diff --git a/README.md b/README.md
index 4918ea1..49ff821 100644
--- a/README.md
+++ b/README.md
@@ -81,13 +81,18 @@ Detect the supported SynthID pixel carrier after installing the pixel runtime:
 ```bash
 uv tool install --force "remove-ai-watermarks[pixels]"
 remove-ai-watermarks detect-synthid image.png
+remove-ai-watermarks detect-synthid resized.png --register-scale
 ```
 
 This detector is positive-only and limited to one measured carrier family in
 the [calibrated image-size range](docs/synthid.md#32-how-our-tool-detects-the-supported-carrier).
-It expects the recovered carrier at its measured 16-pixel sampling scale;
-arbitrary spatial resampling is not registered. `not_detected` or `unsupported`
-is not a clean-image guarantee.
+The fast default expects the recovered carrier at its measured 16-pixel
+sampling scale. `--register-scale` opts into a much slower bounded scale search
+for resized images from 250,000 through 10,000,000 decoded pixels, with both
+sides at least 64 pixels. Its measured positive scale range is approximately
+0.65 through 1.5; 0.5x resizes remain outside reliable detection. `identify`
+keeps the fast default. `not_detected` or `unsupported` is not a clean-image
+guarantee.
 
 For visible watermark removal, install the pixel dependencies:
 
diff --git a/docs/cli.md b/docs/cli.md
index 072cdca..bafee3b 100644
--- a/docs/cli.md
+++ b/docs/cli.md
@@ -64,13 +64,19 @@ TrustMark. Metadata inspection still runs.
 ```bash
 remove-ai-watermarks detect-synthid image.png
 remove-ai-watermarks detect-synthid image.png --json
+remove-ai-watermarks detect-synthid resized.png --register-scale
 ```
 
 The command returns one of `detected`, `not_detected`, or `unsupported`. The
 runtime detector covers one frozen periodic carrier family in the
 [calibrated image-size range](synthid.md#32-how-our-tool-detects-the-supported-carrier)
-and needs the `pixels` extra. It never resizes the input and does not register
-a carrier whose sampling period changed through arbitrary spatial resampling.
+and needs the `pixels` extra. The default never resizes the input and does not
+register a carrier whose sampling period changed through spatial resampling.
+`--register-scale` enables a substantially slower bounded search over measured
+carrier periods for images from 250,000 through 10,000,000 decoded pixels, with
+both sides at least 64 pixels. It is opt-in and is not used by `identify`.
+The measured positive scale range is approximately 0.65 through 1.5; 0.5x
+resizes are not reliably detected.
 It is positive-only: `not_detected` means the score stayed below this detector's
 threshold, while `unsupported` means the image geometry is outside its scope.
 Neither result proves that another SynthID epoch or payload is absent.
diff --git a/docs/known-limitations.md b/docs/known-limitations.md
index 71e1cef..44a4e9d 100644
--- a/docs/known-limitations.md
+++ b/docs/known-limitations.md
@@ -70,13 +70,17 @@ Google does not publish the proprietary SynthID payload decoder. The project
 ships a positive-only detector for one measured periodic image carrier in a
 calibrated image-size range, plus provenance-based recognition
 from Google AI C2PA or current OpenAI C2PA with an explicit watermark action.
-It does not cover images outside that size range, crop, strong JPEG compression,
-video, or future carrier epochs. Arbitrary dimensions inside the range are
-supported only while the recovered carrier retains its measured 16-pixel
-sampling lattice. A spatial resize changes that period; the runtime does not
-yet search fractional periods or infer the pre-resize geometry. A `not_detected`
-or `unsupported` result is not a negative universal verdict, and removal still
-requires the matching provider oracle for confirmation.
+It does not cover images outside the selected mode's size range, crop, strong
+JPEG compression, video, or future carrier epochs. Arbitrary dimensions inside
+the default range are supported only while the recovered carrier retains its
+measured 16-pixel sampling lattice. A spatial resize changes that period. The
+opt-in `detect-synthid --register-scale` mode searches a bounded
+fractional-period range for images from 250,000 through 10,000,000 pixels, but
+requires both sides to be at least 64 pixels. Its measured positive range is
+approximately scale 0.65 through 1.5; a 0.5 resize aliases the carrier with
+ordinary resize lattices and is not reliably detected. It is also slower. A
+`not_detected` or `unsupported` result is not a negative universal verdict, and
+removal still requires the matching provider oracle for confirmation.
 
 For important outputs:
 
diff --git a/docs/module-internals.md b/docs/module-internals.md
index 4044b87..8fca511 100644
--- a/docs/module-internals.md
+++ b/docs/module-internals.md
@@ -500,7 +500,32 @@ the native threshold, compared with an upper bound of 48 when the true source
 geometry was supplied. The complete search still failed its small frozen
 control split: a threshold above 250 development controls accepted two of 150
 final controls. Multiplying the canonical score by spectral-period confidence
-also accepted two. This branch is not a calibrated runtime fallback.
+also accepted two. That baseline was rejected rather than shipped at its
+discovery threshold.
+
+The opt-in `register_scale=True` path is the later calibrated successor. It
+keeps the harmonic search, adds three-level scale-space consistency and quadrant
+agreement. Its calibrated decision requires the canonical winner to be the
+strongest spectral-period candidate, its three-way amplitude to cross the
+threshold for that period bin, and both independent high-frequency template
+bands to correlate by at least `0.075`. The public registered score is the
+minimum normalized margin across those gates, so its decision threshold is
+`1.0`.
+
+The earlier single-threshold version produced 68 false positives among 11,273
+symmetrically downscaled Spaces controls. A top-candidate plus period-bin version
+removed those but still produced 6 false positives among 1,000 source-independent
+Open Images controls. The high-band gate removed them and then accepted none of
+499 previously untouched Open Images reserve controls. The resulting rule
+retained 229 of 355 source-disjoint transformed positives: 0/65 at scale 0.5 and
+229/290 from scale 0.65 through 1.5. The explicit period-8 rescue is rejected
+because resize lattices fully overlap its positive distribution.
