diff --git a/decnet/profiler/behave_shell/_features/__init__.py b/decnet/profiler/behave_shell/_features/__init__.py
index 7cba8bc2..eff85fce 100644
--- a/decnet/profiler/behave_shell/_features/__init__.py
+++ b/decnet/profiler/behave_shell/_features/__init__.py
@@ -25,6 +25,7 @@ from decnet.profiler.behave_shell._features.cognitive import (
     inter_command_latency_class,
 )
 from decnet.profiler.behave_shell._features.environmental import (
+    locale,
     shell_type,
     terminal_multiplexer,
 )
@@ -73,4 +74,5 @@ FEATURES: tuple[FeatureFn, ...] = (
     landing_ritual,
     shell_type,
     terminal_multiplexer,
+    locale,
 )
diff --git a/decnet/profiler/behave_shell/_features/environmental.py b/decnet/profiler/behave_shell/_features/environmental.py
index a96d815b..cdba4722 100644
--- a/decnet/profiler/behave_shell/_features/environmental.py
+++ b/decnet/profiler/behave_shell/_features/environmental.py
@@ -7,12 +7,25 @@ which F.1 / F.3 / E.4 read.
 
 Step F.1: ``environmental.shell_type``.
 Step F.2: ``environmental.terminal_multiplexer``.
+Step F.3: ``environmental.locale``.
 """
 from __future__ import annotations
 
 import collections
+import re
 from typing import Iterator
 
+from decnet_behave_core.spec.envelope import Observation
+
+from decnet.profiler.behave_shell._ctx import SessionContext
+from decnet.profiler.behave_shell._features._emit import make_observation
+from decnet.profiler.behave_shell._parse import PromptLine, strip_ansi
+from decnet.profiler.behave_shell._thresholds import (
+    LOCALE_MIN_VALUE_LENGTH,
+    SHELL_TYPE_MIN_PROMPTS,
+)
+
+
 # Multiplexer fingerprints scanned over RAW output (multiplexer escapes
 # ARE ANSI sequences, so we must NOT strip-ANSI before searching).
 # Sources:
@@ -31,14 +44,42 @@ _SCREEN_MARKERS: tuple[str, ...] = (
     "\x1b]83;",
 )
 
-from decnet_behave_core.spec.envelope import Observation
 
-from decnet.profiler.behave_shell._ctx import SessionContext
-from decnet.profiler.behave_shell._features._emit import make_observation
-from decnet.profiler.behave_shell._parse import PromptLine
-from decnet.profiler.behave_shell._thresholds import (
-    SHELL_TYPE_MIN_PROMPTS,
+# Locale envvar regex: matches `KEY=VALUE` where KEY is one of the
+# three locale envvars and VALUE is a POSIX locale name. The value
+# pattern is intentionally restrictive — letters, underscore for the
+# territory delimiter, optional codeset (.UTF-8 / .utf8), optional
+# modifier (@euro). The trailing `(?=[\s'\"\\$]|$)` anchors the
+# match against shell quoting and end-of-line.
+_LOCALE_VALUE_RE = re.compile(
+    r"(?P<key>LC_ALL|LANG|LC_CTYPE)=(?P<val>[A-Za-z]{2,3}"
+    r"(?:_[A-Za-z]{2,3})?(?:\.[A-Za-z0-9-]+)?(?:@[A-Za-z0-9]+)?|C|POSIX)"
 )
+_LOCALE_KEY_PRIORITY: dict[str, int] = {"LC_ALL": 3, "LANG": 2, "LC_CTYPE": 1}
+
+
+def _to_bcp47(posix_value: str) -> str | None:
+    """Normalise a POSIX locale value to a BCP-47 tag.
+
+    Returns:
+        ``None`` when the value is malformed (caller skips emission).
+        ``"und"`` for ``C`` / ``POSIX`` (BCP-47 'undetermined').
+        Otherwise ``language-REGION`` (e.g. ``en-US``, ``pt-BR``);
+        codeset / modifier suffixes dropped (BCP-47 doesn't carry them).
+    """
+    if posix_value in ("C", "POSIX"):
+        return "und"
+    base = posix_value.split(".", 1)[0].split("@", 1)[0]
+    parts = base.split("_")
+    if not parts or not parts[0].isalpha() or len(parts[0]) < 2:
+        return None
+    lang = parts[0].lower()
+    if len(parts) == 1:
+        return lang
+    region = parts[1]
+    if not region.isalpha():
+        return None
+    return f"{lang}-{region.upper()}"
 
