diff --git a/docs/compaction-refresh/probes/p5_receipt_replay.py b/docs/compaction-refresh/probes/p5_receipt_replay.py
new file mode 100644
index 00000000..76f8d4b0
--- /dev/null
+++ b/docs/compaction-refresh/probes/p5_receipt_replay.py
@@ -0,0 +1,228 @@
+#!/usr/bin/env python3
+"""P5 Gate0 replay probe; BUILT ONLY, execution is Mos-gated.
+
+Run only under fresh-executor authorization:
+  python3 -I -S -B docs/compaction-refresh/probes/p5_receipt_replay.py
+
+Each of the default three isolated runs launches the shipped lease-broker daemon
+in a distinct private temporary directory.  This driver never changes broker
+state directly and does not replace the promote gate: every transition is sent
+over the daemon's real Unix socket.  It proves the shipped order is
+PENDING_DELIVERY -> observe/evidence commit -> consume -> VERIFIED and that a
+consumed challenge cannot be replayed or reopen/renew its lease.
+"""
+
+from __future__ import annotations
+
+import argparse
+import base64
+import importlib.util
+import json
+import os
+import shutil
+import socket
+import subprocess
+import sys
+import tempfile
+import time
+from pathlib import Path
+
+
+HERE = Path(__file__).resolve().parent
+REPOSITORY = HERE.parents[2]
+TOOLS = REPOSITORY / "packages/mosaic/framework/tools/lease-broker"
+DAEMON = TOOLS / "daemon.py"
+FRAGMENTS = TOOLS / "normative_fragments.py"
+
+
+def load_shipped_fragments():
+    if not FRAGMENTS.is_file():
+        raise RuntimeError(f"shipped normative construction missing: {FRAGMENTS}")
+    spec = importlib.util.spec_from_file_location("p5_shipped_normative_fragments", FRAGMENTS)
+    if spec is None or spec.loader is None:
+        raise RuntimeError("unable to load shipped normative construction")
+    module = importlib.util.module_from_spec(spec)
+    spec.loader.exec_module(module)
+    return module
+
+
+def request(socket_path: Path, value: dict[str, object]) -> dict[str, object]:
+    with socket.socket(socket.AF_UNIX, socket.SOCK_STREAM) as connection:
+        connection.settimeout(3.0)
+        connection.connect(str(socket_path))
+        connection.sendall((json.dumps(value, separators=(",", ":")) + "\n").encode())
+        connection.shutdown(socket.SHUT_WR)
+        response = bytearray()
+        while True:
+            chunk = connection.recv(4096)
+            if not chunk:
+                break
+            response.extend(chunk)
+    if not response.endswith(b"\n") or response.count(b"\n") != 1:
+        raise AssertionError(f"unframed broker reply: {bytes(response)!r}")
+    parsed = json.loads(response[:-1])
+    if not isinstance(parsed, dict):
+        raise AssertionError(f"non-object broker reply: {parsed!r}")
+    return parsed
+
+
+def wait_ready(process: subprocess.Popen[str], socket_path: Path) -> None:
+    deadline = time.monotonic() + 5.0
+    while time.monotonic() < deadline:
+        if socket_path.exists():
+            return
+        if process.poll() is not None:
+            output = process.stdout.read() if process.stdout is not None else ""
+            raise RuntimeError(f"shipped daemon exited before READY: {output}")
+        time.sleep(0.02)
+    raise TimeoutError("shipped daemon did not create private probe socket")
+
+
+def expect_refused(reply: dict[str, object], code: str) -> None:
+    if reply != {"ok": False, "code": code}:
+        raise AssertionError(f"expected refusal {code}, got {reply!r}")
+
+
+def run_once(index: int) -> str:
+    fragments = load_shipped_fragments()
+    root = Path(tempfile.mkdtemp(prefix=f"mosaic-p5-replay-{index}-"))
+    os.chmod(root, 0o700)
+    socket_path = root / "broker.sock"
+    state_path = root / "state.json"
+    observer_path = root / "test-observer.json"
+    process = subprocess.Popen(
+        [
+            sys.executable, "-I", "-S", "-B", str(DAEMON), "--socket", str(socket_path),
+            "--state", str(state_path), "--test-observer-file", str(observer_path),
+        ],
+        stdin=subprocess.DEVNULL,
+        stdout=subprocess.PIPE,
+        stderr=subprocess.STDOUT,
+        text=True,
+    )
+    try:
+        wait_ready(process, socket_path)
+        registered = request(socket_path, {"action": "register_anchor", "runtime_generation": 1})
+        if registered.get("ok") is not True or not isinstance(registered.get("session_id"), str):
+            raise AssertionError(f"registration failed: {registered!r}")
+        session_id = registered["session_id"]
+        construction = fragments.build_payload(
+            manifest_version=1,
+            generator_version="p5-replay-probe",
+            fragments=[
+                fragments.NormativeFragment(
+                    "authority/probe",
+                    b"P5 shipped transition driver\n",
+                    "63537df1a6cb0d80195a96757ab11d629e5b5e1f23be167218b84cb195b1c1d6",
+                ),
+            ],
+        )
+        if construction.injectionDecision != "ACCEPTED" or not construction.promotion:
+            raise AssertionError("shipped normative construction refused P5 fixture")
+        binding = {
+            "compaction_epoch": index,
+            "request_epoch": index + 100,
+            "h_source": construction.h_source,
+            "h_payload": construction.h_payload,
+            "schema_version": 1,
+        }
+        construction_request = {
+            "manifest_version": 1,
+            "generator_version": "p5-replay-probe",
+            "fragments": [{
+                "source_id": "authority/probe",
+                "content_base64": base64.b64encode(b"P5 shipped transition driver\n").decode("ascii"),
+                "expected_sha256": "63537df1a6cb0d80195a96757ab11d629e5b5e1f23be167218b84cb195b1c1d6",
+            }],
+        }
+        pending = request(socket_path, {
+            "action": "begin_verification",
+            "session_id": session_id,
+            "runtime_generation": 1,
+            "runtime": "pi",
+            "binding": binding,
+            "construction": construction_request,
+        })
+        if pending.get("ok") is not True or pending.get("state") != "PENDING_VERIFICATION":
+            raise AssertionError(f"shipped pending-delivery transition failed: {pending!r}")
+        challenge = pending.get("receipt_challenge")
+        receipt = pending.get("receipt")
+        if not isinstance(challenge, str) or not isinstance(receipt, str):
+            raise AssertionError(f"shipped broker did not mint a receipt challenge: {pending!r}")
+
+        # Promotion before observation/evidence/consumption is forbidden.
+        expect_refused(request(socket_path, {
+            "action": "promote_lease",
+            "session_id": session_id,
+            "runtime_generation": 1,
+            "receipt_challenge": challenge,
+        }), "INVALID_LEASE_TRANSITION")
+
+        observer_path.write_text(json.dumps({
+            "session_id": session_id,
+            "runtime_generation": 1,
+            "latest_assistant_message": receipt,
+        }), encoding="utf-8")
+        os.chmod(observer_path, 0o600)
+        observed = request(socket_path, {
+            "action": "observe_receipt",
+            "session_id": session_id,
+            "runtime_generation": 1,
+            "receipt_challenge": challenge,
+        })
+        if observed.get("ok") is not True or observed.get("state") != "PENDING_PROMOTION":
+            raise AssertionError(f"shipped evidence transition failed: {observed!r}")
+        durable = json.loads(state_path.read_text(encoding="utf-8"))
+        evidence = durable["tokens"][challenge].get("evidence")
+        if not isinstance(evidence, dict) or not isinstance(evidence.get("h_latest_assistant"), str):
+            raise AssertionError("shipped receipt evidence was not committed before consume/promote")
+
+        promoted = request(socket_path, {
+            "action": "promote_lease",
+            "session_id": session_id,
+            "runtime_generation": 1,
+            "receipt_challenge": challenge,
+        })
+        if promoted.get("ok") is not True or promoted.get("state") != "VERIFIED":
+            raise AssertionError(f"shipped consume-before-promote transition failed: {promoted!r}")
+
+        # T25/T28: the actual consumed challenge, re-presented through the
+        # shipped daemon, can neither be observed again nor re-promote/reopen.
+        expect_refused(request(socket_path, {
+            "action": "observe_receipt",
+            "session_id": session_id,
+            "runtime_generation": 1,
+            "receipt_challenge": challenge,
+        }), "RECEIPT_REPLAY")
+        expect_refused(request(socket_path, {
+            "action": "promote_lease",
+            "session_id": session_id,
+            "runtime_generation": 1,
+            "receipt_challenge": challenge,
+        }), "RECEIPT_REPLAY")
+        return challenge
+    finally:
+        if process.poll() is None:
+            process.terminate()
+            try:
+                process.wait(timeout=3.0)
+            except subprocess.TimeoutExpired:
+                process.kill()
+                process.wait()
+        shutil.rmtree(root, ignore_errors=True)
+
+
+def main() -> None:
+    parser = argparse.ArgumentParser()
+    parser.add_argument("--runs", type=int, default=3)
+    arguments = parser.parse_args()
+    if arguments.runs != 3:
+        raise SystemExit("P5 requires exactly three isolated runs")
+    challenges = [run_once(index) for index in range(arguments.runs)]
+    if len(set(challenges)) != arguments.runs:
+        raise AssertionError("separate shipped cycles did not mint unique challenges")
+    print("P5 receipt replay probe PASS: 3 isolated shipped-daemon runs")
+
+
+if __name__ == "__main__":
+    main()
diff --git a/docs/scratchpads/832-receipt-challenge-protocol.md b/docs/scratchpads/832-receipt-challenge-protocol.md
new file mode 100644
index 00000000..9bc36483
--- /dev/null
+++ b/docs/scratchpads/832-receipt-challenge-protocol.md
@@ -0,0 +1,13 @@
+# #832 Receipt-challenge protocol — build scratchpad
+
+- **Objective:** Deliver WI-5 receipt-challenge protocol ACs T25, T26, T28, and T29 only.
+- **Authority:** BUILD-BRIEF, SPEC-v5, ratification, and red-team hashes verified in STEP-0.
+- **Base:** `e522b22fa4492861b0fcd4a956a8795c54eb9bfe` (`origin/main`).
+- **Constraints:** Byte-build only: no live broker/socket/systemd/tmux mutation. No PR, self-review, or probe fire. T27/T30 are out of scope.
+- **Plan:**
+  1. Add red-first deterministic T26/T29 in-build tests that call shipped normative construction and broker path.
+  2. Add an unexecuted, isolated P5 out-of-process replay harness that drives the shipped daemon and asserts consume-before-promote for T25/T28.
+  3. Implement the broker-minted receipt challenge and exact receipt observation/consume/promote path.
+  4. Run unit, framework-shell, compile, lint, and type checks; push after the required queue guard; report to `mosaic-100`.
+- **Risks:** The standalone harness must drive the real daemon without a divergent fixture. If that is impossible, stop and flag Mos.
+- **Evidence:** Initial RED recorded in `/home/hermes/agent-work/reviews/832-wi5-red-receipt-challenge.log`; initial green checks passed. Remediation RED recorded in `/home/hermes/agent-work/reviews/832-wi5-remediation-red.log` before observer/payload implementation; remediation green passed. Remediation-2 RED recorded in `/home/hermes/agent-work/reviews/832-wi5-remediation2-red.log`: each rejected begin restored prior VERIFIED authority. Remediation-2 GREEN: receipt unittest (5: all `INVALID_CONSTRUCTION`, `PAYLOAD_CONSTRUCTION_REFUSED`, and `PAYLOAD_BINDING_MISMATCH` cases preserve UNVERIFIED and deny the next mutator), normative-fragments unittest (5), state-store regression (10), full mutator-gate acceptance (20, including the real begin → observer → consume → promote path), `py_compile`, Mosaic package lint/typecheck, and targeted Prettier check. The P5 harness remains unfired. Coverage tooling remains unavailable (`python3 -m coverage`: module not installed). Push pending.
diff --git a/packages/mosaic/framework/tools/lease-broker/daemon.py b/packages/mosaic/framework/tools/lease-broker/daemon.py
index 1ac8e22f..8df693ff 100644
--- a/packages/mosaic/framework/tools/lease-broker/daemon.py
+++ b/packages/mosaic/framework/tools/lease-broker/daemon.py
@@ -6,6 +6,7 @@ from __future__ import annotations
 import argparse
 import copy
 import errno
+import hmac
 from concurrent.futures import ThreadPoolExecutor
 import json
 import os
@@ -20,6 +21,15 @@ import time
 from pathlib import Path
 from typing import Final
 
