diff --git a/decnet/orchestrator/drivers/ssh.py b/decnet/orchestrator/drivers/ssh.py
index 276264aa..b94bb0dc 100644
--- a/decnet/orchestrator/drivers/ssh.py
+++ b/decnet/orchestrator/drivers/ssh.py
@@ -26,7 +26,12 @@ from datetime import datetime, timezone
 
 from decnet.logging import get_logger
 from decnet.orchestrator.drivers.base import ActivityDriver, ActivityResult
-from decnet.orchestrator.scheduler import Action, FileAction, TrafficAction
+from decnet.orchestrator.scheduler import (
+    Action,
+    EditAction,
+    FileAction,
+    TrafficAction,
+)
 
 log = get_logger("orchestrator.ssh")
 
@@ -109,6 +114,8 @@ class SSHDriver(ActivityDriver):
             return await self._run_traffic(action)
         if isinstance(action, FileAction):
             return await self._run_file(action)
+        if isinstance(action, EditAction):
+            return await self._run_edit(action)
         raise TypeError(f"unsupported action type: {type(action)!r}")
 
     async def _run_traffic(self, action: TrafficAction) -> ActivityResult:
@@ -135,6 +142,61 @@ class SSHDriver(ActivityDriver):
             )
         return ActivityResult(success=success, payload=payload)
 
+    async def _run_edit(self, action: EditAction) -> ActivityResult:
+        """Mutate an existing synthetic file in place.
+
+        The realism planner already loaded the previous body from the
+        ``synthetic_files`` row, so we don't re-fetch via ``read_file``;
+        the body the planner saw is the body we mutate.  This avoids a
+        TOCTOU window where the file changed between pick and apply
+        (the realism worker is the only writer in the MVP, but the
+        contract should still be tight).
+        """
+        from decnet.realism.bodies import next_iteration as _next_iteration
+        from decnet.realism.taxonomy import ContentClass
+
+        try:
+            cls = ContentClass(action.content_class)
+        except ValueError:
+            return ActivityResult(
+                success=False,
+                payload={
+                    "dst_decky": action.dst_name,
+                    "path": action.path,
+                    "error": f"unknown content_class: {action.content_class!r}",
+                },
+            )
+        try:
+            new_body = _next_iteration(
+                cls, action.persona, action.previous_body,
+            )
+        except KeyError:
+            return ActivityResult(
+                success=False,
+                payload={
+                    "dst_decky": action.dst_name,
+                    "path": action.path,
+                    "error": (
+                        f"content_class={cls!s} does not support edits"
+                    ),
+                },
+            )
+        result = await self.plant_file(
+            action.dst_name,
+            action.path,
+            new_body.encode("utf-8"),
+            mode=0o644,
+            mtime=action.mtime,
+        )
+        # Carry edit-specific metadata through to the orchestrator
+        # event payload so the worker's synthetic_files bump (and the
+        # dashboard's lineage view) sees what actually landed.
+        if result.success:
+            result.payload["new_body"] = new_body
+            result.payload["new_body_bytes"] = len(new_body.encode("utf-8"))
+            result.payload["synthetic_file_uuid"] = action.synthetic_file_uuid
+        return result
+
     async def _run_file(self, action: FileAction) -> ActivityResult:
         # FileAction's content is a string; the realism path uses
         # bytes-typed plant_file so binary blobs (DOCX/PDF, future
diff --git a/decnet/orchestrator/events.py b/decnet/orchestrator/events.py
index c63af0b7..83497c4f 100644
--- a/decnet/orchestrator/events.py
+++ b/decnet/orchestrator/events.py
@@ -6,7 +6,12 @@ from typing import Any
 
 from decnet.bus import topics as _topics
 from decnet.orchestrator.drivers.base import ActivityResult
-from decnet.orchestrator.scheduler import Action, FileAction, TrafficAction
+from decnet.orchestrator.scheduler import (
+    Action,
+    EditAction,
+    FileAction,
+    TrafficAction,
+)
 
