diff --git a/.sdkharness/README.md b/.sdkharness/README.md
index 7f0a0bbf..0bfe3de1 100644
--- a/.sdkharness/README.md
+++ b/.sdkharness/README.md
@@ -14,8 +14,14 @@ SDKHARNESS_RESULT	health	golden-path	PASS	-
 ```
 
 The customer-health, conformance, and resilience executables accept only
-literal IPv4 loopback HTTP simulator URLs. They import `b2sdk` from this exact
-checkout and never use a production B2 endpoint or real credentials.
+literal IPv4 loopback HTTP simulator URLs (`http://127.0.0.1:<port>`; `localhost`,
+`::1` and DNS names are refused). They import `b2sdk` from this exact checkout
+and never use a production B2 endpoint or real credentials: every check uses
+only the fixed simulator credential `test-key-id` / `test-key`, and the health
+check refuses any other `B2_TEST_APPLICATION_KEY*` pair before it reaches the
+SDK. The individual conformance and resilience check files refuse to run at all
+unless they are given a loopback simulator URL, so run them through the
+dispatchers below, never with real `B2_*` values in the environment.
 
 Conformance owns 33 capability checks and resilience owns 16 injected-fault
 checks. Their small dispatchers validate the invocation, run the selected
@@ -23,3 +29,59 @@ repository-owned assertion, and translate its standing verdict into the
 five-field `SDKHARNESS_RESULT` record. The central harness continues to own
 scenario selection, simulator lifecycle, fleet evidence, issue reconciliation,
 reporting, and notification.
+
+## Run one check locally
+
+Nothing here touches B2. You need Python 3.10+ and Node 22+ (for the simulator).
+
+```bash
+# 1. This checkout, in a virtualenv (the checks import b2sdk from here)
+python -m venv .venv && . .venv/bin/activate && pip install -e .
+
+# 2. A local simulator (any one of these; it needs access to backblaze-labs/b2-simulator)
+git clone https://github.com/backblaze-labs/b2-simulator /tmp/b2-simulator
+node /tmp/b2-simulator/bin/simulator/serve.mjs --control > /tmp/sim.out 2>&1 &    # prints the URLs
+SIM_PID=$!
+# ...or use the simulator embedded in the harness: sdkharness/bin/simulator/serve.mjs
+
+# 3. Wait until it has printed all three listener lines (http, https, control), then read them
+for _ in $(seq 100); do
+  [ "$(grep -c '^SIMULATOR-' /tmp/sim.out)" -ge 3 ] && break
+  kill -0 "$SIM_PID" 2>/dev/null || { echo 'simulator exited:'; cat /tmp/sim.out; break; }
+  sleep 0.1
+done
+export SDKHARNESS_SIMULATOR_URL=$(sed -n 's/^SIMULATOR-LISTENING \(http:.*\)/\1/p' /tmp/sim.out)
+export SDKHARNESS_SIMULATOR_HTTPS_URL=$(sed -n 's/^SIMULATOR-LISTENING \(https:.*\)/\1/p' /tmp/sim.out)
+export SDKHARNESS_SIMULATOR_CONTROL_URL=$(sed -n 's/^SIMULATOR-CONTROL \(.*\)/\1/p' /tmp/sim.out)
+export SDKHARNESS_SIMULATOR_CA=/tmp/b2-simulator/bin/simulator/loopback-cert.pem
+```
+
+The standalone simulator also exports the same values as `B2SIM_URL`,
+`B2SIM_HTTPS_URL`, `B2SIM_CONTROL_URL` and `B2SIM_CA` (its `bin/lib/simulator.sh`
+helper); the checks read the `SDKHARNESS_SIMULATOR_*` names above.
+
+```bash
+# conformance (one capability)
+SDKHARNESS_TEST_LEVEL=conformance SDKHARNESS_SCENARIO=files.upload .sdkharness/tests/run-conformance
+
+# resilience (one injected fault; needs the control URL, i.e. serve.mjs --control)
+SDKHARNESS_TEST_LEVEL=resilience SDKHARNESS_SCENARIO=api.backoff_503 .sdkharness/tests/run-resilience
+
+# customer health (needs a bucket in the simulator first)
+python - <<'PY'
+import os
+from b2sdk.v3 import B2Api, InMemoryAccountInfo
+api = B2Api(InMemoryAccountInfo())
+api.authorize_account('test-key-id', 'test-key', realm=os.environ['SDKHARNESS_SIMULATOR_URL'])
+api.create_bucket('sdkharness-healthcheck', 'allPrivate')
+PY
+HEALTHCHECK_REALM_URL=$SDKHARNESS_SIMULATOR_URL B2_TEST_APPLICATION_KEY_ID=test-key-id \
+  B2_TEST_APPLICATION_KEY=test-key B2_BUCKET_NAME=sdkharness-healthcheck \
+  .sdkharness/tests/health-golden-path
+```
+
+When you are done, stop the simulator with `kill "$SIM_PID"`.
+
+Each prints one `SDKHARNESS_RESULT` line. Scenario ids are in `tests.tsv`.
+A `FAIL` for `api.retry_after_503`, `upload.retry_408` is a known SDK finding, not a setup problem.
+
diff --git a/.sdkharness/tests/conformance/bucket.cors b/.sdkharness/tests/conformance/bucket.cors
index 6ad31881..d370b27d 100755
--- a/.sdkharness/tests/conformance/bucket.cors
+++ b/.sdkharness/tests/conformance/bucket.cors
@@ -37,6 +37,9 @@ import os
 import sys
 import uuid
 
+sys.path.insert(0, os.path.join(os.path.dirname(os.path.abspath(__file__)), '..', 'lib'))
+from loopback_guard import refusal  # noqa: E402
+
 SLUG = 'b2-sdk-python'
 CAPABILITY = 'bucket.cors'
 RULE = {
@@ -63,8 +66,6 @@ REASONS = (
 # own server (bin/simulator/serve.mjs), whose URL the runner exports as
 # CONFORMANCE_SIMULATOR_URL; unset, there is nothing to reach: no-realm-option.
 REALMS = {
-    'staging': 'staging',
-    'production': 'production',
     'simulator': os.environ.get('CONFORMANCE_SIMULATOR_URL') or None,
 }
 
@@ -78,7 +79,7 @@ def credential(realm):
     """The (key id, key) pair for a realm: the fixed one at @simulator."""
     if realm == REALMS['simulator']:
         return SIM_CREDENTIAL
-    return os.environ.get('B2_APPLICATION_KEY_ID'), os.environ.get('B2_APPLICATION_KEY')
+    raise AssertionError('conformance checks use only the fixed simulator credential')
 
 
 class Amber(Exception):
@@ -299,7 +300,11 @@ def run(target: str) -> None:
 
 
 def main() -> int:
-    target = os.environ.get('CONFORMANCE_TARGET', 'staging')
+    target = os.environ.get('CONFORMANCE_TARGET', 'simulator')
+    refused = refusal('conformance')
+    if refused:
+        say(target, f'FAIL ({refused})')
+        return 1
     try:
         run(target)
     except Amber as amber:
diff --git a/.sdkharness/tests/conformance/bucket.crud b/.sdkharness/tests/conformance/bucket.crud
index 99283003..de5cd114 100755
--- a/.sdkharness/tests/conformance/bucket.crud
+++ b/.sdkharness/tests/conformance/bucket.crud
@@ -30,6 +30,9 @@ import os
 import sys
 import uuid
 
+sys.path.insert(0, os.path.join(os.path.dirname(os.path.abspath(__file__)), '..', 'lib'))
+from loopback_guard import refusal  # noqa: E402
+
 SLUG = 'b2-sdk-python'
 CAPABILITY = 'bucket.crud'
 
@@ -48,8 +51,6 @@ REASONS = (
 # own server (bin/simulator/serve.mjs), whose URL the runner exports as
 # CONFORMANCE_SIMULATOR_URL; unset, there is nothing to reach: no-realm-option.
 REALMS = {
-    'staging': 'staging',
-    'production': 'production',
     'simulator': os.environ.get('CONFORMANCE_SIMULATOR_URL') or None,
 }
 
@@ -63,7 +64,7 @@ def credential(realm):
     """The (key id, key) pair for a realm: the fixed one at @simulator."""
     if realm == REALMS['simulator']:
         return SIM_CREDENTIAL
-    return os.environ.get('B2_APPLICATION_KEY_ID'), os.environ.get('B2_APPLICATION_KEY')
+    raise AssertionError('conformance checks use only the fixed simulator credential')
 
 
 class Amber(Exception):
@@ -283,7 +284,11 @@ def run(target: str) -> None:
 
 
 def main() -> int:
-    target = os.environ.get('CONFORMANCE_TARGET', 'staging')
+    target = os.environ.get('CONFORMANCE_TARGET', 'simulator')
+    refused = refusal('conformance')
+    if refused:
+        say(target, f'FAIL ({refused})')
+        return 1
     try:
         run(target)
     except Amber as amber:
diff --git a/.sdkharness/tests/conformance/bucket.lifecycle b/.sdkharness/tests/conformance/bucket.lifecycle
index d3b47f0a..a8246e43 100755
--- a/.sdkharness/tests/conformance/bucket.lifecycle
+++ b/.sdkharness/tests/conformance/bucket.lifecycle
@@ -28,6 +28,9 @@ import os
 import sys
 import uuid
 
+sys.path.insert(0, os.path.join(os.path.dirname(os.path.abspath(__file__)), '..', 'lib'))
+from loopback_guard import refusal  # noqa: E402
+
 SLUG = 'b2-sdk-python'
 CAPABILITY = 'bucket.lifecycle'
 RULE = {'fileNamePrefix': 'st/', 'daysFromHidingToDeleting': 1}
@@ -48,8 +51,6 @@ REASONS = (
 # own server (bin/simulator/serve.mjs), whose URL the runner exports as
 # CONFORMANCE_SIMULATOR_URL; unset, there is nothing to reach: no-realm-option.
 REALMS = {
-    'staging': 'staging',
-    'production': 'production',
     'simulator': os.environ.get('CONFORMANCE_SIMULATOR_URL') or None,
 }
 
@@ -63,7 +64,7 @@ def credential(realm):
     """The (key id, key) pair for a realm: the fixed one at @simulator."""
     if realm == REALMS['simulator']:
         return SIM_CREDENTIAL
-    return os.environ.get('B2_APPLICATION_KEY_ID'), os.environ.get('B2_APPLICATION_KEY')
+    raise AssertionError('conformance checks use only the fixed simulator credential')
 
 
 class Amber(Exception):
@@ -271,7 +272,11 @@ def run(target: str) -> None:
 
 
 def main() -> int:
-    target = os.environ.get('CONFORMANCE_TARGET', 'staging')
+    target = os.environ.get('CONFORMANCE_TARGET', 'simulator')
+    refused = refusal('conformance')
+    if refused:
+        say(target, f'FAIL ({refused})')
+        return 1
     try:
         run(target)
     except Amber as amber:
diff --git a/.sdkharness/tests/conformance/bucket.notification_rules b/.sdkharness/tests/conformance/bucket.notification_rules
index cbefb378..62f69093 100755
--- a/.sdkharness/tests/conformance/bucket.notification_rules
+++ b/.sdkharness/tests/conformance/bucket.notification_rules
@@ -28,6 +28,9 @@ import string
 import sys
 import uuid
 
+sys.path.insert(0, os.path.join(os.path.dirname(os.path.abspath(__file__)), '..', 'lib'))
+from loopback_guard import refusal  # noqa: E402
+
 SLUG = 'b2-sdk-python'
 CAPABILITY = 'bucket.notification_rules'
 RULE_NAME = 'sdkharness-conf'
@@ -49,8 +52,6 @@ REASONS = (
 # own server (bin/simulator/serve.mjs), whose URL the runner exports as
 # CONFORMANCE_SIMULATOR_URL; unset, there is nothing to reach: no-realm-option.
 REALMS = {
-    'staging': 'staging',
-    'production': 'production',
     'simulator': os.environ.get('CONFORMANCE_SIMULATOR_URL') or None,
 }
 
@@ -64,7 +65,7 @@ def credential(realm):
     """The (key id, key) pair for a realm: the fixed one at @simulator."""