+
+The separately measured geometry range remains 250,000 through 10,000,000
+decoded pixels with both sides at least 64 pixels. The default path and
+`identify` remain the native fold. A 20-image real-corpus drift check was
+byte-identical after integration. The calibration history and caveats are in the
+linked detector research plan.
 
 ### Portable metadata record
 
diff --git a/docs/python-api.md b/docs/python-api.md
index a30dbb2..51c7b25 100644
--- a/docs/python-api.md
+++ b/docs/python-api.md
@@ -23,15 +23,23 @@ result = raiw.detect_synthid("input.png")
 print(result.status)     # "detected" | "not_detected" | "unsupported"
 print(result.score)      # float for a supported image size, otherwise None
 print(result.threshold)  # frozen operating point
+
+# Opt in when the image may have been spatially resized.
+registered = raiw.detect_synthid("resized.png", register_scale=True)
 ```
 
 The detector is positive-only and covers one measured periodic carrier family
 in the [calibrated image-size range](synthid.md#32-how-our-tool-detects-the-supported-carrier).
-Arbitrary dimensions are accepted inside that range, but arbitrary spatial
-resampling can change the carrier period and is not registered. `not_detected`
-means only that this model did not find its carrier; `unsupported` is kept
-separate from a negative result. Neither is proof that the image contains no
-SynthID watermark.
+Arbitrary dimensions are accepted inside the default range, but spatial
+resampling can change the carrier period. `register_scale=True` enables the
+slower scale-registered operating point over 250,000 through 10,000,000 decoded
+pixels, with both sides at least 64 pixels; the default remains the native-period
+detector used by `identify`. Its score is a normalized multi-gate statistic with
+a threshold of `1.0`, not the native detector's raw template correlation. Scale
+0.5 is outside its reliable positive range.
+`not_detected` means only that the selected model did not find its carrier;
+`unsupported` is kept separate from a negative result. Neither is proof that
+the image contains no SynthID watermark.
 
 ## Remove visible marks
 
diff --git a/docs/supported-signals.md b/docs/supported-signals.md
index c84214a..7437a19 100644
--- a/docs/supported-signals.md
+++ b/docs/supported-signals.md
@@ -132,9 +132,14 @@ calibrated image-size range, available through `detect-synthid`
 and the default pixel pass in `identify` when the `pixels` extra is installed.
 The unchanged fixed threshold accepted none of the public COCO views in both
 an observed-geometry challenge and a generated-geometry challenge covering all
-modulo-16 edge cases. Arbitrary dimensions in the calibrated range are accepted,
-but the input must retain the measured 16-pixel carrier scale: arbitrary spatial
-resampling is not registered. The detector does not attribute a provider locally.
+modulo-16 edge cases. Arbitrary dimensions in the default calibrated range are
+accepted, but the input must retain the measured 16-pixel carrier scale. The
+opt-in `detect-synthid --register-scale` mode performs a slower bounded scale
+search over its separately measured 250,000-through-10,000,000-pixel range and
+requires both sides to be at least 64 pixels. Its measured positive scale range
+is approximately 0.65 through 1.5; 0.5x resizes are not reliably detected.
+`identify` does not enable that search. Neither detector attributes a provider
+locally.
 
 The tool also recognizes presence from supported provenance: Google AI C2PA
 under Google's all-media watermark policy, and current OpenAI C2PA carrying an
diff --git a/docs/synthid-detector-removal-plan.md b/docs/synthid-detector-removal-plan.md
index 1e9f377..ca22e6b 100644
--- a/docs/synthid-detector-removal-plan.md
+++ b/docs/synthid-detector-removal-plan.md
@@ -1331,6 +1331,99 @@ acceptance from 49 to 52 of 60 at its development threshold but accepted the
 same two final controls. This fractional FFT baseline is rejected for runtime
 use; better period estimation alone does not solve multiple-search specificity.
 
+### 2026-08-11: multiscale calibration and opt-in registered detector
+
+The fractional search was revisited as a multiple-search calibration problem,
+not as period estimation alone. A source-disjoint challenge used 200 confirmed
+fixed-carrier positives, 40 independent sources at each scale 0.8, 0.9, 1.1,
+1.2, and 1.333. Controls were 4,900 unique COCO images resized into the positive
+pixel-count domain, split into 2,900 development and 2,000 final items. The ten
+frozen candidates combined exact fractional canonical correlation, quadrant
+agreement, and a three-level scale-space score through mean or minimum rules.
+Pure box-counting dimensions, multifractal width, lacunarity, roughness,
+wavelet-energy slope, and a generic log-Mellin response had already failed the
+smaller pilot and were not promoted to the large challenge.
+
+At the development-only threshold, the three-way mean accepted 177 of 200
+positives versus 163 for canonical correlation alone, with 16 positives rescued
+and two lost in the paired decision. Each produced one final false positive, on
+different source images, so the development threshold was rejected. Recalibrating
+above all 4,900 COCO controls left 176 of 200 three-way positives and 160 of 200
+canonical-only positives. This recalibration is exploratory rather than a new
+validation because the final controls had then been inspected.
+
+A second source-disjoint corpus froze those thresholds before scoring 105 new
+positives across scales 0.5, 0.65, 0.8, 1.0, 1.2, 1.333, and 1.5, plus all 879
+content-deduplicated Kodak and Picsum controls available locally. Natural
+2816x1536 controls had a heavier null tail than upscaled COCO: the three-way
+mean accepted 82 of 105 positives and six controls at the COCO threshold,
+whereas canonical-only accepted 79 positives and ten controls. A threshold
+above all 5,779 controls retained 253 of the first 305 positives for the
+three-way mean. Four coarse pixel-count bins recovered only six more positives,
+so size normalization did not explain the main 0.5x miss.
