tupac: Instructions file for Codex

Spec for “tupac” — a CLI MCP ↔ OpenAI Responses bridge (latest deps, quoting your brief)

---

“i want to build a CLI MCP client app. python, uv, FastMCP, typer, on the openai responses API”

• Name the package tupac.

• Latest libs (all installed with uv add):

  • openai @ latest (o-series ready)
  • fastmcp @ latest — v 2.4.0 adds one-line Client(config) multi-server support ([gofastmcp.com][1])
  • typer @ latest (>= 0.12)
  • rich @ latest (for colour) ([typer.tiangolo.com][2])
  • Python ≥ 3.11

---

“the CLI takes a JSON file and a prompt. the JSON configures MCP servers and a system prompt. the app is a simple loop that…”

CLI (tupac run cfg.json "prompt"):

1. Load config (system_prompt, mcp_servers, model, etc.).
2. Construct mcp_servers array exactly as in Claude’s MCP connector docs ([docs.anthropic.com][3]).
3. Instantiate fastmcp.Client(config) — passing the parsed MCP-server list straight in satisfies the new constructor signature; no manual tool mapping required.
4. Seed messages = [{"role":"system","content":system_prompt}, {"role":"user","content":prompt}].

Loop:

while True:
    resp = client.responses.create(
        model=config.model,
        input=messages,
        mcp_servers=config.mcp_servers,  # Claude-format JSON
        stream=False
    )
    out = resp.output[0]
    if out.type == "mcp_tool_use":
        try:
            result = await mcp.call_tool(out)      # fastmcp handles dispatch
            messages += [
                out,
                {"type":"mcp_tool_result",
                 "tool_use_id": out.id,
                 "is_error": False,
                 "content": result}
            ]
        except Exception as exc:
            messages += [
                out,
                {"type":"mcp_tool_result",
                 "tool_use_id": out.id,
                 "is_error": True,
                 "content": str(exc)}
            ]
        continue          # keep cycling
    break                 # plain assistant text → done

Tool errors are surfaced to the model ( is_error=True ) so it can recover automatically.

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“maps MCP tools to responses API … parameters are the complicated part”

FastMCP now exposes Tool.model_json_schema(); embed that JSON Schema untouched inside each tool definition so the Responses API can validate arguments itself ([gofastmcp.com][1]).

---

“handle all MCP data types properly…”

MCP content	How tupac returns it
TextContent	inline text
ImageContent, PdfContent, AudioContent, VideoContent, any BlobContent	print only the generated file-name (e.g. out_2025-06-07T12-00-01.png) and tell the human “open this file to view”; save bytes to ./outputs/

The canonical list of binary types (images, PDFs, audio, video) is in the MCP Resources spec ([modelcontextprotocol.io][4]).

---

“for resources, maintain a cache… list of compact representations … cache misses include the full text”

Use an LRU keyed by uri. Send two XML blocks (why XML? Anthropic’s prompt-engineering guide emphasises XML tags for structured context) ([docs.anthropic.com][5]):

<resources>
  <resource uri="https://foo" title="Foo doc" type="text"/>
  …
</resources>
<resource_details>
  <resource uri="https://foo"><![CDATA[full text]]></resource>
</resource_details>