Give the length hint a floor, not just a wall

A ceiling alone is a one-sided instruction, and models read it in opposite
directions. A verbose one is held back by it; a terse one has nothing to act
on except "write only as much as the moment needs -- a typical turn is much
shorter" and collapses to two paragraphs. Same prompt, wildly different turn
lengths depending on which model is behind it.

State a floor as well, so the guidance is a band. The two bounds are
deliberately asymmetric -- "must not exceed" for the wall the endpoint
enforces, "should not stop short of" for the floor -- so neither reads as a
number to hit, which is the property the earlier A/B says decides whether
this hint helps or hurts. "Prefer the lower end" inherits the anti-overshoot
job the deleted "much shorter" line was doing, but now with a number under
it, so a terse model lands on the floor instead of at forty words.

Below MIN_LENGTH_FLOOR_WORDS the floor is dropped and the tight-cap wording
is left byte-identical: at a tight cap a short turn is the correct turn, and
that phrasing is the one measured to keep the state block alive (0/6
truncations at cap 250 against 2/6 unhinted). So this only moves loose caps.
MAX_LENGTH_FLOOR_WORDS keeps the share from demanding 555 words minimum at
cap 2400 -- a big cap means long turns are allowed, not compulsory.

Shipped without an A/B run, deliberately. Two things to watch live: whether a
stated range invites landing mid-range on verbose models (drop the share to
~0.25 if so), and whether the state block still survives -- nothing reads
finish_reason yet, so truncation is silent.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01DfMCsN1KBLsTqMkj5hSgrY

backend/tests/test_length_hint.py

@@ -114,8 +114,44 @@ def test_hint_is_phrased_as_a_ceiling_not_a_budget():
     it exists to avoid. The limit framing must survive future prompt edits."""
     hint = builder.length_hint(800, has_ws=True)
     assert "must not exceed" in hint
-    assert "much shorter" in hint, "without this the number still reads as a target"
     assert "under about" not in hint
+    assert "lower end" in hint, "without this the number still reads as a target"
+
+
+def test_hint_states_a_floor_as_well_as_a_ceiling():
+    """A ceiling alone is one-sided: a terse model has nothing to act on but the
+    "only as much as the moment needs" clause and collapses to two paragraphs.
+    The floor is what makes the same prompt land in the same place across models
+    that lean opposite ways."""
+    hint = builder.length_hint(800, has_ws=True)
+    assert "506" in hint and "177" in hint
+    assert "should not stop short of" in hint
+    # Asymmetric on purpose: the wall is a wall, the floor is a floor, and neither
+    # is phrased as a number to hit.
+    assert hint.index("must not exceed") < hint.index("should not stop short of")
+
+
+def test_floor_stays_well_under_the_ceiling():
+    for cap in (400, 800, 1500, 2400):
+        hint = builder.length_hint(cap, has_ws=True)
+        ceiling, floor = (int(n) for n in re.findall(r"(\d+)", hint)[:2])
+        assert floor < ceiling * 0.5
+
+
+def test_floor_is_dropped_when_the_cap_is_too_tight_for_one():
+    """At a tight cap a short turn is the correct turn, and the tight-cap wording
+    is the one measured to keep the state block alive (0/6 truncations at cap 250
+    against 2/6 unhinted) — so it is left exactly as it was."""
+    hint = builder.length_hint(250, has_ws=True)
+    assert "should not stop short of" not in hint
+    assert "much shorter" in hint
+
+
+def test_floor_does_not_grow_without_bound():
+    """A big cap means "long turns are allowed", not "every turn must be an essay":
+    the share alone would demand 555 words minimum at cap 2400."""
+    hint = builder.length_hint(2400, has_ws=True)
+    assert str(builder.MAX_LENGTH_FLOOR_WORDS) in hint
 
 
 def test_no_hint_when_the_cap_is_too_small_to_phrase():