 
 def to_row(action: Action, result: ActivityResult) -> dict[str, Any]:
@@ -31,6 +36,16 @@ def to_row(action: Action, result: ActivityResult) -> dict[str, Any]:
             src_decky_uuid=None,
             dst_decky_uuid=action.dst_uuid,
         )
+    elif isinstance(action, EditAction):
+        # EditAction shares the "file" kind (same dashboard view, same
+        # bus topic family) but action="file:edit" lets queries
+        # discriminate when needed.
+        base.update(
+            kind="file",
+            action=action.description,
+            src_decky_uuid=None,
+            dst_decky_uuid=action.dst_uuid,
+        )
     else:
         raise TypeError(f"unsupported action type: {type(action)!r}")
     return base
@@ -40,7 +55,7 @@ def topic_for(action: Action) -> str:
     """Map an action to its bus topic."""
     if isinstance(action, TrafficAction):
         return _topics.orchestrator(_topics.ORCHESTRATOR_TRAFFIC, action.dst_uuid)
-    if isinstance(action, FileAction):
+    if isinstance(action, (FileAction, EditAction)):
         return _topics.orchestrator(_topics.ORCHESTRATOR_FILE, action.dst_uuid)
     raise TypeError(f"unsupported action type: {type(action)!r}")
 
@@ -48,6 +63,6 @@ def topic_for(action: Action) -> str:
 def event_type_for(action: Action) -> str:
     if isinstance(action, TrafficAction):
         return _topics.ORCHESTRATOR_TRAFFIC
-    if isinstance(action, FileAction):
+    if isinstance(action, (FileAction, EditAction)):
         return _topics.ORCHESTRATOR_FILE
     raise TypeError(f"unsupported action type: {type(action)!r}")
diff --git a/decnet/orchestrator/scheduler.py b/decnet/orchestrator/scheduler.py
index 35bb5703..67233dff 100644
--- a/decnet/orchestrator/scheduler.py
+++ b/decnet/orchestrator/scheduler.py
@@ -58,7 +58,28 @@ class FileAction:
     description: str = "file:create"
 
 
-Action = TrafficAction | FileAction
+@dataclass(frozen=True)
+class EditAction:
+    """Read-modify-write of an existing synthetic file.
+
+    Stage 3b of the realism migration: a previously-planted ``TODO.md``
+    gets a checkbox flipped, a notes file gets a new line appended, a
+    cron log gets a fresh entry tacked on.  ``synthetic_file_uuid`` is
+    the row in ``synthetic_files`` to update; ``previous_body`` is
+    what the planner already saw so the driver doesn't double-fetch.
+    """
+    dst_uuid: str
+    dst_name: str
+    path: str
+    persona: str
+    content_class: str
+    previous_body: str
+    synthetic_file_uuid: str
+    mtime: Optional[datetime] = None
+    description: str = "file:edit"
+
+
+Action = TrafficAction | FileAction | EditAction
 
 
 def _has_ssh(decky: dict[str, Any]) -> bool:
@@ -100,26 +121,64 @@ async def pick_file(
     *,
     now: Optional[datetime] = None,
     rand: Optional[secrets.SystemRandom] = None,
-) -> Optional[FileAction]:
-    """Realism-driven file action.
+) -> Optional[Action]:
+    """Realism-driven file action — create or edit.
 
     Resolves personas per decky (topology pool when the decky has a
     parent topology; global pool otherwise), filters to deckies in any
-    persona's work hours, asks :func:`decnet.realism.planner.pick` to
-    pick the (decky, persona, content_class, path, body, mtime), and
-    maps the resulting :class:`Plan` to a :class:`FileAction` the
+    persona's work hours, optionally fetches an edit candidate from
+    the synthetic_files table, and asks
+    :func:`decnet.realism.planner.pick` to choose between create / edit
+    / leave-alone.  Maps the resulting :class:`Plan` to a
+    :class:`FileAction` (create) or :class:`EditAction` (edit) the
     SSH driver can dispatch.
 
