diff --git a/CHANGELOG.md b/CHANGELOG.md
index c17766f..492e7be 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -112,6 +112,48 @@ least one table — the gate never starves retrieval of all candidates.
 Phase 1-C (smooth sigmoid read-gate threshold) is the remaining
 planned follow-up before the Phase 4 enforcement readiness review.
 
+### Added — issue #116 Phase 1-C: smooth sigmoid read-gate (observability slice)
+
+The memo (§1.4 Stage 2) argues that a hard `--limit N` truncation of
+results is both noise-sensitive (an item whose relevance score
+fluctuates slightly around the cutoff flips in and out
+unpredictably) and non-learnable (zero gradient at the boundary
+gives a feedback loop nothing to update against). The memo's
+recommendation is a smooth sigmoid threshold with learnable slope
+and midpoint, calibrated from operational data.
+
+This release ships the **observability slice** of that
+recommendation — the math module + a `_sigmoid_rank_weight` field
+attached to every result item. Purely additive: rank order is
+unchanged, no items are dropped, the bench harness sees identical
+output structure modulo the new per-item field. Learning the slope
+and midpoint from accumulating outcomes is Phase 4 territory and is
+not implemented here.
+
+- **`agentmemory.sigmoid_gate`** — new module exposing
+  `SigmoidParams`, `sigmoid(x, slope, midpoint)`,
+  `normalize_rank(rank, total)`, `weight_for_rank(rank, total)`,
+  `weights_for_results(total)`, `annotate_with_weights(items)`.
+  Pure math, no DB access. Conservative defaults
+  (`slope=6.0, midpoint=0.5`) chosen per memo §1.4 Stage 2 ("Start
+  with a shallow slope and a conservative midpoint, then tighten
+  as data accumulates").
+
+- **`cmd_search` hookpoint** — annotates each surfaced result item
+  with `_sigmoid_rank_weight` derived from its 1-based rank in the
+  bucket. Failures swallow so a bad gate cannot break search.
+
+- **Tests** — `tests/test_sigmoid_gate.py`. 14 tests covering
+  endpoint behavior, midpoint crossing, monotonicity, extreme
+  overflow handling, SigmoidParams validation, rank normalization,
+  bucket annotation, idempotence (does not overwrite existing
+  weights), and graceful handling of non-dict items.
+
+Phase 4 enforcement readiness review remains the next planned step,
+gated on accumulating 4+ weeks of pathway-log data before deciding
+whether the BG/cerebellum learning loops are calibrated enough to
+flip from shadow to enforcement.
+
 ## [2.7.0] — 2026-05-13 — *Procedural memory layer (Velamj, PR #94)*
 
 The third memory type. brainctl now treats Tulving's 1972 tripartite
diff --git a/src/agentmemory/_impl.py b/src/agentmemory/_impl.py
index 568de0e..eac978a 100644
--- a/src/agentmemory/_impl.py
+++ b/src/agentmemory/_impl.py
@@ -7553,6 +7553,25 @@ def _normalize_bucket_scores(bucket_name):
     elif _debug_mode:
         _out["_debug"] = {"all_signals_informative": True}
 
+    # Issue #116 Phase 1-C: smooth sigmoid read-gate (observability slice).
+    # Annotate each surfaced result item with a sigmoid-weighted
+    # confidence-in-relevance score derived from its 1-based rank
+    # position. Purely additive — does NOT change rank order, does NOT
+    # drop any items. Downstream consumers (BG/cerebellum learning
+    # loops, agents, future sigmoid-aware rerankers) can decide what
+    # to do with the weight. Failures swallow so a bad gate cannot
+    # break search.
+    try:
+        from agentmemory.sigmoid_gate import (
+            annotate_with_weights as _sg_annotate,
+            DEFAULT_PARAMS as _SG_DEFAULTS,
+        )
+        for _bucket_key, _bucket_rows in results.items():
+            if isinstance(_bucket_rows, list) and _bucket_rows:
+                _sg_annotate(_bucket_rows, params=_SG_DEFAULTS)
+    except Exception:  # pragma: no cover — defensive
+        pass
+
     # Issue #116 Phase 1-A: emit one row to retrieval_pathway_log capturing the
     # pathway fingerprint of this retrieval. Best-effort; never blocks the
     # return path. Gated behind BRAINCTL_PATHWAY_LOG env var. See
diff --git a/src/agentmemory/sigmoid_gate.py b/src/agentmemory/sigmoid_gate.py
new file mode 100644
index 0000000..dc546b0
--- /dev/null
+++ b/src/agentmemory/sigmoid_gate.py
@@ -0,0 +1,175 @@
+"""Smooth sigmoid read-gate — issue #116 Phase 1-C (observability slice).