     if realm == REALMS['simulator']:
         return SIM_CREDENTIAL
-    return os.environ.get('B2_APPLICATION_KEY_ID'), os.environ.get('B2_APPLICATION_KEY')
+    raise AssertionError('conformance checks use only the fixed simulator credential')
 
 
 class Amber(Exception):
@@ -291,7 +292,11 @@ def run(target: str) -> None:
 
 
 def main() -> int:
-    target = os.environ.get('CONFORMANCE_TARGET', 'staging')
+    target = os.environ.get('CONFORMANCE_TARGET', 'simulator')
+    refused = refusal('conformance')
+    if refused:
+        say(target, f'FAIL ({refused})')
+        return 1
     try:
         run(target)
     except Amber as amber:
diff --git a/.sdkharness/tests/conformance/bucket.replication_config b/.sdkharness/tests/conformance/bucket.replication_config
index 9164397d..5d5d95c8 100755
--- a/.sdkharness/tests/conformance/bucket.replication_config
+++ b/.sdkharness/tests/conformance/bucket.replication_config
@@ -28,6 +28,9 @@ import os
 import sys
 import uuid
 
+sys.path.insert(0, os.path.join(os.path.dirname(os.path.abspath(__file__)), '..', 'lib'))
+from loopback_guard import refusal  # noqa: E402
+
 SLUG = 'b2-sdk-python'
 CAPABILITY = 'bucket.replication_config'
 RULE_NAME = 'sdkharness-conf'
@@ -60,8 +63,6 @@ REASONS = (
 # own server (bin/simulator/serve.mjs), whose URL the runner exports as
 # CONFORMANCE_SIMULATOR_URL; unset, there is nothing to reach: no-realm-option.
 REALMS = {
-    'staging': 'staging',
-    'production': 'production',
     'simulator': os.environ.get('CONFORMANCE_SIMULATOR_URL') or None,
 }
 
@@ -75,7 +76,7 @@ def credential(realm):
     """The (key id, key) pair for a realm: the fixed one at @simulator."""
     if realm == REALMS['simulator']:
         return SIM_CREDENTIAL
-    return os.environ.get('B2_APPLICATION_KEY_ID'), os.environ.get('B2_APPLICATION_KEY')
+    raise AssertionError('conformance checks use only the fixed simulator credential')
 
 
 class Amber(Exception):
@@ -319,7 +320,11 @@ def run(target: str) -> None:
 
 
 def main() -> int:
-    target = os.environ.get('CONFORMANCE_TARGET', 'staging')
+    target = os.environ.get('CONFORMANCE_TARGET', 'simulator')
+    refused = refusal('conformance')
+    if refused:
+        say(target, f'FAIL ({refused})')
+        return 1
     try:
         run(target)
     except Amber as amber:
diff --git a/.sdkharness/tests/conformance/bucket.replication_helper b/.sdkharness/tests/conformance/bucket.replication_helper
index 7b984022..34483db0 100755
--- a/.sdkharness/tests/conformance/bucket.replication_helper
+++ b/.sdkharness/tests/conformance/bucket.replication_helper
@@ -33,6 +33,9 @@ import os
 import sys
 import uuid
 
+sys.path.insert(0, os.path.join(os.path.dirname(os.path.abspath(__file__)), '..', 'lib'))
+from loopback_guard import refusal  # noqa: E402
+
 SLUG = 'b2-sdk-python'
 CAPABILITY = 'bucket.replication_helper'
 RULE_NAME = 'sdkharness-conf'
@@ -55,8 +58,6 @@ REASONS = (
 # own server (bin/simulator/serve.mjs), whose URL the runner exports as
 # CONFORMANCE_SIMULATOR_URL; unset, there is nothing to reach: no-realm-option.
 REALMS = {
-    'staging': 'staging',
-    'production': 'production',
     'simulator': os.environ.get('CONFORMANCE_SIMULATOR_URL') or None,
 }
 
@@ -70,7 +71,7 @@ def credential(realm):
     """The (key id, key) pair for a realm: the fixed one at @simulator."""
     if realm == REALMS['simulator']:
         return SIM_CREDENTIAL
-    return os.environ.get('B2_APPLICATION_KEY_ID'), os.environ.get('B2_APPLICATION_KEY')
+    raise AssertionError('conformance checks use only the fixed simulator credential')
 
 
 class Amber(Exception):
@@ -335,7 +336,11 @@ def run(target: str) -> None:
 
 
 def main() -> int:
-    target = os.environ.get('CONFORMANCE_TARGET', 'staging')
+    target = os.environ.get('CONFORMANCE_TARGET', 'simulator')
+    refused = refusal('conformance')
+    if refused:
+        say(target, f'FAIL ({refused})')
+        return 1
     try:
         run(target)
     except Amber as amber:
diff --git a/.sdkharness/tests/conformance/client.auth_persistence b/.sdkharness/tests/conformance/client.auth_persistence
index 7ce8c08f..46fb5b40 100755
--- a/.sdkharness/tests/conformance/client.auth_persistence
+++ b/.sdkharness/tests/conformance/client.auth_persistence
@@ -34,6 +34,9 @@ import sys
 import tempfile
 import uuid
 
+sys.path.insert(0, os.path.join(os.path.dirname(os.path.abspath(__file__)), '..', 'lib'))
+from loopback_guard import refusal  # noqa: E402
+
 SLUG = 'b2-sdk-python'
 CAPABILITY = 'client.auth_persistence'
 
@@ -52,8 +55,6 @@ REASONS = (
 # own server (bin/simulator/serve.mjs), whose URL the runner exports as
 # CONFORMANCE_SIMULATOR_URL; unset, there is nothing to reach: no-realm-option.
 REALMS = {
-    'staging': 'staging',
-    'production': 'production',
     'simulator': os.environ.get('CONFORMANCE_SIMULATOR_URL') or None,
 }
 
@@ -67,7 +68,7 @@ def credential(realm):
     """The (key id, key) pair for a realm: the fixed one at @simulator."""
     if realm == REALMS['simulator']:
         return SIM_CREDENTIAL
-    return os.environ.get('B2_APPLICATION_KEY_ID'), os.environ.get('B2_APPLICATION_KEY')
+    raise AssertionError('conformance checks use only the fixed simulator credential')
 
 
 class Amber(Exception):
@@ -334,7 +335,11 @@ def run(target: str) -> None:
 
 
 def main() -> int:
-    target = os.environ.get('CONFORMANCE_TARGET', 'staging')
+    target = os.environ.get('CONFORMANCE_TARGET', 'simulator')
+    refused = refusal('conformance')
+    if refused:
+        say(target, f'FAIL ({refused})')
+        return 1
     try:
         run(target)
     except Amber as amber:
diff --git a/.sdkharness/tests/conformance/client.progress b/.sdkharness/tests/conformance/client.progress
index 830a11fc..8c39d335 100755
--- a/.sdkharness/tests/conformance/client.progress
+++ b/.sdkharness/tests/conformance/client.progress
@@ -29,6 +29,9 @@ import sys
 import threading
 import uuid
 
+sys.path.insert(0, os.path.join(os.path.dirname(os.path.abspath(__file__)), '..', 'lib'))
+from loopback_guard import refusal  # noqa: E402
+
 SLUG = 'b2-sdk-python'
 CAPABILITY = 'client.progress'
 OBJECT_NAME = 'st/progress.bin'
@@ -49,8 +52,6 @@ REASONS = (
 # own server (bin/simulator/serve.mjs), whose URL the runner exports as
 # CONFORMANCE_SIMULATOR_URL; unset, there is nothing to reach: no-realm-option.
 REALMS = {
-    'staging': 'staging',
-    'production': 'production',
     'simulator': os.environ.get('CONFORMANCE_SIMULATOR_URL') or None,
 }
 
@@ -64,7 +65,7 @@ def credential(realm):
     """The (key id, key) pair for a realm: the fixed one at @simulator."""
     if realm == REALMS['simulator']:
         return SIM_CREDENTIAL
-    return os.environ.get('B2_APPLICATION_KEY_ID'), os.environ.get('B2_APPLICATION_KEY')
+    raise AssertionError('conformance checks use only the fixed simulator credential')
 
 
 class Amber(Exception):
@@ -308,7 +309,11 @@ def run(target: str) -> None:
 
 
 def main() -> int:
-    target = os.environ.get('CONFORMANCE_TARGET', 'staging')
+    target = os.environ.get('CONFORMANCE_TARGET', 'simulator')
+    refused = refusal('conformance')
+    if refused:
+        say(target, f'FAIL ({refused})')
+        return 1
     try:
         run(target)
     except Amber as amber:
diff --git a/.sdkharness/tests/conformance/client.simulator b/.sdkharness/tests/conformance/client.simulator
index 17a5959a..1936bfa1 100755
--- a/.sdkharness/tests/conformance/client.simulator
+++ b/.sdkharness/tests/conformance/client.simulator
@@ -32,6 +32,9 @@ import socket
 import sys
 import uuid
 
+sys.path.insert(0, os.path.join(os.path.dirname(os.path.abspath(__file__)), '..', 'lib'))
+from loopback_guard import refusal  # noqa: E402
+
 SLUG = 'b2-sdk-python'
 CAPABILITY = 'client.simulator'
 OBJECT_NAME = 'st/simulator.txt'
@@ -48,10 +51,8 @@ REASONS = (
     'not-claimed',
 )
 
-# A requested target this tool cannot reach maps to a realm or to amber.
-# b2sdk ships no B2 fake reachable through a realm NAME, so `simulator` is
-# `no-realm-option` here -- not a failure of the capability.
-REALMS = {'staging': 'staging', 'production': 'production'}
+# This check never uses a realm or a credential: it drives the RawSimulator that
+# ships inside b2sdk, in-process, with the network disabled.
 
 
 class Amber(Exception):
@@ -128,42 +129,6 @@ def payload_of(size: int) -> bytes:
     return (unit * (size // len(unit) + 1))[:size]
 
 
-def prepare(target):
-    """Realm, credentials and the INSTALLED package -- before any network call."""
-    realm = REALMS.get(target)
-    if realm is None:
-        raise Amber('no-realm-option', f"b2sdk exposes no '{target}' realm")
-
-    if not os.environ.get('B2_APPLICATION_KEY_ID') or not os.environ.get('B2_APPLICATION_KEY'):
-        raise Amber('no-credential', 'B2_APPLICATION_KEY_ID / B2_APPLICATION_KEY unset')
-
-    try:
-        import b2sdk.v3 as b2
-    except ImportError as error:
-        raise Amber('missing-runtime', 'b2sdk is not installed') from error
-
-    return b2, realm
-
-
-def authorize(api, realm):
-    """Authorize against the named realm. Credentials never leave this call."""
-    step(
-        'authenticate',
-        lambda: api.authorize_account(
-            os.environ['B2_APPLICATION_KEY_ID'],
-            os.environ['B2_APPLICATION_KEY'],
-            realm=realm,
-        ),
-    )
-    return api
-
-
-def connect(target, **api_kwargs):
-    """The common case: one authorized B2Api against the requested realm."""
