diff --git a/decnet/profiler/behave_shell/_features/__init__.py b/decnet/profiler/behave_shell/_features/__init__.py
index 838f8a08..c2ba2c49 100644
--- a/decnet/profiler/behave_shell/_features/__init__.py
+++ b/decnet/profiler/behave_shell/_features/__init__.py
@@ -24,6 +24,9 @@ from decnet.profiler.behave_shell._features.cognitive import (
     inter_command_consistency,
     inter_command_latency_class,
 )
+from decnet.profiler.behave_shell._features.environmental import (
+    shell_type,
+)
 from decnet.profiler.behave_shell._features.temporal import (
     escalation_pattern,
     landing_ritual,
@@ -67,4 +70,5 @@ FEATURES: tuple[FeatureFn, ...] = (
     session_duration,
     escalation_pattern,
     landing_ritual,
+    shell_type,
 )
diff --git a/decnet/profiler/behave_shell/_features/environmental.py b/decnet/profiler/behave_shell/_features/environmental.py
new file mode 100644
index 00000000..9583b071
--- /dev/null
+++ b/decnet/profiler/behave_shell/_features/environmental.py
@@ -0,0 +1,75 @@
+"""``environmental.*`` feature functions.
+
+Phase F ships the five environmental primitives plus F.0's shared
+prompt-line detector. F.0 itself emits no primitive — it populates
+``SessionContext.prompt_lines`` and ``Command.followed_by_prompt``
+which F.1 / F.3 / E.4 read.
+
+Step F.1: ``environmental.shell_type``.
+"""
+from __future__ import annotations
+
+import collections
+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
+from decnet.profiler.behave_shell._thresholds import (
+    SHELL_TYPE_MIN_PROMPTS,
+)
+
+
+def _classify_shell_from_prompt(p: PromptLine) -> str:
+    """Map one prompt line to a shell-type label."""
+    suffix = p.suffix_char
+    line = p.raw_line
+    if suffix in ("$", "#"):
+        # bash / sh / dash all share these — collapsed to "bash" per
+        # registry's bash-family stance. zsh CAN be configured to use
+        # $/# but that's the user's PS1 override; default zsh is %.
+        return "bash"
+    if suffix == "%":
+        return "zsh"
+    if suffix == ">":
+        # Disambiguate by line content. powershell's PS1 starts with
+        # "PS "; cmd.exe's prompt typically contains a Windows path
+        # like "C:\". Everything else is fish.
+        if line.lstrip().startswith("PS "):
+            return "powershell"
+        if "C:\\" in line or "c:\\" in line:
+            return "cmd.exe"
+        return "fish"
+    return "bash"  # defensive — _detect_prompt_suffix only emits one of $#%>
+
+
+def shell_type(ctx: SessionContext) -> Iterator[Observation]:
+    """Emit ``environmental.shell_type``.
+
+    Mode of per-prompt-line classification across
+    ``ctx.prompt_lines``. Skip emission when no prompts detected —
+    the registry's enum doesn't admit ``unknown`` and emitting
+    ``bash`` from no observation at all would be dishonest.
+
+    Confidence drops below ``SHELL_TYPE_MIN_PROMPTS`` (3 prompts);
+    above that threshold the vote is solid.
+    """
+    if not ctx.prompt_lines:
+        return
+    votes = collections.Counter(
+        _classify_shell_from_prompt(p) for p in ctx.prompt_lines
+    )
+    value, _ = votes.most_common(1)[0]
+
+    if len(ctx.prompt_lines) < SHELL_TYPE_MIN_PROMPTS:
+        confidence = 0.40
+    else:
+        confidence = 0.75
+    yield make_observation(
+        ctx,
+        primitive="environmental.shell_type",
+        value=value,
+        confidence=confidence,
+    )
diff --git a/decnet/profiler/behave_shell/_thresholds.py b/decnet/profiler/behave_shell/_thresholds.py
index 5bb1f808..3b49d0ae 100644
--- a/decnet/profiler/behave_shell/_thresholds.py
+++ b/decnet/profiler/behave_shell/_thresholds.py
@@ -228,6 +228,11 @@ LANDING_RITUAL_MIN_COMMANDS: int = 3
 PROMPT_SUFFIX_CHARS: frozenset[str] = frozenset({"$", "#", "%", ">"})
 PROMPT_LINE_MAX_CHARS: int = 256
 
+# ── environmental.shell_type (Step F.1) ────────────────────────────────────
+# Below this many detected prompt-lines, drop confidence (sample-size
+# honesty). Above, the shell-type vote is robust.