     Returns ``None`` when no decky has a non-empty persona pool with a
-    persona currently in its active-hours window.
+    persona currently in its active-hours window, or when the planner
+    rolled "leave alone."
     """
     rng = rand or secrets.SystemRandom()
     when = now or datetime.now(timezone.utc)
 
     enriched = await _resolve_personas(deckies, repo)
-    plan = _realism_pick(enriched, when, rand=rng)
+    if not enriched:
+        return None
+
+    # Pre-fetch a single edit candidate from a random eligible decky,
+    # so the planner can decide whether to use it.  We pick the decky
+    # client-side (cheap) and ask the repo for one row; if there's
+    # nothing editable, planner falls back to create.
+    edit_candidate = None
+    if rng.random() < 0.5 and enriched:
+        # Half the ticks consider an edit. Lower than the planner's
+        # 30% edit weight on purpose — the repo lookup is the
+        # expensive part, no point doing it on every tick.
+        candidate_decky = rng.choice(enriched)
+        try:
+            row = await repo.pick_random_synthetic_file_for_edit(
+                candidate_decky["uuid"],
+            )
+        except Exception:  # noqa: BLE001
+            row = None
+        if row is not None:
+            row = {**row, "decky_name": candidate_decky["name"]}
+            edit_candidate = row
+
+    plan = _realism_pick(enriched, when, edit_candidate=edit_candidate, rand=rng)
     if plan is None:
         return None
+
+    if plan.action == "edit":
+        return EditAction(
+            dst_uuid=plan.decky_uuid,
+            dst_name=plan.decky_name,
+            path=plan.target_path,
+            persona=plan.persona,
+            content_class=plan.content_class.value,
+            previous_body=plan.previous_body or "",
+            synthetic_file_uuid=(edit_candidate or {}).get("uuid", ""),
+            mtime=plan.mtime,
+        )
     return FileAction(
         dst_uuid=plan.decky_uuid,
         dst_name=plan.decky_name,
@@ -203,6 +262,7 @@ def _topology_personas(topology: Optional[dict[str, Any]]) -> list[EmailPersona]
 # ``Plan`` from the scheduler keep working through the migration.
 __all__ = [
     "Action",
+    "EditAction",
     "FileAction",
     "Plan",
     "TrafficAction",
diff --git a/decnet/orchestrator/worker.py b/decnet/orchestrator/worker.py
index 3d23afdd..f8e4d4a4 100644
--- a/decnet/orchestrator/worker.py
+++ b/decnet/orchestrator/worker.py
@@ -212,14 +212,24 @@ async def _one_tick(repo: BaseRepository, bus) -> None:
         await _persist_email(repo, action, result, bus)
     else:
         await _persist_event(repo, action, result, bus)
-        if isinstance(action, scheduler.FileAction) and result.success:
-            try:
-                await _record_synthetic_file(repo, action)
-            except Exception as exc:  # noqa: BLE001
-                logger.warning(
-                    "orchestrator: synthetic_files write failed dst=%s path=%s: %s",
-                    action.dst_uuid, action.path, exc,
-                )
+        if result.success:
+            if isinstance(action, scheduler.FileAction):
+                try:
+                    await _record_synthetic_file(repo, action)
+                except Exception as exc:  # noqa: BLE001
+                    logger.warning(
+                        "orchestrator: synthetic_files write failed dst=%s path=%s: %s",
+                        action.dst_uuid, action.path, exc,
+                    )
+            elif isinstance(action, scheduler.EditAction):
+                try:
+                    await _bump_synthetic_file_after_edit(repo, action, result)
+                except Exception as exc:  # noqa: BLE001
+                    logger.warning(
+                        "orchestrator: synthetic_files edit-bump failed "
+                        "dst=%s path=%s: %s",
+                        action.dst_uuid, action.path, exc,
+                    )
 
 
 async def _persist_event(repo, action, result, bus) -> None:
@@ -284,6 +294,41 @@ async def _persist_email(repo, action: EmailAction, result, bus) -> None:
     )
 