-    b2, realm = prepare(target)
-    return b2, authorize(b2.B2Api(b2.InMemoryAccountInfo(), **api_kwargs), realm)
-
-
 def new_bucket(api, **kwargs):
     """An ephemeral bucket, named as the contract requires."""
     name = f'sdkharness-conf-{uuid.uuid4().hex[:12]}'
@@ -281,6 +246,10 @@ def run(target: str) -> None:
 
 def main() -> int:
     target = 'simulator'
+    refused = refusal('conformance')
+    if refused:
+        say(target, f'FAIL ({refused})')
+        return 1
     try:
         run(target)
     except Amber as amber:
diff --git a/.sdkharness/tests/conformance/client.sync b/.sdkharness/tests/conformance/client.sync
index d5ba4269..126049e9 100755
--- a/.sdkharness/tests/conformance/client.sync
+++ b/.sdkharness/tests/conformance/client.sync
@@ -40,6 +40,9 @@ import tempfile
 import time
 import uuid
 
+sys.path.insert(0, os.path.join(os.path.dirname(os.path.abspath(__file__)), '..', 'lib'))
+from loopback_guard import refusal  # noqa: E402
+
 SLUG = 'b2-sdk-python'
 CAPABILITY = 'client.sync'
 REMOTE_PREFIX = 'st/sync'
@@ -60,8 +63,6 @@ REASONS = (
 # own server (bin/simulator/serve.mjs), whose URL the runner exports as
 # CONFORMANCE_SIMULATOR_URL; unset, there is nothing to reach: no-realm-option.
 REALMS = {
-    'staging': 'staging',
-    'production': 'production',
     'simulator': os.environ.get('CONFORMANCE_SIMULATOR_URL') or None,
 }
 
@@ -75,7 +76,7 @@ def credential(realm):
     """The (key id, key) pair for a realm: the fixed one at @simulator."""
     if realm == REALMS['simulator']:
         return SIM_CREDENTIAL
-    return os.environ.get('B2_APPLICATION_KEY_ID'), os.environ.get('B2_APPLICATION_KEY')
+    raise AssertionError('conformance checks use only the fixed simulator credential')
 
 
 class Amber(Exception):
@@ -357,7 +358,11 @@ def run(target: str) -> None:
 
 
 def main() -> int:
-    target = os.environ.get('CONFORMANCE_TARGET', 'staging')
+    target = os.environ.get('CONFORMANCE_TARGET', 'simulator')
+    refused = refusal('conformance')
+    if refused:
+        say(target, f'FAIL ({refused})')
+        return 1
     try:
         run(target)
     except Amber as amber:
diff --git a/.sdkharness/tests/conformance/enc.sse_b2 b/.sdkharness/tests/conformance/enc.sse_b2
index 5f2a10fa..8450614a 100755
--- a/.sdkharness/tests/conformance/enc.sse_b2
+++ b/.sdkharness/tests/conformance/enc.sse_b2
@@ -36,6 +36,9 @@ import os
 import sys
 import uuid
 
+sys.path.insert(0, os.path.join(os.path.dirname(os.path.abspath(__file__)), '..', 'lib'))
+from loopback_guard import refusal  # noqa: E402
+
 SLUG = 'b2-sdk-python'
 CAPABILITY = 'enc.sse_b2'
 OBJECT_NAME = 'st/sse-b2.txt'
@@ -56,8 +59,6 @@ REASONS = (
 # own server (bin/simulator/serve.mjs), whose URL the runner exports as
 # CONFORMANCE_SIMULATOR_URL; unset, there is nothing to reach: no-realm-option.
 REALMS = {
-    'staging': 'staging',
-    'production': 'production',
     'simulator': os.environ.get('CONFORMANCE_SIMULATOR_URL') or None,
 }
 
@@ -71,7 +72,7 @@ def credential(realm):
     """The (key id, key) pair for a realm: the fixed one at @simulator."""
     if realm == REALMS['simulator']:
         return SIM_CREDENTIAL
-    return os.environ.get('B2_APPLICATION_KEY_ID'), os.environ.get('B2_APPLICATION_KEY')
+    raise AssertionError('conformance checks use only the fixed simulator credential')
 
 
 class Amber(Exception):
@@ -284,7 +285,11 @@ def run(target: str) -> None:
 
 
 def main() -> int:
-    target = os.environ.get('CONFORMANCE_TARGET', 'staging')
+    target = os.environ.get('CONFORMANCE_TARGET', 'simulator')
+    refused = refusal('conformance')
+    if refused:
+        say(target, f'FAIL ({refused})')
+        return 1
     try:
         run(target)
     except Amber as amber:
diff --git a/.sdkharness/tests/conformance/enc.sse_c b/.sdkharness/tests/conformance/enc.sse_c
index 11a15409..faf10bf3 100755
--- a/.sdkharness/tests/conformance/enc.sse_c
+++ b/.sdkharness/tests/conformance/enc.sse_c
@@ -54,6 +54,9 @@ import os
 import sys
 import uuid
 
+sys.path.insert(0, os.path.join(os.path.dirname(os.path.abspath(__file__)), '..', 'lib'))
+from loopback_guard import refusal  # noqa: E402
+
 SLUG = 'b2-sdk-python'
 CAPABILITY = 'enc.sse_c'
 OBJECT_NAME = 'st/sse-c.txt'
@@ -75,8 +78,6 @@ REASONS = (
 # own server (bin/simulator/serve.mjs), whose URL the runner exports as
 # CONFORMANCE_SIMULATOR_URL; unset, there is nothing to reach: no-realm-option.
 REALMS = {
-    'staging': 'staging',
-    'production': 'production',
     'simulator': os.environ.get('CONFORMANCE_SIMULATOR_URL') or None,
 }
 
@@ -90,7 +91,7 @@ def credential(realm):
     """The (key id, key) pair for a realm: the fixed one at @simulator."""
     if realm == REALMS['simulator']:
         return SIM_CREDENTIAL
-    return os.environ.get('B2_APPLICATION_KEY_ID'), os.environ.get('B2_APPLICATION_KEY')
+    raise AssertionError('conformance checks use only the fixed simulator credential')
 
 
 class Amber(Exception):
@@ -355,7 +356,11 @@ def run(target: str) -> None:
 
 
 def main() -> int:
-    target = os.environ.get('CONFORMANCE_TARGET', 'staging')
+    target = os.environ.get('CONFORMANCE_TARGET', 'simulator')
+    refused = refusal('conformance')
+    if refused:
+        say(target, f'FAIL ({refused})')
+        return 1
     try:
         run(target)
     except Amber as amber:
diff --git a/.sdkharness/tests/conformance/files.delete_version b/.sdkharness/tests/conformance/files.delete_version
index f9147a9d..17a9f536 100755
--- a/.sdkharness/tests/conformance/files.delete_version
+++ b/.sdkharness/tests/conformance/files.delete_version
@@ -27,6 +27,9 @@ import os
 import sys
 import uuid
 
+sys.path.insert(0, os.path.join(os.path.dirname(os.path.abspath(__file__)), '..', 'lib'))
+from loopback_guard import refusal  # noqa: E402
+
 SLUG = 'b2-sdk-python'
 CAPABILITY = 'files.delete_version'
 PREFIX = 'st/'
@@ -48,8 +51,6 @@ REASONS = (
 # own server (bin/simulator/serve.mjs), whose URL the runner exports as
 # CONFORMANCE_SIMULATOR_URL; unset, there is nothing to reach: no-realm-option.
 REALMS = {
-    'staging': 'staging',
-    'production': 'production',
     'simulator': os.environ.get('CONFORMANCE_SIMULATOR_URL') or None,
 }
 
@@ -63,7 +64,7 @@ def credential(realm):
     """The (key id, key) pair for a realm: the fixed one at @simulator."""
     if realm == REALMS['simulator']:
         return SIM_CREDENTIAL
-    return os.environ.get('B2_APPLICATION_KEY_ID'), os.environ.get('B2_APPLICATION_KEY')
+    raise AssertionError('conformance checks use only the fixed simulator credential')
 
 
 class Amber(Exception):
@@ -270,7 +271,11 @@ def run(target: str) -> None:
 
 
 def main() -> int:
-    target = os.environ.get('CONFORMANCE_TARGET', 'staging')
+    target = os.environ.get('CONFORMANCE_TARGET', 'simulator')
+    refused = refusal('conformance')
+    if refused:
+        say(target, f'FAIL ({refused})')
+        return 1
     try:
         run(target)
     except Amber as amber:
diff --git a/.sdkharness/tests/conformance/files.download_by_id b/.sdkharness/tests/conformance/files.download_by_id
index 67b0aed9..ddc2ab60 100755
--- a/.sdkharness/tests/conformance/files.download_by_id
+++ b/.sdkharness/tests/conformance/files.download_by_id
@@ -27,6 +27,9 @@ import os
 import sys
 import uuid
 
+sys.path.insert(0, os.path.join(os.path.dirname(os.path.abspath(__file__)), '..', 'lib'))
+from loopback_guard import refusal  # noqa: E402
+
 SLUG = 'b2-sdk-python'
 CAPABILITY = 'files.download_by_id'
 OBJECT_NAME = 'st/by-id.txt'
@@ -47,8 +50,6 @@ REASONS = (
 # own server (bin/simulator/serve.mjs), whose URL the runner exports as
 # CONFORMANCE_SIMULATOR_URL; unset, there is nothing to reach: no-realm-option.
 REALMS = {
-    'staging': 'staging',
-    'production': 'production',
     'simulator': os.environ.get('CONFORMANCE_SIMULATOR_URL') or None,
 }
 
@@ -62,7 +63,7 @@ def credential(realm):
     """The (key id, key) pair for a realm: the fixed one at @simulator."""
     if realm == REALMS['simulator']:
         return SIM_CREDENTIAL
-    return os.environ.get('B2_APPLICATION_KEY_ID'), os.environ.get('B2_APPLICATION_KEY')
+    raise AssertionError('conformance checks use only the fixed simulator credential')
 
 
 class Amber(Exception):
@@ -269,7 +270,11 @@ def run(target: str) -> None:
 
 
 def main() -> int:
-    target = os.environ.get('CONFORMANCE_TARGET', 'staging')
+    target = os.environ.get('CONFORMANCE_TARGET', 'simulator')
+    refused = refusal('conformance')
+    if refused:
+        say(target, f'FAIL ({refused})')
+        return 1
     try:
         run(target)
     except Amber as amber:
diff --git a/.sdkharness/tests/conformance/files.download_content b/.sdkharness/tests/conformance/files.download_content
index e77b7acd..eb98001a 100755
--- a/.sdkharness/tests/conformance/files.download_content
+++ b/.sdkharness/tests/conformance/files.download_content
@@ -27,6 +27,9 @@ import os
 import sys
 import uuid
 
+sys.path.insert(0, os.path.join(os.path.dirname(os.path.abspath(__file__)), '..', 'lib'))
+from loopback_guard import refusal  # noqa: E402
+
 SLUG = 'b2-sdk-python'
 CAPABILITY = 'files.download_content'
 OBJECT_NAME = 'st/download.txt'
@@ -47,8 +50,6 @@ REASONS = (
 # own server (bin/simulator/serve.mjs), whose URL the runner exports as
 # CONFORMANCE_SIMULATOR_URL; unset, there is nothing to reach: no-realm-option.
 REALMS = {
-    'staging': 'staging',
-    'production': 'production',
     'simulator': os.environ.get('CONFORMANCE_SIMULATOR_URL') or None,
 }
 
@@ -62,7 +63,7 @@ def credential(realm):
     """The (key id, key) pair for a realm: the fixed one at @simulator."""
     if realm == REALMS['simulator']:
         return SIM_CREDENTIAL
-    return os.environ.get('B2_APPLICATION_KEY_ID'), os.environ.get('B2_APPLICATION_KEY')
+    raise AssertionError('conformance checks use only the fixed simulator credential')
 
 
 class Amber(Exception):
@@ -268,7 +269,11 @@ def run(target: str) -> None:
 
 
 def main() -> int:
-    target = os.environ.get('CONFORMANCE_TARGET', 'staging')
+    target = os.environ.get('CONFORMANCE_TARGET', 'simulator')
+    refused = refusal('conformance')
+    if refused:
+        say(target, f'FAIL ({refused})')
+        return 1
     try:
         run(target)
     except Amber as amber:
diff --git a/.sdkharness/tests/conformance/files.hide b/.sdkharness/tests/conformance/files.hide
index 542ded10..171cf366 100755
--- a/.sdkharness/tests/conformance/files.hide
+++ b/.sdkharness/tests/conformance/files.hide
@@ -33,6 +33,9 @@ import os
 import sys
 import uuid
 
+sys.path.insert(0, os.path.join(os.path.dirname(os.path.abspath(__file__)), '..', 'lib'))
+from loopback_guard import refusal  # noqa: E402
+
 SLUG = 'b2-sdk-python'
 CAPABILITY = 'files.hide'
 PREFIX = 'st/'
@@ -54,8 +57,6 @@ REASONS = (
 # own server (bin/simulator/serve.mjs), whose URL the runner exports as
 # CONFORMANCE_SIMULATOR_URL; unset, there is nothing to reach: no-realm-option.
 REALMS = {
-    'staging': 'staging',
-    'production': 'production',
     'simulator': os.environ.get('CONFORMANCE_SIMULATOR_URL') or None,
 }
 
@@ -69,7 +70,7 @@ def credential(realm):
     """The (key id, key) pair for a realm: the fixed one at @simulator."""
