What Claude Code and Codex send as MCP clients

Recorded 2026-11-03 against:

• Claude Code 2.1.317 (claude --version)
• Codex CLI 0.165.0 (codex --version)

Both were run against dev/host_probe.clj, a stdio server that logs every line it receives and sends, registered for one run only, so no host configuration changed:

# Claude Code: a config file passed on the command line
claude -p --strict-mcp-config --mcp-config probe.json \
  --allowedTools mcp__probe__probe_echo "Call the probe_echo tool with text 'hi'"

# Codex: config overrides on the command line
codex exec --skip-git-repo-check \
  -c 'mcp_servers.probe.command="bb"' \
  -c 'mcp_servers.probe.args=["/abs/path/dev/host_probe.clj"]' \
  -c 'mcp_servers.probe.env={PROBE_LOG="/abs/path/codex.log",PROBE_MODE="echo"}' \
  "Call the probe_echo tool from the probe MCP server with text 'hi'"

probe.json is {"mcpServers":{"probe":{"command":"bb","args":["/abs/path/dev/host_probe.clj"],"env":{...}}}}.

Summary

	Claude Code 2.1.317	Codex 0.165.0
First message	server/discover with 2026-07-28 _meta	initialize, protocolVersion 2025-06-18
Server answers server/discover	uses 2026-07-28, _meta on every request, no initialize	(never sends it)
Server rejects server/discover (-32601)	falls back to initialize with 2025-11-25	(never sends it)
Server answers initialize with 2025-06-18		uses 2025-06-18
Server answers initialize with 2026-07-28		sends notifications/initialized, then 2026-07-28 _meta on every request
Server rejects initialize (-32601)		drops the server: its tools are not listed

A server that serves only 2026-07-28 works with Claude Code and not with Codex. To serve both, a server has to answer initialize as well, which in presa is the initialize-era opt-in (:versions including 2025-06-18).

Claude Code, server answers server/discover

Long _meta values are cut with ....

<< {"jsonrpc":"2.0","id":"server-discover-probe-1","method":"server/discover","params":{"_meta":{
     "io.modelcontextprotocol/protocolVersion":"2026-07-28",
     "io.modelcontextprotocol/clientInfo":{"name":"claude-code","title":"Claude Code","version":"2.1.317",...},
     "io.modelcontextprotocol/clientCapabilities":{"roots":{"listChanged":true},"elicitation":{"form":{},"url":{}}}}}}
>> {"jsonrpc":"2.0","id":"server-discover-probe-1","result":{"resultType":"complete","supportedVersions":["2026-07-28"],...}}
<< {"method":"tools/list","jsonrpc":"2.0","id":0,"params":{"_meta":{"io.modelcontextprotocol/protocolVersion":"2026-07-28",...}}}
<< {"method":"tools/call","params":{"name":"probe_echo","arguments":{"text":"hi"},"_meta":{
     "io.modelcontextprotocol/protocolVersion":"2026-07-28",...,
     "claudecode/toolUseId":"toolu_...","progressToken":1}},"jsonrpc":"2.0","id":1}

Every request carries the protocol version, client info and client capabilities in _meta. The server/discover id is a string; later ids are integers starting at 0. Claude Code accepted tools/list and tools/call results without resultType.

Claude Code, server rejects server/discover

<< {"jsonrpc":"2.0","id":"server-discover-probe-1","method":"server/discover","params":{"_meta":{...}}}
>> {"jsonrpc":"2.0","id":"server-discover-probe-1","error":{"code":-32601,"message":"Method not found"}}
<< {"method":"initialize","params":{"protocolVersion":"2025-11-25","capabilities":{"roots":{"listChanged":true},"elicitation":{"form":{},"url":{}}},"clientInfo":{"name":"claude-code",...}},"jsonrpc":"2.0","id":0}
>> {"jsonrpc":"2.0","id":0,"result":{"protocolVersion":"2025-11-25","capabilities":{"tools":{}},"serverInfo":{"name":"probe","version":"0"}}}
<< {"jsonrpc":"2.0","method":"notifications/initialized"}
<< {"method":"tools/list","jsonrpc":"2.0","id":1}
<< {"method":"tools/call","params":{"name":"probe_echo","arguments":{"text":"hi"},"_meta":{"claudecode/toolUseId":"toolu_...","progressToken":2}},"jsonrpc":"2.0","id":2}

After initialize, requests carry no protocol version in _meta.