+
+At 0.5x, a fixed period-8 branch raised canonical correlation for seven of the
+15 discovery positives but also won the unconstrained search on 629 of 879
+controls. Searching it unconditionally merely raised the null threshold. The
+frozen remedy requires period-8 native correlation of at least 0.35 and takes
+the maximum of the old and period-8 three-way scores. It added four positives
+without an accepted control in the discovery corpus. On the earlier 4,900
+controls, the frozen gate never opened and none crossed the final three-way
+threshold `0.2460603834083705`.
+
+The improvement itself was then tested on 50 additional source-disjoint 0.5x
+positives that influenced neither gate nor threshold. The ungated three-way
+score accepted 6; the frozen gate accepted 17, rescuing 11 and losing none. The
+exact paired sign test was `p = 0.0009765625`. These results support an opt-in
+scale-registered detector over 250,000 through 10,000,000 decoded pixels. The
+runtime also requires both sides to be at least 64 pixels so each canonical
+quadrant can retain a full 16-by-16 tile. These results do not establish a
+universal SynthID decoder: the control maximum helped set the operating point,
+33 of 50 new 0.5x positives still missed, and crop, strong codec changes, other
+carrier epochs, and provider attribution remain outside the claim. The fast
+native fold remains the default and the `identify` path.
+
+That period-8 conclusion was overturned by a later symmetric negative test.
+Every one of the 11,506 known non-Google Spaces controls was downscaled by 0.5,
+matching the positive transformation; 11,273 outputs remained in the registered
+geometry range. The direct analytic 8-by-8 template was dominated by shared
+resize structure: 80.8% of its energy projected onto a 2-by-2 lattice and 94.0%
+onto a 4-by-4 lattice. The production period-8 gate opened on 73 controls, and
+all 73 crossed the registered threshold. They spanned hosted jobs, Firefly,
+China AIGC-labeled generators, Meta-tagged images, OpenAI, and local tools. No
+tested secondary condition separated the 11 rescued positives from those 73
+controls. The explicit period-8 rescue is therefore rejected and not part of
+the runtime detector. The 0.5x rescue result is retained here as an incident
+record against asymmetric transformed-positive evaluation.
+
+The same symmetric corpus also invalidated the main single-threshold registered
+rule: 68 of 11,273 supported controls crossed `0.2460603834083705`. Fixed
+thresholds for nine recovered-period bins eliminated those errors while
+retaining 238 of 355 transformed positives, but one of 4,900 COCO controls then
+crossed through the third spectral candidate. Requiring the canonical winner to
+match the strongest spectral candidate removed that case and all 68 Spaces
+errors, with 239 of 355 positives retained. This version was not shipped: a
+fresh, content-deduplicated 1,000-image Open Images validation corpus produced
+six false positives.
+
+All six Open Images errors concentrated their apparent match in one
+middle-frequency ring. Their correlations disappeared or changed sign in the
+two higher-frequency rings of the 16-by-16 carrier, unlike the accepted Gemini
+positives. Adding a frozen minimum correlation of `0.075` across those two rings
+removed the six development errors and retained 229 of 355 positives. A
+previously untouched reserve of 499 supported Open Images controls then produced
+five amplitude-gate candidates and zero final positives; their maximum
+high-band score was `0.004682175209264222`.
+
+The retained positives by scale were 0/65 at 0.5, 8/15 at 0.65, 53/55 at 0.8,
+33/40 at 0.9, 13/15 at 1.0, 30/40 at 1.1, 38/55 at 1.2, 40/55 at 1.333, and
+14/15 at 1.5. A physically transformed template recovered strong correlations
+for some period-8 positives, but 975 top-candidate Spaces controls fully
+overlapped every tested amplitude, spectral-band, and joint feature. No
+zero-control operating point retained a period-8 positive. The calibrated
+runtime therefore uses the top-candidate, period-bin, and high-band gates and
+makes no 0.5x detection claim.
+
 ### 2026-08-10: OpenAI periodic-carrier challenge
 
 The OpenAI track repeated the Google carrier method without runtime provenance
diff --git a/docs/synthid.md b/docs/synthid.md
index 90af5e1..4974e1d 100644
--- a/docs/synthid.md
+++ b/docs/synthid.md
@@ -483,6 +483,30 @@ less conservative per-period thresholds accepted five final controls. The
 runtime therefore detects arbitrary decoded dimensions only when the carrier
 retains its measured 16-pixel scale.
 
+`remove-ai-watermarks detect-synthid image.png --register-scale` opts into the
+separately calibrated scale-registered detector. It samples the 30 strongest
+template harmonics over fractional periods, reconstructs the three strongest
+canonical candidates, and combines full-frame correlation with quadrant and
+three-level scale-space consistency. A positive result additionally requires
+the canonical winner to be the strongest spectral candidate, a calibrated
+threshold for its recovered period, and agreement in two independent
+high-frequency template bands. The reported score is the minimum normalized
+margin across these requirements; its threshold is `1.0`.
+
+The final rule retained 229 of 355 source-disjoint transformed positives and
+accepted none of 499 previously untouched Open Images controls. It retained
+229 of 290 positives from scale 0.65 through 1.5, but none of 65 at scale 0.5.
+A separate period-8 rescue was rejected because symmetric 0.5x control resizing
+reproduced the same lattice and fully overlapped the positive feature range.
+The measured input range is 250,000 through 10,000,000 decoded pixels.
+Both dimensions must also be at least 64 pixels so every canonical quadrant can
+retain one 16-by-16 carrier tile.
+
+This mode is not the default because the bounded search is materially slower;
+scale registration is explicit and `identify` keeps the native detector. The
+registered mode does not reliably detect 0.5x carriers and does not make the
+detector universal across crop, codecs, carrier epochs, or providers.
+
 A positive result identifies the carrier but does not attribute a provider.
 Provider identity still comes from provenance.
 
diff --git a/src/remove_ai_watermarks/_synthid_registered.py b/src/remove_ai_watermarks/_synthid_registered.py
new file mode 100644
index 0000000..58ecbe3
--- /dev/null
+++ b/src/remove_ai_watermarks/_synthid_registered.py
@@ -0,0 +1,304 @@
+"""Opt-in scale registration for the measured periodic SynthID carrier."""
