feat(client,core): auth surface delta — AuthOptions, issuer ctx/stamp, authErrors module

Surface delta for the 2026-07-28 authorization requirements. All additive;
existing OAuthClientProvider implementers compile unchanged. The new fields
are inert until the behavior changes that follow wire them up.

- Extract the inline auth() options object as exported AuthOptions and add
  iss (RFC 9207 callback parameter) and skipIssuerMetadataValidation
  (RFC 8414 §3.3 opt-out). JSDoc is non-assertive — validation lands in
  the follow-up commit.
- OAuthClientProvider.clientInformation/saveClientInformation/tokens/
  saveTokens accept an optional OAuthClientInformationContext carrying the
  resolved authorization-server issuer so providers can key persisted
  credentials per AS.
- OAuthTokens and OAuthClientInformation gain an optional issuer stamp
  field (core/shared/auth.ts) — the slot the SDK writes before persistence
  so stored credentials are bound to the AS that issued them.
- New packages/client/src/client/authErrors.ts with the OAuthClientFlowError
  base class; the flow-specific error classes from later commits land here.

Claude-Session: https://claude.ai/code/session_01XBib5gRe8AMPPJhySCz3EJ

Author: felixweinberger

.changeset/auth-surface-delta.md

@@ -0,0 +1,6 @@
+---
+'@modelcontextprotocol/client': minor
+'@modelcontextprotocol/core': minor
+---
+
+Add the public surface for the 2026-07-28 authorization requirements. New `AuthOptions` type names the `auth()` options object and adds `iss` and `skipIssuerMetadataValidation` fields. `OAuthClientProvider.clientInformation()` / `.saveClientInformation()` / `.tokens()` / `.saveTokens()` accept an optional `OAuthClientInformationContext` carrying the authorization server's `issuer` so providers can key persisted credentials per authorization server. `OAuthTokens` and `OAuthClientInformation` gain an optional `issuer` stamp field. New `OAuthClientFlowError` base class in `authErrors.ts` for the flow-specific error classes that follow. All changes are additive — existing `OAuthClientProvider` implementations compile unchanged; the new fields are inert until the behavior changes that follow wire them up.

docs/migration.md

@@ -1474,6 +1474,27 @@ The following APIs are unchanged between v1 and v2 (only the import paths change
 `Session not found` — unchanged from v1. Note that this use of `-32001` is an SDK convention, not a spec-assigned error code, and it is expected to be re-derived as error handling for the 2026 protocol revision (`2026-07-28`) is adopted. Avoid hard-coding the `-32001` code in
 client logic; key off the HTTP `404` status instead.
 
+## Authorization (2026-07-28 spec)
+
+The 2026-07-28 protocol revision adds client-side authorization requirements (RFC 9207 `iss` validation, RFC 8414 §3.3 issuer-echo, per-authorization-server credential isolation, scope step-up, DCR `application_type`, and refresh-token guidance). The SDK adds the public surface for these now and will implement the parts that land in SDK code (defaulting them on) as the SEP-2468/2352/2350/837/2207 behavior changes land; the parts that live in your `OAuthClientProvider` implementation, your `clientMetadata`, or your host UI are listed under [Conformance obligations for `OAuthClientProvider` implementers](#conformance-obligations-for-oauthclientprovider-implementers).
+
+### `auth()` options are now `AuthOptions`
+
+The inline options object on `auth()` is now the named `AuthOptions` type, exported from `@modelcontextprotocol/client`. Existing call sites need no change. New fields (both currently inert — the validation behavior they feed lands in the follow-up changes tracked by SEP-2468):
+
+- `iss?: string` — the form-urldecoded `iss` query parameter from the authorization callback. Pass it alongside `authorizationCode`; it is forwarded to RFC 9207 issuer validation once that lands.
+- `skipIssuerMetadataValidation?: boolean` — opt-out for the RFC 8414 §3.3 issuer-echo check during discovery. **Security-weakening**; use only with authorization servers known to publish a mismatched `issuer`.
+
+### `OAuthClientProvider` credential methods receive an `issuer` context
+
+`clientInformation(ctx?)`, `saveClientInformation(info, ctx?)`, `tokens(ctx?)`, and `saveTokens(tokens, ctx?)` now receive an optional `OAuthClientInformationContext` parameter carrying `{ issuer: string }` — the authorization server's `issuer` identifier. Providers that persist credentials should key storage by this value so that credentials registered with one authorization server are never sent to another. Providers with a single credential set may ignore the parameter; existing implementations compile unchanged. The SDK does not yet pass this argument; it begins doing so when the SEP-2352 behavior change lands.
+
+`OAuthTokens` and `OAuthClientInformation` also gain an optional `issuer?: string` field. Once the SEP-2352 behavior change lands the SDK will stamp this onto credentials before calling `saveTokens` / `saveClientInformation`; provider implementations should round-trip it unchanged. The field is currently inert.
+
+### Conformance obligations for `OAuthClientProvider` implementers
+
+<!-- Filled in as the SEP-2468/2352/2350/837/2207 behavior PRs land. -->
+
 ## Using an LLM to migrate your code
 