+# This script is loaded both as an executable and through isolated stdlib tests.
+# Keep its co-located receipt implementation importable in both modes.
+_MODULE_DIRECTORY = str(Path(__file__).resolve().parent)
+if _MODULE_DIRECTORY not in sys.path:
+    sys.path.insert(0, _MODULE_DIRECTORY)
+from normative_fragments import build_payload_from_wire
+from receipt_challenge import is_verbatim_receipt, latest_assistant_digest, receipt_for
+from receipt_observer import FileTestReceiptObserver, ReceiptObserver, UnavailableReceiptObserver
+
 MAX_FRAME: Final = 64 * 1024
 MAX_STATE: Final = 4 * 1024 * 1024
 MAX_PENDING_TOKENS: Final = 256
@@ -90,6 +100,14 @@ def valid_binding(binding: object) -> bool:
     )
 
 
+def valid_receipt_evidence(evidence: object) -> bool:
+    return (
+        isinstance(evidence, dict)
+        and set(evidence) == {"h_latest_assistant"}
+        and is_hex_256(evidence["h_latest_assistant"])
+    )
+
+
 def validate_state(value: object) -> dict[str, object]:
     if not isinstance(value, dict) or set(value) != {"version", "sessions", "tokens"}:
         raise BrokerFailure("STATE_INTEGRITY")
@@ -124,7 +142,11 @@ def validate_state(value: object) -> dict[str, object]:
     for token_value, token in tokens.items():
         if not is_hex_256(token_value) or not isinstance(token, dict):
             raise BrokerFailure("STATE_INTEGRITY")
-        if set(token) != {"session_id", "runtime_generation", "binding", "consumed"}:
+        token_fields = set(token)
+        if token_fields not in (
+            {"session_id", "runtime_generation", "binding", "consumed"},
+            {"session_id", "runtime_generation", "binding", "consumed", "evidence"},
+        ):
             raise BrokerFailure("STATE_INTEGRITY")
         session_id = token["session_id"]
         generation = token["runtime_generation"]
@@ -137,6 +159,9 @@ def validate_state(value: object) -> dict[str, object]:
             raise BrokerFailure("STATE_INTEGRITY")
         if not valid_binding(token["binding"]) or token["consumed"] is not False:
             raise BrokerFailure("STATE_INTEGRITY")
+        evidence = token.get("evidence")
+        if evidence is not None and not valid_receipt_evidence(evidence):
+            raise BrokerFailure("STATE_INTEGRITY")
     return value
 