 
+async def _bump_synthetic_file_after_edit(repo, action, result) -> None:
+    """Patch ``synthetic_files`` after a successful EditAction.
+
+    Bumps ``edit_count`` + ``last_modified`` + ``content_hash`` so the
+    dashboard's lineage view shows the change.  When the row's UUID
+    isn't on the action (planner produced an edit plan from a stale
+    candidate that the repo pruned in between), the update is a no-op
+    — resurrecting a pruned row isn't this layer's job.
+
+    The new body comes from ``result.payload["new_body"]`` (the SSH
+    driver stashes it on success); we re-hash here so the orchestrator,
+    not the driver, owns the canonical hash field.
+    """
+    if not action.synthetic_file_uuid:
+        return
+    new_body = result.payload.get("new_body", "")
+    rows = await repo.list_synthetic_files(decky_uuid=action.dst_uuid, limit=200)
+    existing = next(
+        (r for r in rows if r.get("uuid") == action.synthetic_file_uuid),
+        None,
+    )
+    if existing is None:
+        return  # candidate was pruned mid-flight; skip silently
+    patch: dict = {
+        "last_modified": datetime.now(timezone.utc),
+        "edit_count": int(existing.get("edit_count", 0)) + 1,
+    }
+    if new_body:
+        patch["content_hash"] = hashlib.sha256(
+            new_body.encode("utf-8"),
+        ).hexdigest()
+        patch["last_body"] = new_body[:65536]
+    await repo.update_synthetic_file(action.synthetic_file_uuid, patch)
+
+
 async def _record_synthetic_file(repo, action) -> None:
     """Persist (or patch) a synthetic_files row after a FileAction plant.
 
