diff --git a/.trinity/seals/src_port::trios::crates::trios-chat::rings::CR-CHAT-03::src::ratchet_tree_update_integrity_guard.json b/.trinity/seals/src_port::trios::crates::trios-chat::rings::CR-CHAT-03::src::ratchet_tree_update_integrity_guard.json
new file mode 100644
index 0000000000..027177ba4d
--- /dev/null
+++ b/.trinity/seals/src_port::trios::crates::trios-chat::rings::CR-CHAT-03::src::ratchet_tree_update_integrity_guard.json
@@ -0,0 +1,28 @@
+{
+  "built_by": "t27c-bootstrap@0.5.2+01d0a635f",
+  "closure": [],
+  "config": "gen=c,rust,verilog,zig;tests=zig",
+  "gen_hash_c": "sha256:000e5eab294c0c753504f25195acc809377d7f7a8e95a2b27cf4980a00db9311",
+  "gen_hash_rust": "sha256:b3e126bc2a02b0b09abd2ce4b395f3ef014003e70c92e85fab3b97f4a90659e7",
+  "gen_hash_verilog": "sha256:468becd2cbd4b9d753dd1788c6e233704d0c828736ebad0483ff715e996f967b",
+  "gen_hash_zig": "sha256:f02dfea66b3e4c7e419cf89902d92d00bb0115b6c77dd54ae65b1fa3ae4fa529",
+  "module": "port::trios::crates::trios-chat::rings::CR-CHAT-03::src::ratchet_tree_update_integrity_guard",
+  "ring": 12,
+  "seal_schema": 2,
+  "sealed_at": "2026-10-17T14:00:53Z",
+  "sealed_by": "t27c-bootstrap@0.5.2",
+  "spec_hash": "sha256:768e5ce725e6b6c2a407b5cd0c9628774cb202409c001b2ebcc4ffd8dadaa67d",
+  "spec_path": "specs/port/trios/crates/trios-chat/rings/CR-CHAT-03/src/ratchet_tree_update_integrity_guard.t27",
+  "tests": {
+    "failed": 0,
+    "failing": [],
+    "forced": false,
+    "passed": 15,
+    "total": 15,
+    "vacuous": 0
+  },
+  "toolchain": {
+    "test_runner": "sha256:1982ae948e24d00c5d221ccec12dfce1eb01204b53e37b0f857f93b16d64629d",
+    "zig": "0.16.0"
+  }
+}
\ No newline at end of file
diff --git a/docs/reports/t27b_expectations.json b/docs/reports/t27b_expectations.json
index c041cbab56..3bcce211a8 100644
--- a/docs/reports/t27b_expectations.json
+++ b/docs/reports/t27b_expectations.json
@@ -852,6 +852,7 @@
   {"path": "specs/port/trios/crates/trios-chat/rings/CR-CHAT-01/src/key_package_expiry_guard.t27", "t27b": "pass"},
   {"path": "specs/port/trios/crates/trios-chat/rings/CR-CHAT-02/src/epoch_rollover_wraparound_guard.t27", "t27b": "pass"},
   {"path": "specs/port/trios/crates/trios-chat/rings/CR-CHAT-03/src/ratchet_tree_resolution_guard.t27", "t27b": "pass"},
+  {"path": "specs/port/trios/crates/trios-chat/rings/CR-CHAT-03/src/ratchet_tree_update_integrity_guard.t27", "t27b": "pass"},
   {"path": "specs/port/trios/crates/trios-chat/rings/CR-CHAT-03/src/unmerged_leaves_bound_guard.t27", "t27b": "pass"},
   {"path": "specs/port/trios/crates/trios-chat/rings/CR-CHAT-05/src/envelope_size_distribution_uniformity.t27", "t27b": "pass"},
   {"path": "specs/port/trios/crates/trios-chat/rings/CR-CHAT-07/src/cover_decoy_ratio_governor.t27", "t27b": "pass"},
diff --git a/specs/port/trios/crates/trios-chat/rings/CR-CHAT-03/src/ratchet_tree_update_integrity_guard.t27 b/specs/port/trios/crates/trios-chat/rings/CR-CHAT-03/src/ratchet_tree_update_integrity_guard.t27
new file mode 100644
index 0000000000..9dc54d59f5
--- /dev/null
+++ b/specs/port/trios/crates/trios-chat/rings/CR-CHAT-03/src/ratchet_tree_update_integrity_guard.t27
@@ -0,0 +1,347 @@
+module port::trios::crates::trios-chat::rings::CR-CHAT-03::src::ratchet_tree_update_integrity_guard;
+
+// Port of gHashTag/trios crates/trios-chat/rings/CR-CHAT-03/src/ratchet_tree_update_integrity_guard.rs
+// (CR-CHAT-03 - Ratchet tree update integrity guard, Wave-84
+// Lane A, R-CHAT-3).