     if realm == REALMS['simulator']:
         return SIM_CREDENTIAL
-    return os.environ.get('B2_APPLICATION_KEY_ID'), os.environ.get('B2_APPLICATION_KEY')
+    raise AssertionError('conformance checks use only the fixed simulator credential')
 
 
 class Amber(Exception):
@@ -294,7 +295,11 @@ def run(target: str) -> None:
 
 
 def main() -> int:
-    target = os.environ.get('CONFORMANCE_TARGET', 'staging')
+    target = os.environ.get('CONFORMANCE_TARGET', 'simulator')
+    refused = refusal('conformance')
+    if refused:
+        say(target, f'FAIL ({refused})')
+        return 1
     try:
         run(target)
     except Amber as amber:
diff --git a/.sdkharness/tests/conformance/files.list b/.sdkharness/tests/conformance/files.list
index 0fa637fe..2bcb8a32 100755
--- a/.sdkharness/tests/conformance/files.list
+++ b/.sdkharness/tests/conformance/files.list
@@ -29,6 +29,9 @@ import os
 import sys
 import uuid
 
+sys.path.insert(0, os.path.join(os.path.dirname(os.path.abspath(__file__)), '..', 'lib'))
+from loopback_guard import refusal  # noqa: E402
+
 SLUG = 'b2-sdk-python'
 CAPABILITY = 'files.list'
 PREFIX = 'st/list/'
@@ -51,8 +54,6 @@ REASONS = (
 # own server (bin/simulator/serve.mjs), whose URL the runner exports as
 # CONFORMANCE_SIMULATOR_URL; unset, there is nothing to reach: no-realm-option.
 REALMS = {
-    'staging': 'staging',
-    'production': 'production',
     'simulator': os.environ.get('CONFORMANCE_SIMULATOR_URL') or None,
 }
 
@@ -66,7 +67,7 @@ def credential(realm):
     """The (key id, key) pair for a realm: the fixed one at @simulator."""
     if realm == REALMS['simulator']:
         return SIM_CREDENTIAL
-    return os.environ.get('B2_APPLICATION_KEY_ID'), os.environ.get('B2_APPLICATION_KEY')
+    raise AssertionError('conformance checks use only the fixed simulator credential')
 
 
 class Amber(Exception):
@@ -286,7 +287,11 @@ def run(target: str) -> None:
 
 
 def main() -> int:
-    target = os.environ.get('CONFORMANCE_TARGET', 'staging')
+    target = os.environ.get('CONFORMANCE_TARGET', 'simulator')
+    refused = refusal('conformance')
+    if refused:
+        say(target, f'FAIL ({refused})')
+        return 1
     try:
         run(target)
     except Amber as amber:
diff --git a/.sdkharness/tests/conformance/files.metadata b/.sdkharness/tests/conformance/files.metadata
index 5ac29460..050e00f7 100755
--- a/.sdkharness/tests/conformance/files.metadata
+++ b/.sdkharness/tests/conformance/files.metadata
@@ -33,6 +33,9 @@ import os
 import sys
 import uuid
 
+sys.path.insert(0, os.path.join(os.path.dirname(os.path.abspath(__file__)), '..', 'lib'))
+from loopback_guard import refusal  # noqa: E402
+
 SLUG = 'b2-sdk-python'
 CAPABILITY = 'files.metadata'
 OBJECT_NAME = 'st/metadata.txt'
@@ -56,8 +59,6 @@ REASONS = (
 # own server (bin/simulator/serve.mjs), whose URL the runner exports as
 # CONFORMANCE_SIMULATOR_URL; unset, there is nothing to reach: no-realm-option.
 REALMS = {
-    'staging': 'staging',
-    'production': 'production',
     'simulator': os.environ.get('CONFORMANCE_SIMULATOR_URL') or None,
 }
 
@@ -71,7 +72,7 @@ def credential(realm):
     """The (key id, key) pair for a realm: the fixed one at @simulator."""
     if realm == REALMS['simulator']:
         return SIM_CREDENTIAL
-    return os.environ.get('B2_APPLICATION_KEY_ID'), os.environ.get('B2_APPLICATION_KEY')
+    raise AssertionError('conformance checks use only the fixed simulator credential')
 
 
 class Amber(Exception):
@@ -305,7 +306,11 @@ def run(target: str) -> None:
 
 
 def main() -> int:
-    target = os.environ.get('CONFORMANCE_TARGET', 'staging')
+    target = os.environ.get('CONFORMANCE_TARGET', 'simulator')
+    refused = refusal('conformance')
+    if refused:
+        say(target, f'FAIL ({refused})')
+        return 1
     try:
         run(target)
     except Amber as amber:
diff --git a/.sdkharness/tests/conformance/files.server_side_copy b/.sdkharness/tests/conformance/files.server_side_copy
index 48618d59..8e9d1509 100755
--- a/.sdkharness/tests/conformance/files.server_side_copy
+++ b/.sdkharness/tests/conformance/files.server_side_copy
@@ -28,6 +28,9 @@ import os
 import sys
 import uuid
 
+sys.path.insert(0, os.path.join(os.path.dirname(os.path.abspath(__file__)), '..', 'lib'))
+from loopback_guard import refusal  # noqa: E402
+
 SLUG = 'b2-sdk-python'
 CAPABILITY = 'files.server_side_copy'
 SOURCE_NAME = 'st/copy-source.bin'
@@ -54,8 +57,6 @@ REASONS = (
 # own server (bin/simulator/serve.mjs), whose URL the runner exports as
 # CONFORMANCE_SIMULATOR_URL; unset, there is nothing to reach: no-realm-option.
 REALMS = {
-    'staging': 'staging',
-    'production': 'production',
     'simulator': os.environ.get('CONFORMANCE_SIMULATOR_URL') or None,
 }
 
@@ -69,7 +70,7 @@ def credential(realm):
     """The (key id, key) pair for a realm: the fixed one at @simulator."""
     if realm == REALMS['simulator']:
         return SIM_CREDENTIAL
-    return os.environ.get('B2_APPLICATION_KEY_ID'), os.environ.get('B2_APPLICATION_KEY')
+    raise AssertionError('conformance checks use only the fixed simulator credential')
 
 
 class Amber(Exception):
@@ -288,7 +289,11 @@ def run(target: str) -> None:
 
 
 def main() -> int:
-    target = os.environ.get('CONFORMANCE_TARGET', 'staging')
+    target = os.environ.get('CONFORMANCE_TARGET', 'simulator')
+    refused = refusal('conformance')
+    if refused:
+        say(target, f'FAIL ({refused})')
+        return 1
     try:
         run(target)
     except Amber as amber:
diff --git a/.sdkharness/tests/conformance/files.upload b/.sdkharness/tests/conformance/files.upload
index c6691e2a..574451bc 100755
--- a/.sdkharness/tests/conformance/files.upload
+++ b/.sdkharness/tests/conformance/files.upload
@@ -33,6 +33,9 @@ import os
 import sys
 import uuid
 
+sys.path.insert(0, os.path.join(os.path.dirname(os.path.abspath(__file__)), '..', 'lib'))
+from loopback_guard import refusal  # noqa: E402
+
 SLUG = 'b2-sdk-python'
 CAPABILITY = 'files.upload'
 OBJECT_NAME = 'st/upload.txt'
@@ -53,8 +56,6 @@ REASONS = (
 # own server (bin/simulator/serve.mjs), whose URL the runner exports as
 # CONFORMANCE_SIMULATOR_URL; unset, there is nothing to reach: no-realm-option.
 REALMS = {
-    'staging': 'staging',
-    'production': 'production',
     'simulator': os.environ.get('CONFORMANCE_SIMULATOR_URL') or None,
 }
 
@@ -114,13 +115,7 @@ def run(target: str) -> None:
         )
         raise Amber('no-realm-option', detail)
 
-    if target == 'simulator':
-        key_id, key = SIM_CREDENTIAL
-    else:
-        key_id = os.environ.get('B2_APPLICATION_KEY_ID')
-        key = os.environ.get('B2_APPLICATION_KEY')
-        if not key_id or not key:
-            raise Amber('no-credential', 'B2_APPLICATION_KEY_ID / B2_APPLICATION_KEY unset')
+    key_id, key = SIM_CREDENTIAL
 
     try:
         from b2sdk.v3 import B2Api, InMemoryAccountInfo
@@ -174,7 +169,11 @@ def run(target: str) -> None:
 
 
 def main() -> int:
-    target = os.environ.get('CONFORMANCE_TARGET', 'staging')
+    target = os.environ.get('CONFORMANCE_TARGET', 'simulator')
+    refused = refusal('conformance')
+    if refused:
+        say(target, f'FAIL ({refused})')
+        return 1
     try:
         run(target)
     except Amber as amber:
diff --git a/.sdkharness/tests/conformance/keys.crud b/.sdkharness/tests/conformance/keys.crud
index fd5299d9..69a510d9 100755
--- a/.sdkharness/tests/conformance/keys.crud
+++ b/.sdkharness/tests/conformance/keys.crud
@@ -30,6 +30,9 @@ import os
 import sys
 import uuid
 
+sys.path.insert(0, os.path.join(os.path.dirname(os.path.abspath(__file__)), '..', 'lib'))
+from loopback_guard import refusal  # noqa: E402
+
 SLUG = 'b2-sdk-python'
 CAPABILITY = 'keys.crud'
 CAPABILITIES = ['listBuckets', 'listFiles', 'readFiles']
@@ -49,8 +52,6 @@ REASONS = (
 # own server (bin/simulator/serve.mjs), whose URL the runner exports as
 # CONFORMANCE_SIMULATOR_URL; unset, there is nothing to reach: no-realm-option.
 REALMS = {
-    'staging': 'staging',
-    'production': 'production',
     'simulator': os.environ.get('CONFORMANCE_SIMULATOR_URL') or None,
 }
 
@@ -64,7 +65,7 @@ def credential(realm):
     """The (key id, key) pair for a realm: the fixed one at @simulator."""
     if realm == REALMS['simulator']:
         return SIM_CREDENTIAL
-    return os.environ.get('B2_APPLICATION_KEY_ID'), os.environ.get('B2_APPLICATION_KEY')
+    raise AssertionError('conformance checks use only the fixed simulator credential')
 
 
 class Amber(Exception):
@@ -280,7 +281,11 @@ def run(target: str) -> None:
 
 
 def main() -> int:
-    target = os.environ.get('CONFORMANCE_TARGET', 'staging')
+    target = os.environ.get('CONFORMANCE_TARGET', 'simulator')
+    refused = refusal('conformance')
+    if refused:
+        say(target, f'FAIL ({refused})')
+        return 1
     try:
         run(target)
     except Amber as amber:
diff --git a/.sdkharness/tests/conformance/keys.multi_bucket b/.sdkharness/tests/conformance/keys.multi_bucket
index 988d4018..fe34963c 100755
--- a/.sdkharness/tests/conformance/keys.multi_bucket
+++ b/.sdkharness/tests/conformance/keys.multi_bucket
@@ -29,6 +29,9 @@ import os
 import sys
 import uuid
 
+sys.path.insert(0, os.path.join(os.path.dirname(os.path.abspath(__file__)), '..', 'lib'))
+from loopback_guard import refusal  # noqa: E402
+
 SLUG = 'b2-sdk-python'
 CAPABILITY = 'keys.multi_bucket'
 CAPABILITIES = ['listBuckets', 'listFiles', 'readFiles']
@@ -48,8 +51,6 @@ REASONS = (
 # own server (bin/simulator/serve.mjs), whose URL the runner exports as
 # CONFORMANCE_SIMULATOR_URL; unset, there is nothing to reach: no-realm-option.
 REALMS = {
-    'staging': 'staging',
-    'production': 'production',
     'simulator': os.environ.get('CONFORMANCE_SIMULATOR_URL') or None,
 }
 
@@ -63,7 +64,7 @@ def credential(realm):
     """The (key id, key) pair for a realm: the fixed one at @simulator."""