+
+# The optional numeric libraries do not provide complete types for this path.
+# pyright: reportMissingTypeStubs=false, reportUnknownMemberType=false, reportUnknownVariableType=false, reportUnknownArgumentType=false
+
+from __future__ import annotations
+
+import itertools
+import math
+from dataclasses import dataclass
+from typing import TYPE_CHECKING, Any
+
+import cv2
+import numpy as np
+
+from remove_ai_watermarks.synthid_detector import folded_template_score
+
+if TYPE_CHECKING:
+    from numpy.typing import NDArray
+
+_PYRAMID_SCALES = (0.75, 1.0, 1.25)
+_SEARCH_PERIODS = np.linspace(5.0, 32.0, 541, dtype=np.float64)
+_CANONICAL_PERIODS = np.linspace(7.5, 24.5, 1701, dtype=np.float64)
+_PERIOD_THRESHOLDS = (
+    (7.5, 8.5, 0.3770629524888979),
+    (8.5, 10.0, 0.25174716660523494),
+    (10.0, 12.0, 0.284692023502354),
+    (12.0, 14.0, 0.19794247706938645),
+    (14.0, 16.0, 0.33930082812296375),
+    (16.0, 18.0, 0.28915284982686323),
+    (18.0, 20.0, 0.22885510746595789),
+    (20.0, 22.0, 0.24570317032768269),
+    (22.0, 24.5, 0.3142958338390489),
+)
+REGISTERED_HIGH_BAND_THRESHOLD = 0.075
+
+
+@dataclass(frozen=True)
+class RegisteredComponents:
+    """Calibrated components of one scale-registered decision."""
+
+    raw_score: float
+    amplitude_threshold: float
+    selected_period: float
+    spectral_period: float
+    high_band_score: float
+
+    @property
+    def decision_score(self) -> float:
+        """Return a statistic that reaches one only when every gate passes."""
+        if self.selected_period != self.spectral_period:
+            return 0.0
+        return min(
+            self.raw_score / self.amplitude_threshold,
+            self.high_band_score / REGISTERED_HIGH_BAND_THRESHOLD,
+        )
+
+
+def _resize(pixels: NDArray[Any], width: int, height: int) -> NDArray[Any]:
+    interpolation = cv2.INTER_AREA if width < pixels.shape[1] else cv2.INTER_CUBIC
+    return np.asarray(cv2.resize(pixels, (width, height), interpolation=interpolation))
+
+
+def _template_frequency_features(
+    template: NDArray[Any],
+) -> tuple[NDArray[Any], NDArray[Any], NDArray[Any]]:
+    spectrum = np.fft.fft2(template, axes=(0, 1))
+    power = np.sum(np.abs(spectrum) ** 2, axis=2)
+    power[0, 0] = 0.0
+    indices = np.argsort(power.ravel())[::-1][:30]
+    rows, columns = np.unravel_index(indices, power.shape)
+    height, width = template.shape[:2]
+    signed_rows = np.where(rows <= height // 2, rows, rows - height)
+    signed_columns = np.where(columns <= width // 2, columns, columns - width)
+    harmonics = np.column_stack((signed_rows, signed_columns)).astype(np.float64)
+    return harmonics, spectrum[rows, columns], spectrum
+
+
+def _bilinear_sample(
+    spectrum: NDArray[Any],
+    y: NDArray[Any],
+    x: NDArray[Any],
+) -> NDArray[Any]:
+    height, width = spectrum.shape
+    y_floor = np.floor(y)
+    x_floor = np.floor(x)
+    y0 = y_floor.astype(np.int64) % height
+    x0 = x_floor.astype(np.int64) % width
+    y1 = (y0 + 1) % height
+    x1 = (x0 + 1) % width
+    dy = y - y_floor
+    dx = x - x_floor
+    return (
+        spectrum[y0, x0] * (1.0 - dy) * (1.0 - dx)
+        + spectrum[y1, x0] * dy * (1.0 - dx)
+        + spectrum[y0, x1] * (1.0 - dy) * dx
+        + spectrum[y1, x1] * dy * dx
+    )
+
+
+def _spectral_curve(
+    pixels: NDArray[Any],
+    periods: NDArray[Any],
+    harmonics: NDArray[Any],
+    coefficients: NDArray[Any],
+) -> NDArray[Any]:
+    height, width = pixels.shape[:2]
+    y = (periods[:, None] ** -1) * harmonics[None, :, 0] * height
+    x = (periods[:, None] ** -1) * harmonics[None, :, 1] * width
+    sampled = np.empty((len(periods), len(harmonics), 3), dtype=np.complex128)
+    for channel in range(3):
+        residual = pixels[:, :, channel].astype(np.float32)
+        residual -= cv2.GaussianBlur(
+            residual,
+            (0, 0),
+            sigmaX=1.0,
+            sigmaY=1.0,
+            borderType=cv2.BORDER_REFLECT_101,
+        )
+        spectrum = np.fft.fft2(residual)
+        sampled[:, :, channel] = _bilinear_sample(spectrum, y % height, x % width)
+    numerator = np.real(np.sum(np.conj(coefficients)[None, :, :] * sampled, axis=(1, 2)))
+    denominator = np.linalg.norm(coefficients) * np.linalg.norm(sampled, axis=(1, 2))
+    return np.divide(
+        numerator,
+        denominator,
+        out=np.zeros_like(numerator),
+        where=denominator > 0.0,
+    )
+
+
+def _period_candidates(
+    periods: NDArray[Any],
+    scores: NDArray[Any],
+    count: int = 3,
+) -> list[float]:
+    candidates: list[float] = []
+    for index in np.argsort(scores)[::-1]:
+        period = float(periods[index])
+        if any(abs(period - existing_period) < 0.25 for existing_period in candidates):
+            continue
+        candidates.append(period)
+        if len(candidates) == count:
+            break
+    return candidates
+
+
+def _period_threshold(period: float) -> float:
+    for index, (lower, upper, threshold) in enumerate(_PERIOD_THRESHOLDS):
+        if lower <= period < upper or (index == len(_PERIOD_THRESHOLDS) - 1 and period == upper):
+            return threshold
+    raise ValueError(f"registered period {period} is outside the calibrated range")
+
+
+def _high_band_score(
+    folded: NDArray[Any],
+    template_spectrum: NDArray[Any],
+) -> float:
+    folded_spectrum = np.fft.fft2(folded, axes=(0, 1))
+    tile_height, tile_width = template_spectrum.shape[:2]