 
@@ -275,8 +300,13 @@ class StateStore:
 
 
 class Broker:
-    def __init__(self, store: StateStore) -> None:
+    def __init__(self, store: StateStore, observer: ReceiptObserver | None = None) -> None:
         self.store = store
+        self.observer: ReceiptObserver = observer if observer is not None else UnavailableReceiptObserver()
+        # Set only by begin_verification after its mandatory revoke-first fence.
+        # It is preserved if later cycle admission is refused; all other broker
+        # actions retain the normal snapshot rollback behavior.
+        self._rejected_cycle_fence: tuple[dict[str, object], dict[str, dict[str, object]]] | None = None
         # VERIFIED authority is deliberately volatile: broker restart revokes all
         # leases while preserving WI-1 identity and pending-token integrity.
         self.leases: dict[str, dict[str, object]] = {}
@@ -348,6 +378,9 @@ class Broker:
             "runtime_generation": generation,
             "binding": copy.deepcopy(binding),
             "consumed": False,
+            # Receipt evidence is durably committed before this challenge may
+            # be consumed and promotion made externally visible.
+            "evidence": None,
         }
         return token
 
@@ -362,6 +395,7 @@ class Broker:
             raise StateCommitUncertain()
         previous = copy.deepcopy(self.store.value)
         previous_leases = copy.deepcopy(self.leases)
+        self._rejected_cycle_fence = None
         try:
             response = self._handle(peer, request)
             if self.store.value != previous:
@@ -376,9 +410,18 @@ class Broker:
         except StateCommitUncertain:
             raise
         except Exception:
-            self.store.value = previous
-            self.leases = previous_leases
+            fence = self._rejected_cycle_fence
+            if fence is None:
+                self.store.value = previous
+                self.leases = previous_leases
+            else:
+                # A refused re-verification must never resurrect the preceding
+                # VERIFIED authority. Preserve only this post-revoke fence;
+                # every unrelated partial-write failure still rolls back.
+                self.store.value, self.leases = fence
             raise
+        finally:
+            self._rejected_cycle_fence = None
 
     def _handle(self, peer: tuple[int, int, int], request: dict[str, object]) -> dict[str, object]:
         peer_pid, peer_uid, peer_gid = peer
@@ -430,6 +473,7 @@ class Broker:
             session_id, _ = self.authenticate(peer_pid, request)
             runtime = request.get("runtime")
             binding = request.get("binding")
+            construction = request.get("construction")
             ttl_seconds = request.get("ttl_seconds", MAX_LEASE_TTL_SECONDS)
             if runtime not in READ_ONLY_TOOLS:
                 raise BrokerFailure("INVALID_RUNTIME")
@@ -443,47 +487,117 @@ class Broker:
                 raise BrokerFailure("INVALID_LEASE_TTL")
             # Revoke-first is a broker operation, not advisory adapter order.
             self.revoke_session_authority(session_id)
-            token = self.mint_token(session_id, request["runtime_generation"], binding)
+            # If subsequent construction admission rejects, handle() restores
+            # this fence rather than the pre-cycle VERIFIED snapshot.
+            self._rejected_cycle_fence = (
+                copy.deepcopy(self.store.value), copy.deepcopy(self.leases)
+            )
+            try:
+                constructed = build_payload_from_wire(construction)
+            except ValueError as exc:
+                raise BrokerFailure("INVALID_CONSTRUCTION") from exc
+            if (
+                constructed.injectionDecision != "ACCEPTED"
+                or not constructed.promotion
+                or not isinstance(constructed.h_source, str)
+                or not isinstance(constructed.h_payload, str)
+            ):
+                raise BrokerFailure("PAYLOAD_CONSTRUCTION_REFUSED")
+            if (
+                not hmac.compare_digest(binding["h_source"], constructed.h_source)
+                or not hmac.compare_digest(binding["h_payload"], constructed.h_payload)
+            ):
+                raise BrokerFailure("PAYLOAD_BINDING_MISMATCH")
+            cycle_binding = copy.deepcopy(binding)
+            cycle_binding["runtime_generation"] = request["runtime_generation"]
+            challenge = self.mint_token(session_id, request["runtime_generation"], binding)
             self.leases[session_id] = {
                 "state": LEASE_PENDING,
                 "runtime": runtime,
                 "runtime_generation": request["runtime_generation"],
                 "binding": copy.deepcopy(binding),
-                "promotion_token": token,
+                "receipt_challenge": challenge,
                 "ttl_seconds": ttl_seconds,
             }
+            # The broker constructs both the bound challenge and the delivery
+            # text. The model's role is an exact copy, never hashing its output.
             return {
                 "ok": True,
                 "state": LEASE_PENDING,
-                "promotion_token": token,
+                "receipt_challenge": challenge,
+                "receipt": receipt_for(challenge, cycle_binding),
+                "binding": cycle_binding,
             }
-        if action == "promote_lease":
+        if action == "observe_receipt":
             session_id, _ = self.authenticate(peer_pid, request)
-            promotion_token = request.get("promotion_token")
-            lease = self.leases.get(session_id)
-            if not isinstance(lease, dict) or lease.get("state") != LEASE_PENDING:
-                raise BrokerFailure("INVALID_LEASE_TRANSITION")
-            expected_token = lease.get("promotion_token")
-            if (
-                not isinstance(promotion_token, str)
-                or not isinstance(expected_token, str)
-                or not secrets.compare_digest(promotion_token, expected_token)
-            ):
-                raise BrokerFailure("PROMOTION_TOKEN_MISMATCH")
-            token = self.store.tokens().get(promotion_token)
+            challenge = request.get("receipt_challenge")
+            if "latest_assistant_message" in request:
+                raise BrokerFailure("INVALID_RECEIPT")
+            if not isinstance(challenge, str):
+                raise BrokerFailure("INVALID_RECEIPT")
+            token = self.store.tokens().get(challenge)
             if (
                 not isinstance(token, dict)
                 or token.get("session_id") != session_id
                 or token.get("runtime_generation") != request.get("runtime_generation")
-                or token.get("binding") != lease.get("binding")
                 or token.get("consumed") is not False
             ):
-                raise BrokerFailure("PROMOTION_TOKEN_INVALID")
-            del self.store.tokens()[promotion_token]
+                raise BrokerFailure("RECEIPT_REPLAY")
+            lease = self.leases.get(session_id)
+            if not isinstance(lease, dict) or lease.get("state") != LEASE_PENDING:
+                raise BrokerFailure("RECEIPT_REPLAY")
+            expected_challenge = lease.get("receipt_challenge")
+            binding = lease.get("binding")
+            if (
+                not isinstance(expected_challenge, str)
+                or not secrets.compare_digest(challenge, expected_challenge)
+                or not isinstance(binding, dict)
+                or token.get("binding") != binding
+            ):
+                raise BrokerFailure("RECEIPT_REPLAY")
+            receipt_binding = copy.deepcopy(binding)
+            receipt_binding["runtime_generation"] = request["runtime_generation"]
+            message = self.observer.observe_latest_assistant_message(
+                session_id, str(lease["runtime"]), request["runtime_generation"], receipt_binding
+            )
+            if not isinstance(message, str):
+                raise BrokerFailure("RECEIPT_OBSERVATION_UNAVAILABLE")
+            # This accepts one exact current-cycle assistant entry only. It is
+            # deliberately not a transcript search and rejects quoted/extra text.
+            if not is_verbatim_receipt(message, challenge, receipt_binding):
+                raise BrokerFailure("RECEIPT_MISMATCH")
+            evidence = {"h_latest_assistant": latest_assistant_digest(message)}
+            token["evidence"] = evidence
             lease["state"] = LEASE_PENDING_PROMOTION
+            lease["evidence"] = copy.deepcopy(evidence)
+            return {"ok": True, "state": LEASE_PENDING_PROMOTION}
+        if action == "promote_lease":
+            session_id, _ = self.authenticate(peer_pid, request)
+            challenge = request.get("receipt_challenge")
+            if not isinstance(challenge, str):
+                raise BrokerFailure("INVALID_RECEIPT")
+            lease = self.leases.get(session_id)
+            if not isinstance(lease, dict) or lease.get("state") == LEASE_UNVERIFIED:
+                raise BrokerFailure("INVALID_LEASE_TRANSITION")
+            token = self.store.tokens().get(challenge)
+            if (
+                not isinstance(token, dict)
+                or token.get("session_id") != session_id
+                or token.get("runtime_generation") != request.get("runtime_generation")
+                or token.get("consumed") is not False
+            ):
+                raise BrokerFailure("RECEIPT_REPLAY")
+            if lease.get("state") != LEASE_PENDING_PROMOTION:
+                raise BrokerFailure("INVALID_LEASE_TRANSITION")
+            expected_challenge = lease.get("receipt_challenge")
+            if not isinstance(expected_challenge, str) or not secrets.compare_digest(challenge, expected_challenge):
+                raise BrokerFailure("RECEIPT_REPLAY")
+            if token.get("binding") != lease.get("binding") or not valid_receipt_evidence(token.get("evidence")):
+                raise BrokerFailure("PROMOTION_TOKEN_INVALID")
+            del self.store.tokens()[challenge]
             lease["expires_at"] = time.monotonic() + int(lease["ttl_seconds"])
-            # handle() commits token consumption before finish_promotion() makes
-            # VERIFIED externally visible: promote-last by construction.
+            # handle() commits the evidence-backed consumption before
+            # finish_promotion() makes VERIFIED externally visible: promote-last.
             return {"ok": True, "state": LEASE_PENDING_PROMOTION}
         if action == "revoke_lease":
             session_id, _ = self.authenticate(peer_pid, request)
@@ -604,12 +718,16 @@ def handle_connection(
             return
 