     if realm == REALMS['simulator']:
         return SIM_CREDENTIAL
-    return os.environ.get('B2_APPLICATION_KEY_ID'), os.environ.get('B2_APPLICATION_KEY')
+    raise AssertionError('conformance checks use only the fixed simulator credential')
 
 
 class Amber(Exception):
@@ -309,7 +310,11 @@ def run(target: str) -> None:
 
 
 def main() -> int:
-    target = os.environ.get('CONFORMANCE_TARGET', 'staging')
+    target = os.environ.get('CONFORMANCE_TARGET', 'simulator')
+    refused = refusal('conformance')
+    if refused:
+        say(target, f'FAIL ({refused})')
+        return 1
     try:
         run(target)
     except Amber as amber:
diff --git a/.sdkharness/tests/conformance/large.compose_mixed b/.sdkharness/tests/conformance/large.compose_mixed
index cbd9eae9..e09fb88d 100755
--- a/.sdkharness/tests/conformance/large.compose_mixed
+++ b/.sdkharness/tests/conformance/large.compose_mixed
@@ -35,6 +35,9 @@ import sys
 import tempfile
 import uuid
 
+sys.path.insert(0, os.path.join(os.path.dirname(os.path.abspath(__file__)), '..', 'lib'))
+from loopback_guard import refusal  # noqa: E402
+
 SLUG = 'b2-sdk-python'
 CAPABILITY = 'large.compose_mixed'
 LOCAL_SOURCE = 'st/compose-local-source.bin'
@@ -57,8 +60,6 @@ REASONS = (
 # own server (bin/simulator/serve.mjs), whose URL the runner exports as
 # CONFORMANCE_SIMULATOR_URL; unset, there is nothing to reach: no-realm-option.
 REALMS = {
-    'staging': 'staging',
-    'production': 'production',
     'simulator': os.environ.get('CONFORMANCE_SIMULATOR_URL') or None,
 }
 
@@ -72,7 +73,7 @@ def credential(realm):
     """The (key id, key) pair for a realm: the fixed one at @simulator."""
     if realm == REALMS['simulator']:
         return SIM_CREDENTIAL
-    return os.environ.get('B2_APPLICATION_KEY_ID'), os.environ.get('B2_APPLICATION_KEY')
+    raise AssertionError('conformance checks use only the fixed simulator credential')
 
 
 class Amber(Exception):
@@ -318,7 +319,11 @@ def run(target: str) -> None:
 
 
 def main() -> int:
-    target = os.environ.get('CONFORMANCE_TARGET', 'staging')
+    target = os.environ.get('CONFORMANCE_TARGET', 'simulator')
+    refused = refusal('conformance')
+    if refused:
+        say(target, f'FAIL ({refused})')
+        return 1
     try:
         run(target)
     except Amber as amber:
diff --git a/.sdkharness/tests/conformance/large.concurrent_parts b/.sdkharness/tests/conformance/large.concurrent_parts
index 6151f73a..b0bce90d 100755
--- a/.sdkharness/tests/conformance/large.concurrent_parts
+++ b/.sdkharness/tests/conformance/large.concurrent_parts
@@ -38,6 +38,9 @@ import threading
 import time
 import uuid
 
+sys.path.insert(0, os.path.join(os.path.dirname(os.path.abspath(__file__)), '..', 'lib'))
+from loopback_guard import refusal  # noqa: E402
+
 SLUG = 'b2-sdk-python'
 CAPABILITY = 'large.concurrent_parts'
 OBJECT_SERIAL = 'st/concurrent-serial.bin'
@@ -61,8 +64,6 @@ REASONS = (
 # own server (bin/simulator/serve.mjs), whose URL the runner exports as
 # CONFORMANCE_SIMULATOR_URL; unset, there is nothing to reach: no-realm-option.
 REALMS = {
-    'staging': 'staging',
-    'production': 'production',
     'simulator': os.environ.get('CONFORMANCE_SIMULATOR_URL') or None,
 }
 
@@ -76,7 +77,7 @@ def credential(realm):
     """The (key id, key) pair for a realm: the fixed one at @simulator."""
     if realm == REALMS['simulator']:
         return SIM_CREDENTIAL
-    return os.environ.get('B2_APPLICATION_KEY_ID'), os.environ.get('B2_APPLICATION_KEY')
+    raise AssertionError('conformance checks use only the fixed simulator credential')
 
 
 class Amber(Exception):
@@ -354,7 +355,11 @@ def run(target: str) -> None:
 
 
 def main() -> int:
-    target = os.environ.get('CONFORMANCE_TARGET', 'staging')
+    target = os.environ.get('CONFORMANCE_TARGET', 'simulator')
+    refused = refusal('conformance')
+    if refused:
+        say(target, f'FAIL ({refused})')
+        return 1
     try:
         run(target)
     except Amber as amber:
diff --git a/.sdkharness/tests/conformance/large.multipart b/.sdkharness/tests/conformance/large.multipart
index 253dd948..2ee9bf3e 100755
--- a/.sdkharness/tests/conformance/large.multipart
+++ b/.sdkharness/tests/conformance/large.multipart
@@ -36,6 +36,9 @@ import os
 import sys
 import uuid
 
+sys.path.insert(0, os.path.join(os.path.dirname(os.path.abspath(__file__)), '..', 'lib'))
+from loopback_guard import refusal  # noqa: E402
+
 SLUG = 'b2-sdk-python'
 CAPABILITY = 'large.multipart'
 OBJECT_NAME = 'st/multipart.bin'
@@ -56,8 +59,6 @@ REASONS = (
 # own server (bin/simulator/serve.mjs), whose URL the runner exports as
 # CONFORMANCE_SIMULATOR_URL; unset, there is nothing to reach: no-realm-option.
 REALMS = {
-    'staging': 'staging',
-    'production': 'production',
     'simulator': os.environ.get('CONFORMANCE_SIMULATOR_URL') or None,
 }
 
@@ -71,7 +72,7 @@ def credential(realm):
     """The (key id, key) pair for a realm: the fixed one at @simulator."""
     if realm == REALMS['simulator']:
         return SIM_CREDENTIAL
-    return os.environ.get('B2_APPLICATION_KEY_ID'), os.environ.get('B2_APPLICATION_KEY')
+    raise AssertionError('conformance checks use only the fixed simulator credential')
 
 
 class Amber(Exception):
@@ -342,7 +343,11 @@ def run(target: str) -> None:
 
 
 def main() -> int:
-    target = os.environ.get('CONFORMANCE_TARGET', 'staging')
+    target = os.environ.get('CONFORMANCE_TARGET', 'simulator')
+    refused = refusal('conformance')
+    if refused:
+        say(target, f'FAIL ({refused})')
+        return 1
     try:
         run(target)
     except Amber as amber:
diff --git a/.sdkharness/tests/conformance/large.parallel_download b/.sdkharness/tests/conformance/large.parallel_download
index 554050d7..1df48e88 100755
--- a/.sdkharness/tests/conformance/large.parallel_download
+++ b/.sdkharness/tests/conformance/large.parallel_download
@@ -43,6 +43,9 @@ import threading
 import time
 import uuid
 
+sys.path.insert(0, os.path.join(os.path.dirname(os.path.abspath(__file__)), '..', 'lib'))
+from loopback_guard import refusal  # noqa: E402
+
 SLUG = 'b2-sdk-python'
 CAPABILITY = 'large.parallel_download'
 OBJECT_NAME = 'st/parallel-download.bin'
@@ -65,8 +68,6 @@ REASONS = (
 # own server (bin/simulator/serve.mjs), whose URL the runner exports as
 # CONFORMANCE_SIMULATOR_URL; unset, there is nothing to reach: no-realm-option.
 REALMS = {
-    'staging': 'staging',
-    'production': 'production',
     'simulator': os.environ.get('CONFORMANCE_SIMULATOR_URL') or None,
 }
 
@@ -80,7 +81,7 @@ def credential(realm):
     """The (key id, key) pair for a realm: the fixed one at @simulator."""
     if realm == REALMS['simulator']:
         return SIM_CREDENTIAL
-    return os.environ.get('B2_APPLICATION_KEY_ID'), os.environ.get('B2_APPLICATION_KEY')
+    raise AssertionError('conformance checks use only the fixed simulator credential')
 
 
 class Amber(Exception):
@@ -361,7 +362,11 @@ def run(target: str) -> None:
 
 
 def main() -> int:
-    target = os.environ.get('CONFORMANCE_TARGET', 'staging')
+    target = os.environ.get('CONFORMANCE_TARGET', 'simulator')
+    refused = refusal('conformance')
+    if refused:
+        say(target, f'FAIL ({refused})')
+        return 1
     try:
         run(target)
     except Amber as amber:
diff --git a/.sdkharness/tests/conformance/large.resume b/.sdkharness/tests/conformance/large.resume
index aaba4f4b..3ea0559e 100755
--- a/.sdkharness/tests/conformance/large.resume
+++ b/.sdkharness/tests/conformance/large.resume
@@ -39,6 +39,9 @@ import sys
 import tempfile
 import uuid
 
+sys.path.insert(0, os.path.join(os.path.dirname(os.path.abspath(__file__)), '..', 'lib'))
+from loopback_guard import refusal  # noqa: E402
+
 SLUG = 'b2-sdk-python'
 CAPABILITY = 'large.resume'
 OBJECT_NAME = 'st/resume.bin'
@@ -60,8 +63,6 @@ REASONS = (
 # own server (bin/simulator/serve.mjs), whose URL the runner exports as
 # CONFORMANCE_SIMULATOR_URL; unset, there is nothing to reach: no-realm-option.
 REALMS = {
-    'staging': 'staging',
-    'production': 'production',
     'simulator': os.environ.get('CONFORMANCE_SIMULATOR_URL') or None,
 }
 
@@ -75,7 +76,7 @@ def credential(realm):
     """The (key id, key) pair for a realm: the fixed one at @simulator."""
     if realm == REALMS['simulator']:
         return SIM_CREDENTIAL
-    return os.environ.get('B2_APPLICATION_KEY_ID'), os.environ.get('B2_APPLICATION_KEY')
+    raise AssertionError('conformance checks use only the fixed simulator credential')
 
 
 class Amber(Exception):
@@ -415,7 +416,11 @@ def _caused_by_interruption(error) -> bool:
 
 
 def main() -> int:
-    target = os.environ.get('CONFORMANCE_TARGET', 'staging')
+    target = os.environ.get('CONFORMANCE_TARGET', 'simulator')
+    refused = refusal('conformance')
+    if refused:
+        say(target, f'FAIL ({refused})')
+        return 1
     try:
         run(target)
     except Amber as amber:
diff --git a/.sdkharness/tests/conformance/large.unbound_incremental b/.sdkharness/tests/conformance/large.unbound_incremental
index 30abbfd7..32fcaa5a 100755
--- a/.sdkharness/tests/conformance/large.unbound_incremental
+++ b/.sdkharness/tests/conformance/large.unbound_incremental
@@ -46,6 +46,9 @@ import sys
 import tempfile
 import uuid
 
+sys.path.insert(0, os.path.join(os.path.dirname(os.path.abspath(__file__)), '..', 'lib'))
+from loopback_guard import refusal  # noqa: E402
+
 SLUG = 'b2-sdk-python'
 CAPABILITY = 'large.unbound_incremental'
 UNBOUND_NAME = 'st/unbound.bin'
@@ -68,8 +71,6 @@ REASONS = (
 # own server (bin/simulator/serve.mjs), whose URL the runner exports as
 # CONFORMANCE_SIMULATOR_URL; unset, there is nothing to reach: no-realm-option.
 REALMS = {
-    'staging': 'staging',
-    'production': 'production',
     'simulator': os.environ.get('CONFORMANCE_SIMULATOR_URL') or None,
 }
 
@@ -83,7 +84,7 @@ def credential(realm):
     """The (key id, key) pair for a realm: the fixed one at @simulator."""
