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## Summary - Add a deterministic callback-id path segment to local MCP OAuth redirect URIs before starting authorization. - Derive the callback id from the normalized MCP server URL and encode it as a 12-character URL-safe hash. - Reuse the existing exact callback-path validation so OAuth completion only succeeds on the callback path that was sent in the redirect URI. ## Context Slack thread: https://openai.slack.com/archives/C087WB3AGCR/p1777480566571699 That thread calls out the OAuth mix-up class of issue for MCP servers. The connector/App Connect flow already has a callback_id concept that binds the OAuth callback URL to the MCP app/server identity. Codex desktop's local MCP OAuth flow was still using a generic local callback path like `/callback`, so this PR adds the same shape to the shared local MCP OAuth helper. ## Behavior Before this change, local MCP OAuth used: - default local callback URL: `http://127.0.0.1:<port>/callback` - configured callback URL: `<configured callback URL>` unchanged After this change, Codex appends a deterministic callback-id segment: - default local callback URL: `http://127.0.0.1:<port>/callback/<callback_id>` - configured callback URL: `<configured callback path>/<callback_id>` The local callback server already compares the incoming request path against the path from the redirect URI. By appending the callback id before both authorization and callback validation, callbacks that arrive on the old generic path or a mismatched callback-id path are rejected. The callback id is bound to the MCP endpoint URL, including path and query, so path-based multi-tenant MCP deployments on the same origin do not share a callback path. URL fragments are ignored because they are not sent to the server. The change lives in `codex-rmcp-client`, so it covers both the normal desktop MCP OAuth login path and silent/plugin-triggered MCP OAuth login paths that use the same `perform_oauth_login_*` helpers. ## Scope and non-goals - This does not change the app-server protocol or desktop webview request shape. - This does not implement RFC 9207 `iss` validation; issuer validation is still useful when providers return `iss`. - This does not make arbitrary untrusted MCP servers safe to use. It specifically adds callback URL binding for the local MCP OAuth flow. ## Validation - `cargo fmt --all` - `cargo test -p codex-rmcp-client perform_oauth_login`