 
 def _classify_shell_from_prompt(p: PromptLine) -> str:
@@ -139,3 +180,51 @@ def terminal_multiplexer(ctx: SessionContext) -> Iterator[Observation]:
         value=value,
         confidence=confidence,
     )
+
+
+def locale(ctx: SessionContext) -> Iterator[Observation]:
+    """Emit ``environmental.locale`` (free-string BCP-47 tag).
+
+    Searches the ANSI-stripped output stream for ``LANG=``,
+    ``LC_ALL=``, or ``LC_CTYPE=`` substrings — emitted when the
+    operator runs ``env``, ``locale``, or ``printenv``. Highest-priority
+    key wins (``LC_ALL`` > ``LANG`` > ``LC_CTYPE``); the POSIX value is
+    normalised to BCP-47:
+
+    * ``en_US.UTF-8`` → ``en-US``
+    * ``pt_BR.UTF-8`` → ``pt-BR``
+    * ``C`` / ``POSIX`` → ``und``
+    * malformed → skip emission
+
+    Skip emission when no envvar dump is found in the output —
+    silence rather than fabricating a default.
+    """
+    if not ctx.commands:
+        return
+    # Concatenate output; strip ANSI once (locale values aren't escape
+    # sequences themselves so the strip is safe).
+    raw = "".join(d for _t, _k, d in ctx.output_events)
+    if not raw:
+        return
+    text = strip_ansi(raw)
+
+    best_priority = 0
+    best_value: str | None = None
+    for m in _LOCALE_VALUE_RE.finditer(text):
+        prio = _LOCALE_KEY_PRIORITY[m.group("key")]
+        if prio <= best_priority:
+            continue
+        bcp47 = _to_bcp47(m.group("val"))
+        if bcp47 is None or len(bcp47) < LOCALE_MIN_VALUE_LENGTH:
+            continue
+        best_priority = prio
+        best_value = bcp47
+
+    if best_value is None:
+        return
+    yield make_observation(
+        ctx,
+        primitive="environmental.locale",
+        value=best_value,
+        confidence=0.80,
+    )
diff --git a/decnet/profiler/behave_shell/_thresholds.py b/decnet/profiler/behave_shell/_thresholds.py
index 3b49d0ae..dfc98c5f 100644
--- a/decnet/profiler/behave_shell/_thresholds.py
+++ b/decnet/profiler/behave_shell/_thresholds.py
@@ -233,6 +233,11 @@ PROMPT_LINE_MAX_CHARS: int = 256
 # honesty). Above, the shell-type vote is robust.
 SHELL_TYPE_MIN_PROMPTS: int = 3
 