     if realm == REALMS['simulator']:
         return SIM_CREDENTIAL
-    return os.environ.get('B2_APPLICATION_KEY_ID'), os.environ.get('B2_APPLICATION_KEY')
+    raise AssertionError('conformance checks use only the fixed simulator credential')
 
 
 class Amber(Exception):
@@ -414,7 +415,11 @@ def run(target: str) -> None:
 
 
 def main() -> int:
-    target = os.environ.get('CONFORMANCE_TARGET', 'staging')
+    target = os.environ.get('CONFORMANCE_TARGET', 'simulator')
+    refused = refusal('conformance')
+    if refused:
+        say(target, f'FAIL ({refused})')
+        return 1
     try:
         run(target)
     except Amber as amber:
diff --git a/.sdkharness/tests/conformance/lock.bucket_default b/.sdkharness/tests/conformance/lock.bucket_default
index 8c6743d1..facb2590 100755
--- a/.sdkharness/tests/conformance/lock.bucket_default
+++ b/.sdkharness/tests/conformance/lock.bucket_default
@@ -29,6 +29,9 @@ import sys
 import time
 import uuid
 
+sys.path.insert(0, os.path.join(os.path.dirname(os.path.abspath(__file__)), '..', 'lib'))
+from loopback_guard import refusal  # noqa: E402
+
 SLUG = 'b2-sdk-python'
 CAPABILITY = 'lock.bucket_default'
 OBJECT_NAME = 'st/default-retention.txt'
@@ -51,8 +54,6 @@ REASONS = (
 # own server (bin/simulator/serve.mjs), whose URL the runner exports as
 # CONFORMANCE_SIMULATOR_URL; unset, there is nothing to reach: no-realm-option.
 REALMS = {
-    'staging': 'staging',
-    'production': 'production',
     'simulator': os.environ.get('CONFORMANCE_SIMULATOR_URL') or None,
 }
 
@@ -66,7 +67,7 @@ def credential(realm):
     """The (key id, key) pair for a realm: the fixed one at @simulator."""
     if realm == REALMS['simulator']:
         return SIM_CREDENTIAL
-    return os.environ.get('B2_APPLICATION_KEY_ID'), os.environ.get('B2_APPLICATION_KEY')
+    raise AssertionError('conformance checks use only the fixed simulator credential')
 
 
 class Amber(Exception):
@@ -293,7 +294,11 @@ def run(target: str) -> None:
 
 
 def main() -> int:
-    target = os.environ.get('CONFORMANCE_TARGET', 'staging')
+    target = os.environ.get('CONFORMANCE_TARGET', 'simulator')
+    refused = refusal('conformance')
+    if refused:
+        say(target, f'FAIL ({refused})')
+        return 1
     try:
         run(target)
     except Amber as amber:
diff --git a/.sdkharness/tests/conformance/lock.bypass_governance b/.sdkharness/tests/conformance/lock.bypass_governance
index d386ee37..257777a9 100755
--- a/.sdkharness/tests/conformance/lock.bypass_governance
+++ b/.sdkharness/tests/conformance/lock.bypass_governance
@@ -29,6 +29,9 @@ import sys
 import time
 import uuid
 
+sys.path.insert(0, os.path.join(os.path.dirname(os.path.abspath(__file__)), '..', 'lib'))
+from loopback_guard import refusal  # noqa: E402
+
 SLUG = 'b2-sdk-python'
 CAPABILITY = 'lock.bypass_governance'
 OBJECT_NAME = 'st/bypass-governance.txt'
@@ -50,8 +53,6 @@ REASONS = (
 # own server (bin/simulator/serve.mjs), whose URL the runner exports as
 # CONFORMANCE_SIMULATOR_URL; unset, there is nothing to reach: no-realm-option.
 REALMS = {
-    'staging': 'staging',
-    'production': 'production',
     'simulator': os.environ.get('CONFORMANCE_SIMULATOR_URL') or None,
 }
 
@@ -65,7 +66,7 @@ def credential(realm):
     """The (key id, key) pair for a realm: the fixed one at @simulator."""
     if realm == REALMS['simulator']:
         return SIM_CREDENTIAL
-    return os.environ.get('B2_APPLICATION_KEY_ID'), os.environ.get('B2_APPLICATION_KEY')
+    raise AssertionError('conformance checks use only the fixed simulator credential')
 
 
 class Amber(Exception):
@@ -290,7 +291,11 @@ def run(target: str) -> None:
 
 
 def main() -> int:
-    target = os.environ.get('CONFORMANCE_TARGET', 'staging')
+    target = os.environ.get('CONFORMANCE_TARGET', 'simulator')
+    refused = refusal('conformance')
+    if refused:
+        say(target, f'FAIL ({refused})')
+        return 1
     try:
         run(target)
     except Amber as amber:
diff --git a/.sdkharness/tests/conformance/lock.legal_hold b/.sdkharness/tests/conformance/lock.legal_hold
index 7f179e8c..af39e7fc 100755
--- a/.sdkharness/tests/conformance/lock.legal_hold
+++ b/.sdkharness/tests/conformance/lock.legal_hold
@@ -28,6 +28,9 @@ import os
 import sys
 import uuid
 
+sys.path.insert(0, os.path.join(os.path.dirname(os.path.abspath(__file__)), '..', 'lib'))
+from loopback_guard import refusal  # noqa: E402
+
 SLUG = 'b2-sdk-python'
 CAPABILITY = 'lock.legal_hold'
 OBJECT_NAME = 'st/legal-hold.txt'
@@ -48,8 +51,6 @@ REASONS = (
 # own server (bin/simulator/serve.mjs), whose URL the runner exports as
 # CONFORMANCE_SIMULATOR_URL; unset, there is nothing to reach: no-realm-option.
 REALMS = {
-    'staging': 'staging',
-    'production': 'production',
     'simulator': os.environ.get('CONFORMANCE_SIMULATOR_URL') or None,
 }
 
@@ -63,7 +64,7 @@ def credential(realm):
     """The (key id, key) pair for a realm: the fixed one at @simulator."""
     if realm == REALMS['simulator']:
         return SIM_CREDENTIAL
-    return os.environ.get('B2_APPLICATION_KEY_ID'), os.environ.get('B2_APPLICATION_KEY')
+    raise AssertionError('conformance checks use only the fixed simulator credential')
 
 
 class Amber(Exception):
@@ -296,7 +297,11 @@ def run(target: str) -> None:
 
 
 def main() -> int:
-    target = os.environ.get('CONFORMANCE_TARGET', 'staging')
+    target = os.environ.get('CONFORMANCE_TARGET', 'simulator')
+    refused = refusal('conformance')
+    if refused:
+        say(target, f'FAIL ({refused})')
+        return 1
     try:
         run(target)
     except Amber as amber:
diff --git a/.sdkharness/tests/conformance/lock.per_file_retention b/.sdkharness/tests/conformance/lock.per_file_retention
index fa973ff1..d310bbd6 100755
--- a/.sdkharness/tests/conformance/lock.per_file_retention
+++ b/.sdkharness/tests/conformance/lock.per_file_retention
@@ -31,6 +31,9 @@ import sys
 import time
 import uuid
 
+sys.path.insert(0, os.path.join(os.path.dirname(os.path.abspath(__file__)), '..', 'lib'))
+from loopback_guard import refusal  # noqa: E402
+
 SLUG = 'b2-sdk-python'
 CAPABILITY = 'lock.per_file_retention'
 OBJECT_NAME = 'st/per-file-retention.txt'
@@ -52,8 +55,6 @@ REASONS = (
 # own server (bin/simulator/serve.mjs), whose URL the runner exports as
 # CONFORMANCE_SIMULATOR_URL; unset, there is nothing to reach: no-realm-option.
 REALMS = {
-    'staging': 'staging',
-    'production': 'production',
     'simulator': os.environ.get('CONFORMANCE_SIMULATOR_URL') or None,
 }
 
@@ -67,7 +68,7 @@ def credential(realm):
     """The (key id, key) pair for a realm: the fixed one at @simulator."""
     if realm == REALMS['simulator']:
         return SIM_CREDENTIAL
-    return os.environ.get('B2_APPLICATION_KEY_ID'), os.environ.get('B2_APPLICATION_KEY')
+    raise AssertionError('conformance checks use only the fixed simulator credential')
 
 
 class Amber(Exception):
@@ -283,7 +284,11 @@ def run(target: str) -> None:
 
 
 def main() -> int:
-    target = os.environ.get('CONFORMANCE_TARGET', 'staging')
+    target = os.environ.get('CONFORMANCE_TARGET', 'simulator')
+    refused = refusal('conformance')
+    if refused:
+        say(target, f'FAIL ({refused})')
+        return 1
     try:
         run(target)
     except Amber as amber:
diff --git a/.sdkharness/tests/conformance/urls.native_download b/.sdkharness/tests/conformance/urls.native_download
index 8cfd373c..282d2a02 100755
--- a/.sdkharness/tests/conformance/urls.native_download
+++ b/.sdkharness/tests/conformance/urls.native_download
@@ -40,6 +40,9 @@ import urllib.error
 import urllib.request
 import uuid
 
+sys.path.insert(0, os.path.join(os.path.dirname(os.path.abspath(__file__)), '..', 'lib'))
+from loopback_guard import refusal  # noqa: E402
+
 SLUG = 'b2-sdk-python'
 CAPABILITY = 'urls.native_download'
 PREFIX = 'st/'
@@ -63,8 +66,6 @@ REASONS = (
 # own server (bin/simulator/serve.mjs), whose URL the runner exports as
 # CONFORMANCE_SIMULATOR_URL; unset, there is nothing to reach: no-realm-option.
 REALMS = {
-    'staging': 'staging',
-    'production': 'production',
     'simulator': os.environ.get('CONFORMANCE_SIMULATOR_URL') or None,
 }
 
@@ -78,7 +79,7 @@ def credential(realm):
     """The (key id, key) pair for a realm: the fixed one at @simulator."""
     if realm == REALMS['simulator']:
         return SIM_CREDENTIAL
-    return os.environ.get('B2_APPLICATION_KEY_ID'), os.environ.get('B2_APPLICATION_KEY')
+    raise AssertionError('conformance checks use only the fixed simulator credential')
 
 
 class Amber(Exception):
@@ -327,7 +328,11 @@ def run(target: str) -> None:
 
 
 def main() -> int:
-    target = os.environ.get('CONFORMANCE_TARGET', 'staging')
+    target = os.environ.get('CONFORMANCE_TARGET', 'simulator')
+    refused = refusal('conformance')
+    if refused:
+        say(target, f'FAIL ({refused})')
+        return 1
     try:
         run(target)
     except Amber as amber:
diff --git a/.sdkharness/tests/health-golden-path b/.sdkharness/tests/health-golden-path
index bc625a58..9d150599 100755
--- a/.sdkharness/tests/health-golden-path
+++ b/.sdkharness/tests/health-golden-path
@@ -13,6 +13,7 @@ from urllib.parse import urlsplit
 
 LEVEL = 'health'
 SCENARIO = 'golden-path'
+SIMULATOR_CREDENTIAL = ('test-key-id', 'test-key')
 REPOSITORY_ROOT = Path(__file__).resolve().parents[2]
 
 
@@ -51,7 +52,7 @@ def validate_environment(environment: Mapping[str, str]) -> tuple[str, str, str,
         raise CheckFailure('configuration', 'invalid simulator URL') from error
     if (
         parsed.scheme != 'http'
-        or parsed.hostname not in {'127.0.0.1', '::1'}
+        or parsed.hostname != '127.0.0.1'
         or port is None
         or parsed.username is not None
         or parsed.password is not None
@@ -66,7 +67,12 @@ def validate_environment(environment: Mapping[str, str]) -> tuple[str, str, str,
     bucket_name = environment.get('B2_BUCKET_NAME', '')
     if not key_id or not application_key or not bucket_name:
         raise CheckFailure('configuration', 'required simulator input is missing')
-    return simulator_url, key_id, application_key, bucket_name
+    # Only the simulator's fixed test credential is ever used. A real key in the
+    # environment is refused before it can reach the SDK (and so a loopback
+    # listener); the supplied values are never echoed.
