[Bug]: outside-agents docs don't say which MCP tools need an in-thread caller, and the target_required error doesn't mention allProjects #26515

Before submitting

• I searched existing issues and did not find a duplicate. (Support remote/cloud assistants as MCP clients (create and drive threads in configured projects) #22031 was about letting outside agents create threads and went to Ideas; this is only about documenting what already exists.)
• I included enough detail to reproduce or investigate the problem.

Area

Docs

Steps to reproduce

1. Connect an outside MCP client (signed in through /mcp as mcp-client, Full access) from outside any T3 thread.
2. Call tools/list: create_threads, delegate_task, task_status, task_cancel, request_secret, html_preview, html_render, preview_*, device_*, t3_background_wait, t3_worktree_status and t3_worktree_handoff are all listed.
3. Call any of them: each fails with thread_credential_required ("… acts as the calling T3 thread, so it needs an agent running inside T3 Code").
4. Call t3_thread_list without projectId: it fails with target_required ("Pass projectId: this MCP client is not running inside a T3 thread.").

Expected behavior

docs/user/outside-agents.md says which tools an outside agent can use and which need an in-thread caller, and names the replacements: t3_thread_launch + t3_thread_wait / t3_thread_read instead of create_threads / delegate_task, and t3_thread_list { allProjects: true } or t3_thread_search for listing across projects. The target_required message also mentions allProjects.

Actual behavior

The page only says outside agents "can read projects and threads, start and message threads, and check which providers and models are available". The split exists only in McpToolAccess (actsAsCaller / readsAsCaller). Outside agents find out by trial and error, and the t3_thread_list error points at projectId only, so agents conclude that cross-project listing is impossible.

Impact

Outside agents (hosted connectors, bots) spend calls on tools that cannot work for them, and their operators build workarounds for limits that have built-in answers.

Version or commit

v0.1.4-nightly.20261028.3824; docs on main @ 5211acf.

Environment

Outside MCP client over T3 Connect.

Workaround

Read McpToolAccess.ts and the tool descriptions.

Suggestion

Add a short "What outside agents can and cannot call" section to outside-agents.md. Optionally mark in-thread-only tools in their descriptions, or leave them out of tools/list for mcp-client sessions.