Codex MCP client fails to connect

What version of Codex is running?

0.60.1

What subscription do you have?

ChatGPT Pro

Which model were you using?

gpt-5.1-codex

What platform is your computer?

Darwin 25.1.0 Darwin Kernel Version 25.1.0: Mon Oct 20 19:32:56 PDT 2025; root:xnu-12377.41.6~2/RELEASE_ARM64_T8132 arm64

What issue are you seeing?

I'm trying to connect via the ChatGPT web client and Codex CLI to an MCP server. I've validated that the server is returning a successful POST for authentication, but neither the web client nor Codex CLI follow up with any of the expected GET requests to begin interacting with it.

❯ curl -vN "https://ciso-ng.io/mcp/sse?service_token=$SAT_TOKEN"

• Host ciso-ng.io:443 was resolved.
• IPv6: (none)
• IPv4: 172.235.45.12
• Trying 172.235.45.12:443...
• Connected to ciso-ng.io (172.235.45.12) port 443
• ALPN: curl offers h2,http/1.1
• (304) (OUT), TLS handshake, Client hello (1):
• CAfile: /etc/ssl/cert.pem
• CApath: none
• (304) (IN), TLS handshake, Server hello (2):
• (304) (IN), TLS handshake, Unknown (8):
• (304) (IN), TLS handshake, Certificate (11):
• (304) (IN), TLS handshake, CERT verify (15):
• (304) (IN), TLS handshake, Finished (20):
• (304) (OUT), TLS handshake, Finished (20):
• SSL connection using TLSv1.3 / AEAD-AES256-GCM-SHA384 / [blank] / UNDEF
• ALPN: server accepted h2
• Server certificate:
• subject: CN=ciso-ng.io
• start date: Oct 29 14:52:44 2025 GMT
• expire date: Jan 27 14:52:43 2026 GMT
• subjectAltName: host "ciso-ng.io" matched cert's "ciso-ng.io"
• issuer: C=US; O=Let's Encrypt; CN=E8
• SSL certificate verify ok.
• using HTTP/2
• [HTTP/2] [1] OPENED stream for https://ciso-ng.io/mcp/sse?service_token=<REDACTED>
• [HTTP/2] [1] [:method: GET]
• [HTTP/2] [1] [:scheme: https]
• [HTTP/2] [1] [:authority: ciso-ng.io]
• [HTTP/2] [1] [:path: /mcp/sse?service_token=<REDACTED>
• [HTTP/2] [1] [user-agent: curl/8.7.1]
• [HTTP/2] [1] [accept: /]

GET /mcp/sse?service_token=<REDACTED> Host: ciso-ng.io User-Agent: curl/8.7.1 Accept: /

• Request completely sent off

< HTTP/2 200
< date: Thu, 20 Nov 2025 18:19:10 GMT
< content-type: text/event-stream; charset=utf-8
< cache-control: no-cache
< x-request-duration: 359.703
< x-db-duration: 96.177
< x-db-queries: 215
< x-arango-duration: 0.000
< x-arango-calls: 0
< x-redis-hit-rate: NaN
< x-redis-latency-p99: 3.133
< strict-transport-security: max-age=31536000; includeSubDomains
<
data: {"jsonrpc": "2.0", "method": "session.ready", "params": {"session_id": "129c1efb002e4654ae5005aa0b7a27bd", "initialize": {"protocolVersion": "2025-06-18", "serverInfo": {"name": "grcapp", "title": "CISO NG MCP", "version": "0.1.0"}, "capabilities": {"resources": {"listChanged": false}, "tools": {"listChanged": false}}, "instructions": "Expose tenant-scoped governance data to MCP clients. Use the search and fetch tools for ChatGPT connectors, or the domain-specific tools (queues.list, configuration_items.list, controls.list) plus their cison:// resources for bespoke automations."}}}

Codex-CLI starts up with this error:
⚠ MCP client for our-app-mcp failed to start: MCP startup failed: handshaking with MCP
server failed: connection closed: initialize response

⚠ MCP startup incomplete (failed: our-app-mcp)

I've gone through all of the Codex-recommended procedures to validate that server-side behavior is working. ALl checks pass.

What steps can reproduce the bug?

Connect to the MCP server at https://ciso-ng.io/mcp/ with codex-cli or chatgpt web client.

What is the expected behavior?

Codex-CLI and/or ChatGPT web client successfully auth, then allow the user to interact with the app consistent with the privileges assigned to the service account token.

Additional information

_No response_