+//
+// Every TreeKEM update carries the parent hash binding its
+// node to the tree structure; without the gate an attacker
+// injects an orphan node whose hash sits outside
+// consensus, swaps siblings and re-signs so recipients
+// derive different keys, or replays a stale update from a
+// prior epoch. The rules, in the original's fixed order:
+// the batch ceiling (two hundred fifty-six), then per
+// update the node index inside the tree (one thousand
+// twenty-four), the fresh node (no duplicate in the
+// batch), the claimed parent hash equal to the computed
+// one, the epoch equal to the current one, and the signer
+// an active leaf member.
+//
+// The original returns Result<(), TreeUpdateError>. This
+// port carries the verdict as a named struct (owner
+// decision D5): Ok first, then the six variants in the
+// original's declaration order - HashMismatch,
+// InvalidNodeIndex, EpochMismatch, DuplicateNode,
+// BatchTooLarge, NonMemberSigner - with every payload
+// field: the node index of the per-update rules, the
+// signer leaf of the membership rule, and the expected
+// and got epochs of the epoch rule.
+//
+// The original's BTreeSet of seen nodes becomes a scan
+// over the updates already visited, the member set a scan
+// over the member leaf slice (conventions section 4). The
+// loop fixtures of the original - the node hash as the
+// low byte of its index - arrive as a precomputed table
+// the loop reads, the computed and tabular forms matching
+// exactly (the narrowing cast on the original's counter
+// does not exist in t27).
+
+// Maximum nodes in the tree.
+pub const RTUI_MAX_NODES: u64 = 1024;
+
+// Maximum update batch size.
+pub const RTUI_MAX_BATCH: usize = 256;
+
+// A tree update proposal.
+pub struct TreeUpdate {
+    node_index: u64,
+    parent_hash: [32]u8,
+    computed_hash: [32]u8,
+    epoch: u64,
+    signer_leaf: u64,
+}
+
+// Verdict codes: Ok(()) and TreeUpdateError's variants, in declaration order.
+pub enum RtuiCode {
+    Ok,
+    HashMismatch,
+    InvalidNodeIndex,
+    EpochMismatch,
+    DuplicateNode,
+    BatchTooLarge,
+    NonMemberSigner,
+}
+
+// The verdict of the batch validation, with every payload
+// field of the original's variants.
+pub struct RtuiVerdict {
+    code: RtuiCode,
+    idx: u64,
+    expected: u64,
+    got: u64,
+}
+
+// Whole-form equality over two thirty-two-byte hashes - the
+// original's array equality on the parent binding.
+pub fn hashes32_equal(a: [32]u8, b: [32]u8) -> bool {
+    for i in 0..32 {
+        if (a[i] != b[i]) {
+            return false;
+        }
+    }
+    return true;
+}
+
+// Whether the leaf slice holds this signer - the original's
+// member-set contains, answered by a scan.
+pub fn member_contains(members: []const u64, leaf: u64) -> bool {
+    for i in 0..members.len {
+        if (members[i] == leaf) {
+            return true;
+        }
+    }
+    return false;
+}
+
+// Whether an update before position upto already claimed
+// this node - the original's BTreeSet insert probe,
+// answered by a scan.
+pub fn node_seen_before(updates: []const TreeUpdate, upto: usize, node_index: u64) -> bool {
+    for j in 0..upto {
+        if (updates[j].node_index == node_index) {
+            return true;
+        }
+    }
+    return false;
+}
+
+// [VERIFIED] Validate a batch of tree updates. Returns Ok
+// only when every rule holds, checked in the original's
+// fixed order: the batch ceiling, then per update the node
+// bound, the fresh node, the hash agreement, the epoch
+// agreement and the member signer.
