Snowflake MCP streamable_http fails at initialize with "error decoding response body" while same PAT/server works in Cursor

What version of Codex CLI is running?

0.104.0

What subscription do you have?

Business

Which model were you using?

gpt-5.3-codex

What platform is your computer?

macOS

What terminal emulator and version are you using (if applicable)?

iterm2

What issue are you seeing?

Codex fails to start my Snowflake MCP server during initialize handshake with:

Client error: error decoding response body, when send initialize request

The same Snowflake MCP URL + PAT works in Cursor.

## Environment

• Codex CLI: 0.104.0
• OS: macOS
• Transport: streamable HTTP (url in ~/.codex/config.toml)
• Snowflake MCP server URL:

https://<acct-id>.snowflakecomputing.com/api/v2/databases/...

## Config
~/.codex/config.toml:
```toml
[mcp_servers.snowflake]
url = "https://<acct-id>.snowflakecomputing.com/api/v2/databases/..."
bearer_token_env_var = "SNOWFLAKE_MCP_PAT"
http_headers = { "mcp-protocol-version" = "2025-06-18", "Accept-Encoding" = "identity", "Accept" = "application/json, text/event-stream" }

### What steps can reproduce the bug?

  1. Export SNOWFLAKE_MCP_PAT in shell.
  2. Start Codex from same shell.
  3. Codex startup shows:
      - MCP client for 'snowflake' failed to start
      - handshaking with MCP server failed
      - Client error: error decoding response body, when send initialize request
  4. codex mcp get snowflake --json shows expected URL + bearer env var.
  5. Cursor with same PAT + server works.

### What is the expected behavior?

  - Successful initialize + tools/list from Snowflake MCP server (as in Cursor).

### Additional information

  ## Diagnostics already done

  - Confirmed SNOWFLAKE_MCP_PAT is set in launching shell (echo ${#SNOWFLAKE_MCP_PAT} non-zero).
  - Confirmed Codex reads config:
      - codex mcp get snowflake --json returns correct URL and bearer env var.
  - Tried compatibility headers:
      - Accept-Encoding: identity
      - Accept: application/json, text/event-stream
      - mcp-protocol-version: 2025-06-18
  - No change in error.

  Please advise whether this endpoint shape is currently unsupported by Codex MCP client, or if additional config is required for Snowflake-managed MCP
  servers.