diff --git a/tests/test_payment_boundaries.py b/tests/test_payment_boundaries.py
new file mode 100644
index 0000000..44e37ac
--- /dev/null
+++ b/tests/test_payment_boundaries.py
@@ -0,0 +1,320 @@
+import asyncio
+import json
+from collections.abc import AsyncIterator, Callable, Coroutine
+from contextlib import asynccontextmanager
+from typing import Literal
+
+import httpx
+import pytest
+from mpp import Challenge
+
+from inflowpay import ClientOptions, InflowApiError
+from inflowpay.mpp.buyer import BuyerMethod
+from inflowpay.mpp.seller import Seller as MppSeller
+from inflowpay.x402.buyer import Buyer
+from inflowpay.x402.facilitator import Facilitator
+from inflowpay.x402.seller import Seller as X402Seller
+from test_mpp_seller import CONFIG, ID, TERMS, wire_credential
+from test_runtime_native import native_server
+from test_x402_buyer import REQUIREMENT, RESOURCE, SUPPORTED
+from test_x402_seller import PAYLOAD, REQUIREMENTS
+
+KINDS = ("mpp-buyer", "mpp-seller", "x402-buyer", "x402-facilitator")
+PATHS = {
+    "mpp-buyer": "/v1/transactions/mpp",
+    "mpp-seller": "/v1/mpp/broadcast",
+    "x402-buyer": "/v1/transactions/x402",
+    "x402-facilitator": "/v1/x402/settle",
+}
+
+
+@asynccontextmanager
+async def payment_operation(
+    kind: str, options: ClientOptions
+) -> AsyncIterator[Callable[[], Coroutine[object, object, object]]]:
+    if kind == "mpp-buyer":
+        challenge = Challenge.create(
+            method="inflow",
+            intent="charge",
+            request={**TERMS, "recipient": ID},
+            realm="merchant.example",
+            secret_key="test-only-challenge-key",
+        )
+        async with BuyerMethod(options) as method:
+            yield lambda: method.create_credential(challenge)
+    elif kind == "mpp-seller":
+        async with await MppSeller.create(options) as seller:
+            yield lambda: seller.broadcast(wire_credential(), TERMS)
+    elif kind == "x402-buyer":
+        async with await Buyer.create(options) as buyer:
+            yield lambda: buyer.prepare(REQUIREMENT, RESOURCE)
+    else:
+        assert kind == "x402-facilitator"
+        async with await Facilitator.create(options) as facilitator:
+            yield lambda: facilitator.settle(PAYLOAD, REQUIREMENTS)
+
+
+def reply(writer: asyncio.StreamWriter, status: int, value: object) -> None:
+    body = json.dumps(value).encode()
+    writer.write(
+        f"HTTP/1.1 {status} Test\r\nContent-Length: {len(body)}\r\n"
+        "Content-Type: application/json\r\nSet-Cookie: platform-session=secret\r\n"
+        "Connection: close\r\n\r\n".encode()
+        + body
+    )
+
+
+@pytest.mark.parametrize("kind", KINDS)
+@pytest.mark.parametrize("status", [301, 302, 303, 307, 308])
+async def test_payment_entry_points_refuse_redirects(kind: str, status: int) -> None:
+    forwarded: list[str] = []
+    requests: list[tuple[str, dict[str, str]]] = []
+
+    async def target(
+        path: str, headers: dict[str, str], body: bytes, writer: asyncio.StreamWriter
+    ) -> None:
+        forwarded.append(path)
+        reply(writer, 200, {})
+
+    async with native_server(target) as destination:
+
+        async def platform(
+            path: str, headers: dict[str, str], body: bytes, writer: asyncio.StreamWriter
+        ) -> None:
+            requests.append((path, headers))
+            if path != PATHS[kind]:
+                reply(writer, 200, CONFIG if path.endswith("/config") else SUPPORTED)
+                return
+            writer.write(
+                f"HTTP/1.1 {status} Redirect\r\nLocation: {destination}/stolen\r\n"
+                "Content-Length: 0\r\nConnection: close\r\n\r\n".encode()
+            )
+
+        async with (
+            native_server(platform) as base,
+            payment_operation(kind, ClientOptions(base_url=base, api_key="test-key")) as pay,
+        ):
+            with pytest.raises(InflowApiError) as error:
+                await pay()
+            assert error.value.http_status == status
+    assert forwarded == []
+    assert sum(path == PATHS[kind] for path, _ in requests) == 1
+    assert all(headers["x-api-key"] == "test-key" for _, headers in requests)
+    assert all(
+        "cookie" not in headers and "authorization" not in headers for _, headers in requests
+    )
+
+
+@pytest.mark.parametrize("kind", KINDS)
+@pytest.mark.parametrize("status", [401, 403, 409, 503])
+async def test_payment_error_preservation_and_replay(kind: str, status: int) -> None:
+    requests: list[tuple[dict[str, str], bytes]] = []
+    code = (
+        "SELLER_ACCOUNT_REQUIRED" if status == 403 and kind == "mpp-seller" else "PAYMENT_REJECTED"
+    )
+    message = (
+        "A Seller account is required."
