diff --git a/docs/client/oauth-clients.md b/docs/client/oauth-clients.md
index 08ccdca1df..cd7de35626 100644
--- a/docs/client/oauth-clients.md
+++ b/docs/client/oauth-clients.md
@@ -131,7 +131,7 @@ There is one more no-human situation: the client belongs to an enterprise whose
 
 ## When it fails
 
-When the OAuth flow goes wrong, the provider raises an `OAuthFlowError` from `mcp.client.auth`. It has two subclasses. `OAuthRegistrationError` means the authorization server refused to register you. `OAuthTokenError` means the token endpoint said no. One `except OAuthFlowError:` covers discovery, registration, authorization, and exchange.
+When the OAuth flow goes wrong, the provider raises an `OAuthFlowError` from `mcp.client.auth`. It has two subclasses. `OAuthRegistrationError` means registration did not yield a client you can use: the authorization server refused to register you, or it did register you but with credentials this flow cannot use (for instance an authentication method it does not implement). `OAuthTokenError` means a token could not be obtained: the token endpoint said no, or a stored client record carries an authentication method this client cannot apply, which is reported while building the token request rather than sent. One `except OAuthFlowError:` covers discovery, registration, authorization, and exchange.
 
 Not everything is a flow error. The network can still fail; those are ordinary `httpx2` exceptions and pass through untouched.
 
diff --git a/docs/migration.md b/docs/migration.md
index 5eb7659c23..0f62018584 100644
--- a/docs/migration.md
+++ b/docs/migration.md
@@ -2179,6 +2179,27 @@ client_metadata = OAuthClientMetadata(
 
 Under OIDC, omitting `application_type` defaults to `"web"`, which an authorization server may reject for the `localhost` redirect URIs native clients use; sending `"native"` avoids that. Non-OIDC servers ignore the parameter.
 
+### `OAuthClientInformationFull` no longer subclasses `OAuthClientMetadata`, and parses server-substituted metadata
+
+`OAuthClientMetadata` is the registration request a client sends; `OAuthClientInformationFull` is the authorization server's record of a registered client, parsed from its Dynamic Client Registration response. In v1 the second inherited from the first, which typed the response as though it had to be a request this SDK would send. It does not: [RFC 7591 §3.2.1](https://datatracker.ietf.org/doc/html/rfc7591#section-3.2.1) lets the server "reject or replace any of the client's requested metadata values submitted during the registration and substitute them with suitable values", and real servers return an `application_type` outside OIDC Registration's `web`/`native`, an explicit `null`, a `token_endpoint_auth_method` the SDK does not implement, or an empty `redirect_uris`. The inherited strict types turned each of those into a `ValidationError` on a 2xx response - after the server had already provisioned the client, so the registration was discarded and orphaned.
+
+The two are now siblings over a shared `OAuthClientMetadataBase`. `OAuthClientMetadata` keeps its strict types (the SDK still refuses to *send* an unregistered `application_type`), while `OAuthClientInformationFull` accepts what a server may echo:
+
+```python
+# v1
+class OAuthClientInformationFull(OAuthClientMetadata): ...
+
+# v2
+class OAuthClientMetadata(OAuthClientMetadataBase): ...          # request: strict
+class OAuthClientInformationFull(OAuthClientMetadataBase): ...   # server record: tolerant
+```
+
+On `OAuthClientInformationFull`, `application_type` and `token_endpoint_auth_method` are now `str | None`, `grant_types` is `list[str]`, and `redirect_uris` is optional (`list[AnyUrl] | None`, no minimum length). `client_id` is now required (`str`): [RFC 7591 §3.2.1](https://datatracker.ietf.org/doc/html/rfc7591#section-3.2.1) makes it mandatory in the response, and a record of a registered client without one was never meaningful. Code that only reads these fields is unaffected. Code that relied on `isinstance(client_info, OAuthClientMetadata)`, or passed an `OAuthClientInformationFull` where an `OAuthClientMetadata` is expected, must reference the record type directly. `validate_scope()` and `validate_redirect_uri()` moved with the record: they are methods of `OAuthClientInformationFull` (the type authorization-server code holds) and are no longer available on `OAuthClientMetadata`.
+
+A registration response the server sends is no longer rejected on these fields: a member serialized as a placeholder - an explicit `null`, or `""` - reads as an omitted key, so its default applies. Whether a substituted value is usable is judged where it matters, not at parse. When Dynamic Client Registration completes with credentials the authorization-code flow cannot use - a `token_endpoint_auth_method` other than `none`, `client_secret_post`, or `client_secret_basic` (including `private_key_jwt`, whose assertion that flow has no key to sign), or a secret-based method for which the server issued no `client_secret` - the client raises `OAuthRegistrationError` naming the problem, before the record is stored or authorization begins. Separately, a stored or pre-registered record carrying a method the SDK does not know at all raises `OAuthTokenError` when it reaches the token exchange; `private_key_jwt` on such a record does not raise there, so `PrivateKeyJWTOAuthProvider`, which signs its assertion only in the client-credentials exchange, still recovers from a rejected refresh by exchanging afresh.
+
+The SDK's own registration endpoint now returns all registered metadata in its 201 response (RFC 7591 §3.2.1) - including the client's `application_type`, which v1 dropped from the echo (silently reporting the default in place of a client's `"web"`), and `client_secret_expires_at` (`0` when the secret never expires) whenever a `client_secret` is issued. It also now answers a `private_key_jwt` registration with `400 invalid_client_metadata` rather than confirming a method it authenticates no requests with.
+
 ### Stricter client authentication at `/token` and `/revoke`
 
 v2 hardens client authentication on SDK-hosted authorization servers (`create_auth_routes`) in two ways. Both apply automatically; server code only needs changing if you hand-provision client records.
diff --git a/src/mcp/client/auth/oauth2.py b/src/mcp/client/auth/oauth2.py
index 07e224148a..7dc62b52b9 100644
--- a/src/mcp/client/auth/oauth2.py
+++ b/src/mcp/client/auth/oauth2.py
@@ -11,7 +11,7 @@
 import time
 from collections.abc import AsyncGenerator, Awaitable, Callable
 from dataclasses import dataclass, field
-from typing import Any, Protocol
+from typing import Any, Protocol, get_args
 from urllib.parse import quote, urlencode, urljoin, urlparse
 
 import anyio
@@ -19,7 +19,7 @@
 from mcp_types.version import is_version_at_least
 from pydantic import BaseModel, Field, ValidationError
 
