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MCP Apps can run in both Claude and ChatGPT from a single codebase. The SDK auto-detects the host environment and uses the appropriate transport, though some platform-specific behaviors require attention.

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How it works

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Server side

Use registerAppTool() and registerAppResource() to register your tools and resources. These helper functions automatically generate platform-specific metadata, so you write the registration once and it works on both platforms.

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Client side

Call App.connect() without an explicit transport parameter. The SDK detects whether it’s running in Claude or ChatGPT and uses the appropriate transport automatically.

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Platform differences

While the SDK handles most cross-platform concerns automatically, some behaviors vary between hosts.

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Domain handling

The Resource._meta.ui.domain field format and validation rules are determined by each host platform. For example, hosts may use hash-based subdomains, URL-derived patterns, or other formats. For Claude, compute the Resource._meta.ui.domain value by running this command, replacing https://example.com/mcp with your server URL: 
node -e 'const yourServerUrl = "https://example.com/mcp"; console.log(require("crypto").createHash("sha256").update(yourServerUrl).digest("hex").slice(0,32) + ".claudemcpcontent.com")'
Example output for https://example.com/mcp: 
c3d80a4ed901ee05b21755a88273b4a4.claudemcpcontent.com