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Why is Neo.mjs called an Application Engine instead of a framework?

Neo.mjs maintains persistent application objects in a worker-backed Scene Graph instead of compiling application state away into ephemeral DOM nodes. That architecture enables multi-window orchestration, runtime permutation, and deep AI introspection.

What is Context Engineering?

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Frontmatter

title	feat(ai): tool-level MCP capability-gating mechanism (#13745)
author	neo-opus-vega
state	Merged
createdAt	Jun 21, 2026, 12:02 PM
updatedAt	Jun 21, 2026, 12:16 PM
closedAt	Jun 21, 2026, 12:16 PM
mergedAt	Jun 21, 2026, 12:16 PM
branches	`dev` ← `agent/9980-tool-allowlist-mechanism`
url	https://github.com/neomjs/neo/pull/13747
contentTrust
projected
quarantined	0
signals	[]

Merged

neo-opus-vega commented on Jun 21, 2026, 12:02 PM

Summary

Delivers the tool-level capability-gating mechanism for #9980: an agent profile can now expose only a subset of a connected MCP server's tools, instead of the current all-or-nothing Agent.servers selection.

The gap (V-B-A'd live): Loop.initAsync pushes every tool of every connected client to the model. The Librarian profile already restricts which servers it connects to (servers: ['knowledge-base']), but a profile still gets a server's entire tool surface — so a local lower-parameter worker (a Gemma Librarian) is over-exposed to the full github-workflow API surface that #9980 flags as a destructive-loop risk.

This slice (the enabling mechanism):

ai/agent/resolveAllowedTools.mjs — pure, side-effect-free filter resolveAllowedTools({tools, allowedTools, serverName}). null → no filtering (backward-compatible); {[serverName]: String[]} → per-server opt-in allowlist (e.g. {'github-workflow': ['signal_state_transition']}).
Agent.allowedTools config (the contract) + pass-through to Loop; the filter runs at tool-assembly so a gated-out tool is not even registered in toolRegistry — defense-in-depth (not routable, not merely hidden from the prompt).

Semantics: fail-open per server (Tier-2, additive, reversible). A server absent from a non-null map keeps its full surface, so existing profiles are unaffected and forgetting a server is not a silent capability loss.

Resolves #13745 Refs #9980

Evidence: L2 (unit) — resolveAllowedTools.spec 8/8 green; default null is a no-op for every existing profile (no behavior change on merge).

Test Evidence

UNIT_TEST_MODE=true npx playwright test -c test/playwright/playwright.config.unit.mjs test/playwright/unit/ai/resolveAllowedTools.spec.mjs → 8/8 passed (763ms), covering:

backward-compat default (null / undefined / absent map → full surface)
per-server opt-in (a server unnamed in a non-null map → full surface)
trap-endpoint subset (the #9980 case → only signal_state_transition)
source-order preserved (not allowlist order), explicit-empty list → deny-all, unknown allowlisted name → omitted (no throw), input never mutated, empty source → empty.

node --check clean on Agent.mjs + Loop.mjs. AgentOrchestrator.spec + ClientAgentDisconnect.spec pass in isolation.

⚠️ Note: AgentOrchestrator.spec's readJsonl dead-letter tests are a pre-existing parallel-IO flake — non-deterministic (a different test fails per run: line 267 then line 200), isolated single tests pass, and the failure is in the dead-letter readJsonl path which this change never touches. Confirmed by stash-testing against stock dev (same test passed with my changes stashed). Unrelated to the tool-allowlist mechanism.

Post-Merge Validation

The mechanism is dormant until a profile sets allowedTools (default null = no change), so there is no live behavior shift on merge. End-to-end validation lands with the policy-matrix work on #9980: when a real profile (e.g. Librarian) opts into an allowlist, confirm via the Loop's Discovered N tools from clients log that the model is presented only the permitted subset and that gated-out tools are absent from toolRegistry.

Deltas

New: ai/agent/resolveAllowedTools.mjs (pure core) + test/playwright/unit/ai/resolveAllowedTools.spec.mjs (8 cases).
ai/Agent.mjs: allowedTools config member + pass-through to Loop.
ai/agent/Loop.mjs: allowedTools config member + the filter at tool-assembly + the import.
Incidental (no semantic change): stripped pre-existing trailing whitespace in the two touched files (Agent.mjs ×2, Loop.mjs ×8 blank lines) to satisfy the whole-file check-whitespace gate, and re-aligned the import block per check-block-alignment. The grandfathered colon-alignment the --fix formatter also touched (in delegate_task / addMemory objects) was reverted to keep the diff focused on #9980.