+# ── environmental.locale (Step F.3) ────────────────────────────────────────
+# Below this many characters in the parsed locale value, treat as
+# noise and skip emission (a single 'C' or 'en' is too thin).
+LOCALE_MIN_VALUE_LENGTH: int = 2
+
 # ── motor.keystroke_cadence (Step B.1) ──────────────────────────────────────
 # Typing bursts split at gaps > IKI_THINK_MAX_S so think-pauses between
 # commands don't inflate the within-burst CV. Mirrors the prototype's
diff --git a/tests/profiler/behave_shell/test_environmental_locale.py b/tests/profiler/behave_shell/test_environmental_locale.py
new file mode 100644
index 00000000..fa213bce
--- /dev/null
+++ b/tests/profiler/behave_shell/test_environmental_locale.py
@@ -0,0 +1,95 @@
+"""Step F.3: ``environmental.locale``."""
+from __future__ import annotations
+
+from decnet.profiler.behave_shell import extract_session
+from decnet.profiler.behave_shell._features.environmental import _to_bcp47
+from decnet.profiler.behave_shell._parse import AsciinemaEvent
+
+
+PRIMITIVE = "environmental.locale"
+
+
+def _of(observations: list, primitive: str):
+    obs = [o for o in observations if o.primitive == primitive]
+    assert len(obs) == 1, f"expected exactly one {primitive}, got {len(obs)}"
+    return obs[0]
+
+
+def _typed(text: str, t0: float = 0.0, dt: float = 0.05) -> list[AsciinemaEvent]:
+    return [(t0 + i * dt, "i", c) for i, c in enumerate(text)]
+
+
+# ── _to_bcp47 ──────────────────────────────────────────────────────────────
+
+
+def test_to_bcp47_lang_region() -> None:
+    assert _to_bcp47("en_US.UTF-8") == "en-US"
+    assert _to_bcp47("pt_BR.UTF-8") == "pt-BR"
+    assert _to_bcp47("de_DE@euro") == "de-DE"
+
+
+def test_to_bcp47_language_only() -> None:
+    assert _to_bcp47("fr") == "fr"
+
+
+def test_to_bcp47_c_posix() -> None:
+    assert _to_bcp47("C") == "und"
+    assert _to_bcp47("POSIX") == "und"
+
+
+def test_to_bcp47_malformed() -> None:
+    assert _to_bcp47("X") is None        # too short
+    assert _to_bcp47("en_99") is None    # non-alpha region
+
+
+# ── feature integration ────────────────────────────────────────────────────
+
+
+def test_no_envvar_dump_no_emission() -> None:
+    events: list[AsciinemaEvent] = [
+        *_typed("ls\r"),
+        (0.20, "o", "file1\nfile2\n"),
+    ]
+    out = list(extract_session(events, sid="loc-none"))
+    assert [o for o in out if o.primitive == PRIMITIVE] == []
+
+
+def test_lang_envvar_dump_emits_bcp47() -> None:
+    events: list[AsciinemaEvent] = [
+        *_typed("env\r"),
+        (0.20, "o", "PATH=/usr/bin\nLANG=en_US.UTF-8\nUSER=anti\n"),
+    ]
+    obs = _of(list(extract_session(events, sid="loc-en")), PRIMITIVE)
+    assert obs.value == "en-US"
+
+
+def test_lc_all_takes_precedence_over_lang() -> None:
+    events: list[AsciinemaEvent] = [
+        *_typed("env\r"),
+        (0.20, "o", "LANG=en_US.UTF-8\nLC_ALL=pt_BR.UTF-8\nUSER=anti\n"),
+    ]
+    obs = _of(list(extract_session(events, sid="loc-prec")), PRIMITIVE)
+    assert obs.value == "pt-BR"
+
+
+def test_c_locale_emits_und() -> None:
+    events: list[AsciinemaEvent] = [
+        *_typed("env\r"),
+        (0.20, "o", "LANG=C\nUSER=anti\n"),
+    ]
+    obs = _of(list(extract_session(events, sid="loc-und")), PRIMITIVE)
+    assert obs.value == "und"
+
+
+def test_pii_locale_value_only_no_surrounding_output() -> None:
+    """Surrounding output isn't leaked — only the parsed BCP-47 value."""
+    events: list[AsciinemaEvent] = [
+        *_typed("env\r"),
+        (0.20, "o", "SECRET_TOKEN=abcdef123\nLANG=en_US.UTF-8\n"),
+    ]
+    out = list(extract_session(events, sid="loc-pii"))
+    obs = _of(out, PRIMITIVE)
+    serialised = obs.model_dump_json()
+    assert "SECRET_TOKEN" not in serialised
+    assert "abcdef123" not in serialised
+    assert "en-US" in serialised