set_window_option_refuses_untransportable_values(monkeypatch, capsys, v
         "x\u00a0y",  # non-ASCII whitespace no longer splits server-side (#536)
         "-u",  # a `-`-leading value is stored, not read as a flag (psmux/psmux#583)
         "-flag",
+        *_DASH_SAFE_CONTROLS,
     ],
 )
 def test_set_window_option_accepts_wire_safe_values(monkeypatch, value):
@@ -1747,6 +1761,7 @@ def test_set_window_option_write_failure_warns(monkeypatch, capsys):
         'C:\\a"b\\proj',  # `"` cannot survive the listing's one-quote-pair strip
         "bad\nline",  # the listing is parsed line-by-line
         "",  # an empty write is a silent server-side no-op
+        *_PSMUX_ARGV_SHAPES,
     ],
 )
 def test_set_session_option_refuses_untransportable_values(monkeypatch, capsys, value):
@@ -1775,6 +1790,7 @@ def test_set_session_option_refuses_untransportable_values(monkeypatch, capsys,
         "\\\\srv\\share name\\proj",  # spaced UNC — `\\` no longer collapses (#547)
         "C:\\Users\\O'Brien\\proj",  # unspaced `'` no longer opens a quote
         "-u",  # a `-`-leading value is stored, not read as a flag (psmux/psmux#583)
+        *_DASH_SAFE_CONTROLS,
     ],
 )
 def test_set_session_option_accepts_transportable_values(monkeypatch, value):
diff --git a/tests/test_psmux_live.py b/tests/test_psmux_live.py
index 3e81d3778..bd186880c 100644
--- a/tests/test_psmux_live.py
+++ b/tests/test_psmux_live.py
@@ -968,6 +968,37 @@ def test_adopted_a_dash_leading_option_value_is_stored(probe):
     )
 
 
+@pytest.mark.parametrize(
+    ("value", "outcome"),
+    [
+        ("-x=foo", (0, "-x foo")),  # rewritten, and the write reports success
+        ("-tx", (1, "seed")),  # read as a target flag: refused, the seed stays
+        ("--", (1, "seed")),  # read as the end of options: same
+    ],
+)
+def test_premise_an_argv_flag_shaped_option_value_is_not_stored(probe, value, outcome):
+    # psmux's client argv normalizers rewrite `-X=value` to `-X value` and read
+    # `-t…`/`--` as flags before `set-option` sees its value (psmux/psmux#795).
+    # One row per family, so each retires on its own: red means psmux handles
+    # that shape differently now, and if it reads back verbatim,
+    # _transportable's matching refusal is droppable behind a floor bump. The
+    # write's rc is part of the premise, so a probe whose write never ran
+    # cannot pass on the seed alone.
+    mux, session, _ = probe
+    seeded = mux._run(["set-option", "-t", session, "@k", "seed"], check=False)
+    assert seeded.returncode == 0, f"probe setup: seeding @k failed: {seeded.stderr.strip()!r}"
+    written = mux._run(["set-option", "-t", session, "@k", value], check=False)
+    got = mux._run(["show-options", "-qv", "-t", session, "@k"], check=False)
+    assert got.returncode == 0, f"probe setup: reading @k back failed: {got.stderr.strip()!r}"
+    observed = (written.returncode, got.stdout.strip())
+    assert observed == outcome, (
+        f"psmux now answers the option value {value!r} with (rc, read back) {observed!r}, "
+        f"not {outcome!r} (psmux/psmux#795) — if it reads back verbatim, "
+        "psmux_backend._transportable's refusal of that shape is droppable behind a "
+        "floor bump"
+    )
+
+
 def test_adopted_a_pane_inherits_the_servers_start_path(tmp_path):
     """A pane inherits the PATH of the process that started its server, and a
     bare `psmux` in the pane resolves through it (psmux/psmux#773); 3.3.8
@@ -1365,6 +1396,61 @@ def test_premise_a_port_and_key_pair_is_answered_by_whichever_server_it_names(
             (root / f"{forged}.{suffix}").unlink(missing_ok=True)
 
 
+def test_adopted_a_backend_session_spawns_no_warm_standby(tmp_path, monkeypatch):
+    """A session the backend starts boots its server with ``PSMUX_NO_WARM=1``,
+    so no hidden ``__warm__`` standby joins the registry (#903): a booting
+    server with warm enabled spawns one and publishes ``__warm__.port`` beside
+    its own. Red means the create env lost the variable, or psmux stopped
+    honoring it at boot, and every idle project carries the standby again.
+
+    Its own root, not the module fixture's: the raw probe sessions there boot
+    with warm on, so their standby would answer for this one. And an empty
+    config of its own: an inherited `set -g warm off` would suppress the
+    standby whatever the env says.
+    """
+    mux = PsmuxMultiplexer()
+    if not mux.available():
+        pytest.skip("psmux present but not an admitted version")
+    root = tmp_path / "registry"
+    root.mkdir()
+    monkeypatch.setenv("PSMUX_DATA_DIR", str(root))
+    monkeypatch.delenv("PSMUX_NO_WARM", raising=False)  # the backend must supply it
+    monkeypatch.delenv("PSMUX_WARM_POOL_SIZE", raising=False)
+    config = tmp_path / "empty.conf"
+    config.write_text("", encoding="utf-8")
+    monkeypatch.setenv("PSMUX_CONFIG_FILE", str(config))
+    env = _new_session_env()
+    session = f"bmad-loop-nowarm-probe-{uuid.uuid4().hex[:8]}"
+    standby = root / "__warm__.port"
+    try:
+        mux.new_session(session, tmp_path)
+        assert (root / f"{session}.port").is_file(), "probe setup: the session has no port file"
+        # The standby is spawned during the server's boot, and a server may take
+        # psmux's whole readiness window to register on a loaded box; reading
+        # absence any sooner could pass while a late standby is still coming.
+        deadline = time.monotonic() + psmux_teardown.PSMUX_READY_DEADLINE_S
+        while time.monotonic() < deadline and not standby.exists():
+            time.sleep(0.25)
+        assert not standby.exists(), (
+            "a session the psmux backend started spawned a `__warm__` standby — "
+            "new_session's PSMUX_NO_WARM no longer reaches the server it boots"
+        )
+    finally:
+        _teardown_probe_session(mux, session, env)
+        # A standby that registered late is a server of its own, which the
+        # session teardown never reaches. The root is this test's private
+        # tmp dir, so a kill-server scoped to it cannot touch anyone else's.
+        if standby.exists():
+            assert str(root).startswith(str(tmp_path))
+            subprocess.run(
+                ["psmux", "kill-server"],
+                env={**os.environ, "PSMUX_DATA_DIR": str(root)},
+                capture_output=True,
+                timeout=tmux_base.TMUX_TIMEOUT_S,
+                check=False,
+            )
+
+
 def test_adopted_a_missing_registry_root_self_heals(tmp_path):
     """The derived root is a path under the state root that nothing creates in
     advance, so the design leans on psmux ``create_dir_all``-ing it itself