diff --git a/research/complex-cartesian-grid-screen-seed506af/README.md b/research/complex-cartesian-grid-screen-seed506af/README.md
new file mode 100644
index 0000000000..f16b0bc77b
--- /dev/null
+++ b/research/complex-cartesian-grid-screen-seed506af/README.md
@@ -0,0 +1,104 @@
+# Cartesian source-destination grid blocks: a scoped cost obstruction
+
+**Exact block identity and negative cost screen. No selected-word change or new κ.**
+
+Assignment: seed 506af17109f2b8945b6b47236c53520b, track
+complex-mechanisms, task complex-rank-shape-decoder, priority revision 3,
+scope key cad3ba853d7f324f16fa. Exact base:
+0b1cccf86e2f5132ed286a26ef0a681a495740ea. Selected-record SHA-256:
+10b9e340fd8e354a37a0c4813c90e5de1dd42044b5f6aca4a3fc5a42b685fa32.
+The coordination claim is
+[Discussion #1167 comment 18924220](https://github.com/CrocSwap/integer-mult-bounds/discussions/1167#discussioncomment-18924220).
+
+## Result
+
+Let X₁, …, Xᵣ and Y₁, …, Yᵣ be n × n block operands. Place them in the
+first block column and first block row:
+
+- A(i,1) = Xᵢ
+- B(1,j) = Yⱼ
+- all other blocks are zero.
+
+Then the ordinary block product satisfies C(i,j) = XᵢYⱼ for every ordered
+source-destination pair (i,j). This is a Cartesian-grid mechanism. It differs
+from diagonal batching, one-source fanout, and one-destination gather.
+
+The cost model charges a generic width-r matrix-product child by r^(2−a)
+size-n child units. Computing the r² products separately costs r². The
+recursive-work ratio is
+
+    r^(2−a) / r² = r^(−a).
+
+The selected outer guard permits r ≤ 51. Throughout the target range
+0 ≤ a ≤ 1/99, the largest saving occurs at r = 51 and a = 1/99. The exact
+integer inequality
+
+    51 · 19^99 < 20^99
+
+implies 51^(−1/99) > 19/20. Even this most favorable grid therefore saves
+less than **5%** of its own recursive work. An integer 99th-root certificate
+bounds the maximum saving near **3.8937091284%**. The first integer width
+reaching 5% at a = 1/99 is r = 161, outside the guard.
+
+This is a scoped obstruction for one generic width-r block-child contract.
+Input gathering, source access, copies, helper use, frame changes, output
+storage, dirty-state restoration, hosting, precision, and interface
+transitions are all assigned zero cost. The screen gives the proposed block
+every advantage. It does not establish a replacement ceiling for the selected
+word, prove that its exact provider contains Cartesian grids, exclude a
+different bilinear algorithm for a grid, or change κ. The selected profile
+still needs a separate admissible per-call interface before this mechanism
+can be used.
+
+## Evidence and reproduction
+
+The verifier checks the block identity as formal polynomials for 15 width and
+block-size pairs. It rejects two mutated block placements, proves the 5%
+boundary with integer powers, and certifies the decimal interval with an
+integer 99th-root computation.
+
+From the repository root:
+
+    python3 -I -B research/complex-cartesian-grid-screen-seed506af/verify.py
+    python3 scripts/research_package.py --package research/complex-cartesian-grid-screen-seed506af --check-only
+
+Expected verifier prefix:
+
+    PASS_CARTESIAN_GRID_COST_SCREEN identity_fixtures=15 rank_domain=2..51 exponent_domain=0..1/99
+
+Exact base, selected record, catalogue, and live explanatory sources are
+pinned in SOURCES.json. The changed-scope declaration and evidence are in
+TASK.json.
+
+## Related constructions and limits
+
+- PR #1779 studies diagonal batches of rank-one climbs across distinct
+  destinations.
+- PR #1814 studies three sources feeding one destination.
+- PR #1865 studies one source feeding three destinations.
+- PR #1764 and PR #1836 study joint boundary children at one or two lanes.
+
+Those mechanisms have different output patterns. The recent dependency-DAG
+claim on Discussion #1596 was released after applying the PR #679 support
+obstruction to the diagonal-batch family; this package does not replay or
+extend that audit.
+
+## Open next questions
+
+Could a changed child contract exploit a Cartesian grid through a bilinear
+algorithm cheaper than the generic width-r product while still admitting the
+selected provider's arbitrary dirty state and frame endpoints? This matters
+because only a changed recursive interface could evade the cost ratio above.
+The first step is an exact small-width tensor decomposition with source and
+output supports and a complete copy, frame, and restoration invoice. The
+blocker is that no such native interface or admissible selected-word window
+is supplied here.