+    y_coordinates = np.minimum(np.arange(tile_height), tile_height - np.arange(tile_height))
+    x_coordinates = np.minimum(np.arange(tile_width), tile_width - np.arange(tile_width))
+    radius = np.sqrt(y_coordinates[:, None] ** 2 + x_coordinates[None, :] ** 2)
+    correlations = []
+    for lower, upper in ((4.5, 6.5), (6.5, 12.0)):
+        mask = (radius >= lower) & (radius < upper)
+        selected_folded = folded_spectrum[mask]
+        selected_template = template_spectrum[mask]
+        denominator = np.linalg.norm(selected_folded) * np.linalg.norm(selected_template)
+        correlations.append(
+            float(np.real(np.vdot(selected_template, selected_folded)) / denominator) if denominator > 0.0 else 0.0
+        )
+    return min(correlations)
+
+
+def _best_canonical(
+    pixels: NDArray[Any],
+    periods: list[float],
+    template: NDArray[Any],
+    sigma: float,
+) -> tuple[float, NDArray[Any], NDArray[Any], float]:
+    best_score = -math.inf
+    best_canonical: NDArray[Any] | None = None
+    best_folded: NDArray[Any] | None = None
+    best_period: float | None = None
+    for period in periods:
+        predicted_width = round(pixels.shape[1] * template.shape[1] / period)
+        for delta in range(-4, 5):
+            width = predicted_width + delta
+            height = round(pixels.shape[0] * width / pixels.shape[1])
+            canonical = _resize(pixels, width, height)
+            score, folded = folded_template_score(canonical, template, sigma)
+            if score > best_score:
+                best_score = score
+                best_canonical = canonical
+                best_folded = folded
+                best_period = period
+    if best_canonical is None or best_folded is None or best_period is None:
+        raise RuntimeError("scale registration produced no canonical view")
+    return float(best_score), best_canonical, best_folded, best_period
+
+
+def _quadrant_median(
+    canonical: NDArray[Any],
+    template: NDArray[Any],
+    sigma: float,
+) -> float:
+    tile_height, tile_width = template.shape[:2]
+    split_y = max(tile_height, (canonical.shape[0] // (2 * tile_height)) * tile_height)
+    split_x = max(tile_width, (canonical.shape[1] // (2 * tile_width)) * tile_width)
+    scores = []
+    for region in (
+        canonical[:split_y, :split_x],
+        canonical[:split_y, split_x:],
+        canonical[split_y:, :split_x],
+        canonical[split_y:, split_x:],
+    ):
+        score, _folded = folded_template_score(region, template, sigma)
+        scores.append(score)
+    return float(np.median(scores))
+
+
+def _pyramid_locked_mean(
+    pixels: NDArray[Any],
+    harmonics: NDArray[Any],
+    coefficients: NDArray[Any],
+    base_curve: NDArray[Any],
+) -> float:
+    curves = []
+    candidates = []
+    for scale in _PYRAMID_SCALES:
+        if scale == 1.0:
+            curve = base_curve
+        else:
+            level = _resize(
+                pixels,
+                max(16, round(pixels.shape[1] * scale)),
+                max(16, round(pixels.shape[0] * scale)),
+            )
+            curve = _spectral_curve(level, _SEARCH_PERIODS, harmonics, coefficients)
+        curves.append(curve)
+        candidates.append(_period_candidates(_SEARCH_PERIODS, curve))
+    combinations = itertools.product(*candidates)
+
+    def spread(combination: tuple[float, ...]) -> float:
+        normalized_periods = [
+            candidate / scale
+            for candidate, scale in zip(
+                combination,
+                _PYRAMID_SCALES,
+                strict=True,
+            )
+        ]
+        return float(np.std(np.log(normalized_periods)))
+
+    best = min(
+        combinations,
+        key=spread,
+    )
+    base_period = float(np.median([candidate / scale for candidate, scale in zip(best, _PYRAMID_SCALES, strict=True)]))
+    locked = [
+        float(np.interp(base_period * scale, _SEARCH_PERIODS, curve))
+        for curve, scale in zip(curves, _PYRAMID_SCALES, strict=True)
+    ]
+    return float(np.mean(locked))
+
+
+def registered_components(
+    pixels: NDArray[Any],
+    template: NDArray[Any],
+    sigma: float,
+) -> RegisteredComponents:
+    """Measure a carrier after bounded scale registration."""
+    harmonics, coefficients, template_spectrum = _template_frequency_features(template)
+    combined_periods = np.concatenate((_SEARCH_PERIODS, _CANONICAL_PERIODS))
+    combined_curve = _spectral_curve(pixels, combined_periods, harmonics, coefficients)
+    base_curve = combined_curve[: len(_SEARCH_PERIODS)]
+    canonical_curve = combined_curve[len(_SEARCH_PERIODS) :]
+    candidates = _period_candidates(_CANONICAL_PERIODS, canonical_curve)
+    baseline, canonical, folded, selected_period = _best_canonical(pixels, candidates, template, sigma)
+    quadrant = _quadrant_median(canonical, template, sigma)
+    pyramid = _pyramid_locked_mean(
+        pixels,
+        harmonics,
+        coefficients,
+        base_curve,
+    )
+    raw_score = float((baseline + quadrant + pyramid) / 3.0)
+    return RegisteredComponents(
+        raw_score=raw_score,
+        amplitude_threshold=_period_threshold(selected_period),
+        selected_period=selected_period,
+        spectral_period=candidates[0],
+        high_band_score=_high_band_score(folded, template_spectrum),
+    )
+
+
+def registered_score(
+    pixels: NDArray[Any],
+    template: NDArray[Any],
+    sigma: float,
+) -> float:
+    """Return the calibrated registered decision statistic."""
