diff --git a/decnet/profiler/behave_shell/_ctx.py b/decnet/profiler/behave_shell/_ctx.py
index 1a13cbbf..d290d461 100644
--- a/decnet/profiler/behave_shell/_ctx.py
+++ b/decnet/profiler/behave_shell/_ctx.py
@@ -10,6 +10,7 @@ will fill ``commands`` / ``inter_cmd_iats`` / ``output_per_cmd``.
 """
 from __future__ import annotations
 
+import math
 from dataclasses import dataclass, field
 from typing import Iterable
 
@@ -17,7 +18,9 @@ from decnet.profiler.behave_shell._parse import (
     AsciinemaEvent,
     Command,
     PasteBurst,
+    detect_error_in_output,
     hash_token,
+    strip_ansi,
 )
 from decnet.profiler.behave_shell._thresholds import (
     IKI_THINK_MAX_S,
@@ -219,6 +222,36 @@ def _segment_commands(inputs: list[AsciinemaEvent]) -> tuple[Command, ...]:
     return tuple(cmds)
 
 
+def _annotate_commands_with_output(
+    commands: tuple[Command, ...],
+    outputs: list[AsciinemaEvent],
+) -> tuple[Command, ...]:
+    """Re-emit ``commands`` with ``errored`` / ``output_bytes`` filled.
+
+    The output window for ``commands[i]`` spans from its ``end_ts``
+    (the ``\\r``/``\\n`` that ran it) to the ``start_ts`` of the next
+    command. The last command's window is open-ended (``math.inf``)
+    so output events arriving at or after ``t_end`` are still captured.
+    """
+    if not commands:
+        return commands
+    annotated: list[Command] = []
+    for i, cmd in enumerate(commands):
+        win_end = commands[i + 1].start_ts if i + 1 < len(commands) else math.inf
+        byte_count, errored = _output_window(outputs, cmd.end_ts, win_end)
+        annotated.append(Command(
+            start_ts=cmd.start_ts,
+            end_ts=cmd.end_ts,
+            first_token_hash=cmd.first_token_hash,
+            tab_count=cmd.tab_count,
+            shortcut_count=cmd.shortcut_count,
+            pipe_count=cmd.pipe_count,
+            errored=errored,
+            output_bytes=byte_count,
+        ))
+    return tuple(annotated)
+
+
 def _per_command_iats(
     commands: tuple[Command, ...],
     inputs: list[AsciinemaEvent],
@@ -252,6 +285,32 @@ def _output_bytes_between(
     return sum(len(d) for t, _k, d in outputs if start <= t < end)
 
 
+def _output_window(
+    outputs: list[AsciinemaEvent],
+    start: float,
+    end: float,
+) -> tuple[int, bool]:
+    """Walk output events in ``[start, end)`` once.
+
+    Returns ``(byte_count, errored)``. ``byte_count`` is the raw byte
+    count (pre-strip); ``errored`` is the canonical-error-pattern match
+    over the ANSI-stripped concatenation. The stripped text is dropped
+    on return — PII discipline: only an int and a bool leave this
+    helper. The full output bytes never enter ``Command`` or the
+    ``SessionContext``.
+    """
+    chunks: list[str] = []
+    byte_count = 0
+    for t, _k, d in outputs:
+        if start <= t < end:
+            byte_count += len(d)
+            chunks.append(d)
+    if not chunks:
+        return 0, False
+    stripped = strip_ansi("".join(chunks))
+    return byte_count, detect_error_in_output(stripped)
+
+
 def build_session_context(
     events: Iterable[AsciinemaEvent],
     *,
@@ -290,6 +349,7 @@ def build_session_context(
     typing_bursts = _split_typing_bursts(iats)
     backspace_count, backspace_iats, kill_line_count = _scan_correction_signals(inputs)
     commands = _segment_commands(inputs)
+    commands = _annotate_commands_with_output(commands, outputs)
     inter_cmd_iats = tuple(
         max(0.0, commands[i + 1].start_ts - commands[i].end_ts)
         for i in range(len(commands) - 1)
diff --git a/decnet/profiler/behave_shell/_parse.py b/decnet/profiler/behave_shell/_parse.py
index 1de05ce9..6816b645 100644
--- a/decnet/profiler/behave_shell/_parse.py
+++ b/decnet/profiler/behave_shell/_parse.py
@@ -15,6 +15,7 @@ from __future__ import annotations
 
 import hashlib
 import json
+import re
 from dataclasses import dataclass
 from typing import Iterable, Iterator, Literal, Tuple
 
@@ -22,6 +23,47 @@ EventKind = Literal["i", "o"]
 AsciinemaEvent = Tuple[float, EventKind, str]
 