+
+The memo (issue #116 §1.4 Stage 2) argues that a hard `--limit N`
+truncation of search results is both noise-sensitive (an item whose
+relevance score fluctuates slightly around the cutoff flips in and
+out unpredictably) and non-learnable (zero gradient at the boundary
+gives a feedback loop nothing to update against).
+
+The memo's recommendation is a smooth sigmoid threshold with
+learnable slope and midpoint, calibrated from operational data.
+
+This module ships the **observability slice** of that recommendation:
+
+  - Pure-math sigmoid + per-rank weight helpers.
+  - Conservative default parameters (shallow slope, mid midpoint).
+  - A `weight_for_rank(rank, total)` helper that surfaces a soft
+    confidence-in-relevance per result item without changing the
+    item's actual rank position.
+
+The intended use at this stage is **additive** — `cmd_search`
+attaches a `_sigmoid_rank_weight` field to each returned item so
+downstream consumers (BG/cerebellum learning loops, agents, future
+sigmoid-aware rerankers) can experiment with using it without any
+risk of regressing the existing rank order or the bench harness.
+
+Learning the slope / midpoint from accumulating outcomes is Phase 4
+territory and not implemented here. The conservative defaults are
+chosen so that the surfaced weights are useful as a relative ordering
+signal but do not over-commit to any particular calibration before
+the data is in.
+
+See also:
+  - research/issue-116-audit-vs-origin-main.md §6.2.4 — "Smooth
+    sigmoid read-gate threshold"
+  - research/brainctl-brain-architecture-issue-116.md §1.4 Stage 2
+"""
+from __future__ import annotations
+
+import math
+from dataclasses import dataclass
+from typing import Iterable
+
+# Conservative defaults — chosen for "shallow slope, conservative
+# midpoint" per memo §1.4 Stage 2 ("Start with a shallow slope and
+# a conservative midpoint, then tighten as data accumulates").
+#
+# With slope=6.0 and midpoint=0.5 over normalized rank ∈ [0, 1]:
+#   rank 1 of  5 (normalized 1.0 ) → weight ≈ 0.953
+#   rank 3 of  5 (normalized 0.5 ) → weight = 0.5
+#   rank 5 of  5 (normalized 0.0 ) → weight ≈ 0.047
+#   rank 1 of 10 (normalized 1.0 ) → weight ≈ 0.953
+#   rank 5 of 10 (normalized 0.555) → weight ≈ 0.583
+#   rank 10 of 10 (normalized 0.0) → weight ≈ 0.047
+#
+# A shallower slope produces a flatter weight curve that distributes
+# uncertainty more evenly across positions; a steeper slope sharpens
+# the in/out distinction. The Phase 4 calibration step will fit these
+# from accumulated outcomes.
+DEFAULT_SLOPE = 6.0
+DEFAULT_MIDPOINT = 0.5
+
+
+@dataclass(frozen=True)
+class SigmoidParams:
+    """A (slope, midpoint) pair. Frozen so it can be passed around and
+    cached without surprise mutations."""
+    slope: float = DEFAULT_SLOPE
+    midpoint: float = DEFAULT_MIDPOINT
+
+    def __post_init__(self) -> None:
+        # Defensive — keeps callers honest about what shapes are
+        # meaningful. midpoint must be in (0, 1) so rank-position
+        # normalization stays well-defined; slope must be > 0 so the
+        # function is monotone increasing in x.
+        if not (0.0 < self.midpoint < 1.0):
+            raise ValueError(
+                f"midpoint must be in (0, 1); got {self.midpoint!r}"
+            )
+        if self.slope <= 0.0:
+            raise ValueError(f"slope must be > 0; got {self.slope!r}")
+
+
+DEFAULT_PARAMS = SigmoidParams()
+
+
+def sigmoid(x: float, *, slope: float = DEFAULT_SLOPE,
+            midpoint: float = DEFAULT_MIDPOINT) -> float:
+    """Smooth threshold function.
+
+    Returns a value in (0, 1) that crosses 0.5 at x = midpoint and
+    approaches 1.0 (resp. 0.0) for x well above (resp. below) midpoint.
+
+    The slope controls how sharp the transition is — large slope
+    approximates a hard cutoff, small slope produces a gentle ramp.
+    """
+    # math.exp can overflow for huge negative exponents; guard.
