diff --git a/.cspell/custom-words.txt b/.cspell/custom-words.txt
index ce73c361..d31fe3a0 100644
--- a/.cspell/custom-words.txt
+++ b/.cspell/custom-words.txt
@@ -47,6 +47,7 @@ emvco
 endlocal
 envoyproxy
 esac
+fastmcp
 felixge
 Fiuu
 fontawesome
@@ -66,6 +67,7 @@ gradlew
 Gravitee
 groupcache
 gson
+gwei
 Hashkey
 honnef
 hprof
@@ -83,6 +85,7 @@ jetbrains
 Jetpack
 jvmargs
 Kaia
+keccak
 keepattributes
 keepclassmembers
 Klarna
@@ -93,6 +96,7 @@ ktor
 Ktor
 KXMYBJWNQ
 Lazada
+levelname
 libpeerconnection
 Lightspark
 linenums
@@ -149,6 +153,8 @@ ropeproject
 RPCURL
 Rulebook
 screenreaders
+sdjwt
+sepolia
 setlocal
 sharedpref
 Shopcider
@@ -165,10 +171,12 @@ stablecoins
 stdr
 stretchr
 superfences
+toggleable
 Truelayer
 Trulioo
 udpa
 unmarshal
+usdc
 viewmodel
 vulnz
 Wallex
diff --git a/.github/workflows/linter.yaml b/.github/workflows/linter.yaml
index fea1b9c1..663a5b19 100644
--- a/.github/workflows/linter.yaml
+++ b/.github/workflows/linter.yaml
@@ -4,6 +4,11 @@ on:
   pull_request:
     branches: [main]
 
+permissions:
+  contents: read
+  statuses: write
+  pull-requests: write
+
 jobs:
   build:
     name: Lint Code Base
@@ -11,12 +16,13 @@ jobs:
 
     steps:
       - name: Checkout Code
-        uses: actions/checkout@v5
+        uses: actions/checkout@fbc6f3992d24b796d5a048ff273f7fcc4a7b6c09 # v5.1.0
         with:
           fetch-depth: 0
+          persist-credentials: false
 
       - name: Lint Code Base
-        uses: super-linter/super-linter/slim@v8
+        uses: super-linter/super-linter/slim@4ce20838b8ab83717e78138c5b3a1407148e0918 # v8.7.0
         env:
           DEFAULT_BRANCH: main
           GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
@@ -25,6 +31,7 @@ jobs:
           VALIDATE_ALL_CODEBASE: false
           FILTER_REGEX_EXCLUDE: "^(\\.github/|\\.vscode/|code/samples/).*|CODE_OF_CONDUCT.md|CHANGELOG.md"
           VALIDATE_BIOME_FORMAT: false
+          VALIDATE_BIOME_LINT: false
           VALIDATE_PYTHON_BLACK: false
           VALIDATE_PYTHON_FLAKE8: false
           VALIDATE_PYTHON_ISORT: false
diff --git a/code/samples/python/src/roles/x402_psp_mcp/server.py b/code/samples/python/src/roles/x402_psp_mcp/server.py
index 26370f04..cbfa6e3f 100644
--- a/code/samples/python/src/roles/x402_psp_mcp/server.py
+++ b/code/samples/python/src/roles/x402_psp_mcp/server.py
@@ -113,36 +113,43 @@ def settle_payment(
       agent_provider_pub = JWK.from_json(
           AGENT_PROVIDER_PUB_PATH.read_text(encoding="utf-8")
       )
-    except (OSError, ValueError, json.JSONDecodeError):
-      _logger.warning(
-          "Agent-provider public key not found — skipping SD-JWT verification"
-      )
-  else:
-    _logger.warning(
-        "Agent-provider public key not found — skipping SD-JWT verification"
-    )
+    except Exception:  # noqa: BLE001
+      # Any failure to load or validate the key (missing, unreadable,
+      # malformed JSON, wrong JSON type, or an invalid JWK) must fail
+      # closed via the guard below, so catch broadly rather than
+      # enumerate library exception types (jwcrypto raises JWException,
+      # and a non-object JWK raises TypeError, neither a ValueError).
+      _logger.exception("Failed to load agent-provider public key")
+
+  if not agent_provider_pub:
+    return {
+        "error": "agent_provider_key_missing",
+        "message": (
+            "Agent-provider public key not found; cannot verify payment"
+            " mandate"
+        ),
+    }
 
-  if agent_provider_pub:
-    try:
-      payloads = MandateClient().verify(
-          token=mandate_chain_str,
-          key_or_provider=lambda _token: agent_provider_pub,
-          expected_aud="credential-provider",
-          expected_nonce=payment_nonce,
-      )
-      parsed_chain = PaymentMandateChain.parse(payloads)
-      violations = parsed_chain.verify(
-          expected_open_checkout_hash=open_checkout_hash
-      )
-      if violations:
-        return {
-            "error": "mandate_verification_failed",
-            "message": "; ".join(violations),
-        }
-      _logger.info("SD-JWT mandate chain verified successfully")
-    except Exception as e:
-      _logger.exception("SD-JWT mandate verification failed")
-      return {"error": "mandate_verification_failed", "message": str(e)}
+  try:
+    payloads = MandateClient().verify(
+        token=mandate_chain_str,
+        key_or_provider=lambda _token: agent_provider_pub,
+        expected_aud="credential-provider",
+        expected_nonce=payment_nonce,
+    )
+    parsed_chain = PaymentMandateChain.parse(payloads)
+    violations = parsed_chain.verify(
+        expected_open_checkout_hash=open_checkout_hash
+    )
+    if violations:
+      return {
+          "error": "mandate_verification_failed",
+          "message": "; ".join(violations),
+      }
+    _logger.info("SD-JWT mandate chain verified successfully")
+  except Exception as e:
+    _logger.exception("SD-JWT mandate verification failed")
+    return {"error": "mandate_verification_failed", "message": str(e)}
 