-from mcp.client.auth.exceptions import OAuthFlowError, OAuthTokenError
+from mcp.client.auth.exceptions import OAuthFlowError, OAuthRegistrationError, OAuthTokenError
 from mcp.client.auth.utils import (
     build_oauth_authorization_server_metadata_discovery_urls,
     build_protected_resource_metadata_discovery_urls,
@@ -48,6 +48,7 @@
     OAuthMetadata,
     OAuthToken,
     ProtectedResourceMetadata,
+    TokenEndpointAuthMethod,
 )
 from mcp.shared.auth_utils import (
     calculate_token_expiry,
@@ -58,6 +59,55 @@
 
 logger = logging.getLogger(__name__)
 
+# Methods a registered client's record may carry without a token request being an error,
+# derived from the set the SDK is willing to request so the two cannot drift. `None`/"none"
+# send no client secret. `private_key_jwt` sends none from here either: only
+# `PrivateKeyJWTOAuthProvider` signs the assertion, and only in its client-credentials
+# exchange, so its inherited refresh path must pass through here without raising - a refresh
+# the server then rejects falls back to a fresh client-credentials exchange, which signs.
+# Anything else is a method no client here can apply.
+_KNOWN_TOKEN_ENDPOINT_AUTH_METHODS: tuple[str | None, ...] = (None, *get_args(TokenEndpointAuthMethod))
+
+# Methods that authenticate the token request with the minted `client_secret`; a
+# registration assigning one is only usable if the server issued that secret.
+_SECRET_TOKEN_ENDPOINT_AUTH_METHODS = ("client_secret_post", "client_secret_basic")
+
+# Methods a registration completed by the authorization-code flow can act on. That flow
+# authenticates the token request with the minted client secret (or nothing); it holds no key
+# to sign a `private_key_jwt` assertion, so a server assigning that method has registered a
+# client this flow cannot use. `PrivateKeyJWTOAuthProvider` never registers dynamically.
+_REGISTRATION_USABLE_TOKEN_ENDPOINT_AUTH_METHODS: tuple[str | None, ...] = tuple(
+    method for method in _KNOWN_TOKEN_ENDPOINT_AUTH_METHODS if method != "private_key_jwt"
+)
+
+
+def check_registration_usable(client_info: OAuthClientInformationFull) -> None:
+    """Confirm a registration this flow completed is one it can act on.
+
+    RFC 7591 §3.2.1 lets the authorization server replace requested metadata and leaves it to
+    the client to "check the values in the response to determine if the registration is
+    sufficient for use". Two substitutions make the minted credentials unusable, and both are
+    judged here - before the record is persisted or any interactive authorization begins -
+    rather than surfacing later as an opaque failure at the token endpoint: a token-endpoint
+    auth method the authorization-code flow cannot apply (one it does not implement, or
+    `private_key_jwt`, whose assertion this flow has no key to sign), and a secret-based
+    method the flow could apply but for which the server issued no `client_secret`.
+
+    Raises:
+        OAuthRegistrationError: The server registered the client with a
+            `token_endpoint_auth_method` this flow cannot apply, or with a secret-based
+            method but no `client_secret`.
+    """
+    method = client_info.token_endpoint_auth_method
+    if method not in _REGISTRATION_USABLE_TOKEN_ENDPOINT_AUTH_METHODS:
+        raise OAuthRegistrationError(
+            f"Authorization server registered the client with unsupported token_endpoint_auth_method {method!r}"
+        )
+    if method in _SECRET_TOKEN_ENDPOINT_AUTH_METHODS and client_info.client_secret is None:
+        raise OAuthRegistrationError(
+            f"Authorization server registered the client for {method!r} but issued no client_secret"
+        )
+
 
 class PKCEParameters(BaseModel):
     """PKCE (Proof Key for Code Exchange) parameters."""
@@ -190,6 +240,12 @@ def prepare_token_auth(
 
         Returns:
             Tuple of (updated_data, updated_headers)
+
+        Raises:
+            OAuthTokenError: The client record carries a `token_endpoint_auth_method` this
+                client does not know. A dynamic registration assigning an unusable method is
+                rejected earlier, by `check_registration_usable`; this fires for a stored or
+                pre-registered record that reaches a token request with such a method.
         """
         if headers is None:
             headers = {}  # pragma: no cover
@@ -199,7 +255,7 @@ def prepare_token_auth(
 
         auth_method = self.client_info.token_endpoint_auth_method
 
-        if auth_method == "client_secret_basic" and self.client_info.client_id and self.client_info.client_secret:
+        if auth_method == "client_secret_basic" and self.client_info.client_secret:
             # URL-encode client ID and secret per RFC 6749 Section 2.3.1
             encoded_id = quote(self.client_info.client_id, safe="")
             encoded_secret = quote(self.client_info.client_secret, safe="")
@@ -208,11 +264,14 @@ def prepare_token_auth(
             headers["Authorization"] = f"Basic {encoded_credentials}"
             # Don't include client_secret in body for basic auth
             data = {k: v for k, v in data.items() if k != "client_secret"}
-        elif auth_method == "client_secret_post" and self.client_info.client_id and self.client_info.client_secret:
+        elif auth_method == "client_secret_post" and self.client_info.client_secret:
             # Include client_id and client_secret in request body (RFC 6749 §2.3.1)
             data["client_id"] = self.client_info.client_id
             data["client_secret"] = self.client_info.client_secret
-        # For auth_method == "none", don't add any client_secret
+        elif auth_method not in _KNOWN_TOKEN_ENDPOINT_AUTH_METHODS:
+            raise OAuthTokenError(f"Registered client uses unsupported token_endpoint_auth_method {auth_method!r}")
+        # For "none" (or absent), don't add any client_secret; "private_key_jwt" adds its
+        # assertion in the provider that implements it, not here.
 