diff --git a/decnet/realism/bodies.py b/decnet/realism/bodies.py
index 801337ab..ec96f47c 100644
--- a/decnet/realism/bodies.py
+++ b/decnet/realism/bodies.py
@@ -231,3 +231,120 @@ def make_body(
             f"no body generator registered for content_class={content_class!r}"
         )
     return gen(persona, rng)
+
+
+# ── Edit-in-place mutators ─────────────────────────────────────────────────
+# Stage 3b: deterministic per-class mutations.  The contract: take the
+# previous body bytes, return a plausible *next* iteration (append a
+# line, flip a checkbox, fix a typo).  Append-only for logs; small
+# in-place edits for user content.  LLM enrichment in stage 6 wires
+# next_iteration to ask "what would <persona> write next" with the
+# previous body in the prompt; the deterministic path stays as the
+# fallback.
+
+
+def _edit_todo(
+    prev: str, persona: str, rng: secrets.SystemRandom,
+) -> str:
+    """Flip an unchecked box, append a new item, or both.
+
+    Real TODO files evolve: items get checked off as work happens, new
+    items get added, occasionally a sub-bullet appears under an
+    existing one.  We pick one of those mutations per call.
+    """
+    lines = prev.splitlines()
+    unchecked_indices = [
+        i for i, ln in enumerate(lines) if ln.startswith("- [ ]")
+    ]
+    op = rng.choice(("flip", "append", "both") if unchecked_indices else ("append",))
+    if op in ("flip", "both") and unchecked_indices:
+        idx = rng.choice(unchecked_indices)
+        lines[idx] = lines[idx].replace("- [ ]", "- [x]", 1)
+    if op in ("append", "both"):
+        new_item = rng.choice(_TODO_VERBS)
+        marker = "[x]" if rng.random() < 0.15 else "[ ]"
+        lines.append(f"- {marker} {new_item}")
+    return "\n".join(lines) + ("" if prev.endswith("\n") else "\n")
+
+
+def _edit_note(
+    prev: str, persona: str, rng: secrets.SystemRandom,
+) -> str:
+    """Append one new note line or insert a follow-up under an existing one."""
+    new_line = rng.choice(_NOTE_TEMPLATES)
+    if prev.endswith("\n"):
+        return prev + new_line + "\n"
+    return prev + "\n" + new_line + "\n"
+
+
+def _edit_draft(
+    prev: str, persona: str, rng: secrets.SystemRandom,
+) -> str:
+    """Append a new short paragraph to the existing draft."""
+    addition = (
+        "\nFollow-up: I'll send the deck once finance signs off on the numbers.\n",
+        "\nP.S.: Looping in ops on the rollout sequence — they have context I don't.\n",
+        "\nLet me know if any of this needs another pass.\n",
+    )
+    return prev.rstrip() + "\n" + rng.choice(addition)
+
+
+def _edit_script(
+    prev: str, persona: str, rng: secrets.SystemRandom,
+) -> str:
+    """Append a comment line — scripts evolve via comments and small fixes."""
+    comments = (
+        "# TODO: handle the empty-input case\n",
+        "# 2026-04-27: hardened error path after the prod incident\n",
+        "# noqa: shellcheck disagrees but this is what the runbook says\n",
+    )
+    return prev.rstrip() + "\n" + rng.choice(comments)
+
+
+def _edit_log_cron(
+    prev: str, persona: str, rng: secrets.SystemRandom,
+) -> str:
+    """Append one new cron syslog line — logs only ever grow."""
+    extra = _body_log_cron(persona, rng)
+    return prev.rstrip() + "\n" + extra.splitlines()[-1] + "\n"
+
+
+def _edit_log_daemon(
+    prev: str, persona: str, rng: secrets.SystemRandom,
+) -> str:
+    extra = _body_log_daemon(persona, rng)
+    return prev.rstrip() + "\n" + extra.splitlines()[-1] + "\n"
+
+
+_EDITORS: dict[ContentClass, Callable[[str, str, secrets.SystemRandom], str]] = {
+    ContentClass.NOTE: _edit_note,
+    ContentClass.TODO: _edit_todo,
+    ContentClass.DRAFT: _edit_draft,
+    ContentClass.SCRIPT: _edit_script,
+    ContentClass.LOG_CRON: _edit_log_cron,
+    ContentClass.LOG_DAEMON: _edit_log_daemon,
+}
+
+
+def next_iteration(
+    content_class: ContentClass,
+    persona: str,
+    previous_body: str,
+    *,
+    rand: Optional[secrets.SystemRandom] = None,
+) -> str:
+    """Return the next-iteration body for an edit-in-place mutation.
+
+    Raises :class:`KeyError` for content classes that don't support
+    editing (canary blobs, cache-tmp scratch files, email).  The
+    planner filters those out before producing an :class:`EditAction`,
+    so reaching this branch with an unsupported class is a bug worth
+    surfacing loudly.
+    """
+    rng = rand or secrets.SystemRandom()
+    editor = _EDITORS.get(content_class)
+    if editor is None:
+        raise KeyError(
+            f"content_class={content_class!r} does not support edits"
+        )
+    return editor(previous_body, persona, rng)
diff --git a/decnet/realism/planner.py b/decnet/realism/planner.py
index 1000cdc9..bc2fd4b3 100644
--- a/decnet/realism/planner.py
+++ b/decnet/realism/planner.py
@@ -26,7 +26,7 @@ from typing import Any, Optional, Sequence
 from decnet.realism import bodies, naming
 from decnet.realism.diurnal import in_work_hours, sample_mtime
 from decnet.realism.personas import EmailPersona
-from decnet.realism.taxonomy import ContentClass, Plan
+from decnet.realism.taxonomy import ContentClass, Plan, PlanAction  # noqa: F401
 
 
 # Stage-3 weighted sampling:
@@ -83,16 +83,23 @@ def pick(
     deckies: Sequence[dict[str, Any]],
     now: datetime,
     *,
+    edit_candidate: Optional[dict[str, Any]] = None,
     rand: Optional[secrets.SystemRandom] = None,
 ) -> Optional[Plan]:
     """Return a single :class:`Plan` for the orchestrator's tick.
 