+        if code == "SELLER_ACCOUNT_REQUIRED"
+        else "Payment rejected."
+    )
+
+    async def platform(
+        path: str, headers: dict[str, str], body: bytes, writer: asyncio.StreamWriter
+    ) -> None:
+        if path != PATHS[kind]:
+            reply(writer, 200, CONFIG if path.endswith("/config") else SUPPORTED)
+            return
+        requests.append((headers, body))
+        reply(writer, status, {"errors": [{"code": code, "message": message}]})
+
+    async with (
+        native_server(platform) as base,
+        payment_operation(kind, ClientOptions(base_url=base, api_key="test-key")) as pay,
+    ):
+        with pytest.raises(InflowApiError) as error:
+            await pay()
+    assert (error.value.code, str(error.value), error.value.http_status) == (code, message, status)
+    assert error.value.endpoint == PATHS[kind]
+    assert error.value.body == {"errors": [{"code": code, "message": message}]}
+    assert "set-cookie" not in error.value.headers
+    assert len(requests) == (4 if kind == "mpp-seller" and status == 503 else 1)
+    assert all(body == requests[0][1] for _, body in requests)
+    if kind == "mpp-seller":
+        keys = {headers["idempotency-key"] for headers, _ in requests}
+        assert len(keys) == 1
+
+
+@pytest.mark.parametrize("kind", KINDS)
+async def test_connection_loss_preserves_operation_retry_policy(kind: str) -> None:
+    requests: list[tuple[dict[str, str], bytes]] = []
+
+    async def platform(
+        path: str, headers: dict[str, str], body: bytes, writer: asyncio.StreamWriter
+    ) -> None:
+        if path != PATHS[kind]:
+            reply(writer, 200, CONFIG if path.endswith("/config") else SUPPORTED)
+        else:
+            requests.append((headers, body))
+            # Close after receiving the request, before any response: payment outcome is unknown.
+
+    async with (
+        native_server(platform) as base,
+        payment_operation(kind, ClientOptions(base_url=base, api_key="test-key")) as pay,
+    ):
+        with pytest.raises(InflowApiError) as error:
+            await pay()
+    assert error.value.code == "NETWORK_ERROR"
+    assert len(requests) == (4 if kind == "mpp-seller" else 1)
+    assert all(body == requests[0][1] for _, body in requests)
+    if kind == "mpp-seller":
+        assert len({headers["idempotency-key"] for headers, _ in requests}) == 1
+
+
+@pytest.mark.parametrize("kind", KINDS)
+async def test_payment_cancellation_stops_requests(kind: str) -> None:
+    started = asyncio.Event()
+    calls = 0
+
+    async def platform(
+        path: str, headers: dict[str, str], body: bytes, writer: asyncio.StreamWriter
+    ) -> None:
+        nonlocal calls
+        if path != PATHS[kind]:
+            reply(writer, 200, CONFIG if path.endswith("/config") else SUPPORTED)
+        else:
+            calls += 1
+            started.set()
+            await asyncio.Event().wait()
+
+    async with (
+        native_server(platform) as base,
+        payment_operation(kind, ClientOptions(base_url=base, api_key="test-key")) as pay,
+    ):
+        task = asyncio.create_task(pay())
+        await started.wait()
+        task.cancel()
+        with pytest.raises(asyncio.CancelledError):
+            await task
+    assert calls == 1
+
+
+@pytest.mark.parametrize("kind", KINDS)
+@pytest.mark.parametrize("environment", ["production", "sandbox"])
+async def test_public_clients_use_selected_environment(
+    kind: str, environment: Literal["production", "sandbox"]
+) -> None:
+    urls: list[httpx.URL] = []
+
+    def handle(request: httpx.Request) -> httpx.Response:
+        urls.append(request.url)
+        if request.url.path == PATHS[kind]:
+            return httpx.Response(401)
+        return httpx.Response(
+            200, json=CONFIG if request.url.path.endswith("/config") else SUPPORTED
+        )
+
+    options = ClientOptions(
+        environment=environment, api_key="test-key", transport=httpx.MockTransport(handle)
+    )
+    async with payment_operation(kind, options) as pay:
