diff --git a/src/cmcp_gateway/mcp/proxy.py b/src/cmcp_gateway/mcp/proxy.py
index d08110ac..3f2e089e 100644
--- a/src/cmcp_gateway/mcp/proxy.py
+++ b/src/cmcp_gateway/mcp/proxy.py
@@ -13,6 +13,8 @@
 
 from __future__ import annotations
 
+import hashlib
+import json
 import logging
 from dataclasses import dataclass
 from datetime import UTC, datetime
@@ -94,6 +96,7 @@ def _build_cedar_context(
         entry = self._catalog.lookup(tool_name)
         return {
             "tool_name": tool_name,
+            "arguments": arguments,
             "server_identity": entry.server.url if entry else "",
             "compliance_domain": entry.compliance_domain if entry else "external",
             "baa_covered": (not entry.requires_baa) if entry else False,
@@ -166,6 +169,9 @@ async def call_tool(
         sensitivity_before = self._session.max_sensitivity
         would_have_denied = False
 
+        _payload_bytes = json.dumps(arguments, sort_keys=True, separators=(",", ":")).encode()
+        request_payload_hash = f"sha256:{hashlib.sha256(_payload_bytes).hexdigest()}"
+
         # Step 1: catalog lookup
         entry = self._catalog.lookup(tool_name)
         if entry is None:
@@ -177,7 +183,7 @@ async def call_tool(
                 server_identity=None,
                 policy_decision="deny",
                 policy_rule_matched="catalog_miss",
-                request_payload_hash=None,
+                request_payload_hash=request_payload_hash,
                 session_sensitivity_before=sensitivity_before,
                 session_sensitivity_after=self._session.max_sensitivity,
             )
@@ -216,6 +222,7 @@ async def call_tool(
                 server_identity=entry.server.url,
                 policy_decision="deny",
                 policy_rule_matched=str(exc),
+                request_payload_hash=request_payload_hash,
                 session_sensitivity_before=sensitivity_before,
                 session_sensitivity_after=self._session.max_sensitivity,
             )
@@ -258,6 +265,7 @@ async def call_tool(
                 server_identity=entry.server.url,
                 policy_decision="deny",
                 policy_rule_matched=f"agt_gateway:{type(exc).__name__}",
+                request_payload_hash=request_payload_hash,
                 session_sensitivity_before=sensitivity_before,
                 session_sensitivity_after=self._session.max_sensitivity,
             )
@@ -315,6 +323,7 @@ async def call_tool(
                 server_identity=entry.server.url,
                 policy_decision="deny",
                 policy_rule_matched=egress_deny_reason,
+                request_payload_hash=request_payload_hash,
                 session_sensitivity_before=sensitivity_before,
                 session_sensitivity_after=self._session.max_sensitivity,
             )
@@ -343,6 +352,7 @@ async def call_tool(
             policy_decision=policy_decision,
             policy_rule_matched=policy_rule,
             latency_us=latency_us,
+            request_payload_hash=request_payload_hash,
             session_sensitivity_before=sensitivity_before,
             session_sensitivity_after=self._session.max_sensitivity,
         )
diff --git a/tests/unit/test_mcp_proxy.py b/tests/unit/test_mcp_proxy.py
index 0ff804f1..943be7b8 100644
--- a/tests/unit/test_mcp_proxy.py
+++ b/tests/unit/test_mcp_proxy.py
@@ -192,3 +192,64 @@ async def test_proxy_result_contains_audit_entry_hash():
     proxy, _, chain = _make_proxy()
     result = await proxy.call_tool("c1", "test.tool", {})
     assert result.audit_entry_hash == chain.chain_tip
+
+
+# ── POLICY-004: Cedar context includes arguments ──────────────────────────────
+
+@pytest.mark.asyncio
+async def test_cedar_context_includes_arguments():
+    """POLICY-004 — arguments must appear in the Cedar context so policies can inspect them."""
+    evaluator = _make_evaluator()
+    proxy, _, _ = _make_proxy(evaluator=evaluator)
+    args = {"patient_id": "p-123", "action": "read"}
+    await proxy.call_tool("c1", "test.tool", args)
+    ctx = evaluator.evaluate.call_args[0][0]
+    assert ctx["arguments"] == args
+
+
+# ── POLICY-005: request_payload_hash in all audit entries ────────────────────
+
+@pytest.mark.asyncio
+async def test_audit_payload_hash_present_on_allow():
+    """POLICY-005 — successful calls must record request_payload_hash in the audit entry."""
+    proxy, _, chain = _make_proxy()
+    await proxy.call_tool("c1", "test.tool", {"k": "v"})
+    entry = next(e for e in reversed(chain.entries) if e.entry_type == "tool_call")
+    assert entry.request_payload_hash is not None
+    assert entry.request_payload_hash.startswith("sha256:")
+
+
+@pytest.mark.asyncio
+async def test_audit_payload_hash_present_on_catalog_deny():
+    """POLICY-005 — catalog-miss denials must record request_payload_hash."""
+    proxy, _, chain = _make_proxy()
+    await proxy.call_tool("c1", "ghost.tool", {"x": 1})
+    entry = next(e for e in reversed(chain.entries) if e.entry_type == "tool_call")
+    assert entry.request_payload_hash is not None
+    assert entry.request_payload_hash.startswith("sha256:")
+
+
+@pytest.mark.asyncio
+async def test_audit_payload_hash_present_on_cedar_deny():
+    """POLICY-005 — Cedar policy denials must record request_payload_hash."""
+    evaluator = _make_evaluator(allow=False)
+    proxy, _, chain = _make_proxy(evaluator=evaluator)
+    await proxy.call_tool("c1", "test.tool", {"secret": "leak"})
+    entry = next(e for e in reversed(chain.entries) if e.entry_type == "tool_call")
+    assert entry.request_payload_hash is not None
+    assert entry.request_payload_hash.startswith("sha256:")
+
+
+@pytest.mark.asyncio
+async def test_audit_payload_hash_is_canonical_sha256():
+    """POLICY-005 — payload hash must be sha256 of canonical JSON (sort_keys, no spaces)."""
+    import hashlib
+    import json
+
+    proxy, _, chain = _make_proxy()
+    args = {"b": 2, "a": 1}
+    await proxy.call_tool("c1", "test.tool", args)
+    entry = next(e for e in reversed(chain.entries) if e.entry_type == "tool_call")
+    expected_bytes = json.dumps(args, sort_keys=True, separators=(",", ":")).encode()
+    expected = f"sha256:{hashlib.sha256(expected_bytes).hexdigest()}"
+    assert entry.request_payload_hash == expected