+    z = -slope * (x - midpoint)
+    if z > 500:
+        return 0.0
+    if z < -500:
+        return 1.0
+    return 1.0 / (1.0 + math.exp(z))
+
+
+def normalize_rank(rank: int, total: int) -> float:
+    """Map a 1-based rank position (1=best) inside a result set of
+    `total` items to a normalized score in [0, 1] where 1.0 is the top
+    of the list and 0.0 is the bottom.
+
+    Edge cases:
+      - total <= 0           → 0.5 (degenerate input; refuse to commit)
+      - total == 1, rank == 1 → 1.0 (the only item is the top)
+      - rank outside [1, total] is clamped to that range.
+    """
+    if total <= 0:
+        return 0.5
+    if total == 1:
+        return 1.0
+    if rank < 1:
+        rank = 1
+    elif rank > total:
+        rank = total
+    # rank=1 → 1.0, rank=total → 0.0, linear in between.
+    return (total - rank) / (total - 1)
+
+
+def weight_for_rank(rank: int, total: int,
+                    *, params: SigmoidParams = DEFAULT_PARAMS) -> float:
+    """Sigmoid weight for an item at 1-based `rank` in a result set
+    of `total` items. Composition of `normalize_rank` + `sigmoid`."""
+    return sigmoid(
+        normalize_rank(rank, total),
+        slope=params.slope,
+        midpoint=params.midpoint,
+    )
+
+
+def weights_for_results(total: int,
+                        *, params: SigmoidParams = DEFAULT_PARAMS) -> list[float]:
+    """Compute the sigmoid weight at every rank position 1..total.
+
+    Returns a list of length `total` indexed by zero-based position
+    (i.e. result[0] holds the weight for rank 1). Convenient for
+    callers that hand out a ranked list and want to attach weights
+    in one pass.
+    """
+    if total <= 0:
+        return []
+    return [weight_for_rank(rank, total, params=params)
+            for rank in range(1, total + 1)]
+
+
+def annotate_with_weights(items: Iterable[dict],
+                          *,
+                          weight_key: str = "_sigmoid_rank_weight",
+                          params: SigmoidParams = DEFAULT_PARAMS,
+                          ) -> list[dict]:
+    """Mutate each dict in `items` by attaching a sigmoid weight at
+    `weight_key`, keyed by 1-based rank in iteration order.
+
+    Items that already carry a value at `weight_key` are left
+    untouched — the gate does not overwrite an explicit upstream
+    decision. Returns the same list (passed through) for chaining.
+    """
+    items_list = list(items)
+    total = len(items_list)
+    if total == 0:
+        return items_list
+    for idx, item in enumerate(items_list, start=1):
+        if not isinstance(item, dict):
+            continue
+        if weight_key in item:
+            continue
+        item[weight_key] = weight_for_rank(idx, total, params=params)
+    return items_list
diff --git a/tests/test_sigmoid_gate.py b/tests/test_sigmoid_gate.py
new file mode 100644
index 0000000..f237022
--- /dev/null
+++ b/tests/test_sigmoid_gate.py
@@ -0,0 +1,140 @@
+"""Tests for sigmoid_gate — issue #116 Phase 1-C.
+
+Covers:
+  - SigmoidParams validation
+  - sigmoid() endpoint behavior + midpoint crossing
+  - normalize_rank() edge cases
+  - weight_for_rank() ordering
+  - weights_for_results() shape
+  - annotate_with_weights() mutation + idempotence
+"""
+from __future__ import annotations
+
+import pytest
+
+from agentmemory.sigmoid_gate import (
+    DEFAULT_MIDPOINT,
+    DEFAULT_PARAMS,
+    DEFAULT_SLOPE,
+    SigmoidParams,
+    annotate_with_weights,
+    normalize_rank,
+    sigmoid,
+    weight_for_rank,
+    weights_for_results,
+)
+
+
+def test_sigmoid_crosses_05_at_midpoint():
+    # At x == midpoint, sigmoid output is exactly 0.5 regardless of slope.
+    assert sigmoid(0.5, slope=1.0, midpoint=0.5) == pytest.approx(0.5)
+    assert sigmoid(0.7, slope=12.0, midpoint=0.7) == pytest.approx(0.5)
+
+
+def test_sigmoid_approaches_endpoints():
+    # Far above midpoint → 1; far below → 0.
+    assert sigmoid(100.0, slope=1.0, midpoint=0.0) > 0.999
+    assert sigmoid(-100.0, slope=1.0, midpoint=0.0) < 0.001
+
+
+def test_sigmoid_handles_extreme_overflow():
+    # The internal exp() must not raise on extreme inputs.