+
+## Credit
+
+Credit the selected #1094/#1042 complex supplier lineage and the distinct
+fanout, gather, diagonal-batch, and joint-boundary predecessors above.
+Prepared by Cinna Davis with substantial OpenAI Codex assistance in choosing
+the construction, deriving its cost screen, writing the verifier, and
+packaging the result. New work is Apache-2.0.
+
diff --git a/research/complex-cartesian-grid-screen-seed506af/SOURCES.json b/research/complex-cartesian-grid-screen-seed506af/SOURCES.json
new file mode 100644
index 0000000000..f71ee15a78
--- /dev/null
+++ b/research/complex-cartesian-grid-screen-seed506af/SOURCES.json
@@ -0,0 +1,30 @@
+{
+  "schema_version": 1,
+  "base_commit": "0b1cccf86e2f5132ed286a26ef0a681a495740ea",
+  "files": [
+    {
+      "path": "certificates/selected-result.json",
+      "usage": "provenance",
+      "git_blob": "ae6a99f28aa468649eb66a361e764d769363bf11",
+      "sha256": "10b9e340fd8e354a37a0c4813c90e5de1dd42044b5f6aca4a3fc5a42b685fa32"
+    },
+    {
+      "path": "docs/research/agent-task-catalog.json",
+      "usage": "provenance",
+      "git_blob": "06c96f5579bb67ed1b86d334e78fbed7da6945f1",
+      "sha256": "6686ad58f1fbdbc87134a78901b60fd53a20d3984355e91defbbebc9e9fa8075"
+    },
+    {
+      "path": "docs/research/bottleneck-brief.md",
+      "usage": "input",
+      "git_blob": "9a2c01f65bdb5c6816685bf00c8510ea8db43856",
+      "sha256": "a5e316569672dc88276647d1a7ae91df150b944e1376a975c67af246d94d67c8"
+    },
+    {
+      "path": "research/complex-local-word-ceiling/README.md",
+      "usage": "input",
+      "git_blob": "f5f742f8196da29d2c3f4dffb3c27685ca9401f6",
+      "sha256": "b60be039cb0e03e064a27407d0bce9e58908d793da5ee405c876de5aebfa5f81"
+    }
+  ]
+}
diff --git a/research/complex-cartesian-grid-screen-seed506af/TASK.json b/research/complex-cartesian-grid-screen-seed506af/TASK.json
new file mode 100644
index 0000000000..2dcbcda1cf
--- /dev/null
+++ b/research/complex-cartesian-grid-screen-seed506af/TASK.json
@@ -0,0 +1,79 @@
+{
+  "schema_version": 1,
+  "track": "complex-mechanisms",
+  "seed": "506af17109f2b8945b6b47236c53520b",
+  "scope_key": "cad3ba853d7f324f16fa",
+  "selected_record_sha256": "10b9e340fd8e354a37a0c4813c90e5de1dd42044b5f6aca4a3fc5a42b685fa32",
+  "claim_url": "https://github.com/CrocSwap/integer-mult-bounds/discussions/1167#discussioncomment-18924220",
+  "overlap_snapshot": {
+    "searched_at": "2026-10-14T12:50:35Z",
+    "open_prs": 532,
+    "discussions": 512,
+    "complete_pagination": true,
+    "warnings": [],
+    "exact_grid_matches": 0
+  },
+  "task_id": "complex-rank-shape-decoder",
+  "priority_revision": 3,
+  "base_commit": "0b1cccf86e2f5132ed286a26ef0a681a495740ea",
+  "original_card": {
+    "id": "complex-rank-shape-decoder",
+    "question": "Can a decoder deliver the local paid mass in fewer, larger-rank children (block frame climbs instead of rank-1 to rank-3 steps)?",
+    "starting_points": [
+      "research/complex-local-word-ceiling/README.md",
+      "research/complex-decoder-field-extension-seed44/README.md",
+      "research/complex-frame-pair-decoders/README.md"
+    ],
+    "methods": [
+      "Find one exact small decoding identity whose frame path climbs in blocks, and price its complete per-call profile.",
+      "Bound the best effective child rank achievable for one fixed support alphabet, including copies, frames and restoration."
+    ],
+    "acceptance_gate": "A per-call local profile, with all copied information, frames and restoration counted, whose ceiling exceeds the selected word's by at least 5%; masked or overcomplete supports must change an assumption of the eight-term all-pairs bound.",
+    "first_step": {
+      "command": [
+        "python3",
+        "-B",
+        "research/complex-local-word-ceiling/verify.py"
+      ],
+      "expected": "PASS_COMPLEX_LOCAL_WORD_CEILING",
+      "inspect": "Read the selected word's per-call local profile, its packing ceiling and the lever table; run levers.py for the ideal budget of the lever you target.",
+      "scope": "Positive control and planning budget only; this is not a new construction or full cold replay."