         return data, headers
 
@@ -664,6 +723,7 @@ async def async_auth_flow(self, request: httpx2.Request) -> AsyncGenerator[httpx
                             )
                             registration_response = yield registration_request
                             client_information = await handle_registration_response(registration_response)
+                            check_registration_usable(client_information)
                             # Only record the issuer when the registration above actually targeted
                             # the discovered AS — either via its published registration_endpoint,
                             # or because the resource-origin /register fallback is on the issuer's
diff --git a/src/mcp/client/auth/utils.py b/src/mcp/client/auth/utils.py
index fc87c9e469..31e2e5cade 100644
--- a/src/mcp/client/auth/utils.py
+++ b/src/mcp/client/auth/utils.py
@@ -1,9 +1,11 @@
 import re
+from typing import Any, cast
 from urllib.parse import urljoin, urlparse
 
 from httpx2 import Request, Response
 from mcp_types import LATEST_PROTOCOL_VERSION
 from pydantic import AnyUrl, ValidationError
+from pydantic_core import from_json
 
 from mcp.client.auth import OAuthFlowError, OAuthRegistrationError, OAuthTokenError
 from mcp.shared.auth import (
@@ -299,10 +301,17 @@ async def handle_registration_response(response: Response) -> OAuthClientInforma
 
     try:
         content = await response.aread()
-        client_info = OAuthClientInformationFull.model_validate_json(content)
-        return client_info
-    except ValidationError as e:  # pragma: no cover
-        raise OAuthRegistrationError(f"Invalid registration response: {e}")
+        body = from_json(content)
+        # `issuer` is the SDK's own binding of these credentials to the server they were
+        # registered with (SEP-2352), stamped by the auth flow - never sourced from the
+        # wire, so it is dropped before the body is parsed rather than trusted or cleared.
+        if isinstance(body, dict):
+            cast(dict[str, Any], body).pop("issuer", None)
+        return OAuthClientInformationFull.model_validate(body)
+    except ValueError as e:
+        # `from_json` reports malformed bytes/JSON as ValueError, and pydantic's
+        # ValidationError is itself a ValueError, so both parse layers surface here.
+        raise OAuthRegistrationError(f"Invalid registration response: {e}") from e
 
 
 def is_valid_client_metadata_url(url: str | None) -> bool:
@@ -381,8 +390,8 @@ def create_client_info_from_metadata_url(
 
     Args:
         client_metadata_url: The URL to use as the client_id
-        redirect_uris: The redirect URIs from the client metadata (passed through for
-            compatibility with OAuthClientInformationFull which inherits from OAuthClientMetadata)
+        redirect_uris: The redirect URIs from the client metadata, recorded on the client
+            information alongside the client_id
 
     Returns:
         OAuthClientInformationFull with the URL as client_id
diff --git a/src/mcp/server/auth/handlers/register.py b/src/mcp/server/auth/handlers/register.py
index e565b27383..7fb14b2c43 100644
--- a/src/mcp/server/auth/handlers/register.py
+++ b/src/mcp/server/auth/handlers/register.py
@@ -50,6 +50,18 @@ async def handle(self, request: Request) -> Response:
         # If auth method is None, default to client_secret_post
         if client_metadata.token_endpoint_auth_method is None:
             client_metadata.token_endpoint_auth_method = "client_secret_post"
+        # This server authenticates token requests with the client secret it mints; it holds
+        # no client key to verify a private_key_jwt assertion, so confirming that method would
+        # register a client whose every token request is then rejected. Refuse it instead
+        # (RFC 7591 §3.2.2), before minting credentials the client could never use.
+        if client_metadata.token_endpoint_auth_method == "private_key_jwt":
+            return PydanticJSONResponse(
+                content=RegistrationErrorResponse(
+                    error="invalid_client_metadata",
+                    error_description="token_endpoint_auth_method 'private_key_jwt' is not supported",
+                ),
+                status_code=400,
+            )
 
         client_secret = None
         if client_metadata.token_endpoint_auth_method != "none":  # pragma: no branch
@@ -106,33 +118,27 @@ async def handle(self, request: Request) -> Response:
             )
 
         client_id_issued_at = int(time.time())
-        client_secret_expires_at = (
-            client_id_issued_at + self.options.client_secret_expiry_seconds
-            if self.options.client_secret_expiry_seconds is not None
-            else None
-        )
+        # RFC 7591 §3.2.1: client_secret_expires_at is REQUIRED whenever a client_secret is
+        # issued, with 0 (not omission) meaning it never expires; a public client gets none.
+        client_secret_expires_at = None
+        if client_secret is not None:
+            client_secret_expires_at = (
+                client_id_issued_at + self.options.client_secret_expiry_seconds
+                if self.options.client_secret_expiry_seconds is not None
+                else 0
+            )
 
-        client_info = OAuthClientInformationFull(
-            client_id=client_id,
-            client_id_issued_at=client_id_issued_at,
-            client_secret=client_secret,
-            client_secret_expires_at=client_secret_expires_at,
-            # passthrough information from the client request
-            redirect_uris=client_metadata.redirect_uris,
-            token_endpoint_auth_method=client_metadata.token_endpoint_auth_method,
-            grant_types=client_metadata.grant_types,
-            response_types=client_metadata.response_types,
-            client_name=client_metadata.client_name,
-            client_uri=client_metadata.client_uri,
-            logo_uri=client_metadata.logo_uri,
-            scope=client_metadata.scope,
-            contacts=client_metadata.contacts,
-            tos_uri=client_metadata.tos_uri,
-            policy_uri=client_metadata.policy_uri,
-            jwks_uri=client_metadata.jwks_uri,
-            jwks=client_metadata.jwks,
-            software_id=client_metadata.software_id,
-            software_version=client_metadata.software_version,
+        # RFC 7591 §3.2.1: the response returns all registered metadata about the client, so
+        # the record is the whole validated request plus the credentials minted here - built
+        # from the request's dump so no metadata field can be silently omitted from the echo.
+        client_info = OAuthClientInformationFull.model_validate(
+            {
+                **client_metadata.model_dump(),
+                "client_id": client_id,
+                "client_id_issued_at": client_id_issued_at,
+                "client_secret": client_secret,
+                "client_secret_expires_at": client_secret_expires_at,
+            }
         )
         try:
             # Register client
diff --git a/src/mcp/shared/auth.py b/src/mcp/shared/auth.py
index 2bbf7a715a..881379d381 100644
--- a/src/mcp/shared/auth.py
+++ b/src/mcp/shared/auth.py
@@ -1,10 +1,27 @@
-from typing import Any, Literal
+from typing import Any, Literal, cast
 
-from pydantic import AnyHttpUrl, AnyUrl, BaseModel, ConfigDict, Field, field_validator
+from pydantic import AnyHttpUrl, AnyUrl, BaseModel, ConfigDict, Field, field_validator, model_validator
 
 # RFC 7523 JWT bearer grant; SEP-990 leg 2 uses this to present the ID-JAG.
 JWT_BEARER_GRANT_TYPE = "urn:ietf:params:oauth:grant-type:jwt-bearer"
 