+    if (key_id, application_key) != SIMULATOR_CREDENTIAL:
+        raise CheckFailure('configuration', 'only the fixed simulator credential is accepted')
+    return simulator_url, SIMULATOR_CREDENTIAL[0], SIMULATOR_CREDENTIAL[1], bucket_name
 
 
 def repository_api_factory():
diff --git a/.sdkharness/tests/lib/loopback_guard.py b/.sdkharness/tests/lib/loopback_guard.py
new file mode 100644
index 00000000..2bd38bab
--- /dev/null
+++ b/.sdkharness/tests/lib/loopback_guard.py
@@ -0,0 +1,55 @@
+"""Refuse to run a repository-owned leaf check outside a loopback simulator.
+
+The dispatchers (`run-conformance`, `run-resilience`) already validate their
+invocation. The leaf checks are executable on their own, so each one calls
+`refusal()` first: a leaf that is run directly can only ever talk to a loopback
+simulator with the fixed test credential, never to staging or production with
+ambient `B2_*` credentials.
+"""
+
+from __future__ import annotations
+
+import os
+from urllib.parse import urlsplit
+
+
+def _is_loopback_origin(value: str, scheme: str) -> bool:
+    try:
+        parsed = urlsplit(value)
+        port = parsed.port
+    except ValueError:
+        return False
+    return (
+        parsed.scheme == scheme
+        and parsed.hostname == '127.0.0.1'
+        and port is not None
+        and parsed.username is None
+        and parsed.password is None
+        and parsed.path in {'', '/'}
+        and not parsed.query
+        and not parsed.fragment
+    )
+
+
+def refusal(level: str) -> str | None:
+    """The reason this leaf must not run here, or None when it is safe to run.
+
+    `level` is `conformance` or `resilience`. The environment is the one the
+    dispatcher builds: `<LEVEL>_TARGET`, `<LEVEL>_SIMULATOR_URL`, optionally
+    `<LEVEL>_SIMULATOR_HTTPS_URL`, and `RESILIENCE_CONTROL_URL` for resilience.
+    """
+    prefix = level.upper()
+    target = os.environ.get(f'{prefix}_TARGET', 'simulator')
+    if target != 'simulator':
+        return f'configuration -- only the simulator target is supported, got {target!r}'
+    hint = 'run it through .sdkharness/tests/run-' + level + ' with a loopback simulator'
+    if not _is_loopback_origin(os.environ.get(f'{prefix}_SIMULATOR_URL', ''), 'http'):
+        return f'configuration -- {prefix}_SIMULATOR_URL must be http://127.0.0.1:<port>; {hint}'
+    https_url = os.environ.get(f'{prefix}_SIMULATOR_HTTPS_URL', '')
+    if https_url and not _is_loopback_origin(https_url, 'https'):
+        return f'configuration -- {prefix}_SIMULATOR_HTTPS_URL must be https://127.0.0.1:<port>'
+    if level == 'resilience' and not _is_loopback_origin(
+        os.environ.get('RESILIENCE_CONTROL_URL', ''), 'http'
+    ):
+        return 'configuration -- RESILIENCE_CONTROL_URL must be http://127.0.0.1:<port>'
+    return None
diff --git a/.sdkharness/tests/resilience/api.backoff_503 b/.sdkharness/tests/resilience/api.backoff_503
index 40fc0e4e..680ae01f 100755
--- a/.sdkharness/tests/resilience/api.backoff_503
+++ b/.sdkharness/tests/resilience/api.backoff_503
@@ -28,6 +28,9 @@ import time
 import urllib.request
 import uuid
 
+sys.path.insert(0, os.path.join(os.path.dirname(os.path.abspath(__file__)), '..', 'lib'))
+from loopback_guard import refusal  # noqa: E402
+
 SLUG = 'b2-sdk-python'
 SCENARIO = 'api.backoff_503'
 OBJECT_NAME = 'res/backoff_503.txt'
@@ -155,6 +158,10 @@ def run() -> None:
 
 
 def main() -> int:
+    refused = refusal('resilience')
+    if refused:
+        say(f'FAIL ({refused})')
+        return 1
     try:
         run()
     except Amber as amber:
diff --git a/.sdkharness/tests/resilience/api.retry_after_429 b/.sdkharness/tests/resilience/api.retry_after_429
index 612bad3e..2bad7d67 100755
--- a/.sdkharness/tests/resilience/api.retry_after_429
+++ b/.sdkharness/tests/resilience/api.retry_after_429
@@ -27,6 +27,9 @@ import time
 import urllib.request
 import uuid
 
+sys.path.insert(0, os.path.join(os.path.dirname(os.path.abspath(__file__)), '..', 'lib'))
+from loopback_guard import refusal  # noqa: E402
+
 SLUG = 'b2-sdk-python'
 SCENARIO = 'api.retry_after_429'
 OBJECT_NAME = 'res/retry_after_429.txt'
@@ -159,6 +162,10 @@ def run() -> None:
 
 
 def main() -> int:
+    refused = refusal('resilience')
+    if refused:
+        say(f'FAIL ({refused})')
+        return 1
     try:
         run()
     except Amber as amber:
diff --git a/.sdkharness/tests/resilience/api.retry_after_503 b/.sdkharness/tests/resilience/api.retry_after_503
index fd1c9cc3..79e5037b 100755
--- a/.sdkharness/tests/resilience/api.retry_after_503
+++ b/.sdkharness/tests/resilience/api.retry_after_503
@@ -28,6 +28,9 @@ import time
 import urllib.request
 import uuid
 
+sys.path.insert(0, os.path.join(os.path.dirname(os.path.abspath(__file__)), '..', 'lib'))
+from loopback_guard import refusal  # noqa: E402
+
 SLUG = 'b2-sdk-python'
 SCENARIO = 'api.retry_after_503'
 OBJECT_NAME = 'res/retry_after_503.txt'
@@ -160,6 +163,10 @@ def run() -> None:
 
 
 def main() -> int:
+    refused = refusal('resilience')
+    if refused:
+        say(f'FAIL ({refused})')
+        return 1
     try:
         run()
     except Amber as amber:
diff --git a/.sdkharness/tests/resilience/auth.clock_expiry b/.sdkharness/tests/resilience/auth.clock_expiry
index fc25ec10..542035af 100755
--- a/.sdkharness/tests/resilience/auth.clock_expiry
+++ b/.sdkharness/tests/resilience/auth.clock_expiry
@@ -34,6 +34,9 @@ import sys
 import urllib.request
 import uuid
 
+sys.path.insert(0, os.path.join(os.path.dirname(os.path.abspath(__file__)), '..', 'lib'))
+from loopback_guard import refusal  # noqa: E402
+
 SLUG = 'b2-sdk-python'
 SCENARIO = 'auth.clock_expiry'
 OBJECT_NAME = 'res/clock_expiry.txt'
@@ -160,6 +163,10 @@ def run() -> None:
 
 
 def main() -> int:
+    refused = refusal('resilience')
+    if refused:
+        say(f'FAIL ({refused})')
+        return 1
     try:
         run()
     except Amber as amber:
diff --git a/.sdkharness/tests/resilience/auth.expired_401 b/.sdkharness/tests/resilience/auth.expired_401
index 08470d54..a3af2919 100755
--- a/.sdkharness/tests/resilience/auth.expired_401
+++ b/.sdkharness/tests/resilience/auth.expired_401
@@ -29,6 +29,9 @@ import sys
 import urllib.request
 import uuid
 
+sys.path.insert(0, os.path.join(os.path.dirname(os.path.abspath(__file__)), '..', 'lib'))
+from loopback_guard import refusal  # noqa: E402
+
 SLUG = 'b2-sdk-python'
 SCENARIO = 'auth.expired_401'
 OBJECT_NAME = 'res/expired_401.txt'
@@ -145,6 +148,10 @@ def run() -> None:
 
 
 def main() -> int:
+    refused = refusal('resilience')
+    if refused:
+        say(f'FAIL ({refused})')
+        return 1
     try:
         run()
     except Amber as amber:
diff --git a/.sdkharness/tests/resilience/download.retry_503 b/.sdkharness/tests/resilience/download.retry_503
index ca51ccb0..eac3982a 100755
--- a/.sdkharness/tests/resilience/download.retry_503
+++ b/.sdkharness/tests/resilience/download.retry_503
@@ -28,6 +28,9 @@ import sys
 import urllib.request
 import uuid
 
+sys.path.insert(0, os.path.join(os.path.dirname(os.path.abspath(__file__)), '..', 'lib'))
+from loopback_guard import refusal  # noqa: E402
+
 SLUG = 'b2-sdk-python'
 SCENARIO = 'download.retry_503'
 OBJECT_NAME = 'res/download_retry_503.bin'
@@ -148,6 +151,10 @@ def run() -> None:
 
 
 def main() -> int:
+    refused = refusal('resilience')
+    if refused:
+        say(f'FAIL ({refused})')
+        return 1
     try:
         run()
     except Amber as amber:
diff --git a/.sdkharness/tests/resilience/part.retry_503 b/.sdkharness/tests/resilience/part.retry_503
index 855180c3..d8cdbe47 100755
--- a/.sdkharness/tests/resilience/part.retry_503
+++ b/.sdkharness/tests/resilience/part.retry_503
@@ -33,6 +33,9 @@ import sys
 import urllib.request
 import uuid
 
+sys.path.insert(0, os.path.join(os.path.dirname(os.path.abspath(__file__)), '..', 'lib'))
+from loopback_guard import refusal  # noqa: E402
+
 SLUG = 'b2-sdk-python'
 SCENARIO = 'part.retry_503'
 OBJECT_NAME = 'res/part_retry_503.bin'
@@ -182,6 +185,10 @@ def run() -> None:
 
 
 def main() -> int:
+    refused = refusal('resilience')
+    if refused:
+        say(f'FAIL ({refused})')
+        return 1
     try:
         run()
     except Amber as amber:
diff --git a/.sdkharness/tests/resilience/upload.cap_exceeded_403 b/.sdkharness/tests/resilience/upload.cap_exceeded_403
index b7b62dce..2f0dd353 100755
--- a/.sdkharness/tests/resilience/upload.cap_exceeded_403
+++ b/.sdkharness/tests/resilience/upload.cap_exceeded_403
@@ -27,6 +27,9 @@ import sys
 import urllib.request
 import uuid
 
+sys.path.insert(0, os.path.join(os.path.dirname(os.path.abspath(__file__)), '..', 'lib'))
+from loopback_guard import refusal  # noqa: E402
+
 SLUG = 'b2-sdk-python'
 SCENARIO = 'upload.cap_exceeded_403'
 OBJECT_NAME = 'res/cap_exceeded.txt'
@@ -154,6 +157,10 @@ def run() -> None:
 
 
 def main() -> int:
+    refused = refusal('resilience')
+    if refused:
+        say(f'FAIL ({refused})')
+        return 1
     try:
         run()
     except Amber as amber:
diff --git a/.sdkharness/tests/resilience/upload.expired_token_401 b/.sdkharness/tests/resilience/upload.expired_token_401
index 38dc1618..a5dc9bb7 100755
--- a/.sdkharness/tests/resilience/upload.expired_token_401
+++ b/.sdkharness/tests/resilience/upload.expired_token_401
@@ -31,6 +31,9 @@ import sys
 import urllib.request
 import uuid
 
+sys.path.insert(0, os.path.join(os.path.dirname(os.path.abspath(__file__)), '..', 'lib'))
+from loopback_guard import refusal  # noqa: E402
+
 SLUG = 'b2-sdk-python'
 SCENARIO = 'upload.expired_token_401'
 OBJECT_NAME = 'res/expired_token_401.txt'
@@ -162,6 +165,10 @@ def run() -> None:
 
 
 def main() -> int:
+    refused = refusal('resilience')
+    if refused:
+        say(f'FAIL ({refused})')
+        return 1
     try:
         run()
     except Amber as amber:
diff --git a/.sdkharness/tests/resilience/upload.get_url_503 b/.sdkharness/tests/resilience/upload.get_url_503
index d5068ff0..abebbb3b 100755
--- a/.sdkharness/tests/resilience/upload.get_url_503
+++ b/.sdkharness/tests/resilience/upload.get_url_503
@@ -30,6 +30,9 @@ import sys
 import urllib.request
 import uuid
 
+sys.path.insert(0, os.path.join(os.path.dirname(os.path.abspath(__file__)), '..', 'lib'))
+from loopback_guard import refusal  # noqa: E402
+
