MCP client: include the calling Agent's session id in tools/call params._meta

[Liang-HZ]

What

The MCP client bridge sends { name, arguments } as the tools/call params and leaves _meta empty, so an MCP server can tell which process is calling but never which conversation. For any server holding per-conversation state — browser automation with tab groups, editors, anything with sessions — every conversation in the harness ends up in one bucket.

Why it looks like an oversight rather than a decision

_meta is a first-class optional field on every MCP request:

// BaseRequestParamsSchema
z.object({ _meta: RequestMetaSchema.optional() })

and the other clients already populate it — ZCode sends session_id, Trae sends chatSessionId, Antigravity sends antigravity.google/conversation_id, Codex sends thread_id, WorkBuddy sends workbuddy.ai/conversationId.

Frames traced from 0.2.0-rc.2 on macOS carry only protocolVersion / clientInfo / clientCapabilities:

{"io.modelcontextprotocol/protocolVersion":"2026-07-28",
 "io.modelcontextprotocol/clientInfo":{"name":"dsh-mcp-client","version":"0.0.1"},
 "io.modelcontextprotocol/clientCapabilities":{}}

The identity is already in scope

The agent loop already sets it on the execution:

• packages/core/tools/src/index.ts — ToolExecutionInput.agent: "The agent on whose behalf the call runs (set by the agent loop)"
• packages/core/agent/src/types.ts — Agent.id is the SessionId

and the tools/call closure already receives that execution:

// packages/mcp/mcp-client/src/tools.ts
call: (args, execution) => client.callTool(
  { name: tool.name, arguments: args },
  { signal: execution.signal, timeout: opts.toolCallTimeoutMs, toolDefinition: tool },
),

so it is roughly a three-line change:

call: (args, execution) => client.callTool(
  execution.agent === undefined
    ? { name: tool.name, arguments: args }
    : { name: tool.name, arguments: args, _meta: { session_id: execution.agent.id } },
  { /* unchanged */ },
),

Omitting _meta entirely (rather than sending a null) keeps "no agent on this dispatch" distinguishable from "agent with an unknown id".

One naming request

A bare session_id would be nice, because that is the key ZCode already sends — servers can then read one key for both clients.

Workaround available today

Since this repo does not take external PRs, I built the same change as an extract-and-patch plugin so servers can rely on it in the meantime: https://github.com/Liang-HZ/dsh-plugin-mcp-session-meta

It becomes obsolete the moment this lands, which is rather the point of posting here. Happy to test a build if anyone wants to try one.