+# Token-endpoint client authentication methods this SDK's clients request, and the set
+# `OAuthContext.prepare_token_auth` recognizes on a registered client (`private_key_jwt` is
+# applied by `PrivateKeyJWTOAuthProvider`; the rest send a client secret or nothing).
+TokenEndpointAuthMethod = Literal["none", "client_secret_post", "client_secret_basic", "private_key_jwt"]
+
+# grant_types a client requests when it does not specify its own (RFC 7591 §2).
+DEFAULT_GRANT_TYPES = ["authorization_code", "refresh_token"]
+
+
+def _empty_str_to_none(v: object) -> object:
+    # RFC 7591 §2 marks these URL fields OPTIONAL; a "" placeholder means absent, so it
+    # must not fail AnyHttpUrl validation. (The registered-client record applies the same
+    # rule to every member; this coercion serves the request model.)
+    if v == "":
+        return None
+    return v
+
 
 class OAuthToken(BaseModel):
     """See https://datatracker.ietf.org/doc/html/rfc6749#section-5.1"""
@@ -47,32 +64,21 @@ def __init__(self, message: str):
         self.message = message
 
 
-class OAuthClientMetadata(BaseModel):
-    """RFC 7591 OAuth 2.0 Dynamic Client Registration Metadata.
+class OAuthClientMetadataBase(BaseModel):
+    """RFC 7591 OAuth 2.0 Dynamic Client Registration metadata shared verbatim by the
+    registration request (`OAuthClientMetadata`) and the authorization server's record of a
+    registered client (`OAuthClientInformationFull`). Fields whose acceptable values differ
+    between the two - what this SDK sends versus what a third-party server may echo - are
+    declared on each model rather than here.
     See https://datatracker.ietf.org/doc/html/rfc7591#section-2
     """
 
     model_config = ConfigDict(url_preserve_empty_path=True)
 
-    redirect_uris: list[AnyUrl] | None = Field(..., min_length=1)
-    # supported auth methods for the token endpoint
-    token_endpoint_auth_method: (
-        Literal["none", "client_secret_post", "client_secret_basic", "private_key_jwt"] | None
-    ) = None
-    # supported grant_types of this implementation
-    grant_types: list[
-        Literal["authorization_code", "refresh_token", "urn:ietf:params:oauth:grant-type:jwt-bearer"] | str
-    ] = [
-        "authorization_code",
-        "refresh_token",
-    ]
     # The MCP spec requires the "code" response type, but OAuth
     # servers may also return additional types they support
     response_types: list[str] = ["code"]
     scope: str | None = None
-    # SEP-837: OIDC application_type. Defaults to "native" since MCP clients typically use
-    # loopback redirect URIs; set "web" for remote browser-based clients on a non-local host.
-    application_type: Literal["web", "native"] = "native"
 
     # these fields are currently unused, but we support & store them for potential
     # future use
@@ -97,13 +103,76 @@ class OAuthClientMetadata(BaseModel):
     )
     @classmethod
     def _empty_string_optional_url_to_none(cls, v: object) -> object:
-        # RFC 7591 §2 marks these URL fields OPTIONAL. Some authorization servers
-        # echo omitted metadata back as "" instead of dropping the keys, which
-        # AnyHttpUrl would otherwise reject — throwing away an otherwise valid
-        # registration response. Treat "" as absent.
-        if v == "":
-            return None
-        return v
+        # These URL fields are OPTIONAL; an echoed "" would otherwise fail AnyHttpUrl
+        # and throw away an otherwise valid registration response.
+        return _empty_str_to_none(v)
+
+
+class OAuthClientMetadata(OAuthClientMetadataBase):
+    """RFC 7591 OAuth 2.0 Dynamic Client Registration request metadata: what an MCP
+    client sends when it registers. Field values are narrowed to what this SDK will put
+    on the wire; parsing the authorization server's response is `OAuthClientInformationFull`'s
+    job. See https://datatracker.ietf.org/doc/html/rfc7591#section-2
+    """
+
+    redirect_uris: list[AnyUrl] | None = Field(..., min_length=1)
+    # supported auth methods for the token endpoint
+    token_endpoint_auth_method: TokenEndpointAuthMethod | None = None
+    # supported grant_types of this implementation
+    grant_types: list[
+        Literal["authorization_code", "refresh_token", "urn:ietf:params:oauth:grant-type:jwt-bearer"] | str
+    ] = list(DEFAULT_GRANT_TYPES)
+    # SEP-837: OIDC application_type. Defaults to "native" since MCP clients typically use
+    # loopback redirect URIs; set "web" for remote browser-based clients on a non-local host.
+    application_type: Literal["web", "native"] = "native"
+
+
+class OAuthClientInformationFull(OAuthClientMetadataBase):
+    """RFC 7591 OAuth 2.0 Dynamic Client Registration client information response
+    (client information plus metadata) - the authorization server's record of a
+    registered client. See https://datatracker.ietf.org/doc/html/rfc7591#section-3.2.1
+
+    A third-party authorization server "MAY reject or replace any of the client's
+    requested metadata values submitted during the registration and substitute them with
+    suitable values", so `application_type`, `token_endpoint_auth_method`, and `grant_types`
+    are typed to accept any string the server echoes, and `redirect_uris` may be absent or
+    empty. A member the server serializes as a placeholder - an explicit `null`, or `""` -
+    is read as an omitted key, so the field's default applies rather than the parse failing.
+    Whether a substituted value is usable is decided where the value is used, not at parse.
+    `redirect_uris` elements are still parsed as URLs, as the authorization server compares
+    them against a client's requested `redirect_uri`.
+    """
+
+    redirect_uris: list[AnyUrl] | None = None
+    # RFC 7591 §3.2.1: the server may assign an auth method other than the one requested,
+    # including methods this SDK does not implement, or omit it.
+    token_endpoint_auth_method: str | None = None
+    grant_types: list[str] = list(DEFAULT_GRANT_TYPES)
+    # SEP-837: OIDC application_type. OIDC Registration §2 defines "web" and "native", but
+    # servers echo other strings or an explicit null; the value is informational here.
+    application_type: str | None = None
+
+    # RFC 7591 §3.2.1: client_id is REQUIRED in a client information response - a body
+    # without one is not a registration, whatever else it echoes.
+    client_id: str
+    client_secret: str | None = None
+    client_id_issued_at: int | None = None
+    client_secret_expires_at: int | None = None
+    # SEP-2352: the issuer these credentials were registered with, recorded by the SDK (not an
+    # RFC 7591 field) to detect authorization-server migration and avoid cross-AS credential reuse.
+    issuer: str | None = None
+
+    @model_validator(mode="before")
+    @classmethod
+    def _placeholder_members_read_as_omitted(cls, data: object) -> object:
+        # Servers dump unset members of their client record as null, or echo them as "",
+        # instead of omitting the keys. Either placeholder would otherwise fail the parse of a
+        # list field (or read "" as an unrecognized method) and discard an already-provisioned
+        # registration; a placeholder and an absent key mean the same thing.
+        if isinstance(data, dict):
+            members = cast(dict[str, Any], data)
+            return {key: value for key, value in members.items() if value is not None and value != ""}
+        return data
 
