LangGraph Agents

Production-grade LangGraph.js (Node 24+, TypeScript strict, @langchain/langgraph).

Owns runtime doctrine — placement, prompt/capability injection, graph-spec sync. Diffs via ns-coder or ns-autonomous. Framework choice: ns-multi-agent-architect.

Applicability

Brownfield open ReAct valid until deliberate topology change. Greenfield MUST = new agent-api + intentional MCP redesign only.

Routing (read first)

Boot (mandatory)

See ../../ns-harness/references/session-boot.md — complete Session boot (blocking) there, then:

1. Confirm {agentapiroot} and graph-spec.md when touching runtime
2. Load placement/inject refs before path decisions (references/placement-and-domains.md, references/prompt-and-capability-injection.md)
3. Continue this skill

Success: placement + inject doctrine + project rules. Failure: invented folders or external frameworks.

When to use

Core doctrine

LangGraph = control flow. MCP/local tools = capabilities under graph. Small graph state (refs, summaries). Checkpointer = full state; context window = LLM view — separate.

System prompt: compose base_invariant (motor) + injected (product persona) per LLM invoke. Never persist composed system/persona text in graph state, checkpointer, or durable messages. Summary SystemMessage at index 0 ≠ full system — references/prompt-and-capability-injection.md, references/message-content-blocks.md.

Locale: conversation-observed turnLocale (detection-first from human messages ± intent slots); configurable.locale weak hint only; Intl formatters in code — not fixed bootstrap locale. references/evidence-and-fidelity.md, templates/snippets/conversation-locale.ts.snippet.

Operator progress (JSON planner hops): greenfield streamingsse with a planner/analyst that emits structured executionPlan (no bindTools on that hop) MUST persist userFacingIntent (or analysis.userFacingIntent) + executionPlan on AgentState. userFacingIntent language MUST match the current user message (last HumanMessage) — not English unless that message is English; not product default locale. Machine intent stays English for audit. Emit SSE thinking from that field at node entry of the next hop — not responsestreaming, not in durable messages. Hop 0 uses generic copy from conversation/presentation/ (or locale) in the same operator language. Open ReAct + ToolNode uses toolstarted / toolfinished only. Details: templates/contracts/planner-contract.md, references/streaming-and-hitl.md.

Three capability kinds bind to the model:

Wire names must match ^[a-zA-Z0-9_-]{1,128}$ (use __ separators; colons only in internal ids). Colon in a new wire name is Critical on review.

Pre-change gates

Before new file or inject/bind change: complete all three. No code until posted.

1. Placement Decision Block

### Placement Decision Block

- Artifact: …
- Type: …
- Target path: …
- Layer: …
- Refs: placement-and-domains.md
- do_not_create_under: […]

Full matrix: references/placement-and-domains.md.

2. Prompt / Capability plan

### Prompt / Capability plan

- Compose: base_invariant + injected (rebuild per invoke; not in state/checkpointer/durable messages)
- Motor (`base_invariant`): [gather-no-Markdown / sole-writer / tool discipline / JSON planner userFacingIntent is SSE not Markdown / …]
- Product (`injected`): canonical path + persona/tone notes; mode-resolved: yes/no; modes: [...]; resolver: ...
- System layers touched: […]
- Canonical prompt path: …
- Session overlay: yes/no
- Bind list: […]
- Auto-inject skills: […] (exclusive of bind for same id)
- Truncate caps: tool vs skill body
- Bind parity: …
- Spec paths to sync: […]

Full doctrine: references/prompt-and-capability-injection.md.

3. Spec Sync Gate

Nodes, edges, state, capabilities, recursion_limit, or wire names change: update graph-spec.md same delivery. Stale archive ≠ SoT — sync spec to intended runtime; do not force-fit live code to stale archive.

Reference map

Load on demand — do not memorize whole files.

Templates: templates/agent-runtime/ (greenfield tree), templates/graph-spec.md, templates/contracts/, templates/snippets/ (brownfield patches).

Session inputs

Orphan recovery (brownfield first)

Runtime disorganized or team blocked:

1. Read references/orphan-recovery-checklist.md; score project.
2. Gap report: structure, placement, context window, inject/bind parity, MCP governance, HTTP, spec sync.
3. Ordered fix plan (one phase per message if large).
4. Implement via ns-coder.

No new graph nodes or MCP servers until layout + governance baselines pass.

Build workflow (greenfield or post-recovery)

Phase 0 — Spec gate

If graph-spec.md is missing, create it from templates/graph-spec.md. Minimum sections: locked header (framework, architecture, interactionmode), domain ownership, prompt composition, state schema, nodes table, edges, interrupts, memory, capability bind/inject table, recursionlimit, HTTP routes.

If the user has no architecture decision yet, stop and invoke ns-multi-agent-architect first.

Phase 1 — Skeleton

If {agentapiroot}/package.json is missing, run scripts/bootstrap-agent-runtime.mjs (references/bootstrap-agent-runtime.md). Then npm install && npm test in {agentapiroot}.

Do not assemble the tree from templates/snippets/. Do not copy another product. After copy: align src/graph/ with graph-spec.md; put domain in conversation/ + config/ via later phases / ns-coder.

Brownfield existing tree: align per references/runtime-layout.md — snippets for patches only.

