diff --git a/packages/mosaic/framework/tools/lease-broker/normative_fragments.py b/packages/mosaic/framework/tools/lease-broker/normative_fragments.py
new file mode 100644
index 00000000..0d1c118e
--- /dev/null
+++ b/packages/mosaic/framework/tools/lease-broker/normative_fragments.py
@@ -0,0 +1,176 @@
+#!/usr/bin/env python3
+"""Fail-closed construction of verbatim-hashed normative fragments.
+
+This is the single construction path for the Claude and Pi adapters.  The
+payload contains only versioned source metadata and exact validated source
+bytes.  ``h_payload`` is derived afterwards and is deliberately not representable
+as a payload input, preventing cryptographic self-reference.
+"""
+
+from __future__ import annotations
+
+import hashlib
+import hmac
+import struct
+from collections.abc import Sequence
+from typing import Final
+
+MAX_FRAGMENT_BYTES: Final = 64 * 1024
+HASH_DOMAIN_SEPARATOR: Final = b"MOSAIC/H_PAYLOAD/v1\x00"
+SOURCE_DOMAIN_SEPARATOR: Final = b"MOSAIC/H_SOURCE/v1\x00"
+_PAYLOAD_VERSION_LABEL: Final = b"MOSAIC/B_PAYLOAD/v1"
+
+
+class NormativeFragment:
+    """A source identity, its expected digest, and its exact resolved bytes."""
+
+    def __init__(self, source_id: str, content: bytes | None, expected_sha256: str) -> None:
+        self.source_id = source_id
+        self.content = content
+        self.expected_sha256 = expected_sha256
+
+
+class ConstructionResult:
+    """A source-admission decision and, only when admitted, derived payload values.
+
+    ``promotion`` means the construction has produced the only values a later
+    receipt protocol may use to attempt promotion. It never performs broker
+    promotion itself. A REFUSED result has no payload/hash values, so it cannot
+    advance to that later protocol.
+    """
+
+    def __init__(
+        self,
+        injection_decision: str,
+        promotion: bool,
+        source_reason: str | None,
+        b_payload: bytes | None,
+        h_payload: str | None,
+        h_source: str | None,
+    ) -> None:
+        self.injectionDecision = injection_decision
+        self.promotion = promotion
+        self.source_reason = source_reason
+        self.b_payload = b_payload
+        self.h_payload = h_payload
+        self.h_source = h_source
+
+
+def length_frame(parts: Sequence[bytes]) -> bytes:
+    """Encode a finite ordered byte sequence with unambiguous 64-bit framing."""
+
+    framed = bytearray(struct.pack(">Q", len(parts)))
+    for part in parts:
+        if not isinstance(part, bytes):
+            raise TypeError("length framing requires bytes")
+        framed.extend(struct.pack(">Q", len(part)))
+        framed.extend(part)
+    return bytes(framed)
+
+
+def _sha256_hex(payload: bytes) -> str:
+    return hashlib.sha256(payload).hexdigest()
+
+
+def _valid_digest(value: object) -> bool:
+    return (
+        isinstance(value, str)
+        and len(value) == 64
+        and all(character in "0123456789abcdef" for character in value)
+    )
+
+
+def _source_reason(fragment: NormativeFragment) -> str | None:
+    if not isinstance(fragment.source_id, str) or not fragment.source_id:
+        return "missing"
+    if fragment.content is None:
+        return "missing"
+    if not isinstance(fragment.content, bytes):
+        return "missing"
+    if len(fragment.content) > MAX_FRAGMENT_BYTES:
+        return "oversize"
+    if not _valid_digest(fragment.expected_sha256):
+        return "hash-mismatch"
+    actual = _sha256_hex(fragment.content)
+    if not hmac.compare_digest(actual, fragment.expected_sha256):
+        return "hash-mismatch"
+    return None
+
+
+def _refused(reason: str) -> ConstructionResult:
+    return ConstructionResult(
+        injection_decision="REFUSED",
+        promotion=False,
+        source_reason=reason,
+        b_payload=None,
+        h_payload=None,
+        h_source=None,
+    )
+
+
+def build_payload(
+    *,
+    manifest_version: int,
+    generator_version: str,
+    fragments: Sequence[NormativeFragment],
+) -> ConstructionResult:
+    """Construct B_payload then H_payload after fail-closed source validation.
+
+    The sequence order is caller-supplied resolved-source order and is encoded
+    directly. Changing source order, source identity, metadata, or any exact
+    fragment byte therefore changes the framed B_payload and its derived hash.
+    """
+
+    if type(manifest_version) is not int or manifest_version < 0:
+        return _refused("missing")
+    if not isinstance(generator_version, str) or not generator_version:
+        return _refused("missing")
+
+    validated = list(fragments)
+    if not validated:
+        return _refused("missing")
+    for fragment in validated:
+        if not isinstance(fragment, NormativeFragment):
+            return _refused("missing")
+        reason = _source_reason(fragment)
+        if reason is not None:
+            return _refused(reason)
+
+    source_identities = [
+        length_frame([
+            fragment.source_id.encode("utf-8"),
+            fragment.expected_sha256.encode("ascii"),
+        ])
+        for fragment in validated
+    ]
+    h_source = _sha256_hex(SOURCE_DOMAIN_SEPARATOR + length_frame(source_identities))
+    payload_parts = [
+        _PAYLOAD_VERSION_LABEL,
+        str(manifest_version).encode("ascii"),
+        generator_version.encode("utf-8"),
+        *source_identities,
+        *(fragment.content for fragment in validated),
+    ]
+    # h_payload is intentionally not an argument or field in payload_parts.
+    b_payload = length_frame(payload_parts)
+    h_payload = _sha256_hex(HASH_DOMAIN_SEPARATOR + length_frame([b_payload]))
+    return ConstructionResult(
+        injection_decision="ACCEPTED",
+        promotion=True,
+        source_reason=None,
+        b_payload=b_payload,
+        h_payload=h_payload,
+        h_source=h_source,
+    )
+
+
+def build_for_claude(**kwargs: object) -> ConstructionResult:
+    """Claude adapter entrypoint; delegates to the sole shared constructor."""
+
+    return build_payload(**kwargs)  # type: ignore[arg-type]
+
+
+def build_for_pi(**kwargs: object) -> ConstructionResult:
+    """Pi adapter entrypoint; delegates to the sole shared constructor."""
+
+    return build_payload(**kwargs)  # type: ignore[arg-type]