 SLUG = 'b2-sdk-python'
 SCENARIO = 'upload.get_url_503'
 OBJECT_NAME = 'res/get_url_503.txt'
@@ -153,6 +156,10 @@ def run() -> None:
 
 
 def main() -> int:
+    refused = refusal('resilience')
+    if refused:
+        say(f'FAIL ({refused})')
+        return 1
     try:
         run()
     except Amber as amber:
diff --git a/.sdkharness/tests/resilience/upload.reset_before_response b/.sdkharness/tests/resilience/upload.reset_before_response
index 7505c5f2..1903e153 100755
--- a/.sdkharness/tests/resilience/upload.reset_before_response
+++ b/.sdkharness/tests/resilience/upload.reset_before_response
@@ -31,6 +31,9 @@ import sys
 import urllib.request
 import uuid
 
+sys.path.insert(0, os.path.join(os.path.dirname(os.path.abspath(__file__)), '..', 'lib'))
+from loopback_guard import refusal  # noqa: E402
+
 SLUG = 'b2-sdk-python'
 SCENARIO = 'upload.reset_before_response'
 OBJECT_NAME = 'res/reset_before_response.txt'
@@ -175,6 +178,10 @@ def run() -> None:
 
 
 def main() -> int:
+    refused = refusal('resilience')
+    if refused:
+        say(f'FAIL ({refused})')
+        return 1
     try:
         run()
     except Amber as amber:
diff --git a/.sdkharness/tests/resilience/upload.reset_mid_request b/.sdkharness/tests/resilience/upload.reset_mid_request
index 36210e16..a4e11642 100755
--- a/.sdkharness/tests/resilience/upload.reset_mid_request
+++ b/.sdkharness/tests/resilience/upload.reset_mid_request
@@ -30,6 +30,9 @@ import sys
 import urllib.request
 import uuid
 
+sys.path.insert(0, os.path.join(os.path.dirname(os.path.abspath(__file__)), '..', 'lib'))
+from loopback_guard import refusal  # noqa: E402
+
 SLUG = 'b2-sdk-python'
 SCENARIO = 'upload.reset_mid_request'
 OBJECT_NAME = 'res/reset_mid_request.txt'
@@ -163,6 +166,10 @@ def run() -> None:
 
 
 def main() -> int:
+    refused = refusal('resilience')
+    if refused:
+        say(f'FAIL ({refused})')
+        return 1
     try:
         run()
     except Amber as amber:
diff --git a/.sdkharness/tests/resilience/upload.retry_408 b/.sdkharness/tests/resilience/upload.retry_408
index 3a9e5c71..2399a255 100755
--- a/.sdkharness/tests/resilience/upload.retry_408
+++ b/.sdkharness/tests/resilience/upload.retry_408
@@ -31,6 +31,9 @@ import sys
 import urllib.request
 import uuid
 
+sys.path.insert(0, os.path.join(os.path.dirname(os.path.abspath(__file__)), '..', 'lib'))
+from loopback_guard import refusal  # noqa: E402
+
 SLUG = 'b2-sdk-python'
 SCENARIO = 'upload.retry_408'
 OBJECT_NAME = 'res/retry_408.txt'
@@ -160,6 +163,10 @@ def run() -> None:
 
 
 def main() -> int:
+    refused = refusal('resilience')
+    if refused:
+        say(f'FAIL ({refused})')
+        return 1
     try:
         run()
     except Amber as amber:
diff --git a/.sdkharness/tests/resilience/upload.retry_500 b/.sdkharness/tests/resilience/upload.retry_500
index 7af24478..84a476a7 100755
--- a/.sdkharness/tests/resilience/upload.retry_500
+++ b/.sdkharness/tests/resilience/upload.retry_500
@@ -30,6 +30,9 @@ import sys
 import urllib.request
 import uuid
 
+sys.path.insert(0, os.path.join(os.path.dirname(os.path.abspath(__file__)), '..', 'lib'))
+from loopback_guard import refusal  # noqa: E402
+
 SLUG = 'b2-sdk-python'
 SCENARIO = 'upload.retry_500'
 OBJECT_NAME = 'res/retry_500.txt'
@@ -159,6 +162,10 @@ def run() -> None:
 
 
 def main() -> int:
+    refused = refusal('resilience')
+    if refused:
+        say(f'FAIL ({refused})')
+        return 1
     try:
         run()
     except Amber as amber:
diff --git a/.sdkharness/tests/resilience/upload.retry_503 b/.sdkharness/tests/resilience/upload.retry_503
index c6ed4332..65708c51 100755
--- a/.sdkharness/tests/resilience/upload.retry_503
+++ b/.sdkharness/tests/resilience/upload.retry_503
@@ -34,6 +34,9 @@ import sys
 import urllib.request
 import uuid
 
+sys.path.insert(0, os.path.join(os.path.dirname(os.path.abspath(__file__)), '..', 'lib'))
+from loopback_guard import refusal  # noqa: E402
+
 SLUG = 'b2-sdk-python'
 SCENARIO = 'upload.retry_503'
 OBJECT_NAME = 'res/retry_503.txt'
@@ -130,6 +133,10 @@ def run() -> None:
 
 
 def main() -> int:
+    refused = refusal('resilience')
+    if refused:
+        say(f'FAIL ({refused})')
+        return 1
     try:
         run()
     except Amber as amber:
diff --git a/.sdkharness/tests/resilience/upload.stall b/.sdkharness/tests/resilience/upload.stall
index a29f8ff7..7c1949a5 100755
--- a/.sdkharness/tests/resilience/upload.stall
+++ b/.sdkharness/tests/resilience/upload.stall
@@ -27,6 +27,9 @@ import sys
 import urllib.request
 import uuid
 
+sys.path.insert(0, os.path.join(os.path.dirname(os.path.abspath(__file__)), '..', 'lib'))
+from loopback_guard import refusal  # noqa: E402
+
 SLUG = 'b2-sdk-python'
 SCENARIO = 'upload.stall'
 OBJECT_NAME = 'res/stall.txt'
@@ -146,6 +149,10 @@ def run() -> None:
 
 
 def main() -> int:
+    refused = refusal('resilience')
+    if refused:
+        say(f'FAIL ({refused})')
+        return 1
     try:
         run()
     except Amber as amber:
diff --git a/changelog.d/+sdkharness-loopback-guard.infrastructure.md b/changelog.d/+sdkharness-loopback-guard.infrastructure.md
new file mode 100644
index 00000000..074913df
--- /dev/null
+++ b/changelog.d/+sdkharness-loopback-guard.infrastructure.md
@@ -0,0 +1 @@
+Repository-owned sdkharness conformance and resilience leaf checks now refuse to run unless they are given a loopback simulator URL, and always use the fixed simulator credential, so running one directly can no longer reach staging or production with ambient `B2_*` credentials.
diff --git a/test/unit/test_sdkharness_conformance_resilience_contract.py b/test/unit/test_sdkharness_conformance_resilience_contract.py
index d3f7d1d1..e133e44f 100644
--- a/test/unit/test_sdkharness_conformance_resilience_contract.py
+++ b/test/unit/test_sdkharness_conformance_resilience_contract.py
@@ -106,3 +106,69 @@ def test_child_environment_scrubs_real_b2_credentials_and_pins_checkout():
     assert child['B2_APPLICATION_KEY'] == 'test-key'
     assert 'B2_OPERATOR_EXTRA' not in child
     assert child['PYTHONPATH'] == str(contract.REPOSITORY_ROOT)
+
+
+LEAF_ROOT = Path(__file__).parents[2] / '.sdkharness' / 'tests'
+LEAVES = sorted(
+    path for level in ('conformance', 'resilience') for path in (LEAF_ROOT / level).iterdir()
+)
+
+
+def run_leaf(path, extra_environment):
+    import os
+    import subprocess
+    import sys
+
+    env = {
+        name: value
+        for name, value in os.environ.items()
+        if not name.startswith(('B2_', 'CONFORMANCE_', 'RESILIENCE_', 'SDKHARNESS_'))
+    }
+    env.update(extra_environment)
+    return subprocess.run(
+        [sys.executable, str(path)],
+        env=env,
+        stdin=subprocess.DEVNULL,
+        capture_output=True,
+        text=True,
+        timeout=60,
+        check=False,
+    )
+
+
+@pytest.mark.parametrize('leaf', LEAVES, ids=lambda path: f'{path.parent.name}/{path.name}')
+def test_leaf_checks_refuse_ambient_staging_runs(leaf):
+    level = leaf.parent.name
+    prefix = level.upper()
+    ambient = {
+        'B2_APPLICATION_KEY_ID': 'real-looking-key-id',
+        'B2_APPLICATION_KEY': 'real-looking-key',
+        f'{prefix}_TARGET': 'staging',
+    }
+    refused = run_leaf(leaf, ambient)
+    assert refused.returncode == 1
+    assert 'FAIL (configuration' in refused.stdout
+    assert 'real-looking' not in refused.stdout + refused.stderr
+
+
+@pytest.mark.parametrize('leaf', LEAVES[:2] + LEAVES[-2:], ids=lambda path: path.name)
+def test_leaf_checks_refuse_missing_or_non_loopback_simulator(leaf):
+    prefix = leaf.parent.name.upper()
+    for url in ('', 'http://192.0.2.10:8180', 'http://localhost:8180', 'https://127.0.0.1:8180'):
+        values = {f'{prefix}_SIMULATOR_URL': url}
+        if prefix == 'RESILIENCE':
+            values['RESILIENCE_CONTROL_URL'] = 'http://127.0.0.1:8181'
+        refused = run_leaf(leaf, values)
+        assert refused.returncode == 1, url
+        assert 'FAIL (configuration' in refused.stdout, url
+
+
+def test_leaf_checks_name_no_external_realm_or_ambient_credentials():
+    for leaf in LEAVES:
+        text = leaf.read_text()
+        assert "'staging'" not in text, leaf.name
+        # client.simulator names a realm only for the in-process RawSimulator (no network)
+        assert leaf.name == 'client.simulator' or "'production'" not in text, leaf.name
+        assert "os.environ.get('B2_APPLICATION_KEY" not in text, leaf.name
+        assert "os.environ['B2_APPLICATION_KEY" not in text, leaf.name
+        assert 'refusal(' in text, leaf.name
diff --git a/test/unit/test_sdkharness_contract.py b/test/unit/test_sdkharness_contract.py
index 16116b2e..89bde631 100644
--- a/test/unit/test_sdkharness_contract.py
+++ b/test/unit/test_sdkharness_contract.py
@@ -107,6 +107,7 @@ def test_customer_health_lifecycle():
         'http://127.0.0.1/',
         'http://user@127.0.0.1:8180/',
         'http://127.0.0.1:8180/not-root',
+        'http://[::1]:8180/',
     ],
 )
 def test_rejects_non_loopback_simulator_contract(url):
@@ -132,3 +133,29 @@ def test_failure_result_is_one_credential_safe_record(monkeypatch, capsys):
         'configuration: required simulator input is missing\n'
     )
     assert 'test-key' not in output
+
+
+@pytest.mark.parametrize(
+    'key_id, key',
+    [
+        ('005realkeyid0000000000000', 'K005realapplicationkey00000000000'),
+        ('test-key-id', 'K005realapplicationkey00000000000'),
+        ('005realkeyid0000000000000', 'test-key'),
+    ],
+)
+def test_ambient_real_looking_credentials_never_reach_the_sdk(key_id, key):
+    contract = load_contract()
+    environment = valid_environment()
+    environment['B2_TEST_APPLICATION_KEY_ID'] = key_id
+    environment['B2_TEST_APPLICATION_KEY'] = key
+
+    class ForbiddenApi:
+        def authorize_account(self, *args, **kwargs):  # pragma: no cover - must not run
+            raise AssertionError('a non-simulator credential reached the SDK')
+
+    with pytest.raises(contract.CheckFailure) as raised:
+        contract.run_health(environment, api_factory=lambda: ForbiddenApi())
+    assert raised.value.step == 'configuration'
+    assert raised.value.detail == 'only the fixed simulator credential is accepted'
+    assert 'K005' not in str(raised.value.detail)
+    assert '005real' not in str(raised.value.detail)