     def validate_scope(self, requested_scope: str | None) -> list[str] | None:
         if requested_scope is None:
@@ -118,27 +187,15 @@ def validate_scope(self, requested_scope: str | None) -> list[str] | None:
     def validate_redirect_uri(self, redirect_uri: AnyUrl | None) -> AnyUrl:
         if redirect_uri is not None:
             # Validate redirect_uri against client's registered redirect URIs
-            if self.redirect_uris is None or redirect_uri not in self.redirect_uris:
+            if not self.redirect_uris or redirect_uri not in self.redirect_uris:
                 raise InvalidRedirectUriError(f"Redirect URI '{redirect_uri}' not registered for client")
             return redirect_uri
-        elif self.redirect_uris is not None and len(self.redirect_uris) == 1:
+        elif self.redirect_uris and len(self.redirect_uris) == 1:
             return self.redirect_uris[0]
         else:
-            raise InvalidRedirectUriError("redirect_uri must be specified when client has multiple registered URIs")
-
-
-class OAuthClientInformationFull(OAuthClientMetadata):
-    """RFC 7591 OAuth 2.0 Dynamic Client Registration full response
-    (client information plus metadata).
-    """
-
-    client_id: str | None = None
-    client_secret: str | None = None
-    client_id_issued_at: int | None = None
-    client_secret_expires_at: int | None = None
-    # SEP-2352: the issuer these credentials were registered with, recorded by the SDK (not an
-    # RFC 7591 field) to detect authorization-server migration and avoid cross-AS credential reuse.
-    issuer: str | None = None
+            raise InvalidRedirectUriError(
+                "redirect_uri must be specified unless the client has exactly one registered URI"
+            )
 
 
 class OAuthMetadata(BaseModel):
diff --git a/tests/client/auth/extensions/test_client_credentials.py b/tests/client/auth/extensions/test_client_credentials.py
index cb8fd5e2b6..16336f8002 100644
--- a/tests/client/auth/extensions/test_client_credentials.py
+++ b/tests/client/auth/extensions/test_client_credentials.py
@@ -149,44 +149,6 @@ async def test_exchange_token_client_secret_post_includes_client_id(self, mock_s
         # Should NOT have Basic auth header
         assert "Authorization" not in request.headers
 
-    @pytest.mark.anyio
-    async def test_exchange_token_client_secret_post_without_client_id(self, mock_storage: MockTokenStorage):
-        """Test client_secret_post skips body credentials when client_id is None."""
-        provider = ClientCredentialsOAuthProvider(
-            server_url="https://api.example.com/v1/mcp",
-            storage=mock_storage,
-            client_id="placeholder",
-            client_secret="test-client-secret",
-            token_endpoint_auth_method="client_secret_post",
-            scope="read write",
-        )
-        await provider._initialize()
-        provider.context.oauth_metadata = OAuthMetadata(
-            issuer=AnyHttpUrl("https://api.example.com"),
-            authorization_endpoint=AnyHttpUrl("https://api.example.com/authorize"),
-            token_endpoint=AnyHttpUrl("https://api.example.com/token"),
-        )
-        provider.context.protocol_version = "2025-06-18"
-        # Override client_info to have client_id=None (edge case)
-        provider.context.client_info = OAuthClientInformationFull(
-            redirect_uris=None,
-            client_id=None,
-            client_secret="test-client-secret",
-            grant_types=["client_credentials"],
-            token_endpoint_auth_method="client_secret_post",
-            scope="read write",
-        )
-
-        request = await provider._perform_authorization()
-
-        content = urllib.parse.unquote_plus(request.content.decode())
-        assert "grant_type=client_credentials" in content
-        # Neither client_id nor client_secret should be in body since client_id is None
-        # (RFC 6749 §2.3.1 requires both for client_secret_post)
-        assert "client_id=" not in content
-        assert "client_secret=" not in content
-        assert "Authorization" not in request.headers
-
     @pytest.mark.anyio
     async def test_exchange_token_without_scopes(self, mock_storage: MockTokenStorage):
         """Test token exchange without scopes."""
diff --git a/tests/client/test_auth.py b/tests/client/test_auth.py
index 6bea7bcf70..be96cc8eec 100644
--- a/tests/client/test_auth.py
+++ b/tests/client/test_auth.py
@@ -12,7 +12,7 @@
 from pydantic import AnyHttpUrl, AnyUrl
 
 from mcp.client.auth import OAuthClientProvider, PKCEParameters
-from mcp.client.auth.exceptions import OAuthFlowError, OAuthTokenError
+from mcp.client.auth.exceptions import OAuthFlowError, OAuthRegistrationError, OAuthTokenError
 from mcp.client.auth.utils import (
     build_oauth_authorization_server_metadata_discovery_urls,
     build_protected_resource_metadata_discovery_urls,
@@ -1008,6 +1008,91 @@ def text(self):
         assert "Registration failed: 400" in str(exc_info.value)
 