 
-def serve(socket_path: Path, state_path: Path) -> None:
+def serve(
+    socket_path: Path,
+    state_path: Path,
+    observer: ReceiptObserver | None = None,
+) -> None:
     secure_parent(socket_path)
     if socket_path.exists() or socket_path.is_symlink():
         raise BrokerFailure("SOCKET_ALREADY_EXISTS")
     store = StateStore(state_path)
-    broker = Broker(store)
+    broker = Broker(store, observer)
     broker_lock = threading.Lock()
     slots = threading.BoundedSemaphore(MAX_IN_FLIGHT_CONNECTIONS)
     fatal_lock = threading.Lock()
@@ -689,8 +807,14 @@ def main() -> None:
     parser = argparse.ArgumentParser()
     parser.add_argument("--socket", required=True, type=Path)
     parser.add_argument("--state", required=True, type=Path)
+    parser.add_argument("--test-observer-file", type=Path)
     arguments = parser.parse_args()
-    serve(arguments.socket, arguments.state)
+    observer = (
+        FileTestReceiptObserver(arguments.test_observer_file)
+        if arguments.test_observer_file is not None
+        else None
+    )
+    serve(arguments.socket, arguments.state, observer)
 
 
 if __name__ == "__main__":
diff --git a/packages/mosaic/framework/tools/lease-broker/normative_fragments.py b/packages/mosaic/framework/tools/lease-broker/normative_fragments.py
index 0d1c118e..76b39226 100644
--- a/packages/mosaic/framework/tools/lease-broker/normative_fragments.py
+++ b/packages/mosaic/framework/tools/lease-broker/normative_fragments.py
@@ -9,6 +9,7 @@ as a payload input, preventing cryptographic self-reference.
 
 from __future__ import annotations
 
+import base64
 import hashlib
 import hmac
 import struct
@@ -164,6 +165,44 @@ def build_payload(
     )
 
 
+def build_payload_from_wire(value: object) -> ConstructionResult:
+    """Decode a bounded broker request then invoke the sole payload builder.
+
+    Wire inputs contain only source bytes and their claimed source digests. The
+    authoritative ``build_payload`` implementation remains the only code that
+    admits those bytes and derives ``h_source``/``h_payload``.
+    """
+
+    if not isinstance(value, dict) or set(value) != {
+        "manifest_version", "generator_version", "fragments"
+    }:
+        raise ValueError("invalid construction")
+    raw_fragments = value["fragments"]
+    if not isinstance(raw_fragments, list):
+        raise ValueError("invalid construction")
+    fragments: list[NormativeFragment] = []
+    for raw_fragment in raw_fragments:
+        if not isinstance(raw_fragment, dict) or set(raw_fragment) != {
+            "source_id", "content_base64", "expected_sha256"
+        }:
+            raise ValueError("invalid construction")
+        source_id = raw_fragment["source_id"]
+        encoded = raw_fragment["content_base64"]
+        expected_sha256 = raw_fragment["expected_sha256"]
+        if not isinstance(source_id, str) or not isinstance(encoded, str) or not isinstance(expected_sha256, str):
+            raise ValueError("invalid construction")
+        try:
+            content = base64.b64decode(encoded.encode("ascii"), validate=True)
+        except (UnicodeEncodeError, ValueError) as exc:
+            raise ValueError("invalid construction") from exc
+        fragments.append(NormativeFragment(source_id, content, expected_sha256))
+    return build_payload(
+        manifest_version=value["manifest_version"],
+        generator_version=value["generator_version"],
+        fragments=fragments,
+    )
+
+
 def build_for_claude(**kwargs: object) -> ConstructionResult:
     """Claude adapter entrypoint; delegates to the sole shared constructor."""
 