+pub fn validate_tree_updates(updates: []const TreeUpdate, current_epoch: u64, member_leaves: []const u64) -> RtuiVerdict {
+    if (updates.len > RTUI_MAX_BATCH) {
+        return RtuiVerdict { code: RtuiCode.BatchTooLarge, idx: 0, expected: 0, got: 0, };
+    }
+    for i in 0..updates.len {
+        if (updates[i].node_index >= RTUI_MAX_NODES) {
+            return RtuiVerdict { code: RtuiCode.InvalidNodeIndex, idx: updates[i].node_index, expected: 0, got: 0, };
+        }
+        if (node_seen_before(updates, i, updates[i].node_index)) {
+            return RtuiVerdict { code: RtuiCode.DuplicateNode, idx: updates[i].node_index, expected: 0, got: 0, };
+        }
+        if (!hashes32_equal(updates[i].parent_hash, updates[i].computed_hash)) {
+            return RtuiVerdict { code: RtuiCode.HashMismatch, idx: updates[i].node_index, expected: 0, got: 0, };
+        }
+        if (updates[i].epoch != current_epoch) {
+            return RtuiVerdict { code: RtuiCode.EpochMismatch, idx: 0, expected: current_epoch, got: updates[i].epoch, };
+        }
+        if (!member_contains(member_leaves, updates[i].signer_leaf)) {
+            return RtuiVerdict { code: RtuiCode.NonMemberSigner, idx: updates[i].signer_leaf, expected: 0, got: 0, };
+        }
+    }
+    return RtuiVerdict { code: RtuiCode.Ok, idx: 0, expected: 0, got: 0, };
+}
+
+// The fixture members: the even leaves zero through six.
+pub const MEMBERS: [4]u64 = [4]u64{0, 2, 4, 6,};
+
+// The all-double-F-hex hash of the mismatch probe.
+pub const HASH_FF: [32]u8 = [32]u8{255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255,}
+
+// The low-byte table of the loop fixtures: the node hash
+// reads the low byte of its index.
+pub const RTUI_LOW: [257]u8 = [257]u8{0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 80, 81, 82, 83, 84, 85, 86, 87, 88, 89, 90, 91, 92, 93, 94, 95, 96, 97, 98, 99, 100, 101, 102, 103, 104, 105, 106, 107, 108, 109, 110, 111, 112, 113, 114, 115, 116, 117, 118, 119, 120, 121, 122, 123, 124, 125, 126, 127, 128, 129, 130, 131, 132, 133, 134, 135, 136, 137, 138, 139, 140, 141, 142, 143, 144, 145, 146, 147, 148, 149, 150, 151, 152, 153, 154, 155, 156, 157, 158, 159, 160, 161, 162, 163, 164, 165, 166, 167, 168, 169, 170, 171, 172, 173, 174, 175, 176, 177, 178, 179, 180, 181, 182, 183, 184, 185, 186, 187, 188, 189, 190, 191, 192, 193, 194, 195, 196, 197, 198, 199, 200, 201, 202, 203, 204, 205, 206, 207, 208, 209, 210, 211, 212, 213, 214, 215, 216, 217, 218, 219, 220, 221, 222, 223, 224, 225, 226, 227, 228, 229, 230, 231, 232, 233, 234, 235, 236, 237, 238, 239, 240, 241, 242, 243, 244, 245, 246, 247, 248, 249, 250, 251, 252, 253, 254, 255, 0,}
+
+// The fixture builder the tests share: an update whose
+// parent and computed hashes agree, both the low byte of
+// its node index.
+fn make_update(node_index: u64, epoch: u64, signer_leaf: u64) -> TreeUpdate {
+    var h: [32]u8 = [32]u8{0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,};
+    const slot: usize = @as(usize, node_index % 256);
+    for j in 0..32 {
+        h[j] = RTUI_LOW[slot];
+    }
+    return TreeUpdate { node_index: node_index, parent_hash: h, computed_hash: h, epoch: epoch, signer_leaf: signer_leaf, };
+}
+
+// **RTUI-01** - the parent hash mismatch rejected: the
+// claimed double-F-hex of the computed low byte, the
+// verdict naming the node.