+SHELL_TYPE_MIN_PROMPTS: int = 3
+
 # ── 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_shell_type.py b/tests/profiler/behave_shell/test_environmental_shell_type.py
new file mode 100644
index 00000000..8fd90b40
--- /dev/null
+++ b/tests/profiler/behave_shell/test_environmental_shell_type.py
@@ -0,0 +1,87 @@
+"""Step F.1: ``environmental.shell_type``."""
+from __future__ import annotations
+
+from decnet.profiler.behave_shell import extract_session
+from decnet.profiler.behave_shell._parse import AsciinemaEvent
+
+
+PRIMITIVE = "environmental.shell_type"
+
+
+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)]
+
+
+def _session(prompt_lines: list[str]) -> list[AsciinemaEvent]:
+    """Build a synthetic session: one ``ls`` per prompt, prompt printed
+    as the post-execution output of that command."""
+    events: list[AsciinemaEvent] = []
+    for i, prompt in enumerate(prompt_lines):
+        events.extend(_typed("ls\r", t0=i * 1.0))
+        events.append((i * 1.0 + 0.5, "o", f"out\n{prompt}"))
+    return events
+
+
+def test_no_prompts_no_emission() -> None:
+    events = _typed("ls\r", t0=0.0) + [(0.5, "o", "file1\n")]
+    out = list(extract_session(events, sid="sht-noprompt"))
+    assert [o for o in out if o.primitive == PRIMITIVE] == []
+
+
+def test_bash_prompt_emits_bash() -> None:
+    out = list(extract_session(_session(["anti@host:~$ "] * 5), sid="sht-bash"))
+    assert _of(out, PRIMITIVE).value == "bash"
+
+
+def test_root_prompt_still_bash() -> None:
+    """# is bash root, not a separate shell."""
+    out = list(extract_session(_session(["root@host:/etc# "] * 5), sid="sht-root"))
+    assert _of(out, PRIMITIVE).value == "bash"
+
+
+def test_zsh_prompt_emits_zsh() -> None:
+    out = list(extract_session(_session(["host% "] * 5), sid="sht-zsh"))
+    assert _of(out, PRIMITIVE).value == "zsh"
+
+
+def test_fish_prompt_emits_fish() -> None:
+    out = list(extract_session(_session(["anti@host ~> "] * 5), sid="sht-fish"))
+    assert _of(out, PRIMITIVE).value == "fish"
+
+
+def test_powershell_prompt_emits_powershell() -> None:
+    out = list(extract_session(
+        _session(["PS C:\\Users\\anti> "] * 5), sid="sht-ps",
+    ))
+    assert _of(out, PRIMITIVE).value == "powershell"
+
+
+def test_cmd_exe_prompt_emits_cmd_exe() -> None:
+    out = list(extract_session(_session(["C:\\Users\\anti>"] * 5), sid="sht-cmd"))
+    assert _of(out, PRIMITIVE).value == "cmd.exe"
+
+
+def test_majority_wins() -> None:
+    """Mixed prompts, bash majority → bash."""
+    out = list(extract_session(_session([
+        "anti@host:~$ ",
+        "anti@host:~$ ",
+        "anti@host:~$ ",
+        "host% ",  # one zsh outlier
+        "anti@host:~$ ",
+    ]), sid="sht-mix"))
+    assert _of(out, PRIMITIVE).value == "bash"
+
+
+def test_few_prompts_low_confidence() -> None:
+    short = list(extract_session(_session(["anti@host:~$ "] * 2), sid="sht-short"))
+    full = list(extract_session(_session(["anti@host:~$ "] * 6), sid="sht-full"))
+    s = _of(short, PRIMITIVE)
+    f = _of(full, PRIMITIVE)
+    assert s.confidence < f.confidence