diff --git a/packages/mosaic/framework/tools/lease-broker/receipt_challenge.py b/packages/mosaic/framework/tools/lease-broker/receipt_challenge.py
new file mode 100644
index 00000000..4c2f5c3d
--- /dev/null
+++ b/packages/mosaic/framework/tools/lease-broker/receipt_challenge.py
@@ -0,0 +1,43 @@
+#!/usr/bin/env python3
+"""Broker-side construction and verification for one-time receipt challenges."""
+
+from __future__ import annotations
+
+import hashlib
+import hmac
+import struct
+from typing import Final
+
+
+LATEST_ASSISTANT_DOMAIN_SEPARATOR: Final = b"MOSAIC/H_LATEST_ASSISTANT/v1\x00"
+
+
+def _length_frame(value: bytes) -> bytes:
+    return struct.pack(">Q", len(value)) + value
+
+
+def receipt_for(challenge: str, binding: dict[str, object]) -> str:
+    """Return the sole receipt text a model may copy for this broker cycle."""
+
+    return (
+        "MOSAIC-RECEIPT{"
+        f"challenge={challenge}; "
+        f"H_payload={binding['h_payload']}; "
+        f"gen={binding['runtime_generation']}; "
+        f"cep={binding['compaction_epoch']}"
+        "}"
+    )
+
+
+def is_verbatim_receipt(message: str, challenge: str, binding: dict[str, object]) -> bool:
+    """Require the exact one current-cycle receipt, not a transcript substring."""
+
+    expected = receipt_for(challenge, binding)
+    return hmac.compare_digest(message, expected)
+
+
+def latest_assistant_digest(message: str) -> str:
+    """Record the broker-computed digest of the exact observed assistant entry."""
+
+    encoded = message.encode("utf-8")
+    return hashlib.sha256(LATEST_ASSISTANT_DOMAIN_SEPARATOR + _length_frame(encoded)).hexdigest()
diff --git a/packages/mosaic/framework/tools/lease-broker/receipt_observer.py b/packages/mosaic/framework/tools/lease-broker/receipt_observer.py
new file mode 100644
index 00000000..49cac951
--- /dev/null
+++ b/packages/mosaic/framework/tools/lease-broker/receipt_observer.py
@@ -0,0 +1,106 @@
+#!/usr/bin/env python3
+"""Trusted latest-assistant-message observer boundary for receipt promotion.
+
+Runtime adapters must implement ``observe_latest_assistant_message`` directly:
+Claude selects the exact latest assistant entry and Pi selects ``message_end``.
+The broker accepts no observed message through its request protocol.
+"""
+
+from __future__ import annotations
+
+import json
+import os
+import stat
+from pathlib import Path
+from typing import Protocol
+
+
+class ReceiptObserver(Protocol):
+    def observe_latest_assistant_message(
+        self,
+        session_id: str,
+        runtime: str,
+        runtime_generation: int,
+        binding: dict[str, object],
+    ) -> str | None: ...
+
+
+class UnavailableReceiptObserver:
+    """Production-safe default until a runtime adapter injects an observer."""
+
+    def observe_latest_assistant_message(
+        self,
+        _session_id: str,
+        _runtime: str,
+        _runtime_generation: int,
+        _binding: dict[str, object],
+    ) -> str | None:
+        return None
+
+
+class TestReceiptObserver:
+    """Deterministic controlled observer used only by byte-build tests."""
+
+    def __init__(self) -> None:
+        self._messages: dict[tuple[str, int], str] = {}
+
+    def record_latest_assistant_message(
+        self, session_id: str, runtime_generation: int, message: str
+    ) -> None:
+        self._messages[(session_id, runtime_generation)] = message
+
+    def observe_latest_assistant_message(
+        self,
+        session_id: str,
+        _runtime: str,
+        runtime_generation: int,
+        _binding: dict[str, object],
+    ) -> str | None:
+        return self._messages.get((session_id, runtime_generation))
+
+
+class FileTestReceiptObserver:
+    """Private fixture-file observer for isolated out-of-process test drivers."""
+
+    def __init__(self, path: Path) -> None:
+        self.path = path
+
+    def observe_latest_assistant_message(
+        self,
+        session_id: str,
+        _runtime: str,
+        runtime_generation: int,
+        _binding: dict[str, object],
+    ) -> str | None:
+        flags = os.O_RDONLY | getattr(os, "O_CLOEXEC", 0) | getattr(os, "O_NOFOLLOW", 0)
+        try:
+            descriptor = os.open(self.path, flags)
+        except OSError:
+            return None
+        try:
+            metadata = os.fstat(descriptor)
+            if (
+                not stat.S_ISREG(metadata.st_mode)
+                or stat.S_IMODE(metadata.st_mode) != 0o600
+                or metadata.st_uid != os.geteuid()
+                or metadata.st_size > 64 * 1024
+            ):
+                return None
+            raw = os.read(descriptor, 64 * 1024 + 1)
+        finally:
+            os.close(descriptor)
+        if len(raw) > 64 * 1024:
+            return None
+        try:
+            value = json.loads(raw)
+        except (json.JSONDecodeError, UnicodeDecodeError):
+            return None
+        if (
+            not isinstance(value, dict)
+            or set(value) != {"session_id", "runtime_generation", "latest_assistant_message"}
+            or value["session_id"] != session_id
+            or value["runtime_generation"] != runtime_generation
+            or not isinstance(value["latest_assistant_message"], str)
+        ):
+            return None
+        return value["latest_assistant_message"]
diff --git a/packages/mosaic/package.json b/packages/mosaic/package.json
index 20bb7384..c7096afe 100644
--- a/packages/mosaic/package.json
+++ b/packages/mosaic/package.json
@@ -25,7 +25,7 @@
     "lint": "eslint src",
     "typecheck": "tsc --noEmit",
     "test": "vitest run --passWithNoTests && pnpm run test:framework-shell",
-    "test:framework-shell": "python3 src/lease-broker/daemon_deadline_unittest.py && python3 src/lease-broker/normative_fragments_unittest.py && python3 src/mutator-gate/runtime_tools_unittest.py && python3 src/mutator-gate/runtime_launch_guard_unittest.py && python3 framework/tools/lease-broker/check-runtime-launches.py --root ../.. && bash framework/tools/codex/test-pr-diff-context.sh"
+    "test:framework-shell": "python3 src/lease-broker/daemon_deadline_unittest.py && python3 src/lease-broker/normative_fragments_unittest.py && python3 src/lease-broker/receipt_challenge_unittest.py && python3 src/mutator-gate/runtime_tools_unittest.py && python3 src/mutator-gate/runtime_launch_guard_unittest.py && python3 framework/tools/lease-broker/check-runtime-launches.py --root ../.. && bash framework/tools/codex/test-pr-diff-context.sh"
   },
   "dependencies": {
     "@mosaicstack/brain": "workspace:*",
diff --git a/packages/mosaic/src/lease-broker/broker-test-client.ts b/packages/mosaic/src/lease-broker/broker-test-client.ts
index b3aeddbc..8b0fa995 100644
--- a/packages/mosaic/src/lease-broker/broker-test-client.ts
+++ b/packages/mosaic/src/lease-broker/broker-test-client.ts
@@ -1,4 +1,5 @@
 import { createHash } from 'node:crypto';
+import { chmod, writeFile } from 'node:fs/promises';
 import { createConnection, type Socket } from 'node:net';
 