+test rtui_01_hash_mismatch_rejected {
+    var u: TreeUpdate = make_update(1, 5, 0);
+    u.parent_hash = HASH_FF;
+    const batch: [1]TreeUpdate = [1]TreeUpdate{u,};
+    const v = validate_tree_updates(&batch, 5, &MEMBERS);
+    assert(v.code == RtuiCode.HashMismatch);
+    assert(v.idx == 1);
+}
+
+// **RTUI-02** - the invalid node index rejected: the node
+// ceiling itself.
+test rtui_02_invalid_node_rejected {
+    const u: TreeUpdate = make_update(1024, 5, 0);
+    const batch: [1]TreeUpdate = [1]TreeUpdate{u,};
+    const v = validate_tree_updates(&batch, 5, &MEMBERS);
+    assert(v.code == RtuiCode.InvalidNodeIndex);
+    assert(v.idx == 1024);
+}
+
+// **RTUI-03** - the epoch mismatch rejected: four of a
+// current five, the verdict carrying both.
+test rtui_03_epoch_mismatch_rejected {
+    const u: TreeUpdate = make_update(1, 4, 0);
+    const batch: [1]TreeUpdate = [1]TreeUpdate{u,};
+    const v = validate_tree_updates(&batch, 5, &MEMBERS);
+    assert(v.code == RtuiCode.EpochMismatch);
+    assert(v.expected == 5);
+    assert(v.got == 4);
+}
+
+// **RTUI-04** - the duplicate node rejected: node one
+// updated twice in one batch.
+test rtui_04_duplicate_rejected {
+    const batch: [2]TreeUpdate = [2]TreeUpdate{
+        make_update(1, 5, 0),
+        make_update(1, 5, 2),
+    };
+    const v = validate_tree_updates(&batch, 5, &MEMBERS);
+    assert(v.code == RtuiCode.DuplicateNode);
+    assert(v.idx == 1);
+}
+
+// **RTUI-05** - the batch too large rejected: two hundred
+// fifty-seven of a ceiling of two hundred fifty-six,
+// built in a loop like the original's.
+test rtui_05_batch_too_large_rejected {
+    var batch: [257]TreeUpdate;
+    for i in 0..257 {
+        batch[i] = make_update(@as(u64, i), 5, 0);
+    }
+    const v = validate_tree_updates(&batch, 5, &MEMBERS);
+    assert(v.code == RtuiCode.BatchTooLarge);
+}
+
+// **RTUI-06** - the non-member signer rejected: ninety-nine
+// of the even members.
+test rtui_06_non_member_rejected {
+    const u: TreeUpdate = make_update(1, 5, 99);
+    const batch: [1]TreeUpdate = [1]TreeUpdate{u,};
+    const v = validate_tree_updates(&batch, 5, &MEMBERS);
+    assert(v.code == RtuiCode.NonMemberSigner);
+    assert(v.idx == 99);
+}
+
+// **RTUI-07** - the valid batch accepted.
+test rtui_07_valid_accepted {
+    const batch: [2]TreeUpdate = [2]TreeUpdate{
+        make_update(1, 5, 0),
+        make_update(3, 5, 2),
+    };
+    const v = validate_tree_updates(&batch, 5, &MEMBERS);
+    assert(v.code == RtuiCode.Ok);
+}
+
+// **RTUI-08** - the empty batch accepted.
+test rtui_08_empty_accepted {
+    const batch: [0]TreeUpdate = [0]TreeUpdate{};
+    const v = validate_tree_updates(&batch, 5, &MEMBERS);
+    assert(v.code == RtuiCode.Ok);
+}
+
+// **RTUI-09** - a single update accepted.
+test rtui_09_single_accepted {
+    const batch: [1]TreeUpdate = [1]TreeUpdate{
+        make_update(1, 5, 0),
+    };
+    const v = validate_tree_updates(&batch, 5, &MEMBERS);
+    assert(v.code == RtuiCode.Ok);
+}
+
+// **RTUI-10** - the maximum batch accepted: exactly two
+// hundred fifty-six distinct nodes.