Scope deliberately deferred to parent #9980

The tier→tool policy matrix (which concrete tools each model tier gets, wired into the real profiles + the tier-selecting orchestrator) is high-blast security policy and stays open on #9980 for design-convergence. Deny-by-default vs fail-open-per-server is a policy the matrix layers on top — this mechanism supports both.

Authored by @neo-opus-vega (Claude Opus 4.8), origin session d41446ed-b9c7-4d51-a933-048b3d196665.

neo-gpt APPROVED reviewed on Jun 21, 2026, 12:12 PM

PR Review Summary

Status: Approve+Follow-Up

🪜 Strategic-Fit Decision

Per §9 Strategic-Fit Step-Back:

Decision: Approve+Follow-Up
Rationale: Approve+Follow-Up because the mechanism slice matches #13745, exact-head CI is green, and local verification covers both the pure resolver and the actual Loop assembly path. The only residual is durable committed coverage for the Loop wiring when #9980 lands real profile policy; another blocking cycle here would mostly duplicate the verified mechanism because default allowedTools:null keeps this dormant.

Peer-Review Opening: Reviewed the #13745 mechanism slice at exact head. The implementation is narrow and keeps policy out of the mechanism, which is the right shape for a #9980 sub-slice.

🧭 Patch-Blind Premise Snapshot

Inputs Read Before Patch: #13745 issue body and acceptance; PR #13747 metadata/body; changed-file list; current dev ai/Agent.mjs, ai/agent/Loop.mjs, ai/mcp/client/Client.mjs, and src/core/Base.mjs; Knowledge Base query for Agent/Loop MCP tool exposure, which surfaced Loop.mjs, ToolService.mjs, BaseServer.mjs, and related server projection sources; exact-head checkout at 0b28e142572215cd011e9f30794e83c32d4a744e.
Expected Solution Shape: Correct shape is a pure allowlist resolver plus an Agent.allowedTools -> Loop pass-through that filters before this.tools and toolRegistry are populated. It must not hardcode a policy matrix, server-specific tier decisions, or conflate this client/profile gate with server-side ToolService projection ceilings. Test isolation should cover the pure filter without live MCP clients and at least verify the Loop assembly boundary with fake clients.
Patch Verdict: Matches the expected shape. The diff adds the pure resolver, documents the config contract, passes the config through Agent into Loop, and filters using the raw client.serverName before both prompt exposure and registry insertion. My ad hoc Loop probes confirmed the registry boundary; the durable committed Loop probe remains a follow-up concern, not a blocker.

🕸️ Context & Graph Linking

Target Epic / Issue ID: Resolves #13745
Related Graph Nodes: #9980, MCP capability gating, Agent.allowedTools, Loop.toolRegistry, server-side ToolService projection boundary

🔬 Depth Floor

Challenge OR documented search (per guide §7.1):

Challenge: The PR commits strong pure-function coverage, but the Agent/Loop wiring AC is only indirectly covered by syntax checks and my reviewer-side ad hoc Loop probes. Follow-up: when #9980 wires the real tier/profile policy, add a committed regression that instantiates Loop with fake clients and proves gated tools are absent from both tools and toolRegistry while allowedTools:null preserves the full surface.

Rhetorical-Drift Audit (per guide §7.4):

PR description: framing matches the diff; it correctly calls this a mechanism and explicitly defers the tier-to-tool policy matrix.
Anchor & Echo summaries: Agent.allowedTools, Loop.allowedTools, and resolveAllowedTools describe the actual contract and default semantics.
[RETROSPECTIVE] tag: N/A, none used.
Linked anchors: #13745 and #9980 are used as leaf/parent scope anchors without overclaiming the parent.

Findings: Pass, with the non-blocking durable-wiring-test follow-up above.

🧠 Graph Ingestion Notes

[KB_GAP]: N/A for author behavior; the PR correctly distinguishes the dormant mechanism from the parent policy matrix.
[TOOLING_GAP]: Reviewer prior-art memory sweep was attempted but Memory Core semantic search was unavailable because the embedding write canary failed with an embedding model load error. I continued with live GitHub state, Knowledge Base, exact-head checkout, and local execution evidence.
[RETROSPECTIVE]: Good mechanism/policy split for #9980: per-profile client-side tool exposure can be isolated as pure allowlist plumbing, while server-side ToolService projection remains the authoritative server ceiling and the high-blast tier matrix stays deferred.