 
+@pytest.mark.anyio
+async def test_registration_response_with_substituted_metadata_yields_the_credentials():
+    """A 201 whose echoed metadata differs from the request still registers the client.
+
+    The authorization server returned an application_type outside OIDC Registration's set,
+    a null redirect_uris, and an auth method the SDK does not implement. RFC 7591 §3.2.1
+    permits the server to substitute values; the client keeps the credentials it minted.
+    """
+    body = (
+        b'{"client_id": "issued-id", "client_secret": "issued-secret", '
+        b'"application_type": "confidential", "redirect_uris": null, '
+        b'"token_endpoint_auth_method": "client_secret_jwt"}'
+    )
+    response = httpx2.Response(201, content=body)
+
+    client_info = await handle_registration_response(response)
+
+    assert client_info.client_id == "issued-id"
+    assert client_info.client_secret == "issued-secret"
+    assert client_info.application_type == "confidential"
+
+
+@pytest.mark.anyio
+@pytest.mark.parametrize("echoed_issuer", ["https://not-the-flow.example", 12345], ids=["string", "not-a-string"])
+async def test_registration_response_does_not_seed_the_issuer_binding_from_the_body(echoed_issuer: object):
+    """The issuer binding (SEP-2352) is the SDK's record of which server it registered with,
+    stamped by the auth flow; an "issuer" member in the untrusted response body is dropped
+    before parsing - never populating the binding, and never failing the parse either, so a
+    mismatched or malformed value cannot discard the credentials on every 401."""
+    body = json.dumps({"client_id": "issued-id", "issuer": echoed_issuer}).encode()
+
+    client_info = await handle_registration_response(httpx2.Response(201, content=body))
+
+    assert client_info.client_id == "issued-id"
+    assert client_info.issuer is None
+
+
+@pytest.mark.anyio
+@pytest.mark.parametrize(
+    "content",
+    [b"<html>not json</html>", b'["json", "but", "not", "an", "object"]', '{"client_id": "café"}'.encode("latin-1")],
+    ids=["not-json", "not-an-object", "not-utf8"],
+)
+async def test_a_2xx_body_that_is_not_client_information_is_an_oauth_registration_error(content: bytes):
+    """A success status whose body is not client information - unparseable, not an object, or
+    not valid UTF-8 - surfaces as OAuthRegistrationError rather than a raw parse failure, so a
+    single OAuthFlowError handler still covers registration."""
+    response = httpx2.Response(201, content=content)
+
+    with pytest.raises(OAuthRegistrationError):
+        await handle_registration_response(response)
+
+
+@pytest.mark.anyio
+async def test_token_exchange_reports_an_unimplemented_registered_auth_method(oauth_provider: OAuthClientProvider):
+    """A server-assigned auth method the SDK cannot apply (RFC 7591 §3.2.1 lets the server
+    substitute one) is reported at the token exchange rather than sending the request
+    unauthenticated for the server to reject as invalid_client."""
+    oauth_provider.context.client_info = OAuthClientInformationFull(
+        client_id="registered-id",
+        client_secret="registered-secret",
+        token_endpoint_auth_method="client_secret_jwt",
+    )
+
+    with pytest.raises(OAuthTokenError):
+        await oauth_provider._exchange_token_authorization_code("test_auth_code", "test_verifier")
+
+
+def test_prepare_token_auth_leaves_a_private_key_jwt_client_to_its_provider(oauth_provider: OAuthClientProvider):
+    """private_key_jwt is recognized, so the base leaves the request untouched rather than
+    raising - PrivateKeyJWTOAuthProvider's inherited refresh path passes through here, and a
+    refresh the server then rejects (no assertion is signed on it) falls back to a fresh,
+    signed client-credentials exchange instead of aborting the flow."""
+    oauth_provider.context.client_info = OAuthClientInformationFull(
+        client_id="registered-id",
+        client_secret="registered-secret",
+        token_endpoint_auth_method="private_key_jwt",
+    )
+
+    data, headers = oauth_provider.context.prepare_token_auth({"grant_type": "refresh_token"}, {})
+
+    assert data == {"grant_type": "refresh_token"}
+    assert headers == {}
+
+
 class TestCreateClientRegistrationRequest:
     """Test client registration request creation."""
 
diff --git a/tests/interaction/_requirements.py b/tests/interaction/_requirements.py
index 17bc3a4b5a..fbc7c13846 100644
--- a/tests/interaction/_requirements.py
+++ b/tests/interaction/_requirements.py
@@ -2940,6 +2940,17 @@ def __post_init__(self) -> None:
         transports=("streamable-http",),
         note="Auth is enforced at the HTTP layer; Cache-Control is an HTTP header.",
     ),
+    "hosting:auth:as:register-echo": Requirement(
+        source="sdk",
+        behavior=(
+            "The bundled registration endpoint returns all registered metadata about the client "
+            "in its 201 response (RFC 7591 §3.2.1) - the client's `application_type` rather than a "
+            "substituted default, and `client_secret_expires_at` (0 when the secret never expires) "
+            "whenever a `client_secret` is issued."
+        ),
+        transports=("streamable-http",),
+        note="Auth is enforced at the HTTP layer; the bundled AS is an ASGI app.",
+    ),
     "hosting:auth:as:register-error-response": Requirement(
         source="sdk",
         behavior=(
@@ -3660,6 +3671,19 @@ def __post_init__(self) -> None:
         transports=("streamable-http",),
         note="OAuth is HTTP-only.",
     ),
+    "client-auth:dcr:substituted-metadata": Requirement(
+        source="sdk",
+        behavior=(
+            "A 201 registration response whose echoed metadata the server substituted (RFC 7591 §3.2.1) - "
+            "an unregistered application_type, null redirect_uris, extra grant types - completes the flow; "
+            "substituted credentials the authorization-code flow cannot apply (an unimplemented "
+            "token_endpoint_auth_method; private_key_jwt, whose assertion it has no key to sign; or a "
+            "secret-based method with no client_secret issued) are instead reported as an "
+            "OAuthRegistrationError before the record is persisted or authorization begins."
+        ),
+        transports=("streamable-http",),
+        note="OAuth is HTTP-only.",
+    ),
     "client-auth:dcr": Requirement(
         source=f"{SPEC_BASE_URL}/basic/authorization#dynamic-client-registration",
         behavior=(
diff --git a/tests/interaction/auth/test_as_handlers.py b/tests/interaction/auth/test_as_handlers.py
index f59478b49a..1876cd7181 100644
--- a/tests/interaction/auth/test_as_handlers.py
+++ b/tests/interaction/auth/test_as_handlers.py
@@ -239,10 +239,42 @@ async def test_registration_with_invalid_metadata_is_rejected_with_400(
 
     bad_scope = await http.post("/register", json=body | {"scope": "forbidden"})
     assert bad_scope.status_code == 400
-    body = bad_scope.json()
-    assert body["error"] == "invalid_client_metadata"
+    bad_scope_body = bad_scope.json()
+    assert bad_scope_body["error"] == "invalid_client_metadata"
     # The description embeds a set difference whose ordering is not stable, so assert the prefix.
-    assert body["error_description"].startswith("Requested scopes are not valid: ")
+    assert bad_scope_body["error_description"].startswith("Requested scopes are not valid: ")
+
+    # The server holds no client key to verify a private_key_jwt assertion, so it refuses to
+    # confirm a registration whose every token request it would then reject (RFC 7591 §3.2.2).
+    unsignable = await http.post("/register", json=body | {"token_endpoint_auth_method": "private_key_jwt"})
+    assert unsignable.status_code == 400
+    assert unsignable.json() == snapshot(
+        {
+            "error": "invalid_client_metadata",
+            "error_description": "token_endpoint_auth_method 'private_key_jwt' is not supported",
+        }
+    )
+
+
+@requirement("hosting:auth:as:register-echo")
+@pytest.mark.parametrize("application_type", ["web", "native"])
+async def test_registration_response_echoes_the_registered_application_type(
+    as_app: tuple[httpx2.AsyncClient, InMemoryAuthorizationServerProvider],
+    application_type: str,
+) -> None:
+    """The 201 body reflects the application_type the client registered (RFC 7591 §3.2.1)."""
+    http, _ = as_app
+    body = oauth_client_metadata().model_dump(mode="json", exclude_none=True)
+
+    response = await http.post("/register", json=body | {"application_type": application_type})
+
+    assert response.status_code == 201
+    echoed = response.json()
+    assert echoed["application_type"] == application_type
+    # A secret was issued and no expiry is configured, so RFC 7591 §3.2.1 requires the
+    # response to carry client_secret_expires_at, with 0 (present, not omitted) for "never".
+    assert echoed["client_secret"]
+    assert echoed["client_secret_expires_at"] == 0
 
