Ephemeral tool injection

[alshdavid]

Issue

Currently, ACP supports tool use through MCP.

While that is useful, it adds a significant amount of complexity to the orchestrator in cases where state cannot be shared between the orchestrator and the MCP server.

Example Use Case

My exact use case is building a headless agent harness orchestrator;

flowchart LR
    subgraph host [Host Machine]
        A[Host Orchestrator]
    end 

    subgraph container [Podman Container]
        B[Container Orchestrator] --> C
        C[ACP Server] --> B
        C -.-> D
        D[Codebase] -.-> C
    end

    A <-.->|stdio| B

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The agent needs access to, for example, GitHub to raise a PR or Jira to update a ticket status. I don't pass the container tokens for Github/Jira/etc, instead the Container Orchestrator exposes tools to the agent harness (ACP server) that enable these actions.

Ideally, when spawning the agent in ACP mode, I would define my tools custom tools. This ensures the pipeline from the ACP Server -> Container Orchestrator -> Host Orchestrator uses the same circuit.

flowchart LR
    B[Container Orchestrator] -->|stdin| C
    C[ACP Server] -->|stdout| B

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The issue with MCP is that MCP servers are either external processes or http servers. This means my Container Orchestrator need to, separately, spin up an http server for MCP - which adds a second communication pathway internally and unnecessary complexity.

flowchart LR
    B[Container Orchestrator] -->|stdin| C
    C[ACP Server] -->|stdout ACP| B
    C[ACP Server] -->|http MCP| B

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That is, if http MCP even works - it's unreliable at best. Worst case scenario I will need to develop a separate CLI tool as an MCP that connects to my Container Orchestrator over http (like you see with pretty much all MCP tools today)

flowchart LR
    B[Container Orchestrator] -->|stdin| C
    C[ACP Server] -->|stdout ACP| B
    C <-->|stdio| D
    D[MCP Proxy] <-->|http| B

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Proposal

Incorporate ephemeral tool injection modeled after the codex-cli app-server implementation.

{
  "jsonrpc": "2.0",
  "id": 1,
  "method": "initialize",
  "params": {
    "clientInfo": { "name": "my-custom-harness", "version": "1.0.0" }
  }
}

{
  "jsonrpc": "2.0",
  "id": 2,
  "method": "thread/start",
  "params": {
    "dynamic_tools": [
      {
        "name": "adder",
        "description": "Adds two numbers together and returns the sum.",
        "inputSchema": {
          "type": "object",
          "properties": {
            "a": { "type": "number", "description": "The first number" },
            "a_val": { "type": "number", "description": "First operand" },
            "b": { "type": "number", "description": "The second number" }
          },
          "required": ["a", "b"]
        }
      }
    ]
  }
}

{
  "jsonrpc": "2.0",
  "id": 3,
  "method": "turn/start",
  "params": {
    "threadId": "thr_abc123",
    "input": [
      {
        "type": "userMessage",
        "content": [
          { "type": "text", "text": "What is 42 plus 58? Use the adder tool." }
        ]
      }
    ]
  }
}