 An LLM-optimized version of this guide is available at [`docs/migration-SKILL.md`](migration-SKILL.md). It contains dense mapping tables designed for tools like Claude Code to mechanically apply all the changes described above. You can paste it into your LLM context or load it as

packages/client/src/client/auth.ts

@@ -82,6 +82,22 @@ export interface AuthProvider {
     onUnauthorized?(ctx: UnauthorizedContext): Promise<void>;
 }
 
+/**
+ * Context passed to the credential-persistence methods on
+ * {@linkcode OAuthClientProvider} — `clientInformation` / `saveClientInformation`
+ * and `tokens` / `saveTokens`. Carries the resolved authorization-server `issuer`
+ * so provider implementations can key persisted credentials per authorization
+ * server (RFC 6749 §2.2 — client identifiers are unique to the AS that issued
+ * them). Providers that store a single credential set may ignore it.
+ */
+export interface OAuthClientInformationContext {
+    /**
+     * The authorization server's `issuer` identifier from its validated metadata
+     * document, used as the binding key for persisted credentials.
+     */
+    issuer: string;
+}
+
 /**
  * Type guard distinguishing `OAuthClientProvider` from a minimal `AuthProvider`.
  * Transports use this at construction time to classify the `authProvider` option.
@@ -167,8 +183,14 @@ export interface OAuthClientProvider {
      * Loads information about this OAuth client, as registered already with the
      * server, or returns `undefined` if the client is not registered with the
      * server.
+     *
+     * @param ctx - Carries the resolved authorization-server `issuer`. Providers
+     *   that persist credentials per authorization server should return the entry
+     *   keyed by `ctx.issuer`. Providers with a single credential set may ignore it.
      */
-    clientInformation(): OAuthClientInformationMixed | undefined | Promise<OAuthClientInformationMixed | undefined>;
+    clientInformation(
+        ctx?: OAuthClientInformationContext
+    ): OAuthClientInformationMixed | undefined | Promise<OAuthClientInformationMixed | undefined>;
 
     /**
      * If implemented, this permits the OAuth client to dynamically register with
@@ -177,20 +199,32 @@ export interface OAuthClientProvider {
      *
      * This method is not required to be implemented if client information is
      * statically known (e.g., pre-registered).
+     *
+     * @param ctx - Carries the resolved authorization-server `issuer`. Providers
+     *   that persist credentials per authorization server should store the entry
+     *   keyed by `ctx.issuer`.
      */
-    saveClientInformation?(clientInformation: OAuthClientInformationMixed): void | Promise<void>;
+    saveClientInformation?(clientInformation: OAuthClientInformationMixed, ctx?: OAuthClientInformationContext): void | Promise<void>;
 
     /**
      * Loads any existing OAuth tokens for the current session, or returns
      * `undefined` if there are no saved tokens.
+     *
+     * @param ctx - Carries the resolved authorization-server `issuer`. Providers
+     *   that persist tokens per authorization server should return the entry
+     *   keyed by `ctx.issuer`. Providers with a single token set may ignore it.
      */
-    tokens(): OAuthTokens | undefined | Promise<OAuthTokens | undefined>;
+    tokens(ctx?: OAuthClientInformationContext): OAuthTokens | undefined | Promise<OAuthTokens | undefined>;
 
     /**
      * Stores new OAuth tokens for the current session, after a successful
      * authorization.
+     *
+     * @param ctx - Carries the resolved authorization-server `issuer`. Providers
+     *   that persist tokens per authorization server should store the entry
+     *   keyed by `ctx.issuer`.
      */
-    saveTokens(tokens: OAuthTokens): void | Promise<void>;
+    saveTokens(tokens: OAuthTokens, ctx?: OAuthClientInformationContext): void | Promise<void>;
 
     /**
      * Invoked to redirect the user agent to the given URL to begin the authorization flow.
@@ -531,22 +565,50 @@ export async function parseErrorResponse(input: Response | string): Promise<OAut
     }
 }
 