 
 @requirement("hosting:auth:as:redirect-uri-binding")
diff --git a/tests/interaction/auth/test_discovery.py b/tests/interaction/auth/test_discovery.py
index dc9f3794af..28685bf8cd 100644
--- a/tests/interaction/auth/test_discovery.py
+++ b/tests/interaction/auth/test_discovery.py
@@ -17,14 +17,16 @@
 import pytest
 from inline_snapshot import snapshot
 from mcp_types import ListToolsResult, Tool
-from pydantic import AnyHttpUrl
+from pydantic import AnyHttpUrl, AnyUrl
 
 from mcp.client.auth import OAuthFlowError, OAuthRegistrationError
 from mcp.server import Server, ServerRequestContext
-from mcp.shared.auth import OAuthMetadata, ProtectedResourceMetadata
+from mcp.shared.auth import OAuthClientInformationFull, OAuthMetadata, ProtectedResourceMetadata
 from tests.interaction._connect import BASE_URL, mounted_app
 from tests.interaction._requirements import requirement
 from tests.interaction.auth._harness import (
+    REDIRECT_URI,
+    InMemoryTokenStorage,
     RecordedRequest,
     auth_settings,
     connect_with_oauth,
@@ -174,6 +176,91 @@ async def test_a_400_from_the_registration_endpoint_surfaces_as_a_registration_e
     assert [r.path for r in recorded if r.path in ("/authorize", "/token")] == []
 
 
+@requirement("client-auth:dcr:substituted-metadata")
+async def test_a_registration_response_with_substituted_metadata_completes_the_flow() -> None:
+    """A 201 whose echoed metadata differs from the request still yields a working client.
+
+    The shim replaces the real `/register` with a body that echoes an `application_type`
+    outside OIDC Registration's set, a null `redirect_uris`, and an extra grant type - a
+    substitution RFC 7591 §3.2.1 permits. The registration proceeds and, after `/register`
+    stopped answering, the client authorizes and exchanges a token as normal.
+    """
+    recorded, on_request = record_requests()
+    provider = InMemoryAuthorizationServerProvider()
+    server = Server("guarded", on_list_tools=list_tools)
+    client_id = "substituted-client"
+    provider.clients[client_id] = OAuthClientInformationFull(
+        client_id=client_id,
+        client_secret="s3cr3t",
+        redirect_uris=[AnyUrl(REDIRECT_URI)],
+        token_endpoint_auth_method="client_secret_post",
+        scope="mcp",
+    )
+    body = json.dumps(
+        {
+            "client_id": client_id,
+            "client_secret": "s3cr3t",
+            "token_endpoint_auth_method": "client_secret_post",
+            "application_type": "confidential",
+            "redirect_uris": None,
+            "grant_types": ["authorization_code", "refresh_token", "client_credentials"],
+        }
+    ).encode()
+    app_shim = shim(serve={"/register": (201, body)})
+    storage = InMemoryTokenStorage()
+
+    with anyio.fail_after(5):
+        async with connect_with_oauth(
+            server, provider=provider, storage=storage, app_shim=app_shim, on_request=on_request
+        ) as (client, _):
+            result = await client.list_tools()
+
+    assert result.tools[0].name == snapshot("probe")
+    assert storage.client_info is not None
+    assert storage.client_info.client_id == client_id
+    assert storage.client_info.application_type == "confidential"
+    assert [r.path for r in recorded].index("/register") < [r.path for r in recorded].index("/token")
+
+
+@requirement("client-auth:dcr:substituted-metadata")
+@pytest.mark.parametrize(
+    "credentials",
+    [
+        pytest.param(
+            {"client_secret": "s3cr3t", "token_endpoint_auth_method": "client_secret_jwt"}, id="unimplemented"
+        ),
+        pytest.param({"client_secret": "s3cr3t", "token_endpoint_auth_method": "private_key_jwt"}, id="unsignable"),
+        pytest.param({"token_endpoint_auth_method": "client_secret_post"}, id="secret-not-issued"),
+    ],
+)
+async def test_a_registration_assigning_unusable_credentials_surfaces_as_a_registration_error(
+    credentials: dict[str, str],
+) -> None:
+    """A 201 assigning credentials this flow cannot apply is a registration error.
+
+    RFC 7591 §3.2.1 leaves it to the client to judge whether a substituted value makes the
+    registration usable. The authorization-code flow authenticates with the minted secret, so
+    an unimplemented method, `private_key_jwt` (an assertion it has no key to sign), and a
+    secret-based method with no secret issued are all unusable; each is reported before
+    the record is stored or any authorize/token request is made.
+    """
+    recorded, on_request = record_requests()
+    provider = InMemoryAuthorizationServerProvider()
+    server = Server("guarded", on_list_tools=list_tools)
+    body = json.dumps({"client_id": "unusable", **credentials}).encode()
+    app_shim = shim(serve={"/register": (201, body)})
+    storage = InMemoryTokenStorage()
+
+    with anyio.fail_after(5):
+        with pytest.RaisesGroup(pytest.RaisesExc(OAuthRegistrationError), flatten_subgroups=True):
+            await connect_with_oauth(
+                server, provider=provider, storage=storage, app_shim=app_shim, on_request=on_request
+            ).__aenter__()
+
+    assert storage.client_info is None
+    assert [r.path for r in recorded if r.path in ("/authorize", "/token")] == []
+
+
 @requirement("client-auth:prm-resource-mismatch")
 async def test_prm_with_a_mismatched_resource_aborts_the_flow_before_authorize() -> None:
     """A PRM document whose `resource` does not cover the server URL aborts the flow.
diff --git a/tests/shared/test_auth.py b/tests/shared/test_auth.py
index 7463bc5a8a..5286b93834 100644
--- a/tests/shared/test_auth.py
+++ b/tests/shared/test_auth.py
@@ -1,9 +1,9 @@
 """Tests for OAuth 2.0 shared code."""
 