 const DEFAULT_TIMEOUT_MS = 3_000;
@@ -185,3 +186,51 @@ export async function requestBrokerReply<T extends object>(
     options,
   );
 }
+
+export interface ReceiptChallengeCycle {
+  sessionId: string;
+  runtimeGeneration: number;
+  receiptChallenge: string;
+  receipt: string;
+}
+
+export interface ReceiptChallengeReply {
+  ok: boolean;
+  code?: string;
+  state?: 'UNVERIFIED' | 'PENDING_VERIFICATION' | 'PENDING_PROMOTION' | 'VERIFIED';
+}
+
+/**
+ * Complete the shipped begin -> trusted-observer -> consume -> promote path.
+ * The private fixture is read by the daemon's injected test observer; the
+ * observation request itself never carries assistant-message content.
+ */
+export async function observeAndPromoteReceiptChallenge(
+  socketPath: string,
+  observerFixturePath: string,
+  cycle: ReceiptChallengeCycle,
+): Promise<ReceiptChallengeReply> {
+  await writeFile(
+    observerFixturePath,
+    `${JSON.stringify({
+      session_id: cycle.sessionId,
+      runtime_generation: cycle.runtimeGeneration,
+      latest_assistant_message: cycle.receipt,
+    })}\n`,
+    { encoding: 'utf8', mode: 0o600 },
+  );
+  await chmod(observerFixturePath, 0o600);
+  const observed = await requestBrokerReply<ReceiptChallengeReply>(socketPath, {
+    action: 'observe_receipt',
+    session_id: cycle.sessionId,
+    runtime_generation: cycle.runtimeGeneration,
+    receipt_challenge: cycle.receiptChallenge,
+  });
+  if (observed.ok !== true || observed.state !== 'PENDING_PROMOTION') return observed;
+  return await requestBrokerReply<ReceiptChallengeReply>(socketPath, {
+    action: 'promote_lease',
+    session_id: cycle.sessionId,
+    runtime_generation: cycle.runtimeGeneration,
+    receipt_challenge: cycle.receiptChallenge,
+  });
+}
diff --git a/packages/mosaic/src/lease-broker/receipt_challenge_unittest.py b/packages/mosaic/src/lease-broker/receipt_challenge_unittest.py
new file mode 100644
index 00000000..a6c9fdb9
--- /dev/null
+++ b/packages/mosaic/src/lease-broker/receipt_challenge_unittest.py
@@ -0,0 +1,227 @@
+#!/usr/bin/env python3
+"""RED-first contracts for the shipped receipt challenge and observer seam."""
+
+from __future__ import annotations
+
+import base64
+import copy
+import hashlib
+import importlib.util
+import os
+import tempfile
+import unittest
+from pathlib import Path
+
+
+TOOLS = Path(__file__).parents[2] / "framework/tools/lease-broker"
+DAEMON_PATH = TOOLS / "daemon.py"
+FRAGMENTS_PATH = TOOLS / "normative_fragments.py"
+OBSERVER_PATH = TOOLS / "receipt_observer.py"
+
+
+def load_module(name: str, path: Path):
+    assert path.is_file(), f"shipped module is missing: {path}"
+    spec = importlib.util.spec_from_file_location(name, path)
+    if spec is None or spec.loader is None:
+        raise RuntimeError(f"unable to load {name}")
+    module = importlib.util.module_from_spec(spec)
+    spec.loader.exec_module(module)
+    return module
+
+
+DAEMON = load_module("lease_broker_receipt_daemon", DAEMON_PATH)
+FRAGMENTS = load_module("lease_broker_normative_fragments", FRAGMENTS_PATH)
+
+
+class BrokerFixture(unittest.TestCase):
+    def setUp(self) -> None:
+        self.temporary = tempfile.TemporaryDirectory()
+        root = Path(self.temporary.name)
+        os.chmod(root, 0o700)
+        self.peer = (os.getpid(), os.getuid(), os.getgid())
+        self.broker = DAEMON.Broker(DAEMON.StateStore(root / "state.json"))
+        registered = self.broker.handle(self.peer, {
+            "action": "register_anchor",
+            "runtime_generation": 7,
+        })
+        self.session_id = registered["session_id"]
+        self.assertIsInstance(self.session_id, str)
+
+    def tearDown(self) -> None:
+        self.temporary.cleanup()
+
+    def construction(self) -> tuple[dict[str, object], dict[str, object]]:
+        content = b"Constitution\n"
+        expected_sha256 = hashlib.sha256(content).hexdigest()
+        construction = {
+            "manifest_version": 1,
+            "generator_version": "wi5-receipt-test",
+            "fragments": [{
+                "source_id": "authority/constitution",
+                "content_base64": base64.b64encode(content).decode("ascii"),
+                "expected_sha256": expected_sha256,
+            }],
+        }
+        result = FRAGMENTS.build_payload(
+            manifest_version=construction["manifest_version"],
+            generator_version=construction["generator_version"],
+            fragments=[FRAGMENTS.NormativeFragment("authority/constitution", content, expected_sha256)],
+        )
+        self.assertEqual(result.injectionDecision, "ACCEPTED")
+        self.assertTrue(result.promotion)
+        return construction, {
+            "compaction_epoch": 3,
+            "request_epoch": 8,
+            "h_source": result.h_source,
+            "h_payload": result.h_payload,
+            "schema_version": 1,
+        }
+
+    def begin(self, binding: dict[str, object], construction: dict[str, object]) -> dict[str, object]:
+        response = self.broker.handle(self.peer, {
+            "action": "begin_verification",
+            "session_id": self.session_id,
+            "runtime_generation": 7,
+            "runtime": "pi",
+            "binding": binding,
+            "construction": construction,
+        })
+        self.assertEqual(response["state"], DAEMON.LEASE_PENDING)
+        self.assertIsInstance(response.get("receipt_challenge"), str)
+        self.assertIsInstance(response.get("receipt"), str)
+        return response
+
+
+class BuildPayloadAdmissionTest(BrokerFixture):
+    def test_b3_forged_h_source_or_h_payload_is_refused_against_shipped_build_payload(self) -> None:
+        construction, trusted = self.construction()
+        for field in ("h_source", "h_payload"):
+            with self.subTest(field=field):
+                forged = dict(trusted)
+                forged[field] = "f" * 64
+                with self.assertRaisesRegex(DAEMON.BrokerFailure, "PAYLOAD_BINDING_MISMATCH"):
+                    self.begin(forged, construction)
+
+
+class ReceiptObserverTest(BrokerFixture):
+    def setUp(self) -> None:
+        super().setUp()
+        observers = load_module("lease_broker_test_observer", OBSERVER_PATH)
+        self.observer = observers.TestReceiptObserver()
+        self.broker = DAEMON.Broker(self.broker.store, observer=self.observer)
+
+    def record(self, message: str) -> None:
+        self.observer.record_latest_assistant_message(self.session_id, 7, message)
+
+    def observe(self, challenge: str, **untrusted: object) -> dict[str, object]:
+        return self.broker.handle(self.peer, {
+            "action": "observe_receipt",
+            "session_id": self.session_id,
+            "runtime_generation": 7,
+            "receipt_challenge": challenge,
+            **untrusted,
+        })
+
+    def promote(self, challenge: str) -> dict[str, object]:
+        return self.broker.handle(self.peer, {
+            "action": "promote_lease",
+            "session_id": self.session_id,
+            "runtime_generation": 7,
+            "receipt_challenge": challenge,
+        })
+
+    def test_b2_echoed_request_observation_is_refused_but_observer_source_promotes(self) -> None:
+        construction, binding = self.construction()
+        cycle = self.begin(binding, construction)
+        challenge = cycle["receipt_challenge"]
+        receipt = cycle["receipt"]
+
+        with self.assertRaisesRegex(DAEMON.BrokerFailure, "INVALID_RECEIPT"):
+            self.observe(challenge, latest_assistant_message=receipt)
+        with self.assertRaisesRegex(DAEMON.BrokerFailure, "RECEIPT_OBSERVATION_UNAVAILABLE"):
+            self.observe(challenge)
+
+        self.record(receipt)
+        self.assertEqual(self.observe(challenge)["state"], DAEMON.LEASE_PENDING_PROMOTION)
+        self.assertEqual(self.promote(challenge)["state"], DAEMON.LEASE_VERIFIED)
+        self.assertEqual(self.broker.handle(self.peer, {
+            "action": "authorize_tool",
+            "session_id": self.session_id,
+            "runtime_generation": 7,
+            "runtime": "pi",
+            "tool_name": "bash",
+        })["decision"], "allow")
+
+    def test_rejected_begin_keeps_revoke_first_fence_for_all_construction_refusals(self) -> None:
+        construction, binding = self.construction()
+        refusal_cases = {
+            "INVALID_CONSTRUCTION": {"bad": "construction"},
+            "PAYLOAD_CONSTRUCTION_REFUSED": {
+                **construction,
+                "fragments": [{
+                    **construction["fragments"][0],
+                    "expected_sha256": "0" * 64,
+                }],
+            },
+            "PAYLOAD_BINDING_MISMATCH": None,
+        }
+        for expected_code, rejected_construction in refusal_cases.items():
+            with self.subTest(expected_code=expected_code):
+                verified = self.begin(binding, construction)
+                self.record(verified["receipt"])
+                self.observe(verified["receipt_challenge"])
+                self.assertEqual(self.promote(verified["receipt_challenge"])["state"], DAEMON.LEASE_VERIFIED)
+
+                rejected_binding = copy.deepcopy(binding)
+                if expected_code == "PAYLOAD_BINDING_MISMATCH":
+                    rejected_binding["h_payload"] = "f" * 64
+                    rejected_construction = construction
+                with self.assertRaisesRegex(DAEMON.BrokerFailure, expected_code):
+                    self.begin(rejected_binding, rejected_construction)
+
+                self.assertEqual(
+                    self.broker.leases[self.session_id]["state"], DAEMON.LEASE_UNVERIFIED
+                )
+                denied = self.broker.handle(self.peer, {
+                    "action": "authorize_tool",
+                    "session_id": self.session_id,
+                    "runtime_generation": 7,
+                    "runtime": "pi",
+                    "tool_name": "bash",
+                })
+                self.assertEqual(denied["decision"], "deny")
+                self.assertEqual(denied["state"], DAEMON.LEASE_UNVERIFIED)
+
+    def test_t26_stale_epoch_receipt_cannot_promote_against_shipped_binding(self) -> None:
+        construction, stale_binding = self.construction()
+        stale = self.begin(stale_binding, construction)
+        current_binding = dict(stale_binding)
+        current_binding["compaction_epoch"] = 4
+        current_binding["request_epoch"] = 9
+        current = self.begin(current_binding, construction)
+        self.assertNotEqual(stale["receipt_challenge"], current["receipt_challenge"])
+
+        self.record(stale["receipt"])
+        with self.assertRaisesRegex(DAEMON.BrokerFailure, "RECEIPT_MISMATCH"):
+            self.observe(current["receipt_challenge"])
+        with self.assertRaisesRegex(DAEMON.BrokerFailure, "INVALID_LEASE_TRANSITION"):
+            self.promote(current["receipt_challenge"])
+
+    def test_t29_altered_model_hash_cannot_promote_against_shipped_binding(self) -> None:
+        construction, binding = self.construction()
+        cycle = self.begin(binding, construction)
+        expected = cycle["receipt"]
+        altered_hash = "f" * 64
+        self.assertNotEqual(altered_hash, cycle["binding"]["h_payload"])
+        altered = expected.replace(cycle["binding"]["h_payload"], altered_hash, 1)
+        self.assertNotEqual(altered, expected)
+
+        self.record(altered)
+        with self.assertRaisesRegex(DAEMON.BrokerFailure, "RECEIPT_MISMATCH"):
+            self.observe(cycle["receipt_challenge"])
+        with self.assertRaisesRegex(DAEMON.BrokerFailure, "INVALID_LEASE_TRANSITION"):
+            self.promote(cycle["receipt_challenge"])
+
+
+if __name__ == "__main__":
+    unittest.main()
diff --git a/packages/mosaic/src/mutator-gate/mutator-gate.acceptance.spec.ts b/packages/mosaic/src/mutator-gate/mutator-gate.acceptance.spec.ts
index a334c623..ca809b18 100644
--- a/packages/mosaic/src/mutator-gate/mutator-gate.acceptance.spec.ts
+++ b/packages/mosaic/src/mutator-gate/mutator-gate.acceptance.spec.ts
@@ -6,19 +6,31 @@ import { spawn, spawnSync, type ChildProcess } from 'node:child_process';
 import { afterEach, describe, expect, test } from 'vitest';
 