 
+# CSI / OSC / SGR / single-char escape sweeper. One pass, then we drop the
+# stripped text on the floor — only the boolean error verdict (and the byte
+# count, computed before stripping) leaves the helper. Full prompt-string
+# parsing lives in Phase F.0; this is the slice cognitive.error_resilience.*
+# needs to ship correctly.
+_ANSI_RE = re.compile(
+    r"""
+    \x1B            # ESC
+    (?:
+        \[ [0-?]* [ -/]* [@-~]   # CSI
+      | \] [^\x07\x1B]* (?:\x07|\x1B\\)?   # OSC, ST-or-BEL terminated
+      | [@-Z\\-_]                # 2-byte escapes (ESC followed by 0x40-0x5F)
+    )
+    """,
+    re.VERBOSE,
+)
+
+
+def strip_ansi(data: str) -> str:
+    """Remove ANSI escape sequences. Used pre-error-pattern match."""
+    return _ANSI_RE.sub("", data)
+
+
+# Canonical bash/sh error fingerprints. v0.1 heuristic — Phase F.0's prompt
+# parser will subsume this with PS1 + exit-code sniff. Any change here must
+# leave the calibration grid green.
+_OUTPUT_ERROR_PATTERNS: tuple[re.Pattern[str], ...] = (
+    re.compile(r"command not found"),
+    re.compile(r"No such file or directory"),
+    re.compile(r"Permission denied"),
+    re.compile(r": cannot "),
+    re.compile(r"Operation not permitted"),
+    re.compile(r"syntax error near unexpected token"),
+)
+
+
+def detect_error_in_output(stripped: str) -> bool:
+    """True if any canonical error fingerprint matches the stripped output."""
+    return any(p.search(stripped) for p in _OUTPUT_ERROR_PATTERNS)
+
+
 @dataclass(frozen=True, slots=True)
 class PasteBurst:
     """Contiguous run of paste-class input events."""
@@ -53,6 +95,16 @@ class Command:
     byte sweep. They feed the ``motor.shell_mastery.*`` primitives
     (Phase C). The raw bytes themselves are read once during the
     sweep and discarded — only the counters are retained.
+
+    ``errored`` (Step D.0) is set when the output stream between this
+    command and the next contains a canonical bash/sh error fingerprint
+    (see :func:`detect_error_in_output`). ``output_bytes`` is the byte
+    count of that same window. Both are populated in the segmentation
+    walk; the underlying output text is stripped of ANSI then matched,
+    and the stripped text is discarded — only the bool and the int
+    leave the segmentation pass. Drives the ``cognitive.error_resilience.*``
+    family (Phase D) and the ``error_rate`` term of
+    ``cognitive.cognitive_load``.
     """
 
     start_ts: float
@@ -61,6 +113,8 @@ class Command:
     tab_count: int = 0
     shortcut_count: int = 0
     pipe_count: int = 0
+    errored: bool = False
+    output_bytes: int = 0
 
 
 def hash_token(token: str) -> str:
diff --git a/decnet/profiler/behave_shell/_thresholds.py b/decnet/profiler/behave_shell/_thresholds.py
index 82ccadd3..a337efc8 100644
--- a/decnet/profiler/behave_shell/_thresholds.py
+++ b/decnet/profiler/behave_shell/_thresholds.py
@@ -76,6 +76,17 @@ FEEDBACK_MIN_PAIRS: int = 5
 PAUSE_CV_METRONOMIC_MAX: float = 0.40
 PAUSE_CV_BIMODAL_MIN: float = 1.50
 