+/**
+ * Options for {@linkcode auth}. The full OAuth flow orchestrator's input.
+ */
+export interface AuthOptions {
+    /** The MCP server URL — the protected resource the flow authorizes against. */
+    serverUrl: string | URL;
+    /**
+     * The authorization code returned by the authorization server on the redirect
+     * callback. When set, {@linkcode auth} exchanges it for tokens; when unset,
+     * {@linkcode auth} runs discovery and either refreshes or initiates redirect.
+     */
+    authorizationCode?: string;
+    /**
+     * The form-urldecoded `iss` query parameter from the authorization callback,
+     * if present. Passed through to RFC 9207 §2.4 issuer validation alongside
+     * `authorizationCode`. The validation behavior is wired up in a follow-up
+     * change; this field is currently inert.
+     */
+    iss?: string;
+    /** Scope to request; computed by Scope Selection Strategy when omitted. */
+    scope?: string;
+    /** Explicit `resource_metadata` URL from a `WWW-Authenticate` challenge. */
+    resourceMetadataUrl?: URL;
+    /** Custom `fetch` implementation. */
+    fetchFn?: FetchLike;
+    /**
+     * Opt-out for the RFC 8414 §3.3 issuer-echo check during authorization
+     * server discovery. Disabling it is **security-weakening** and intended only
+     * for authorization servers known to publish a mismatched `issuer`. The
+     * check itself is wired up in a follow-up change; this flag is currently
+     * inert.
+     *
+     * @default false
+     */
+    skipIssuerMetadataValidation?: boolean;
+}
+
 /**
  * Orchestrates the full auth flow with a server.
  *
  * This can be used as a single entry point for all authorization functionality,
  * instead of linking together the other lower-level functions in this module.
  */
-export async function auth(
-    provider: OAuthClientProvider,
-    options: {
-        serverUrl: string | URL;
-        authorizationCode?: string;
-        scope?: string;
-        resourceMetadataUrl?: URL;
-        fetchFn?: FetchLike;
-    }
-): Promise<AuthResult> {
+export async function auth(provider: OAuthClientProvider, options: AuthOptions): Promise<AuthResult> {
     try {
         return await authInternal(provider, options);
     } catch (error) {
@@ -600,19 +662,7 @@ export function determineScope(options: {
 
 async function authInternal(
     provider: OAuthClientProvider,
-    {
-        serverUrl,
-        authorizationCode,
-        scope,
-        resourceMetadataUrl,
-        fetchFn
-    }: {
-        serverUrl: string | URL;
-        authorizationCode?: string;
-        scope?: string;
-        resourceMetadataUrl?: URL;
-        fetchFn?: FetchLike;
-    }
+    { serverUrl, authorizationCode, scope, resourceMetadataUrl, fetchFn }: AuthOptions
 ): Promise<AuthResult> {
     // Check if the provider has cached discovery state to skip discovery
     const cachedState = await provider.discoveryState?.();

packages/client/src/client/authErrors.ts

@@ -0,0 +1,24 @@
+/**
+ * Error classes thrown by the OAuth client flow ({@linkcode auth} and helpers).
+ *
+ * Each behavior change in the 2026-07-28 authorization requirements adds its
+ * dedicated error class to this module so callers can `instanceof`-dispatch on
+ * the failure mode without string-matching messages.
+ */
+
+/**
+ * Base class for the OAuth-client-flow error family. Concrete subclasses are
+ * added to this module alongside the SEP-2468/837/2207/2350/2352 behavior
+ * changes that throw them, so callers can catch the whole family with a single
+ * `instanceof OAuthClientFlowError` guard once those land.
+ *
+ * @remarks Nothing in the SDK throws this base class directly. In the release
+ * that introduces it no subclass exists yet — the guard is a forward-compat
+ * hook and will not match anything until the first behavior change ships.
+ */
+export class OAuthClientFlowError extends Error {
+    constructor(message: string) {
+        super(message);
+        this.name = new.target.name;
+    }
+}

packages/client/src/index.ts

@@ -8,9 +8,11 @@
 
 export type {
     AddClientAuthentication,
+    AuthOptions,
     AuthProvider,
     AuthResult,
     ClientAuthMethod,
+    OAuthClientInformationContext,
     OAuthClientProvider,
     OAuthDiscoveryState,
     OAuthServerInfo
@@ -37,6 +39,7 @@ export {
     UnauthorizedError,
     validateClientMetadataUrl
 } from './client/auth.js';
+export { OAuthClientFlowError } from './client/authErrors.js';
 export type {
     AssertionCallback,
     ClientCredentialsProviderOptions,

packages/core/src/shared/auth.ts

@@ -135,7 +135,14 @@ export const OAuthTokensSchema = z
         token_type: z.string(),
         expires_in: z.coerce.number().optional(),
         scope: z.string().optional(),
-        refresh_token: z.string().optional()
+        refresh_token: z.string().optional(),
+        /**
+         * SDK-stamped authorization-server `issuer` identifier these tokens were
+         * obtained from. Not part of the RFC 6749 wire response; the client SDK
+         * writes it before persistence so stored tokens are bound to the AS that
+         * issued them.
+         */
+        issuer: z.string().optional()
     })
     .strip();
 
@@ -205,7 +212,14 @@ export const OAuthClientInformationSchema = z
         client_id: z.string(),
         client_secret: z.string().optional(),
         client_id_issued_at: z.number().optional(),
-        client_secret_expires_at: z.number().optional()
+        client_secret_expires_at: z.number().optional(),
+        /**
+         * SDK-stamped authorization-server `issuer` identifier this client was
+         * registered with. Not part of the RFC 7591 wire response; the client SDK
+         * writes it before persistence so stored client credentials are bound to
+         * the AS that issued them.
+         */
+        issuer: z.string().optional()
     })
     .strip();