 import { launchClaudex, type ClaudexHarnessAdapter } from '../commands/claudex.js';
-import { requestBrokerReply } from '../lease-broker/broker-test-client.js';
+import {
+  observeAndPromoteReceiptChallenge,
+  requestBrokerReply,
+} from '../lease-broker/broker-test-client.js';
 
 interface BrokerReply {
   ok: boolean;
   code?: string;
   decision?: 'allow' | 'deny';
-  state?: 'UNVERIFIED' | 'PENDING_VERIFICATION' | 'VERIFIED';
+  state?: 'UNVERIFIED' | 'PENDING_VERIFICATION' | 'PENDING_PROMOTION' | 'VERIFIED';
   session_id?: string;
-  promotion_token?: string;
+  receipt_challenge?: string;
+  receipt?: string;
+}
+
+interface PendingReceiptCycle {
+  sessionId: string;
+  runtimeGeneration: number;
+  receiptChallenge: string;
+  receipt: string;
 }
 
 interface BrokerPaths {
   socket: string;
+  observerFixture: string;
 }
 
 const frameworkRoot = new URL('../../framework/', import.meta.url).pathname;
@@ -38,14 +50,29 @@ const remediationHandlerPath = join(frameworkRoot, 'tools/qa/remediation-hook-ha
 const children: ChildProcess[] = [];
 const temporaryRoots: string[] = [];
 
+const construction = {
+  manifest_version: 1,
+  generator_version: 'mutator-gate-acceptance',
+  fragments: [
+    {
+      source_id: 'authority/mutator-gate-acceptance',
+      content_base64: 'bXV0YXRvci1nYXRlIGFjY2VwdGFuY2UK',
+      expected_sha256: 'cc3de191821d48037f60b4d006fce74b5dd394d39fb5c8bf681c28889ff0e623',
+    },
+  ],
+};
+
 const binding = (compaction_epoch = 1) => ({
   compaction_epoch,
   request_epoch: 0,
-  h_source: 'a'.repeat(64),
-  h_payload: 'b'.repeat(64),
+  h_source: '3c8fc6733d6a2bdc001ed9277d636d7cfac037ae48ed7d54203180a3839dc7a6',
+  h_payload: '42da889037c29c3a41397df0f86465ffd121bdb8cd18ad0d7c3df259ab502d3e',
   schema_version: 1,
 });
 
+const pendingReceiptCycles = new Map<string, PendingReceiptCycle>();
+const observerFixtures = new Map<string, string>();
+
 async function request(socketPath: string, requestValue: object): Promise<BrokerReply> {
   return await requestBrokerReply<BrokerReply>(socketPath, requestValue);
 }
@@ -54,9 +81,18 @@ async function startBroker(): Promise<BrokerPaths> {
   const root = await mkdtemp(join(tmpdir(), 'mosaic-mutator-gate-'));
   await chmod(root, 0o700);
   const socket = join(root, 'broker.sock');
+  const observerFixture = join(root, 'test-observer.json');
   const child = spawn(
     'python3',
-    [daemonPath, '--socket', socket, '--state', join(root, 'state.json')],
+    [
+      daemonPath,
+      '--socket',
+      socket,
+      '--state',
+      join(root, 'state.json'),
+      '--test-observer-file',
+      observerFixture,
+    ],
     {
       stdio: ['ignore', 'pipe', 'pipe'],
     },
@@ -72,7 +108,8 @@ async function startBroker(): Promise<BrokerPaths> {
     );
     child.stdout?.once('data', () => resolve());
   });
-  return { socket };
+  observerFixtures.set(socket, observerFixture);
+  return { socket, observerFixture };
 }
 
 interface RuntimeLaunchEntry {
@@ -166,28 +203,43 @@ async function beginVerification(
   ttl_seconds = 300,
   compactionEpoch = 1,
 ): Promise<BrokerReply> {
-  return await request(socket, {
+  const reply = await request(socket, {
     action: 'begin_verification',
     session_id,
     runtime_generation,
     runtime,
     ttl_seconds,
     binding: binding(compactionEpoch),
+    construction,
   });
+  if (typeof reply.receipt_challenge === 'string' && typeof reply.receipt === 'string') {
+    pendingReceiptCycles.set(reply.receipt_challenge, {
+      sessionId: session_id,
+      runtimeGeneration: runtime_generation,
+      receiptChallenge: reply.receipt_challenge,
+      receipt: reply.receipt,
+    });
+  }
+  return reply;
 }
 
