diff --git a/core/loop/cue.py b/core/loop/cue.py
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+"""P3-A (GenAI lesson 04): prompt cue guidance for structured / stepwise output.
+
+Lesson 04 (prompt engineering) stresses that explicit *cues* — short
+signposts telling the model what shape the answer should take — cut down on
+free-form filler and keep multi-step work on rails. DeepCode already carries
+heavy system-prompt scaffolding; this module adds one *thin, model-facing*
+cue injected as transient turn context on the routed request.
+
+Design constraints
+-----------------
+- The cue must NOT be appended to every provider request. The runner replays
+  the routed request view as the prefix of its compaction summarizer call
+  (the dsh rule — prefix/KV cache reuse). Injecting different text on the
+  compaction path would desynchronize that prefix. We therefore attach the
+  cue via ``transient_context_messages``, which the runner applies to routed
+  requests and never to the summarizer prefix.
+- The cue is advisory prose, not a hard schema: it must never contradict a
+  caller-provided JSON schema or structured-output mode. It only biases
+  *default* behavior (stepwise work, cite-before-claim, no filler).
+
+Enable: env ``DEEPCODE_PROMPT_CUE`` (default on). Set ``0``/``false``/``off``
+to disable.
+"""
+
+from __future__ import annotations
+
+import os
+from typing import Any
+
+_CUE_TEXT = (
+    "Work stepwise: state each step as you take it, use the available tools "
+    "to verify claims before asserting them, and keep the final answer "
+    "tight — no restating the task, no filler. If you cite a retrieved "
+    "memory or tool result, reference it by its label ([n] / tool name)."
+)
+
+
+def prompt_cue_enabled() -> bool:
+    """Whether the prompt cue is on (env ``DEEPCODE_PROMPT_CUE``; default on)."""
+    value = os.environ.get("DEEPCODE_PROMPT_CUE", "").strip().lower()
+    if not value:
+        return True
+    return value not in {"0", "false", "off", "no"}
+
+
+def build_cue_context() -> tuple[dict[str, Any], ...]:
+    """Transient turn-context messages carrying the cue, or empty when off.
+
+    Returns a single ``user``-role message so the runner's
+    ``_with_transient_context`` places it immediately before the canonical
+    user request (user-priority turn context), never inside system-level
+    instructions.
+    """
+    if not prompt_cue_enabled():
+        return ()
+    return ({"role": "user", "content": _CUE_TEXT},)
+
+
+__all__ = [
+    "_CUE_TEXT",
+    "build_cue_context",
+    "prompt_cue_enabled",
+]