fix: register ACP MCP tools as native callable tools via cpython proxy

[sethkarten]

No-Ticket: Evaluation-driven ACP compatibility fix; no Linear ticket was created.

Problem

When verifiers sends MCP tool servers through the ACP session/new(mcp_servers=...) protocol, prime-agent stores them correctly but only adds prose guidance to the system prompt. The model must infer the correct await mcp.call_tool(...) incantation — a reasoning step that often fails, causing fallback to website scraping via cpython.

Solution

Register native ToolDefinition entries for each ACP MCP server. The model sees typed, callable tool entries in its tool list. Execution routes through the IPython kernel via await mcp.call_tool(...) — consistent with prime-agent's cpython-first architecture.

Each server gets two tools:

• mcp_list_tools_{server} — discover tool names and schemas
• mcp_call_{server} — call a tool with JSON arguments

Changes

File	Change
tools/acp-mcp.ts	NEW — proxy ToolDefinitions that route through mcp.call_tool() in the kernel
tools/index.ts	Export createAcpMcpToolDefinitions
agent-session.ts	Wire tools into _rebuildRuntimeForAcpMcpServers
mcp-manager.ts	Add getAcpServers() public getter

---

Note

Medium Risk
Changes tool registration, active tool sets, and system prompts for ACP sessions; incorrect teardown or naming could break MCP access or hide tools from the model, but behavior is covered by new regression tests.

Overview
ACP-supplied MCP servers are no longer only documented in the system prompt as generic mcp.list_tools / mcp.call_tool examples. Each ACP server now gets two first-class agent tools—mcp_list_tools_{server} and mcp_call_{server}—implemented in acp-mcp.ts and executed through the IPython kernel’s MCP API.

agent-session builds and registers these definitions when ACP servers are set or the runtime is rebuilt, adds them to the active tool set, and tears them out of the registry, allowed names, and prompt on releaseAcpMcpServers. Registration fails without the built-in cpython provisioner or if generated tool names collide with base, custom, or extension tools.

mcp-manager splits listing: getAcpServers() for session-scoped ACP configs and getEnabledPersistentGenericServers() for user-declared servers, so ACP servers are excluded from generic MCP prose in the system prompt while persistent servers still are.

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Note

Register ACP MCP tools as native callable tools via cpython proxy

• ACP MCP servers now expose two native agent tools per server (list-tools and call-tools) that execute MCP operations through the session's cpython IPython kernel instead of being treated as generic MCP servers.
• acp-mcp.ts generates deterministic tool names, validates server names against an identifier pattern, rejects duplicates, and passes call arguments into the kernel as JSON-decoded Python data.
• agent-session.ts tracks ACP tool definitions separately, validates tool-name conflicts against base/custom/extension tools, rebuilds the runtime and tool registry on ACP server changes, and cleans up names and definitions on release.
• mcp-manager.ts splits ACP servers from persistent generic servers via getAcpServers and getEnabledPersistentGenericServers.
• Behavioral Change: ACP MCP registration now requires the built-in cpython provisioner and rejects tool-name conflicts; the system prompt no longer emits generic MCP-server instructions for ACP-supplied servers.

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