-    Stage-3 policy: create-only.  Stage 3b extends with the
-    create/edit/leave roll and the synthetic_files lookup for edits.
+    Stage-3b policy: weighted action roll — 60% create, 30% edit, 10%
+    "leave alone" (planner returns ``None`` to skip).  When the roll
+    is "edit" and *edit_candidate* is set (a row from
+    :meth:`BaseRepository.pick_random_synthetic_file_for_edit`), we
+    return an edit Plan; otherwise we fall through to create.
 
-    Returns ``None`` when no eligible (decky, persona) pair exists —
-    the orchestrator treats that as "skip this tick" the same way the
-    pre-realism scheduler did.
+    The orchestrator scheduler is responsible for fetching the edit
+    candidate before calling — keeps this function pure-of-DB and
+    test-friendly.
+
+    Returns ``None`` when no eligible (decky, persona) pair exists or
+    when the action roll lands on "leave alone."
     """
     rng = rand or secrets.SystemRandom()
 
@@ -100,12 +107,18 @@ def pick(
     if not eligible:
         return None
 
+    # Action roll.  Edit only fires when there's a candidate from the
+    # repo — otherwise we either re-roll to create or skip.
+    roll = rng.random()
+    if roll < 0.10:
+        return None  # "leave alone" — quiet tick is realism too
+    if roll < 0.40 and edit_candidate is not None:
+        return _edit_plan(edit_candidate, now, rng)
+
     decky, persona = rng.choice(eligible)
 