 async function promote(
   socket: string,
   session_id: string,
-  promotion_token: string,
+  receipt_challenge: string,
   runtime_generation = 1,
 ): Promise<BrokerReply> {
-  return await request(socket, {
-    action: 'promote_lease',
-    session_id,
-    runtime_generation,
-    promotion_token,
-  });
+  const cycle = pendingReceiptCycles.get(receipt_challenge);
+  const observerFixture = observerFixtures.get(socket);
+  if (cycle === undefined || observerFixture === undefined) {
+    return await request(socket, {
+      action: 'promote_lease',
+      session_id,
+      runtime_generation,
+      receipt_challenge,
+    });
+  }
+  return await observeAndPromoteReceiptChallenge(socket, observerFixture, cycle);
 }
 
 async function authorize(
@@ -249,13 +301,13 @@ describe('whole mutator-class lease gate', () => {
 
     const pending = await beginVerification(socket, sessionId, 'claude');
     expect(pending).toMatchObject({ ok: true, state: 'PENDING_VERIFICATION' });
-    expect(pending.promotion_token).toMatch(/^[a-f0-9]{64}$/);
+    expect(pending.receipt_challenge).toMatch(/^[a-f0-9]{64}$/);
     expect(await authorize(socket, sessionId, 'claude', 'Write')).toMatchObject({
       ok: false,
       decision: 'deny',
     });
 
-    expect(await promote(socket, sessionId, pending.promotion_token!)).toMatchObject({
+    expect(await promote(socket, sessionId, pending.receipt_challenge!)).toMatchObject({
       ok: true,
       state: 'VERIFIED',
     });
@@ -271,9 +323,9 @@ describe('whole mutator-class lease gate', () => {
       ok: false,
       decision: 'deny',
     });
-    expect(await promote(socket, sessionId, pending.promotion_token!)).toMatchObject({
+    expect(await promote(socket, sessionId, pending.receipt_challenge!)).toMatchObject({
       ok: false,
-      code: 'PROMOTION_TOKEN_MISMATCH',
+      code: 'RECEIPT_REPLAY',
     });
   });
 
@@ -401,7 +453,7 @@ describe('whole mutator-class lease gate', () => {
     const { socket } = await startBroker();
     const sessionId = await register(socket);
     const pending = await beginVerification(socket, sessionId, 'claude', 1, 1);
-    await promote(socket, sessionId, pending.promotion_token!);
+    await promote(socket, sessionId, pending.receipt_challenge!);
 
     // Intentionally invoke neither compaction observer: this is the amended
     // D2-v5 bounded residual, not a fail-closed path.
@@ -431,7 +483,7 @@ describe('whole mutator-class lease gate', () => {
     const { socket } = await startBroker();
     const sessionId = await register(socket);
     const pending = await beginVerification(socket, sessionId, 'claude');
-    await promote(socket, sessionId, pending.promotion_token!);
+    await promote(socket, sessionId, pending.receipt_challenge!);
 
     const revoked = spawnSync('python3', [revokerPath, '--runtime', 'claude', '--reason', reason], {
       encoding: 'utf8',
@@ -454,7 +506,7 @@ describe('whole mutator-class lease gate', () => {
     const { socket } = await startBroker();
     const observerSessionId = await register(socket);
     const observerPending = await beginVerification(socket, observerSessionId, 'claude');
-    await promote(socket, observerSessionId, observerPending.promotion_token!);
+    await promote(socket, observerSessionId, observerPending.receipt_challenge!);
 
     expect(await authorize(socket, observerSessionId, 'claude', 'Bash')).toMatchObject({
       ok: true,
@@ -464,12 +516,10 @@ describe('whole mutator-class lease gate', () => {
     const retriedPromotion = await promote(
       socket,
       observerSessionId,
-      observerPending.promotion_token!,
+      observerPending.receipt_challenge!,
     );
     expect(retriedPromotion.ok).toBe(false);
-    expect(['PROMOTION_TOKEN_MISMATCH', 'INVALID_LEASE_TRANSITION']).toContain(
-      retriedPromotion.code,
-    );
+    expect(retriedPromotion.code).toBe('RECEIPT_REPLAY');
 
     const revoked = spawnSync(
       'python3',
@@ -495,7 +545,7 @@ describe('whole mutator-class lease gate', () => {
     const expirySessionId = await register(expirySocket);
     expect(expirySessionId).not.toBe(observerSessionId);
     const expiryPending = await beginVerification(expirySocket, expirySessionId, 'claude', 1, 1);
-    await promote(expirySocket, expirySessionId, expiryPending.promotion_token!);
+    await promote(expirySocket, expirySessionId, expiryPending.receipt_challenge!);
     expect(await authorize(expirySocket, expirySessionId, 'claude', 'Bash')).toMatchObject({
       ok: true,
       decision: 'allow',
@@ -514,7 +564,7 @@ describe('whole mutator-class lease gate', () => {
     const { socket } = await startBroker();
     const sessionId = await register(socket);
     const pending = await beginVerification(socket, sessionId, 'claude');
-    await promote(socket, sessionId, pending.promotion_token!);
+    await promote(socket, sessionId, pending.receipt_challenge!);
 
     const root = await mkdtemp(join(tmpdir(), 'mosaic-observer-fence-'));
     temporaryRoots.push(root);
@@ -547,7 +597,7 @@ describe('whole mutator-class lease gate', () => {
     const { socket } = await startBroker();
     const sessionId = await register(socket);
     const pending = await beginVerification(socket, sessionId, 'pi');
-    await promote(socket, sessionId, pending.promotion_token!);
+    await promote(socket, sessionId, pending.receipt_challenge!);
     const anchorPid = process.pid;
 
     const root = await mkdtemp(join(tmpdir(), 'mosaic-generation-bump-'));
@@ -612,7 +662,7 @@ describe('whole mutator-class lease gate', () => {
     const { socket } = await startBroker();
     const sessionId = await register(socket);
     const pending = await beginVerification(socket, sessionId, 'claude', 1, 1);
-    await promote(socket, sessionId, pending.promotion_token!);
+    await promote(socket, sessionId, pending.receipt_challenge!);
 
     expect(await authorize(socket, sessionId, 'claude', 'Bash')).toMatchObject({
       ok: true,
@@ -626,7 +676,7 @@ describe('whole mutator-class lease gate', () => {
     });
 
     const refreshed = await beginVerification(socket, sessionId, 'claude', 1, 300, 2);
-    await promote(socket, sessionId, refreshed.promotion_token!);
+    await promote(socket, sessionId, refreshed.receipt_challenge!);
     expect(
       await request(socket, {
         action: 'revoke_lease',
@@ -646,7 +696,7 @@ describe('whole mutator-class lease gate', () => {
     const { socket } = await startBroker();
     const sessionId = await register(socket);
     const pending = await beginVerification(socket, sessionId, 'pi');
-    await promote(socket, sessionId, pending.promotion_token!);
+    await promote(socket, sessionId, pending.receipt_challenge!);
 
     expect(await authorize(socket, sessionId, 'pi', 'bash', 2)).toMatchObject({
       ok: false,
@@ -859,7 +909,7 @@ raise SystemExit(0 if len(session_id) == 64 and hook_present and observers_prese
     expect(runRuntimeGate(socket, sessionId, 'pi', 'unknown_custom_tool').status).toBe(2);
 
     const pending = await beginVerification(socket, sessionId, 'claude');
-    await promote(socket, sessionId, pending.promotion_token!);
+    await promote(socket, sessionId, pending.receipt_challenge!);
     expect(runRuntimeGate(socket, sessionId, 'claude', 'Bash').status).toBe(0);
 
     const settings = JSON.parse(await readFile(claudeSettingsPath, 'utf8')) as {