+test rtui_10_max_batch_accepted {
+    var batch: [256]TreeUpdate;
+    for i in 0..256 {
+        batch[i] = make_update(@as(u64, i), 5, 0);
+    }
+    const v = validate_tree_updates(&batch, 5, &MEMBERS);
+    assert(v.code == RtuiCode.Ok);
+}
+
+// The helpers have direct tests (coverage: Untested 0).
+test hashes32_equal_answers {
+    const u: TreeUpdate = make_update(1, 5, 0);
+    assert(hashes32_equal(u.parent_hash, u.computed_hash));
+    assert(!hashes32_equal(u.parent_hash, HASH_FF));
+}
+
+test member_contains_answers {
+    assert(member_contains(&MEMBERS, 0));
+    assert(member_contains(&MEMBERS, 6));
+    assert(!member_contains(&MEMBERS, 99));
+    const empty: [0]u64 = [0]u64{};
+    assert(!member_contains(&empty, 0));
+}
+
+test node_seen_before_answers {
+    const batch: [3]TreeUpdate = [3]TreeUpdate{
+        make_update(1, 5, 0),
+        make_update(3, 5, 0),
+        make_update(1, 5, 0),
+    };
+    assert(!node_seen_before(&batch, 0, 1));
+    assert(!node_seen_before(&batch, 1, 3));
+    assert(node_seen_before(&batch, 2, 1));
+    assert(!node_seen_before(&batch, 2, 5));
+}
+
+// The helper builder has a direct test (coverage: the
+// fixture form reads the table exactly).
+test make_update_answers {
+    const u: TreeUpdate = make_update(1, 5, 0);
+    assert(u.node_index == 1);
+    assert(u.epoch == 5);
+    assert(u.signer_leaf == 0);
+    assert(u.parent_hash[0] == 1);
+    assert(u.computed_hash[0] == 1);
+    const big: TreeUpdate = make_update(300, 5, 0);
+    assert(big.parent_hash[0] == 44);
+}
+
+// Rule order is the original's fixed ladder: the batch
+// ceiling over a flooded batch, then per update the node
+// bound over the duplicate over the hash agreement over
+// the epoch over the membership.
+test rule_order_matches_original {
+    var flooded: [257]TreeUpdate;
+    for i in 0..257 {
+        flooded[i] = make_update(1024, 99, 99);
+    }
+    const v1 = validate_tree_updates(&flooded, 5, &MEMBERS);
+    assert(v1.code == RtuiCode.BatchTooLarge);
+
+    const stacked: [1]TreeUpdate = [1]TreeUpdate{
+        make_update(1024, 99, 99),
+    };
+    const v2 = validate_tree_updates(&stacked, 5, &MEMBERS);
+    assert(v2.code == RtuiCode.InvalidNodeIndex);
+
+    const dup_pair: [2]TreeUpdate = [2]TreeUpdate{
+        make_update(1, 5, 0),
+        make_update(1, 99, 99),
+    };
+    const v3 = validate_tree_updates(&dup_pair, 5, &MEMBERS);
+    assert(v3.code == RtuiCode.DuplicateNode);
+
+    var lying: TreeUpdate = make_update(1, 99, 99);
+    lying.parent_hash = HASH_FF;
+    const lying_batch: [1]TreeUpdate = [1]TreeUpdate{lying,};
+    const v4 = validate_tree_updates(&lying_batch, 5, &MEMBERS);
+    assert(v4.code == RtuiCode.HashMismatch);
+
+    const stale: [1]TreeUpdate = [1]TreeUpdate{
+        make_update(1, 99, 99),
+    };
+    const v5 = validate_tree_updates(&stale, 5, &MEMBERS);
+    assert(v5.code == RtuiCode.EpochMismatch);
+    assert(v5.expected == 5);
+    assert(v5.got == 99);
+
+    const outsider: [1]TreeUpdate = [1]TreeUpdate{
+        make_update(1, 5, 99),
+    };
+    const v6 = validate_tree_updates(&outsider, 5, &MEMBERS);
+    assert(v6.code == RtuiCode.NonMemberSigner);
+}