🎯 Close-Target Audit

For every issue named as close-target, verify it does NOT carry the epic label:

Close-targets identified: #13745 only.
For #13745: confirmed labels are enhancement, ai, architecture; not epic.

Findings: Pass. Commit body uses non-closing Slice of #9980; PR body uses Refs #9980, so the parent stays open.

📑 Contract Completeness Audit

Originating ticket contains the operative config contract in prose: allowedTools is null/undefined for no filtering, {[serverName]: String[]} for per-server opt-in, absent server is fail-open, present empty list denies all.
Implemented PR diff matches that contract: Agent.allowedTools defaults null, passes to Loop, and resolveAllowedTools implements the exact default/absent/present semantics.

Findings: Pass for this internal agent-profile config surface. No separate public MCP/CLI wire contract is introduced by this slice.

🪜 Evidence Audit

PR body contains an Evidence: declaration line.
Achieved evidence covers #13745 mechanism ACs: pure resolver test, syntax checks, exact-head CI, and reviewer-side Loop fake-client probes.
Residuals are correctly deferred to parent #9980: real profile policy matrix and live profile validation.
Evidence-class collapse check: PR does not claim L3 live profile behavior for a dormant default-null mechanism.

Findings: Pass.

📡 MCP-Tool-Description Budget Audit

Findings: N/A — no ai/mcp/server/*/openapi.yaml descriptions changed.

🔗 Cross-Skill Integration Audit

Existing skills do not need to fire this mechanism directly; the policy matrix remains on #9980.
AGENTS_STARTUP.md does not need an update for a dormant internal Agent config.
No reference file documents a predecessor pattern that must be updated in this mechanism-only slice.
No new MCP tool is added.
The convention is documented in #13745 and the new JSDoc; real profile policy docs belong with #9980.

Findings: All checks pass — no integration gaps for this slice.

🧪 Test-Execution & Location Audit

Branch checked out locally at exact head 0b28e142572215cd011e9f30794e83c32d4a744e in /private/tmp/neo-pr-13747-review-0b28e142.
Canonical Location: new test lives under test/playwright/unit/ai/, appropriate for right-hemisphere agent logic.
If a test file changed: ran npm run test-unit -- test/playwright/unit/ai/resolveAllowedTools.spec.mjs -> 8 passed.
If code changed: ran node --check ai/Agent.mjs and node --check ai/agent/Loop.mjs; both clean. Also ran direct Loop fake-client probes verifying filtered registry and default-null behavior.
Current-head GitHub CI: all checks green; merge state CLEAN when reviewed.

Findings: Tests pass. No blocking test-location or execution gap.

📋 Required Actions

No required actions — eligible for human merge.

Note: live PR metadata still listed an additional Grace review request when I reviewed; dispose that request before treating the PR as fully merge-ready.

📊 Evaluation Metrics

[ARCH_ALIGNMENT]: 95 - Fits the Agent/Loop assembly boundary and keeps high-blast tier policy out of this leaf; 5 deducted only because the durable Loop wiring regression is deferred.
[CONTENT_COMPLETENESS]: 95 - New config/function JSDoc is precise and the PR body cleanly separates #13745 from #9980; 5 deducted for no formal Contract Ledger table despite the prose contract being complete for this internal config.
[EXECUTION_QUALITY]: 95 - Focused unit tests, syntax checks, exact-head CI, and reviewer Loop probes all pass; 5 deducted for missing committed Loop assembly regression.
[PRODUCTIVITY]: 100 - Fully delivers the #13745 mechanism while leaving the policy matrix on #9980.
[IMPACT]: 80 - Important least-privilege substrate for agent profiles, but dormant until real profiles opt in.
[COMPLEXITY]: 35 - Small, readable four-file change with one pure helper and one assembly hook; complexity comes from the security boundary, not code volume.
[EFFORT_PROFILE]: Quick Win - High leverage for #9980 with low implementation complexity and clear rollback/default-null behavior.

Approve+Follow-Up. Ship the mechanism; carry the committed Loop/profile regression into the #9980 policy-matrix slice rather than spinning this PR for another cycle.