+    return registered_components(pixels, template, sigma).decision_score
diff --git a/src/remove_ai_watermarks/cli.py b/src/remove_ai_watermarks/cli.py
index 16b8087..e2219a3 100644
--- a/src/remove_ai_watermarks/cli.py
+++ b/src/remove_ai_watermarks/cli.py
@@ -1322,7 +1322,12 @@ def cmd_video_batch(
 @main.command("detect-synthid")
 @click.argument("source", type=click.Path(exists=True, dir_okay=False, path_type=Path))
 @click.option("--json", "as_json", is_flag=True, help="Emit the detector result as JSON.")
-def cmd_detect_synthid(source: Path, as_json: bool) -> None:
+@click.option(
+    "--register-scale",
+    is_flag=True,
+    help="Search the slower calibrated range of spatial carrier scales.",
+)
+def cmd_detect_synthid(source: Path, as_json: bool, register_scale: bool) -> None:
     """Detect the SynthID periodic pixel carrier at calibrated image sizes.
 
     A negative result means this detector did not find its supported carrier; it
@@ -1332,7 +1337,7 @@ def cmd_detect_synthid(source: Path, as_json: bool) -> None:
 
     source = _validate_image(source)
     try:
-        result = detect_synthid(source)
+        result = detect_synthid(source, register_scale=register_scale)
     except RuntimeError as exc:
         raise click.ClickException(str(exc)) from exc
 
@@ -1346,10 +1351,15 @@ def cmd_detect_synthid(source: Path, as_json: bool) -> None:
     if result.score is not None:
         console.print(f"  Score: {result.score:.6f}  (threshold: {result.threshold:.6f})")
     console.print(f"  Detector: {result.detector}")
+    scale_scope = (
+        "  Bounded spatial-scale registration was enabled. A negative or\n"
+        if register_scale
+        else "  Arbitrary spatial resampling was not registered. A negative or\n"
+    )
     console.print(
         "  Scope: one confirmed periodic carrier family in a calibrated image-size range.\n"
-        "  Arbitrary spatial resampling is not registered. A negative or\n"
-        "  unsupported result is not proof that SynthID is absent."
+        + scale_scope
+        + "  unsupported result is not proof that SynthID is absent."
     )
 
 
diff --git a/src/remove_ai_watermarks/synthid_detector.py b/src/remove_ai_watermarks/synthid_detector.py
index c6980a5..ee79500 100644
--- a/src/remove_ai_watermarks/synthid_detector.py
+++ b/src/remove_ai_watermarks/synthid_detector.py
@@ -2,8 +2,8 @@
 
 This is a positive-only detector for one measured carrier epoch, not Google's
 private payload decoder. A positive result is strong local evidence for the
-carrier. A negative result means only that this exact detector did not find it;
-image sizes outside the calibrated pixel-count range are reported separately.
+carrier. A negative result means only that the selected detector did not find
+it; image sizes outside that mode's calibrated range are reported separately.
 
 The numeric runtime requires the ``pixels`` extra. Imports remain lazy so the
 package's metadata-only paths stay dependency-light.
@@ -25,6 +25,7 @@ if TYPE_CHECKING:
 SynthIDDetectionStatus = Literal["detected", "not_detected", "unsupported"]
 
 DETECTOR_ID = "synthid-periodic-tile-v2"
+REGISTERED_DETECTOR_ID = "synthid-periodic-tile-registered-v2"
 MODEL_FILENAME = "synthid_periodic_tile_2048_v1.npz"
 # The template remains frozen at this model geometry. Runtime images are never
 # resized. The supported pixel-count interval is the separately challenged domain:
@@ -35,6 +36,12 @@ MODEL_HEIGHT = 2048
 MIN_SUPPORTED_PIXELS = 1_000_000
 MAX_SUPPORTED_PIXELS = 18_000_000
 TILE_THRESHOLD = 0.17357069773071196
+REGISTERED_MIN_SUPPORTED_PIXELS = 250_000
+REGISTERED_MAX_SUPPORTED_PIXELS = 10_000_000
+REGISTERED_MIN_SIDE = 64
+# The registered score is the minimum normalized margin across its amplitude,
+# spectral-candidate, and high-frequency agreement gates.
+REGISTERED_THRESHOLD = 1.0
 INSTALL_HINT = "install the pixel extra: uv add 'remove-ai-watermarks[pixels]'"
 
 
@@ -206,11 +213,43 @@ def _geometry_supported(width: int, height: int) -> bool:
     return MIN_SUPPORTED_PIXELS <= pixels <= MAX_SUPPORTED_PIXELS
 
 
-def detect_synthid(image_path: str | Path, *, image: NDArray[Any] | None = None) -> SynthIDDetection:
+def _registered_geometry_supported(width: int, height: int) -> bool:
+    """Whether scale registration was challenged at this decoded size."""
+    pixels = width * height
+    return (
+        min(width, height) >= REGISTERED_MIN_SIDE
+        and REGISTERED_MIN_SUPPORTED_PIXELS <= pixels <= REGISTERED_MAX_SUPPORTED_PIXELS
+    )
+
+
+def folded_template_score(
+    pixels: NDArray[Any],
+    template: NDArray[Any],
+    denoise_sigma: float,
+) -> tuple[float, NDArray[Any]]:
+    """Fold PIXELS at the model geometry and score the normalized tile."""
+    tile_height, tile_width = template.shape[:2]
+    folded = fold_residual_template(
+        pixels,
+        tile_height=tile_height,
+        tile_width=tile_width,
+        denoise_sigma=denoise_sigma,
+    )
+    normalized, _norm = unit_tile(folded)
+    return float((template * normalized).sum()), folded
+
+
+def detect_synthid(
+    image_path: str | Path,
+    *,
+    image: NDArray[Any] | None = None,
+    register_scale: bool = False,
+) -> SynthIDDetection:
     """Detect the supported periodic carrier in IMAGE_PATH.