+    },
+    "first_experiment": "Before any synthesis, write the candidate per-call local profile (and centre charge / unpaid coordinates if changed) and screen it with ceiling.ceiling(); reject it if even the ideal ceiling does not beat the selected word's by your stated margin. Ideal budget: rank-16 delivery is 2.31x (levers.py).",
+    "bottleneck_link": "Rank-1 children are about half of recursive cost; the selected word's effective child rank is about 3.9. Larger-rank delivery lowers recursive cost per unit of paid mass.",
+    "allowed_variations": [
+      "One masked, overcomplete or block decoder family with exact labels and supports.",
+      "A scoped lower bound on effective rank for one alphabet.",
+      "If the coverage law, template or guard changes, derive an applicable fully charged replacement bound instead of applying the incumbent ceiling outside its scope."
+    ],
+    "scope_change_triggers": [
+      "Repacking idle remainders of an unchanged word (covered by the packing ceiling).",
+      "Repeating the eight-term all-pairs search inside its proved scope."
+    ],
+    "overlap_terms": [
+      "rank shape",
+      "block climb",
+      "masked decoder",
+      "overcomplete",
+      "effective rank"
+    ]
+  },
+  "scope_status": "changed",
+  "actual_scope": "Test the Cartesian source-destination grid identity A_(i,1)=X_i, B_(1,j)=Y_j, which produces r^2 block products from one generic width-r matrix-product child. Bound its recursive-work saving for every 2<=r<=51 and 0<=a<=1/99, granting all data movement and interface expenses for free.",
+  "scope_change_reason": "The catalogue card starts from one-dimensional block frame climbs. This experiment changes the output geometry to a full Cartesian grid and derives a separate quadratic matrix-product cost model, so the incumbent linear paid-mass ceiling is not applied outside its hypotheses.",
+  "bottleneck_link": "A larger-rank child can reduce recursive work only if its larger block output offsets the full r^(2-a) matrix-product charge. The exact best-case grid ratio is at least 19/20 throughout the target exponent range under the rank-51 guard, below the task's 5% screening threshold even before interface costs.",
+  "budget_minutes": 60,
+  "stop_rule": "Bound every width r=2..51 at 0<=a<=1/99. Stop before selected-word synthesis if the free-interface generic block's exact best-case saving is below 5%; restart only for a changed child contract or cost law.",
+  "outcome": "negative-result",
+  "evidence": "verify.py passes 15 exact formal-polynomial identity fixtures, two block-placement mutation controls, exact integer-power tests for the 5% threshold, and an integer 99th-root decimal bracket. Maximum saving is <5%; first width reaching 5% is 161.",
+  "author": "Cinna Davis",
+  "ai_assistance": "Substantial OpenAI Codex assistance in selecting the mechanism, deriving the cost bound, implementing exact controls, and writing the package.",
+  "open_questions": [
+    {
+      "question": "Can a changed child contract use a cheaper bilinear algorithm for a Cartesian grid while preserving the selected provider's arbitrary dirty state and frame endpoints?",
+      "why_it_matters": "A different admitted interface is the only route from this grid geometry to a recursive-work improvement beyond the generic width-r cost bound.",
+      "first_step": "Check an exact small-width tensor decomposition and compute its full source, copy, frame, output, and restoration invoice.",
+      "blocked_by": "No such native child contract or admissible selected-word window is supplied in this result."
+    }
+  ]
+}
diff --git a/research/complex-cartesian-grid-screen-seed506af/VERIFY.json b/research/complex-cartesian-grid-screen-seed506af/VERIFY.json
new file mode 100644
index 0000000000..c945aa3460
--- /dev/null
+++ b/research/complex-cartesian-grid-screen-seed506af/VERIFY.json
@@ -0,0 +1,22 @@
+{
+  "schema_version": 1,
+  "status": "ready",
+  "python": "3.12",
+  "requirements": [],
+  "inputs": [
+    "docs/research/bottleneck-brief.md",
+    "research/complex-local-word-ceiling/README.md"
+  ],
+  "commands": [
+    {
+      "argv": [
+        "{python}",
+        "-I",
+        "-B",
+        "{package}/verify.py"
+      ],
+      "timeout_seconds": 120,
+      "checks": "15 exact formal-polynomial grid identities; two wrong-placement rejections; exact 51-versus-5-percent and 160/161 threshold inequalities; certified integer 99th-root decimal bracket"
+    }
+  ]
+}
diff --git a/research/complex-cartesian-grid-screen-seed506af/verify.py b/research/complex-cartesian-grid-screen-seed506af/verify.py
new file mode 100644
index 0000000000..cfb52e88e8
--- /dev/null
+++ b/research/complex-cartesian-grid-screen-seed506af/verify.py
@@ -0,0 +1,131 @@
+#!/usr/bin/env python3
+"""Exact source-destination grid identity and optimistic recursive-cost screen."""