Phase 2 — State and persistence

After bootstrap, verify these exist; do not re-scaffold. Fill only gaps vs graph-spec.md.

• AgentState with messages reducer (Annotation.Root or Zod + MessagesZodMeta).
• PostgresSaver in dev/prod; MemorySaver only in tests/setup.ts.
• Every invoke/stream: configurable.thread_id via buildRunConfig.
• JSON planner/analyst (no bindTools on that hop): declare executionPlan + userFacingIntent (or nested on analysis) in graph-spec.md state schema — templates/snippets/state.ts.snippet.

Phase 3 — LLM and messages

• Provider config in src/llm/ (infra only — no domain prompts).
• Prefer JSON mode + Zod parse for structured turns; avoid withStructuredOutput on OpenAI-compatible local servers.
• Normalize provider output via contentBlocks / content_blocks — see references/message-content-blocks.md.

Phase 4 — Context window (mandatory)

Implement per references/context-window-and-tokens.md:

• trimMessagesForLlm before every LLM call.
• normalizeMcpToolResult then truncateToolOutput before ToolMessage enters state.
• Separate CONTEXTSKILLBODYMAXCHARS for skill bodies (snippet skillBodyMaxChars).
• Optional summarizeOlderMessages with persisted compaction (RemoveMessage + rewrite) in the same agent-node return.

Never pass raw state.messages to the model.

Phase 5 — Capabilities

1. Local StructuredTools in src/tools/.
2. MCP: governed client — discovery → local allowlist → wire names → singleton client lifecycle (references/mcp-complex-access.md).
3. Skills: skills/*.md auto-discovered → use_skill__{id} or auto-inject (exclusive per id).

Apply references/capability-governance.md and references/prompt-and-capability-injection.md before bindTools. Enforce bind parity. Wire per-turn tool/MCP budgets from templates/snippets/tool-budget.ts.snippet when MCP or external tools are bound.

Phase 6 — HTTP and interaction mode

Brownfield missing dev-chat: recommend add — not Critical. Postman synced with live routes.

Phase 7 — Observability

Wire references/observability.md: initDb, runStorage, logLlmCall, logToolExecution, persist turn_decisions. LangSmith and OTel are opt-in.

Phase 8 — Evals and review

• Add suites per references/evals-and-gates.md.
• Run npm run build && npm test in {agentapiroot}.
• Invoke ns-reviewer on the diff; ask it to verify placement, inject, wire-name, and bind-parity anti-patterns when the diff touches agent-api.

Maintenance workflow

Ongoing work (not greenfield):

1. Three Pre-change gates
2. graph-spec.md matches intended graph after change
3. Layer: graph node, conversation, MCP, memory, HTTP
4. Read matching reference before edit
5. Minimal diff via ns-coder with placement/inject handoff
6. Postman on HTTP route change
7. Re-run orphan checklist items touched

MCP complex access (quick rules)

Multiple MCP servers, overlapping or large catalogs:

• Discovery filter — allowlisted tools/list before bind
• Local classify — read | write | destructive | admin; never trust server read-only flags
• Singleton client — one MultiServerMCPClient per process; no per-request stdio in prod
• Transport — Streamable HTTP deployed; stdio local single-user dev only
• Errors — MCP isError: true = recoverable ToolMessage status: "error"; protocol fail may abort with HTTP/SSE failed
• Secrets — env or configurable / request payload only; never state or checkpointer

Full: references/mcp-complex-access.md.

Handoff to ns-coder

When implementation is approved, delegate with:

## LangGraph implementation task

- Root: {agent_api_root}
- Spec: path/to/graph-spec.md
- Phase: [number and name from this skill]
- target_paths: […]
- layer: […]
- do_not_create_under: […]
- injection_notes: [layers / bind vs auto-inject / caps]
- spec_paths_to_sync: […]
- References to apply: [list]
- Acceptance: build + test pass; orphan checklist items [n] resolved; placement + inject + bind parity verified
- Review: ns-reviewer after tests — must check placement, inject, wire names (`:`), bind parity (load ns-langgraph-agents anti-patterns when diff touches agent-api)

Stay here for diagnosis, spec, placement, governance design, and greenfield bootstrap copy. ns-coder for feature diffs. Version-scoped features: ns-spec-driven.

Stop conditions

Related skills (ownership)

Forbidden

• Emitting planner userFacingIntent as response_streaming or as Markdown in messages (SSE thinking only; composer remains sole Markdown writer)
• Writing userFacingIntent in a language other than the current user message (e.g. English progress when the operator wrote Portuguese)
• Persisting composed system/persona prompt (base_invariant + injected) — or secrets/API keys — in graph state, checkpointer, or durable messages (rebuild system text per invoke)
• Treating bootstrap / .env / configurable.locale as primary locale SoT, or persisting sticky thread locale (use conversation-observed turnLocale + Intl)
• Passing unbounded tool/MCP output into state.messages
• Applying tool/MCP truncate caps to skill bodies (use CONTEXTSKILLBODYMAXCHARS)
• Trusting MCP tool metadata for security classification
• Spawning stdio MCP subprocesses per HTTP request in production
• memory/ compiling StateGraph
• Domain qualify/conversation prompts in src/llm/
• Locale/presentation under graph/
• Nudge as fake HumanMessage
• Tests under src/
• Greenfield tree from snippets or another product — use templates/agent-runtime/ only