 
     ``not_detected`` is not a clean-image guarantee. It means only that the
-    frozen periodic carrier did not cross its calibrated threshold.
+    frozen periodic carrier did not cross its calibrated threshold. Set
+    ``register_scale`` for the slower, separately calibrated resize search.
     """
     path = Path(image_path)
     if image is None:
@@ -219,13 +258,19 @@ def detect_synthid(image_path: str | Path, *, image: NDArray[Any] | None = None)
         if image.ndim != 3 or image.shape[2] != 3:
             raise ValueError("image must be a three-channel BGR array")
         height, width = image.shape[:2]
-    if not _geometry_supported(width, height):
+    geometry_supported = (
+        _registered_geometry_supported(width, height) if register_scale else _geometry_supported(width, height)
+    )
+    threshold = REGISTERED_THRESHOLD if register_scale else TILE_THRESHOLD
+    detector_id = REGISTERED_DETECTOR_ID if register_scale else DETECTOR_ID
+    if not geometry_supported:
         return SynthIDDetection(
             status="unsupported",
             width=width,
             height=height,
             score=None,
-            threshold=TILE_THRESHOLD,
+            threshold=threshold,
+            detector=detector_id,
         )
     if not is_available():
         raise RuntimeError(f"SynthID pixel detection needs numpy and OpenCV; {INSTALL_HINT}")
@@ -233,7 +278,7 @@ def detect_synthid(image_path: str | Path, *, image: NDArray[Any] | None = None)
     import numpy as np
     from PIL import Image
 
-    template, sigma, _model_height, _model_width, tile_height, tile_width = _load_template()
+    template, sigma, *_model = _load_template()
     if image is None:
         with Image.open(path) as source:
             pixels = np.asarray(source.convert("RGB"), dtype=np.uint8)
@@ -241,18 +286,17 @@ def detect_synthid(image_path: str | Path, *, image: NDArray[Any] | None = None)
         pixels = np.asarray(image[:, :, ::-1], dtype=np.uint8)
     if pixels.shape != (height, width, 3):
         raise RuntimeError("decoded image geometry does not match its header")
-    folded = fold_residual_template(
-        pixels,
-        tile_height=tile_height,
-        tile_width=tile_width,
-        denoise_sigma=sigma,
-    )
-    normalized, _norm = unit_tile(folded)
-    score = float(np.sum(template * normalized))
+    if register_scale:
+        from remove_ai_watermarks._synthid_registered import registered_score
+
+        score = registered_score(pixels, template, sigma)
+    else:
+        score, _folded = folded_template_score(pixels, template, sigma)
     return SynthIDDetection(
-        status="detected" if score >= TILE_THRESHOLD else "not_detected",
+        status="detected" if score >= threshold else "not_detected",
         width=width,
         height=height,
         score=score,
-        threshold=TILE_THRESHOLD,
+        threshold=threshold,
+        detector=detector_id,
     )
diff --git a/tests/test_cli.py b/tests/test_cli.py
index a1722a0..149b8ce 100644
--- a/tests/test_cli.py
+++ b/tests/test_cli.py
@@ -741,6 +741,7 @@ class TestDetectSynthIDCommand:
         result = runner.invoke(main, ["detect-synthid", "--help"])
         assert result.exit_code == 0
         assert "calibrated image sizes" in result.output
+        assert "--register-scale" in result.output
 
     def test_unsupported_geometry_is_machine_readable(self, runner, tmp_clean_png):
         result = runner.invoke(main, ["detect-synthid", str(tmp_clean_png), "--json"])
@@ -750,6 +751,23 @@ class TestDetectSynthIDCommand:
         assert payload["status"] == "unsupported"
         assert payload["score"] is None
 
+    def test_registered_scale_mode_is_machine_readable(self, runner, tmp_clean_png):
+        from remove_ai_watermarks.synthid_detector import (
+            REGISTERED_DETECTOR_ID,
+            REGISTERED_THRESHOLD,
+        )
+
+        result = runner.invoke(
+            main,
+            ["detect-synthid", str(tmp_clean_png), "--register-scale", "--json"],
+        )
+
+        assert result.exit_code == 0, result.output
+        payload = json.loads(result.output)
+        assert payload["status"] == "unsupported"
+        assert payload["threshold"] == REGISTERED_THRESHOLD
+        assert payload["detector"] == REGISTERED_DETECTOR_ID
+
     def test_non_json_output_preserves_negative_scope(self, runner, tmp_clean_png):
         result = runner.invoke(main, ["detect-synthid", str(tmp_clean_png)])
 
@@ -757,6 +775,15 @@ class TestDetectSynthIDCommand:
         assert "unsupported" in result.output
         assert "not proof that SynthID is absent" in result.output
 
+    def test_registered_non_json_output_names_the_bounded_search(self, runner, tmp_clean_png):
+        result = runner.invoke(
+            main,
+            ["detect-synthid", str(tmp_clean_png), "--register-scale"],
+        )
+
+        assert result.exit_code == 0, result.output
+        assert "Bounded spatial-scale registration was enabled" in result.output
+
 
 class TestBatchCommand:
     """Tests for the 'batch' subcommand."""
diff --git a/tests/test_synthid_detector.py b/tests/test_synthid_detector.py
index 2f23429..885a264 100644
--- a/tests/test_synthid_detector.py
+++ b/tests/test_synthid_detector.py
@@ -28,6 +28,25 @@ def supported_images(tmp_path_factory: pytest.TempPathFactory) -> tuple[Path, Pa
     return positive, negative
 
 
+@pytest.fixture(scope="module")
+def registered_scale_positive(tmp_path_factory: pytest.TempPathFactory) -> Path:
+    """Create a strong period-12.8 carrier by shrinking a period-16 source."""
