[Bug]: Antigravity ACP drops delegate_task, t3_thread_launch, and schedule_task over ~40 KB MCP schema budget and lacks T3_ACP_MCP_NODE fallback #36088

Before submitting

• I searched existing issues and did not find a duplicate.
• I included enough detail to reproduce or investigate the problem.

Area

apps/server

Steps to reproduce

1. Start T3 Code (v0.1.10-nightly.20261113.4616) with Antigravity (Antigravity ACP Server v1.5.8, e.g. gemini-4-carbon, full-access) and at least one other enabled provider instance (e.g. Claude or Codex).
2. Open a thread on the Antigravity provider and ask the agent to delegate a subtask to another provider/model via delegate_task (or launch a worktree thread via t3_thread_launch, or create a schedule via schedule_task).
3. Inspect:
   • the schema files written to <T3CODE_HOME>/userdata/providers/antigravity/<id>/antigravity-acp/mcp/t3-code/*.json,
   • the <mcp_servers> block and native tool declarations exposed to the Antigravity model, and
   • the shell environment inside Antigravity's run_command tool (T3_ACP_MCP_NODE).

Expected behavior

delegate_task, t3_thread_launch, and schedule_task are callable from Antigravity threads — either as native eager MCP tools (mcp_t3-code_delegate_task, etc.) or via the documented T3_ACP_MCP_NODE ... acp-mcp-call terminal fallback.

Actual behavior

Antigravity threads can call mcp_t3-code_orchestrator_capabilities, mcp_t3-code_task_status, mcp_t3-code_task_cancel, and mcp_t3-code_create_threads (63 out of 66 t3-code MCP tools), but the exact 3 largest MCP tools — delegate_task, t3_thread_launch, and schedule_task — are completely missing and uncallable, and the T3_ACP_MCP_NODE terminal fallback is also absent:

1. Antigravity ACP ~40 KB eager MCP budget drops the 3 largest schemas, and call_mcp_tool lazy loading is absent in ACP mode:

   • All 66 tool schemas (47,956 minified bytes total) are received from T3's MCP server and written to <appDataDir>/mcp/t3-code/*.json on session/new and session/resume.
   • Complex nested anyOf / additionalProperties schemas are not rejected on structure (create_threads.json at depth 13 with anyOf + additionalProperties is 1,242 B minified and does load eagerly as mcp_t3-code_create_threads).
   • Instead, sorting all 66 t3-code tool schemas by minified byte size shows a sharp cutoff right at 40,052 bytes:
     • The 63 smallest tools (<= 1,668 B each, up through html_render.json at 1,668 B) sum to 40,052 minified bytes and are all registered under Eager: as mcp_t3-code_<name>.
     • The 3 largest tools spill past that ~40 KB eager budget:
       • delegate_task.json (1,938 B minified)
       • t3_thread_launch.json (2,611 B minified)
       • schedule_task.json (3,355 B minified)
   • Although Antigravity's <mcp_servers> prompt preamble mentions Lazy: tools and call_mcp_tool, Antigravity ACP Server v1.5.8 neither emits a Lazy: section nor registers call_mcp_tool in tool declarations, so any tool exceeding the ~40 KB eager budget silently disappears.

2. T3_ACP_MCP_NODE terminal fallback is never injected into Antigravity's shell:

   • T3_CODE_ACP_MCP_FALLBACK_INSTRUCTIONS (packages/provider-core/src/server/orchestrationInstructions.ts) instructs ACP agents to fall back to ELECTRON_RUN_AS_NODE=1 "$T3_ACP_MCP_NODE" ... acp-mcp-call delegate_task ... when T3 tools are absent.
   • However, environmentForSession in packages/provider-acp/src/server/clientTerminals.ts only injects T3_ACP_MCP_NODE when the ACP agent delegates shell execution back to the client via ACP terminal/create.
   • Antigravity ACP executes its built-in run_command tool directly inside its own Go language-server process rather than calling ACP terminal/create, so T3_ACP_MCP_NODE / T3_ACP_MCP_ENTRYPOINT are unset in the shell.

As a result, an Antigravity thread can inspect cross-provider models via orchestrator_capabilities, yet has no mechanism to call delegate_task, t3_thread_launch, or schedule_task (and its native invoke_subagent only accepts inherit | flash_lite | flash | pro).

Impact

Major degradation or frequent failure

Version or commit

t3 v0.1.10-nightly.20261113.4616

Environment

Windows (pwsh), T3 Code Nightly v0.1.10-nightly.20261113.4616 Desktop, Antigravity ACP Server v1.5.8 (gemini-4-carbon, full-access)

Logs or stack traces

Minified JSON schema byte sizes in <T3CODE_HOME>/userdata/providers/antigravity/<id>/antigravity-acp/mcp/t3-code/*.json (all 66 written on session start, only 63 registered in <mcp_servers> Eager:):

create_threads               minified=1242B  missing=False
preview_navigate             minified=1296B  missing=False
preview_open                 minified=1521B  missing=False
t3_project_update            minified=1541B  missing=False
preview_act                  minified=1599B  missing=False
html_render                  minified=1668B  missing=False
delegate_task                minified=1938B  missing=True
t3_thread_launch             minified=2611B  missing=True
schedule_task                minified=3355B  missing=True

Total 63 eager minified bytes: 40052
Total all 66 minified bytes:   47956

Shell environment check inside Antigravity run_command (T3_ACP_MCP_* absent, only T3CODE_CLI_PATH from desktop launcher is present):

AGY_ACP_FORCE_FILE_STORAGE
PLAYWRIGHT_MCP_EXECUTABLE_PATH
...
T3CODE_CLI_PATH

Suggested fix direction / Workaround

• Potential server-side fix: Compacting the t3-code MCP tool schemas / descriptions by ~8 KB total (or omitting/compacting verbose parameter descriptions when serving tools/list to Antigravity ACP) would keep all 66 tools under Antigravity's ~40 KB eager MCP cap so delegate_task, t3_thread_launch, and schedule_task register natively. Alternatively/additionally, expose the ACP MCP fallback env/bridge in a way reachable by ACP providers that run their own shell tools instead of terminal/create.
• Current workaround: Switch the orchestrating thread's provider to Claude or Codex before calling delegate_task, t3_thread_launch, or schedule_task.