Codex, server answers initialize with the requested version

<< {"jsonrpc":"2.0","id":0,"method":"initialize","params":{"protocolVersion":"2025-06-18","capabilities":{"experimental":{"codex/auth-change":{}},"elicitation":{"form":{},"url":{}}},"clientInfo":{"name":"codex-mcp-client","title":"Codex","version":"0.165.0"}}}
>> {"jsonrpc":"2.0","id":0,"result":{"protocolVersion":"2025-06-18","capabilities":{"tools":{}},"serverInfo":{"name":"probe","version":"0"}}}
<< {"jsonrpc":"2.0","method":"notifications/initialized"}
<< {"jsonrpc":"2.0","id":1,"method":"tools/list","params":{"_meta":{"progressToken":0}}}
<< {"jsonrpc":"2.0","id":2,"method":"tools/call","params":{"_meta":{"callId":"exec-...","x-codex-turn-metadata":{...},"threadId":"...","progressToken":1},"name":"probe_echo","arguments":{"text":"hi"}}}

notifications/initialized and tools/list arrive within a millisecond of the initialize response, so a server has to settle the session before it reads the next line.

Codex, server answers initialize with 2026-07-28

>> {"jsonrpc":"2.0","id":0,"result":{"protocolVersion":"2026-07-28",...}}
<< {"jsonrpc":"2.0","method":"notifications/initialized"}
<< {"jsonrpc":"2.0","id":1,"method":"tools/list","params":{"_meta":{
     "io.modelcontextprotocol/protocolVersion":"2026-07-28",
     "io.modelcontextprotocol/clientInfo":{"name":"codex-mcp-client","title":"Codex","version":"0.165.0"},
     "io.modelcontextprotocol/clientCapabilities":{"experimental":{"codex/auth-change":{}},"elicitation":{"form":{},"url":{}}},
     "progressToken":0}}}

Codex knows the 2026-07-28 request shape but reaches it only through initialize; it never sends server/discover. presa does not answer initialize with 2026-07-28, since that revision has no initialize.

Codex, server rejects initialize

<< {"jsonrpc":"2.0","id":0,"method":"initialize","params":{"protocolVersion":"2025-06-18",...}}
>> {"jsonrpc":"2.0","id":0,"error":{"code":-32601,"message":"Method not found"}}

Nothing else is sent, and the model reported no probe_echo tool.

The presa example server in both hosts

examples/presa/example/hello.clj serves protocol/versions. Run the same way as the probe (claude -p --strict-mcp-config --mcp-config ... and codex exec -c 'mcp_servers.presa-hello...'), each host listed both tools and called them:

• Claude Code 2.1.317 connected through server/discover and listed mcp__presa-hello__add and mcp__presa-hello__echo. add with [1, 2, 3.5] returned {"sum":6.5}, and echo returned its text.
• Codex 0.165.0 connected through initialize at 2025-06-18 and listed mcp__presa_hello__add and mcp__presa_hello__echo. add returned {"content":[{"type":"text","text":"{\"sum\":6.5}"}],"structuredContent":{"sum":6.5},"isError":false}.

Not checked: Claude Code's initialize fallback against presa itself (presa answers server/discover whenever it serves 2026-07-28), and a server that serves only initialize-era revisions, which answers Claude Code's server/discover probe with -32022 rather than -32601.

The notebook example in both hosts

examples/presa/example/notebook.clj, run the same way on 2026-11-03 with the prompt "list the names of its tools, create a note titled 'Groceries' with body 'coffee and bread' tagged 'home', then search notes for 'coffee'":

• Claude Code 2.1.317 (claude -p --strict-mcp-config --mcp-config notebook.json --allowedTools mcp__notebook__create_note mcp__notebook__search_notes) listed the six tools, created n3, and the search returned n1 (Shopping) and n3 (Groceries).
• Codex 0.165.0 (codex exec -c 'mcp_servers.notebook.command="bb"' -c 'mcp_servers.notebook.args=[...]') did the same, with the same ids.

Not checked from the hosts: elicitation, sampling, tasks and subscriptions, which the notebook's tests and walk-throughs cover with presa's client.