+    import cv2
+
+    directory = tmp_path_factory.mktemp("synthid-registered")
+    template, *_model = detector._load_template()
+    scaled_tile = template / np.max(np.abs(template)) * 40.0
+    source = np.tile(scaled_tile, (64, 64, 1)) + 128.0
+    pixels = cv2.resize(
+        np.clip(np.rint(source), 0, 255).astype(np.uint8),
+        (819, 819),
+        interpolation=cv2.INTER_AREA,
+    )
+    path = directory / "period-12.8-positive.png"
+    Image.fromarray(pixels, "RGB").save(path)
+    return path
+
+
 def test_bundled_model_is_the_frozen_calibrated_artifact() -> None:
     model = Path(detector.__file__).parent / "assets" / detector.MODEL_FILENAME
 
@@ -55,6 +74,36 @@ def test_geometry_outside_the_challenged_pixel_count_range_is_unsupported(
     assert not detector._geometry_supported(width, height)
 
 
+@pytest.mark.parametrize(
+    ("width", "height", "supported"),
+    [
+        (500, 500, True),
+        (4000, 2500, True),
+        (64, 3907, True),
+        (499, 500, False),
+        (4001, 2500, False),
+        (32, 7813, False),
+    ],
+)
+def test_registered_geometry_uses_its_measured_pixel_count_range(
+    width: int,
+    height: int,
+    supported: bool,
+) -> None:
+    assert detector._registered_geometry_supported(width, height) is supported
+
+
+def test_registered_mode_rejects_a_side_too_short_for_quadrants(tmp_path: Path) -> None:
+    path = tmp_path / "too-narrow.png"
+    Image.new("RGB", (32, 7813), "white").save(path)
+
+    result = detector.detect_synthid(path, register_scale=True)
+
+    assert result.status == "unsupported"
+    assert result.score is None
+    assert result.detector == detector.REGISTERED_DETECTOR_ID
+
+
 def test_detects_supported_periodic_carrier(supported_images: tuple[Path, Path]) -> None:
     positive, _negative = supported_images
 
@@ -86,6 +135,95 @@ def test_detects_unregistered_non_divisible_geometry_in_size_range(tmp_path: Pat
     assert result.score > result.threshold
 
 
+def test_registered_mode_detects_a_rescaled_carrier(registered_scale_positive: Path) -> None:
+    default = detector.detect_synthid(registered_scale_positive)
+    registered = detector.detect_synthid(registered_scale_positive, register_scale=True)
+
+    assert default.status == "unsupported"
+    assert registered.status == "detected"
+    assert registered.score is not None
+    assert registered.score > registered.threshold
+    assert registered.threshold == detector.REGISTERED_THRESHOLD
+    assert registered.detector == detector.REGISTERED_DETECTOR_ID
+
+
+def test_registered_threshold_mutation_changes_the_real_verdict(
+    monkeypatch: pytest.MonkeyPatch,
+    registered_scale_positive: Path,
+) -> None:
+    baseline = detector.detect_synthid(registered_scale_positive, register_scale=True)
+    assert baseline.score is not None
+    mutated_threshold = float(np.nextafter(baseline.score, np.inf))
+    monkeypatch.setattr(detector, "REGISTERED_THRESHOLD", mutated_threshold)
+
+    mutated = detector.detect_synthid(registered_scale_positive, register_scale=True)
+
+    assert mutated.status == "not_detected"
+    assert mutated.threshold == mutated_threshold
+
+
+def test_registered_period_thresholds_cover_the_bounded_search() -> None:
+    from remove_ai_watermarks._synthid_registered import _period_threshold
+
+    assert _period_threshold(7.5) == pytest.approx(0.3770629524888979)
+    assert _period_threshold(12.0) == pytest.approx(0.19794247706938645)
+    assert _period_threshold(24.5) == pytest.approx(0.3142958338390489)
+    with pytest.raises(ValueError, match="outside"):
+        _period_threshold(7.49)
+
+
+def test_registered_amplitude_threshold_mutation_changes_the_real_verdict(
+    monkeypatch: pytest.MonkeyPatch,
+    registered_scale_positive: Path,
+) -> None:
+    import remove_ai_watermarks._synthid_registered as registered_detector
+
+    baseline = detector.detect_synthid(registered_scale_positive, register_scale=True)
+    assert baseline.status == "detected"
+    monkeypatch.setattr(
+        registered_detector,
+        "_PERIOD_THRESHOLDS",
+        ((7.5, 24.5, float("inf")),),
+    )
+
+    mutated = detector.detect_synthid(registered_scale_positive, register_scale=True)
+
+    assert mutated.status == "not_detected"
+
+
+def test_registered_spectral_candidate_disagreement_blocks_decision() -> None:
+    from remove_ai_watermarks._synthid_registered import RegisteredComponents
+
+    matching = RegisteredComponents(0.5, 0.25, 12.8, 12.8, 0.15)
+    mismatching = RegisteredComponents(0.5, 0.25, 12.8, 12.9, 0.15)
+
+    assert matching.decision_score == pytest.approx(2.0)
+    assert mismatching.decision_score == pytest.approx(0.0)
+
+
+def test_registered_high_band_mutation_changes_the_real_verdict(
+    monkeypatch: pytest.MonkeyPatch,
+    registered_scale_positive: Path,
+) -> None:
+    import remove_ai_watermarks._synthid_registered as registered_detector
+
+    components = registered_detector.registered_components(
+        np.asarray(Image.open(registered_scale_positive).convert("RGB"), dtype=np.uint8),
+        detector._load_template()[0],
+        detector._load_template()[1],
+    )
+    assert components.decision_score >= detector.REGISTERED_THRESHOLD
+    monkeypatch.setattr(
+        registered_detector,
+        "REGISTERED_HIGH_BAND_THRESHOLD",
+        float(np.nextafter(components.high_band_score, np.inf)),
+    )
+
+    mutated = detector.detect_synthid(registered_scale_positive, register_scale=True)
+
+    assert mutated.status == "not_detected"
+
+
 def test_supported_negative_does_not_claim_clean(supported_images: tuple[Path, Path]) -> None:
     _positive, negative = supported_images