 import pytest
-from pydantic import ValidationError
+from pydantic import AnyUrl, ValidationError
 
-from mcp.shared.auth import OAuthClientInformationFull, OAuthClientMetadata, OAuthMetadata
+from mcp.shared.auth import InvalidRedirectUriError, OAuthClientInformationFull, OAuthClientMetadata, OAuthMetadata
 
 
 def test_oauth():
@@ -110,8 +110,8 @@ def test_valid_url_passes_through_unchanged():
 
 
 def test_information_full_inherits_coercion():
-    """OAuthClientInformationFull subclasses OAuthClientMetadata, so the
-    same coercion applies to DCR responses parsed via the full model."""
+    """OAuthClientInformationFull shares the metadata base, so the same
+    coercion applies to DCR responses parsed via the full model."""
     data = {
         "client_id": "abc123",
         "redirect_uris": ["https://example.com/callback"],
@@ -130,6 +130,107 @@ def test_information_full_inherits_coercion():
     assert info.jwks_uri is None
 
 
+# RFC 7591 §3.2.1 lets the authorization server reject or replace any requested metadata
+# value in its registration response. Real servers echo values outside the sets the client
+# would send (an unregistered application_type, an explicit null, an auth method the SDK
+# does not implement, an empty redirect_uris array); a parse failure there discards a
+# registration whose client_id the server has already provisioned.
+
+
+@pytest.mark.parametrize(
+    "substituted",
+    [
+        pytest.param({"application_type": "confidential"}, id="unregistered-application-type"),
+        pytest.param({"application_type": ""}, id="empty-application-type"),
+        pytest.param({"application_type": None}, id="null-application-type"),
+        pytest.param({"token_endpoint_auth_method": "client_secret_jwt"}, id="unimplemented-auth-method"),
+        pytest.param({"grant_types": ["authorization_code", "client_credentials"]}, id="extra-grant-type"),
+        pytest.param({"redirect_uris": []}, id="empty-redirect-uris"),
+    ],
+)
+def test_client_information_accepts_server_substituted_metadata(substituted: dict[str, object]):
+    data = {"client_id": "abc123", "client_secret": "s3cr3t", **substituted}
+    info = OAuthClientInformationFull.model_validate(data)
+    assert info.client_id == "abc123"
+    assert info.client_secret == "s3cr3t"
+
+
+def test_client_information_without_echoed_metadata_still_parses():
+    """A response holding only the credentials the server minted is a usable registration."""
+    info = OAuthClientInformationFull.model_validate({"client_id": "abc123"})
+    assert info.client_id == "abc123"
+    assert info.redirect_uris is None
+    assert info.application_type is None
+
+
+def test_every_request_metadata_field_exists_on_the_client_record():
+    """The registration handler builds its 201 echo from the request's dump; every request
+    field must exist on the record so none can be silently dropped from the response."""
+    assert set(OAuthClientMetadata.model_fields) <= set(OAuthClientInformationFull.model_fields)
+
+
+def test_a_registration_response_without_a_client_id_is_rejected():
+    """RFC 7591 §3.2.1 makes client_id REQUIRED; a body without one is not a registration,
+    however permissive the parse is about the metadata around it."""
+    with pytest.raises(ValidationError):
+        OAuthClientInformationFull.model_validate({"application_type": "web"})
+
+
+@pytest.mark.parametrize("placeholder", [None, ""], ids=["null", "empty-string"])
+@pytest.mark.parametrize(
+    "member",
+    ["grant_types", "response_types", "redirect_uris", "application_type", "token_endpoint_auth_method", "scope"],
+)
+def test_client_information_reads_a_placeholder_member_as_an_omitted_key(member: str, placeholder: object):
+    """A server that dumps unset members as null, or echoes them as "", still yields a
+    usable registration: a placeholder and an absent key mean the same, so the field's
+    default applies - including for list fields, where the placeholder is not a valid list."""
+    info = OAuthClientInformationFull.model_validate({"client_id": "abc123", member: placeholder})
+    defaults = OAuthClientInformationFull.model_validate({"client_id": "abc123"})
+    assert getattr(info, member) == getattr(defaults, member)
+
+
+def test_a_placeholder_client_id_is_a_missing_client_id():
+    """The placeholder rule applies to the credential too: an empty client_id is no client_id,
+    so the body is rejected rather than parsing as a registration with an empty identifier."""
+    with pytest.raises(ValidationError):
+        OAuthClientInformationFull.model_validate({"client_id": ""})
+
+
+def test_client_information_that_is_not_an_object_still_fails_the_parse():
+    """The null-as-omitted coercion only touches JSON objects; a body that is not one is
+    passed through and rejected as a normal validation failure rather than swallowed."""
+    with pytest.raises(ValidationError):
+        OAuthClientInformationFull.model_validate("not-an-object")
+
+
+@pytest.mark.parametrize("redirect_uris", [None, []], ids=["absent", "empty"])
+@pytest.mark.parametrize(
+    "redirect_uri", [None, AnyUrl("https://example.com/callback")], ids=["unspecified", "specified"]
+)
+def test_client_with_no_registered_redirect_uris_cannot_resolve_a_redirect(
+    redirect_uris: list[str] | None, redirect_uri: AnyUrl | None
+):
+    """With no registered redirect URIs (absent or empty), no redirect resolves - neither a
+    supplied one (nothing to match against) nor an unspecified one (no single default)."""
+    info = OAuthClientInformationFull.model_validate({"client_id": "abc123", "redirect_uris": redirect_uris})
+    with pytest.raises(InvalidRedirectUriError):
+        info.validate_redirect_uri(redirect_uri)
+
+
+def test_request_metadata_restricts_application_type_to_the_values_the_sdk_sends():
+    """What the SDK sends stays narrow even though what it accepts back is wide."""
+    with pytest.raises(ValidationError):
+        OAuthClientMetadata.model_validate(
+            {"redirect_uris": ["https://example.com/callback"], "application_type": "confidential"}
+        )
+
+
+def test_request_metadata_requires_at_least_one_redirect_uri():
+    with pytest.raises(ValidationError):
+        OAuthClientMetadata.model_validate({"redirect_uris": []})
+
+
 def test_invalid_non_empty_url_still_rejected():
     """Coercion must only touch empty strings — garbage URLs still raise."""
     data = {