+# ── output error-signal helper (Step D.0) ──────────────────────────────────
+# The canonical bash/sh error fingerprints live in ``_parse.py`` as
+# ``_OUTPUT_ERROR_PATTERNS`` (compiled regexes). They're not threshold
+# numbers, so they live next to the helper that uses them rather than
+# here. This v0.1 heuristic will be subsumed by Phase F.0's prompt
+# parser (PS1 echo + exit-code sniff), at which point this comment and
+# the patterns block move to ``_parse.py``'s prompt section. Until then,
+# any drift in registry value definitions for ``error_resilience.*`` or
+# ``cognitive_load`` must be reflected by editing the patterns tuple
+# (not a constant, so no boundary-band logic applies).
+
 # ── 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_command_error_detection.py b/tests/profiler/behave_shell/test_command_error_detection.py
new file mode 100644
index 00000000..25757a6d
--- /dev/null
+++ b/tests/profiler/behave_shell/test_command_error_detection.py
@@ -0,0 +1,134 @@
+"""Step D.0: per-command error-signal helper.
+
+The helper is infrastructure (no primitive emit) — these tests pin
+``Command.errored`` and ``Command.output_bytes`` semantics directly via
+``build_session_context``. The Phase D primitives (D.1, D.5–D.7) all
+read the same fields, so any drift here breaks four downstream
+primitives at once.
+"""
+from __future__ import annotations
+
+from decnet.profiler.behave_shell import extract_session
+from decnet.profiler.behave_shell._ctx import build_session_context
+from decnet.profiler.behave_shell._parse import (
+    AsciinemaEvent,
+    detect_error_in_output,
+    strip_ansi,
+)
+
+
+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)]
+
+
+# ── strip_ansi ──────────────────────────────────────────────────────────────
+
+
+def test_strip_ansi_removes_csi_sgr() -> None:
+    assert strip_ansi("\x1b[31mPermission denied\x1b[0m") == "Permission denied"
+
+
+def test_strip_ansi_removes_osc_with_bel() -> None:
+    # OSC: ESC ] ... BEL — terminal title escape
+    assert strip_ansi("\x1b]0;title\x07hello") == "hello"
+
+
+def test_strip_ansi_passthrough_clean_text() -> None:
+    assert strip_ansi("plain output\nwith newline") == "plain output\nwith newline"
+
+
+# ── detect_error_in_output ──────────────────────────────────────────────────
+
+
+def test_detect_error_command_not_found() -> None:
+    assert detect_error_in_output("bash: foo: command not found") is True
+
+
+def test_detect_error_no_such_file() -> None:
+    assert detect_error_in_output("ls: /nope: No such file or directory") is True
+
+
+def test_detect_error_permission_denied() -> None:
+    assert detect_error_in_output("cat: /etc/shadow: Permission denied") is True
+
+
+def test_detect_error_cannot_access() -> None:
+    assert detect_error_in_output("ls: cannot access '/x': No such file") is True
+
+
+def test_detect_error_clean_output() -> None:
+    assert detect_error_in_output("total 12\ndrwxr-xr-x  3 user user 4096 May  3 12:00 .") is False
+
+
+# ── Command.errored / output_bytes wired through build_session_context ──────
+
+
+def test_command_clean_output_not_errored() -> None:
+    events: list[AsciinemaEvent] = [
+        *_typed("ls\r"),
+        (0.20, "o", "file1\nfile2\n"),
+    ]
+    ctx = build_session_context(events, sid="d0-clean", source="test")
+    assert len(ctx.commands) == 1
+    assert ctx.commands[0].errored is False
+    assert ctx.commands[0].output_bytes == len("file1\nfile2\n")
+
+
+def test_command_with_error_pattern_marked_errored() -> None:
+    events: list[AsciinemaEvent] = [
+        *_typed("foo\r"),
+        (0.20, "o", "bash: foo: command not found\n"),
+    ]
+    ctx = build_session_context(events, sid="d0-err", source="test")
+    assert ctx.commands[0].errored is True
+    assert ctx.commands[0].output_bytes == len("bash: foo: command not found\n")
+
+
+def test_command_with_ansi_wrapped_error_marked_errored() -> None:
+    """ANSI strip must run before pattern match (red-coloured `Permission denied`)."""
+    events: list[AsciinemaEvent] = [
+        *_typed("cat /etc/shadow\r"),
+        (1.50, "o", "\x1b[31mcat: /etc/shadow: Permission denied\x1b[0m\n"),
+    ]
+    ctx = build_session_context(events, sid="d0-ansi", source="test")
+    assert ctx.commands[0].errored is True
+
+
+def test_last_command_output_window_extends_to_t_end() -> None:
+    """The last command's window has no ``commands[i+1]`` — it spans to t_end."""
+    events: list[AsciinemaEvent] = [
+        *_typed("ls\r", t0=0.0),
+        *_typed("foo\r", t0=1.0),
+        (1.50, "o", "bash: foo: command not found\n"),
+    ]
+    ctx = build_session_context(events, sid="d0-last", source="test")
+    assert len(ctx.commands) == 2
+    assert ctx.commands[0].errored is False
+    assert ctx.commands[1].errored is True
+
+
+def test_no_output_events_no_errored() -> None:
+    """A shard with no ``'o'`` events emits clean ``errored=False`` per command."""
+    events: list[AsciinemaEvent] = _typed("ls\r")
+    ctx = build_session_context(events, sid="d0-noout", source="test")
+    assert ctx.commands[0].errored is False
+    assert ctx.commands[0].output_bytes == 0
+
+
+# ── PII regression ──────────────────────────────────────────────────────────
+
+
+def test_pii_no_output_bodies_in_observations() -> None:
+    """Output bytes containing operator-identifying strings must not leak.
+
+    The error pattern triggers ``errored=True``; the surrounding output
+    contains the literal ``secret_payload_xyz`` token. No observation may
+    serialise that token, since the engine only retains a bool + an int.
+    """
+    events: list[AsciinemaEvent] = [
+        *_typed("foo\r"),
+        (0.20, "o", "secret_payload_xyz\nbash: foo: command not found\n"),
+    ]
+    out = list(extract_session(events, sid="d0-pii"))
+    for obs in out:
+        assert "secret_payload_xyz" not in obs.model_dump_json()