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MCP can make a SaaS product’s tools available inside both ChatGPT and Claude, but it does not create one shared sign-in across the two hosts. Each host has its own connector setup and authorization path. For a product team, the useful lesson is to build one well-protected MCP service while treating each chat app as a separate client—with explicit permissions, reliable token renewal, and a network endpoint that the host can reach.
What “one sign-in” can—and cannot—mean
The Model Context Protocol (MCP) gives an AI host a standard way to discover and call tools exposed by a service. It does not supply user identity or require every server to use OAuth. The MCP authorization specification says, “Authorization is OPTIONAL for MCP implementations.” For a protected HTTP server, however, it describes OAuth-based authorization, metadata discovery, and bearer tokens on resource requests. Read the MCP authorization specification.
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In practice, “single sign-in” is best understood as a user authorizing access to your SaaS within a particular host or connector. The available ChatGPT and Claude documentation does not establish that authorization in one automatically signs the user into the other. Design for a common SaaS identity and authorization system, but expect separate consent and token handling for each host.
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How ChatGPT and Claude differ
| Decision | ChatGPT | Claude |
|---|---|---|
| Connector setup and administration | Custom MCP apps are created, tested, and published through developer mode on ChatGPT web for Business and Enterprise/Edu customers. Full MCP support, including write or modify actions, is described as rolling out in beta for Business, Enterprise, and Edu. Availability and interface may change. | Remote connectors can be configured by Team and Enterprise owners; Pro and Max users have user-facing setup. Plan features and interface may change. |
| Sign-in and consent | OAuth configuration depends on the server’s discovery metadata and registered redirect URI. ChatGPT’s current guidance covers how it discovers OAuth information; consult the authentication guide for the applicable setup. | Setup offers authentication choices such as “Sign in now,” “Sign in when needed,” or “No sign in,” along with OAuth client identity choices. Users can enable or disable a configured connector for each conversation. |
| Token renewal | OpenAI advises checking discovery metadata for offline_access or an equivalent and verifying that the provider actually issues refresh tokens. Without refresh support, users may need to authenticate again after access expires. |
The user-facing connector documentation describes sign-in choices. The separate Messages API connector contract makes the API caller responsible for refreshing the OAuth access token; that API behavior should not be assumed to describe the user-facing flow. |
| Where the server must be reachable | Remote servers must be reachable by the relevant ChatGPT product. OpenAI documents Secure MCP Tunnel for private or on-premises servers in supported products; that option is distinct from a publicly reachable remote server. | Remote connector requests come from Anthropic’s cloud infrastructure, not directly from the user’s local machine. A server available only on a private network or blocked from that infrastructure will not work through this route. |
| Tool access and scope | Workspace controls include testing and publishing; supported MCP actions can include writes or modifications. Keep the exposed tools and their authorization scope deliberate. | Connector activation can be controlled per conversation. Tool permissions should match the user’s granted access and the actions exposed by the SaaS. |
For current setup details, see OpenAI’s ChatGPT developer mode and MCP apps guide, its authentication guidance, and its MCP servers documentation. For Claude, see the remote MCP connector setup guide.
#1 Best Overall
Build one service, then integrate each host separately
1. Decide which actions the MCP server should expose
Start with the customer tasks that make sense inside a conversation, then expose only the corresponding tools. Reading account information and changing account data carry different risks. If a tool can create, update, or delete records, ensure the server checks the signed-in user’s permissions when the tool runs; a tool’s presence in a chat interface is not itself authorization.
2. Protect the HTTP endpoint with an appropriate identity flow
For a protected remote MCP server, use the OAuth mechanisms described in the specification and the current host guidance. The protocol is not an identity provider, and OAuth is not mandatory for every implementation. Prefer an established identity provider over building authentication from scratch; the right configuration depends on the SaaS’s user model and the host’s current requirements.
For ChatGPT, follow OpenAI’s current instructions for protected-resource metadata, the WWW-Authenticate challenge, and the redirect URI to register. These details can be implementation-sensitive, so use the live guide rather than copying endpoint assumptions from an older integration.
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3. Treat refresh as part of the sign-in design
A successful first authorization is not enough if the connection must keep working after an access token expires. For ChatGPT, check the provider’s discovery metadata for offline_access or an equivalent and confirm that refresh tokens are issued. If they are not, the user may have to sign in again when access expires. For Claude’s Messages API connector, the API caller supplies an OAuth access token and is responsible for refreshing it; do not conflate that API contract with Claude’s user-facing connector.
4. Confirm the endpoint is reachable from the host
Test reachability from the infrastructure making the request, not just from a developer’s laptop. Claude’s remote connector runs from Anthropic’s cloud infrastructure, so a private-only endpoint or firewall rule that excludes it prevents connection. OpenAI documents Secure MCP Tunnel for private and on-premises servers in supported products; check whether that product and deployment path fit your use case.
5. Configure and validate each host independently
Set up the connector in each product using its current interface and administration model. In ChatGPT, developer mode and publishing are workspace-oriented for the documented business and education plans. In Claude, the setup path depends on plan and may allow per-conversation activation. Test the complete path for each host: discovery, sign-in, consent, tool invocation, authorization enforcement, and behavior when a token expires or is revoked.
What to test before calling it done
- Identity boundaries: Confirm that a user can access only the SaaS data their account permits, even when a request comes through an AI tool.
- Write actions: Verify that changes require the intended authorization and that the tool exposes no broader capability than the user needs.
- Consent and revocation: Check what the user sees when granting access and verify that revoking access stops subsequent calls.
- Token expiry: Exercise refresh where supported and observe the expected reauthentication path where it is not.
- Network access: Confirm each host can reach the endpoint or that a supported private-server mechanism is configured.
- Host-specific administration: Check who can configure, test, publish, and enable the connector in each product.
Keep the two Claude integration surfaces distinct
Claude’s custom remote connector is a user-facing product flow. The Claude Messages API MCP connector is a separate developer integration: the API caller provides the OAuth access token and handles refresh, and the documented API connector supports tool calls over remote HTTP transports rather than direct local STDIO servers. Use the Messages API MCP connector documentation when building against that API, not as a substitute for the user-facing connector instructions.
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ChatGPT’s documented workspace publishing and testing controls suit teams that need workspace-level administration; Claude’s documented connector activation can also be managed per conversation. Private-network requirements matter too: Claude’s remote route depends on Anthropic cloud reachability, while OpenAI documents a tunnel option for private servers in supported products. Neither host’s flow proves cross-host single sign-in. The right setup is the one that preserves your SaaS’s authorization rules while matching how your users and administrators need to manage access.
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