     # User vs system content — biased toward user (realism wins are
-    # bigger there).  Once stage 3b ships edit-in-place, the edit
-    # branch will reuse the same content_class as the existing row;
-    # the create branch picks fresh here.
+    # bigger there).
     if rng.random() < 0.7:
         content_class = _weighted_pick(_USER_CLASS_WEIGHTS, rng)
     else:
@@ -130,3 +143,48 @@ def pick(
             f"window={persona.active_hours}",
         ),
     )
+
+
+def _edit_plan(
+    candidate: dict[str, Any],
+    now: datetime,
+    rng: secrets.SystemRandom,
+) -> Optional[Plan]:
+    """Build an edit-action :class:`Plan` from a synthetic_files row.
+
+    The candidate dict is the shape :meth:`BaseRepository.list_synthetic_files`
+    returns — we only need ``decky_uuid``, ``path``, ``persona``,
+    ``content_class``, ``last_body``, ``uuid``.  Returns ``None`` if
+    the candidate's content_class is somehow not editable (defensive
+    — the repo query already filters those out).
+    """
+    try:
+        cls = ContentClass(candidate["content_class"])
+    except (KeyError, ValueError):
+        return None
+    if cls.is_canary() or cls == ContentClass.CACHE_TMP:
+        return None
+    # mtime: edits bump forward by ~hours-to-days, but never past now.
+    # We model as "the file was edited some time after creation but
+    # before now" — sample_mtime with a tighter cap keeps it recent.
+    edit_mtime = sample_mtime(
+        "00:00-00:00", now, rand=rng,
+        backdate_min_hours=1.0, backdate_max_days=2.0,
+    )
+    return Plan(
+        decky_uuid=candidate["decky_uuid"],
+        decky_name=candidate.get("decky_name", ""),
+        persona=candidate.get("persona", ""),
+        content_class=cls,
+        action="edit",
+        target_path=candidate["path"],
+        mtime=edit_mtime,
+        body_hint=None,  # edit uses previous_body, not a fresh hint
+        previous_body=candidate.get("last_body", ""),
+        notes=(
+            f"persona={candidate.get('persona', '')}",
+            f"class={cls.value}",
+            "action=edit",
+            f"synthetic_file_uuid={candidate.get('uuid', '')}",
+        ),
+    )
diff --git a/tests/realism/test_edit.py b/tests/realism/test_edit.py
new file mode 100644
index 00000000..f6d58700
--- /dev/null
+++ b/tests/realism/test_edit.py
@@ -0,0 +1,98 @@
+"""next_iteration mutators per content class.
+
+Stage 3b — read-modify-write contract: each editor takes a previous
+body and returns a plausible next iteration.  Append-only for logs;
+small in-place edits for user content.
+"""
+from __future__ import annotations
+
+import random
+
+import pytest
+
+from decnet.realism.bodies import next_iteration
+from decnet.realism.taxonomy import ContentClass
+
+
+def test_todo_edit_can_flip_an_unchecked_box() -> None:
+    prev = "- [ ] rotate keys\n- [ ] review pr\n"
+    seen_flip = False
+    for seed in range(40):
+        new = next_iteration(
+            ContentClass.TODO, "admin", prev, rand=random.Random(seed),
+        )
+        if "[x]" in new and "rotate" in new and "[x] rotate" in new:
+            seen_flip = True
+        if "[x]" in new and "[x] review" in new:
+            seen_flip = True
+        if seen_flip:
+            break
+    assert seen_flip, "no checkbox flip across 40 seeds — mutator broken"
+
+
+def test_todo_edit_grows_or_holds_line_count() -> None:
+    prev = "- [ ] rotate keys\n"
+    new = next_iteration(
+        ContentClass.TODO, "admin", prev, rand=random.Random(0),
+    )
+    # Mutators may flip a box (same line count) or append (more lines)
+    # — but never shrink the file.
+    assert len(new.splitlines()) >= len(prev.splitlines())
+
+
+def test_log_cron_edit_is_append_only() -> None:
+    prev = (
+        "Apr 27 09:00:01 hostname CRON[1234]: (root) CMD (run-parts /etc/cron.daily)\n"
+    )
+    new = next_iteration(
+        ContentClass.LOG_CRON, "admin", prev, rand=random.Random(0),
+    )
+    assert new.startswith(prev.rstrip())
+    assert len(new.splitlines()) > len(prev.splitlines())
+
+
+def test_log_daemon_edit_is_append_only() -> None:
+    prev = "Apr 27 09:00:01 hostname systemd[1]: Started Daily apt download activities.\n"
+    new = next_iteration(
+        ContentClass.LOG_DAEMON, "admin", prev, rand=random.Random(0),
+    )
+    assert new.startswith(prev.rstrip())
+
+
+def test_note_edit_grows_the_body() -> None:
+    prev = "remember to ping the on-call\n"
+    new = next_iteration(
+        ContentClass.NOTE, "admin", prev, rand=random.Random(0),
+    )
+    assert prev in new
+    assert len(new) > len(prev)
+
+
+def test_draft_edit_appends_paragraph() -> None:
+    prev = "Hi team,\n\nQuick update.\n"
+    new = next_iteration(
+        ContentClass.DRAFT, "admin", prev, rand=random.Random(0),
+    )
+    assert new.startswith(prev.rstrip())
+    assert len(new) > len(prev)
+
+
+def test_script_edit_appends_comment() -> None:
+    prev = "#!/usr/bin/env bash\nset -e\necho 'hi'\n"
+    new = next_iteration(
+        ContentClass.SCRIPT, "admin", prev, rand=random.Random(0),
+    )
+    assert new.startswith(prev.rstrip())
+    # New tail must be a comment (the editor's contract); never a
+    # silently-injected new exec line.
+    new_tail = new[len(prev.rstrip()):].strip()
+    assert new_tail.startswith("#")
+
+
+@pytest.mark.parametrize("cls", [
+    ContentClass.CACHE_TMP, ContentClass.EMAIL,
+    ContentClass.CANARY_AWS_CREDS, ContentClass.CANARY_HONEYDOC,
+])
+def test_unsupported_classes_raise_in_edit(cls: ContentClass) -> None:
+    with pytest.raises(KeyError):
+        next_iteration(cls, "admin", "anything")