   # 1. Verify Binding: Hash(Mandate) == Nonce
   _logger.info("Step 1: Verifying Binding")
diff --git a/code/samples/python/tests/conftest.py b/code/samples/python/tests/conftest.py
new file mode 100644
index 00000000..50e880ac
--- /dev/null
+++ b/code/samples/python/tests/conftest.py
@@ -0,0 +1,15 @@
+"""Pytest configuration for the samples test-suite.
+
+Adds the samples ``src`` directory to ``sys.path`` so role modules such as
+``roles.x402_psp_mcp.server`` and the shared ``common`` package import
+cleanly when the suite runs from ``code/samples/python``.
+"""
+
+import sys
+
+from pathlib import Path
+
+
+_SRC = Path(__file__).resolve().parent.parent / 'src'
+if str(_SRC) not in sys.path:
+    sys.path.insert(0, str(_SRC))
diff --git a/code/samples/python/tests/roles/x402_psp_mcp/settle_payment_failclosed_tests.py b/code/samples/python/tests/roles/x402_psp_mcp/settle_payment_failclosed_tests.py
new file mode 100644
index 00000000..52d2b890
--- /dev/null
+++ b/code/samples/python/tests/roles/x402_psp_mcp/settle_payment_failclosed_tests.py
@@ -0,0 +1,81 @@
+"""Regression tests for the x402 PSP fail-closed fix (AP2 #309).
+
+When the agent-provider public key cannot be loaded, ``settle_payment``
+must fail closed and return ``agent_provider_key_missing`` instead of
+silently skipping SD-JWT mandate verification. A second test proves that
+verification still runs (and can fail) when the key IS present, so the
+fix did not disable the check. A third test covers key files that parse as
+JSON but are not a usable key (an invalid JWK object, which raises
+``jwcrypto`` ``JWException``, and a non-object JSON value, which raises
+``TypeError``) -- neither is a ``ValueError``/``JSONDecodeError``, and both
+must fail closed rather than propagate.
+"""
+
+import json
+
+import pytest
+
+from jwcrypto.jwk import JWK
+from roles.x402_psp_mcp import server
+from roles.x402_psp_mcp.server import settle_payment
+
+
+def _minimal_bundle():
+    """Return the smallest bundle that reaches the key-loading guard."""
+    return {
+        'payment_mandate_chain': 'not-a-valid-sd-jwt',
+        'eip_3009_payload': {'authorization': {'nonce': '0x00'}},
+        'payment_nonce': 'nonce-abc',
+    }
+
+
+def test_settle_payment_fails_closed_when_key_missing(monkeypatch, tmp_path):
+    """Missing agent-provider key must fail closed, not skip verification."""
+    missing = tmp_path / 'missing.pub'
+    monkeypatch.setattr(server, 'AGENT_PROVIDER_PUB_PATH', missing)
+
+    result = settle_payment(payment_token=json.dumps(_minimal_bundle()))
+
+    assert result['error'] == 'agent_provider_key_missing'
+
+
+def test_settle_payment_still_verifies_when_key_present(monkeypatch, tmp_path):
+    """With the key present, verification still runs and rejects bad input."""
+    pub_path = tmp_path / 'present.pub'
+    public_jwk = JWK.generate(kty='EC', crv='P-256').export_public()
+    pub_path.write_text(public_jwk, encoding='utf-8')
+    monkeypatch.setattr(server, 'AGENT_PROVIDER_PUB_PATH', pub_path)
+
+    result = settle_payment(payment_token=json.dumps(_minimal_bundle()))
+
+    assert result['error'] == 'mandate_verification_failed'
+
+
+@pytest.mark.parametrize(
+    'key_content',
+    [
+        '{}',  # valid JSON object, invalid JWK -> jwcrypto JWException
+        '[]',  # non-object JSON -> TypeError
+        'null',  # non-object JSON -> TypeError
+        '123',  # non-object JSON -> TypeError
+        '',  # empty file -> JSONDecodeError
+    ],
+)
+def test_settle_payment_fails_closed_when_key_is_unusable(
+    monkeypatch, tmp_path, key_content
+):
+    """A file that is not a usable key must fail closed, not raise.
+
+    ``JWK.from_json`` raises different exception types for these inputs
+    (``JWException`` for an invalid JWK object, ``TypeError`` for a
+    non-object JSON value, ``JSONDecodeError`` for empty/garbage text).
+    None is a ``ValueError``, so the key-load guard must catch broadly
+    and return ``agent_provider_key_missing`` for every one of them.
+    """
+    bad_path = tmp_path / 'unusable.pub'
+    bad_path.write_text(key_content, encoding='utf-8')
+    monkeypatch.setattr(server, 'AGENT_PROVIDER_PUB_PATH', bad_path)
+
+    result = settle_payment(payment_token=json.dumps(_minimal_bundle()))
+
+    assert result['error'] == 'agent_provider_key_missing'