+
+from decimal import Decimal, getcontext
+from fractions import Fraction
+
+
+def add_poly(dst, src):
+    for monomial, coefficient in src.items():
+        dst[monomial] = dst.get(monomial, 0) + coefficient
+        if dst[monomial] == 0:
+            del dst[monomial]
+
+
+def mul_poly(left, right):
+    out = {}
+    for a, ca in left.items():
+        for b, cb in right.items():
+            monomial = tuple(sorted(a + b))
+            out[monomial] = out.get(monomial, 0) + ca * cb
+    return out
+
+
+def formal_grid(r, n, bad_b_row=False, permute_y=False):
+    """Return the exact scalar-polynomial product of the sparse block inputs."""
+    size = r * n
+    left = [[{} for _ in range(size)] for _ in range(size)]
+    right = [[{} for _ in range(size)] for _ in range(size)]
+
+    for i in range(r):
+        for p in range(n):
+            for q in range(n):
+                left[i * n + p][q] = {(('x', i, p, q),): 1}
+
+    row_offset = n if bad_b_row else 0
+    for j in range(r):
+        y_index = (j + 1) % r if permute_y else j
+        for p in range(n):
+            for q in range(n):
+                right[row_offset + p][j * n + q] = {(('y', y_index, p, q),): 1}
+
+    product = [[{} for _ in range(size)] for _ in range(size)]
+    for i in range(size):
+        for k in range(size):
+            if not left[i][k]:
+                continue
+            for j in range(size):
+                if right[k][j]:
+                    add_poly(product[i][j], mul_poly(left[i][k], right[k][j]))
+    return product
+
+
+def expected_grid(r, n):
+    size = r * n
+    expected = [[{} for _ in range(size)] for _ in range(size)]
+    for i in range(r):
+        for j in range(r):
+            for p in range(n):
+                for q in range(n):
+                    out = expected[i * n + p][j * n + q]
+                    for k in range(n):
+                        monomial = tuple(sorted((('x', i, p, k), ('y', j, k, q))))
+                        out[monomial] = 1
+    return expected
+
+
+def integer_nth_root_floor(value, degree):
+    lo, hi = 0, 1
+    while hi**degree <= value:
+        hi *= 2
+    while hi - lo > 1:
+        mid = (lo + hi) // 2
+        if mid**degree <= value:
+            lo = mid
+        else:
+            hi = mid
+    return lo
+
+
+def decimal_fraction(value, digits=32):
+    getcontext().prec = digits + 8
+    return format(Decimal(value.numerator) / Decimal(value.denominator), f'.{digits}f')
+
+
+def main():
+    identity_fixtures = 0
+    for r in range(2, 7):
+        for n in range(1, 4):
+            want = expected_grid(r, n)
+            got = formal_grid(r, n)
+            assert got == want, (r, n, 'Cartesian block identity')
+            identity_fixtures += 1
+
+    want = expected_grid(3, 2)
+    assert formal_grid(3, 2, bad_b_row=True) != want
+    assert formal_grid(3, 2, permute_y=True) != want
+
+    # A generic width-r product at exponent 2-a costs r^(2-a) child units;
+    # the r^2 separate products cost r^2. The best ratio in the bounded domain
+    # occurs at r=51 and a=1/99.
+    numerator = 20**99
+    denominator = 19**99
+    assert 51 * denominator < numerator
+    assert 160 * denominator < numerator <= 161 * denominator
+
+    # Certify a decimal bracket for 51^(-1/99) using only integer arithmetic.
+    digits = 30
+    scale = 10**digits
+    power_scale = scale**99
+    root_floor = integer_nth_root_floor(power_scale // 51, 99)
+    assert 51 * root_floor**99 <= power_scale
+    assert power_scale < 51 * (root_floor + 1) ** 99
+    ratio_lo = Fraction(root_floor, scale)
+    ratio_hi = Fraction(root_floor + 1, scale)
+    saving_lo = 100 * (1 - ratio_hi)
+    saving_hi = 100 * (1 - ratio_lo)
+    assert saving_hi < 5
+
+    print(
+        'PASS_CARTESIAN_GRID_COST_SCREEN',
+        f'identity_fixtures={identity_fixtures}',
+        'rank_domain=2..51',
+        'exponent_domain=0..1/99',
+        'max_saving_percent=(' + decimal_fraction(saving_lo) + ',' + decimal_fraction(saving_hi) + ')',
+        'first_width_at_5_percent=161',
+        'controls=4',
+    )
+
+
+if __name__ == '__main__':
+    main()