+        with pytest.raises(InflowApiError):
+            await pay()
+    assert urls
+    assert all(
+        url.scheme == "https"
+        and url.host
+        == ("api.inflowpay.ai" if environment == "production" else "sandbox.inflowpay.ai")
+        for url in urls
+    )
+
+
+@pytest.mark.parametrize("kind", KINDS)
+async def test_payment_errors_do_not_expose_api_key_or_credentials(kind: str) -> None:
+    secret = "test-only-private-api-key"
+
+    def handle(request: httpx.Request) -> httpx.Response:
+        if request.url.path != PATHS[kind]:
+            return httpx.Response(
+                200, json=CONFIG if request.url.path.endswith("/config") else SUPPORTED
+            )
+        return httpx.Response(
+            403,
+            json={
+                "errors": [{"code": "DENIED", "message": f"Denied {secret}"}],
+                "diagnostics": {"credential": "payment-proof", "signature": "signed-payload"},
+            },
+            headers={"X-Request-ID": "trace-id", "X-Info": secret, "Set-Cookie": "secret-cookie"},
+        )
+
+    async with payment_operation(
+        kind, ClientOptions(api_key=secret, transport=httpx.MockTransport(handle))
+    ) as pay:
+        with pytest.raises(InflowApiError) as error:
+            await pay()
+    assert str(error.value) == "Denied [REDACTED]"
+    assert error.value.code == "DENIED"
+    assert error.value.request_id == "trace-id"
+    assert error.value.headers["x-info"] == "[REDACTED]"
+    assert "set-cookie" not in error.value.headers
+    assert error.value.body == {
+        "errors": [{"code": "DENIED", "message": "Denied [REDACTED]"}],
+        "diagnostics": {"credential": "[REDACTED]", "signature": "[REDACTED]"},
+    }
+
+
+@pytest.mark.parametrize("protocol", ["mpp", "x402"])
+async def test_seller_setup_preserves_account_role_error(protocol: str) -> None:
+    paths: list[str] = []
+    message = (
+        "The supplied credentials belong to a Developer account. "
+        "This endpoint requires a Seller account."
+    )
+
+    def handle(request: httpx.Request) -> httpx.Response:
+        paths.append(request.url.path)
+        if request.url.path.endswith("/config"):
+            return httpx.Response(
+                403, json={"errors": [{"code": "SELLER_ACCOUNT_REQUIRED", "message": message}]}
+            )
+        return httpx.Response(200, json=SUPPORTED)
+
+    options = ClientOptions(api_key="developer-key", transport=httpx.MockTransport(handle))
+    with pytest.raises(InflowApiError) as error:
+        if protocol == "mpp":
+            await MppSeller.create(options)
+        else:
+            await X402Seller.create(options)
+    assert error.value.code == "SELLER_ACCOUNT_REQUIRED"
+    assert str(error.value) == message
+    assert paths.count(f"/v1/{protocol}/config") == 1
+
+
+@pytest.mark.parametrize("kind", ["mpp-buyer", "x402-buyer"])
+async def test_buyer_preserves_bearer_authentication(kind: str) -> None:
+    requests: list[httpx.Request] = []
+    tokens: list[str] = []
+
+    async def token() -> str:
+        value = f"test-only-token-{len(tokens)}"
+        tokens.append(value)
+        return value
+
+    def handle(request: httpx.Request) -> httpx.Response:
+        requests.append(request)
+        return (
+            httpx.Response(401)
+            if request.url.path == PATHS[kind]
+            else httpx.Response(200, json=SUPPORTED)
+        )
+
+    async with payment_operation(
+        kind, ClientOptions(access_token=token, transport=httpx.MockTransport(handle))
+    ) as pay:
+        with pytest.raises(InflowApiError) as error:
+            await pay()
+    assert error.value.http_status == 401
+    assert len(requests) == len(tokens)
+    assert len([request for request in requests if request.url.path == PATHS[kind]]) == 1
+    for request, value in zip(requests, tokens, strict=True):
+        assert request.headers["authorization"] == f"Bearer {value}"
+        assert "x-api-key" not in request.headers