+    assert sigmoid(1e9, slope=1e6, midpoint=0.0) == pytest.approx(1.0)
+    assert sigmoid(-1e9, slope=1e6, midpoint=0.0) == pytest.approx(0.0)
+
+
+def test_sigmoid_monotone_increasing():
+    # Strictly monotone in x for fixed positive slope.
+    samples = [sigmoid(x / 10.0) for x in range(0, 11)]
+    for a, b in zip(samples, samples[1:]):
+        assert a < b
+
+
+def test_sigmoid_params_validates_midpoint():
+    with pytest.raises(ValueError):
+        SigmoidParams(slope=1.0, midpoint=0.0)
+    with pytest.raises(ValueError):
+        SigmoidParams(slope=1.0, midpoint=1.0)
+    with pytest.raises(ValueError):
+        SigmoidParams(slope=1.0, midpoint=-0.1)
+
+
+def test_sigmoid_params_validates_slope():
+    with pytest.raises(ValueError):
+        SigmoidParams(slope=0.0, midpoint=0.5)
+    with pytest.raises(ValueError):
+        SigmoidParams(slope=-1.0, midpoint=0.5)
+
+
+def test_normalize_rank_endpoints_and_edges():
+    # rank=1 (best) maps to 1.0; rank=total maps to 0.0
+    assert normalize_rank(1, 10) == 1.0
+    assert normalize_rank(10, 10) == 0.0
+    # Linear interior — rank 5 of 9 should be exactly 0.5
+    assert normalize_rank(5, 9) == pytest.approx(0.5)
+    # Singletons
+    assert normalize_rank(1, 1) == 1.0
+    # Degenerate / clamping
+    assert normalize_rank(5, 0) == 0.5
+    assert normalize_rank(-3, 10) == 1.0
+    assert normalize_rank(99, 10) == 0.0
+
+
+def test_weight_for_rank_orders_with_position():
+    # In a 10-item set, rank 1 must outweigh rank 10 (strictly).
+    w_top = weight_for_rank(1, 10)
+    w_bot = weight_for_rank(10, 10)
+    assert w_top > w_bot
+    # And the middle item lands near 0.5 with default midpoint=0.5.
+    w_mid = weight_for_rank(5, 9)
+    assert w_mid == pytest.approx(0.5, abs=0.01)
+
+
+def test_weights_for_results_shape_and_monotone():
+    weights = weights_for_results(10)
+    assert len(weights) == 10
+    # Strictly decreasing along rank order (best-first).
+    for a, b in zip(weights, weights[1:]):
+        assert a > b
+    # All in (0, 1).
+    for w in weights:
+        assert 0.0 < w < 1.0
+
+
+def test_weights_for_results_empty():
+    assert weights_for_results(0) == []
+    assert weights_for_results(-3) == []
+
+
+def test_annotate_with_weights_mutates_in_place_and_preserves_order():
+    items = [{"id": 1}, {"id": 2}, {"id": 3}]
+    out = annotate_with_weights(items)
+    assert out is not items  # returns a list copy
+    assert [d["id"] for d in out] == [1, 2, 3]
+    # Weights present, monotone-decreasing
+    weights = [d["_sigmoid_rank_weight"] for d in out]
+    assert all(0.0 < w < 1.0 for w in weights)
+    for a, b in zip(weights, weights[1:]):
+        assert a > b
+
+
+def test_annotate_does_not_overwrite_existing_weight():
+    items = [{"id": 1, "_sigmoid_rank_weight": 0.42}, {"id": 2}]
+    out = annotate_with_weights(items)
+    assert out[0]["_sigmoid_rank_weight"] == 0.42  # untouched
+    assert "_sigmoid_rank_weight" in out[1]         # new
+
+
+def test_annotate_skips_non_dict_items():
+    items = [{"id": 1}, "not-a-dict", {"id": 3}]
+    out = annotate_with_weights(items)
+    assert "_sigmoid_rank_weight" in out[0]
+    assert out[1] == "not-a-dict"
+    assert "_sigmoid_rank_weight" in out[2]
+
+
+def test_default_params_have_sane_values():
+    assert DEFAULT_PARAMS.slope == DEFAULT_SLOPE
+    assert DEFAULT_PARAMS.midpoint == DEFAULT_MIDPOINT
+    # And the defaults must satisfy the constructor's validation.
+    SigmoidParams(slope=DEFAULT_SLOPE, midpoint=DEFAULT_MIDPOINT)