LangSmith Deployment Middleware - Nevermined Documentation

A Starlette ASGI middleware (PaymentMiddleware) that gates a LangGraph agent deployed to LangSmith Deployment (formerly LangGraph Platform) with the Nevermined x402 payment flow. Use the build_payment_app factory for one-line wiring via the http.app field in langgraph.json. For tool-time gating inside an agent (@requires_payment decorator), see LangChain Integration. The two integrations are complementary — that page is for protecting tools the agent calls; this page is for protecting the agent’s HTTP entry point.

Installation

pip install payments-py[langsmith]

The [langsmith] extra pulls fastapi, starlette, and langsmith.

Exports

from payments_py.langsmith import (
    PaymentMiddleware,       # Starlette BaseHTTPMiddleware subclass
    RouteConfig,             # dataclass for per-route pricing
    X402_HEADERS,            # dict of x402 v2 header names
    build_payment_app,       # FastAPI app factory (recommended)
)

`build_payment_app`

The recommended entry point — returns a FastAPI app pre-wired with PaymentMiddleware. Mount the returned app in langgraph.json’s http.app field.

Signature

def build_payment_app(
    payments: Payments,
    routes: Optional[Dict[str, Union[RouteConfig, dict]]] = None,
) -> FastAPI: ...

Why FastAPI and not plain Starlette

langgraph-api versions prior to a known internal OpenAPI fix crash on plain Starlette http.app wrappers — update_openapi_spec falls through to Starlette’s SchemaGenerator which YAML-parses internal endpoint docstrings and chokes on them. app.openapi() (FastAPI’s own generator) takes a clean path. build_payment_app returns a FastAPI app so users do not need to know about this upstream bug. The middleware class itself (PaymentMiddleware) is a starlette.middleware.base.BaseHTTPMiddleware and works on both Starlette and FastAPI — only the outer app wrapper matters.

Example

# nvm_app.py
import os
from payments_py import Payments, PaymentOptions
from payments_py.langsmith import build_payment_app, RouteConfig

payments = Payments.get_instance(
    PaymentOptions(
        nvm_api_key=os.environ["NVM_API_KEY"],
        environment=os.environ.get("NVM_ENVIRONMENT", "sandbox"),
    )
)

app = build_payment_app(
    payments=payments,
    routes={
        "POST /threads/{thread_id}/runs/wait": RouteConfig(
            plan_id=os.environ["NVM_PLAN_ID"],
            credits=int(os.environ.get("NVM_CREDITS_PER_INVOKE", "1")),
        ),
    },
)

// langgraph.json
{
  "graphs": { "my_agent": "./src/agent.py:graph" },
  "http": { "app": "./nvm_app.py:app" },
  "env": ".env"
}

langgraph dev and langgraph up both honor the http.app field — the middleware composes around LangSmith Deployment’s built-in routes (/runs, /threads/{id}/runs, /assistants, etc.).

Lifecycle

The middleware implements the canonical x402 verify-then-work-then-settle ordering inside one HTTP cycle:

Request in
  ├─ PaymentMiddleware.dispatch:
  │     ├─ resolve scheme + network from plan metadata (cached)
  │     ├─ build the x402 PaymentRequired envelope
  │     ├─ read payment-signature header
  │     │     └─ missing → 402 + envelope in payment-required header
  │     ├─ facilitator.verify_permissions(...)
  │     │     └─ invalid → 402 + envelope in payment-required header
  │     ├─ stash PaymentContext on request.state
  │     ├─ await call_next(request)            ← agent runs
  │     ├─ if response is 2xx:
  │     │     ├─ facilitator.settle_permissions(...)
  │     │     ├─ on success → attach settlement receipt to payment-response header
  │     │     └─ on failure → log + return response unchanged at 200
  │     └─ else: skip settle (no charge for failed runs)
  │
Response out

Agent exceptions propagate naturally to the ASGI runtime as 5xx — buyers are not charged for failed runs. Settle failures after a successful 2xx response are logged at ERROR and do not surface to the client (the buyer already received the value).

`RouteConfig`

Per-route pricing. Routes that don’t match an incoming request pass through ungated.

Fields

Field	Type	Default	Purpose
`plan_id`	`str`	required	The Nevermined plan ID gating this route
`credits`	`int` or `Callable[[Request], int \| Awaitable[int]]`	`1`	Static or dynamic credits to charge
`agent_id`	`str \| None`	`None`	Optional — surfaces in the envelope and on `nvm.*` metadata for per-agent reconciliation
`network`	`str \| None`	`None`	Override the auto-resolved network
`scheme`	`str \| None`	`None`	Override the auto-resolved scheme
`description`	`str \| None`	`None`	Free-text description for the envelope
`mime_type`	`str \| None`	`None`	Expected response MIME type

Route matching

Routes are keyed as "METHOD /path". Path parameters can use either Starlette :param or FastAPI/LangGraph {param} syntax — both match by position against the incoming request path. Examples that all match POST /threads/abc-123/runs/wait:

"POST /threads/{thread_id}/runs/wait"
"POST /threads/:thread_id/runs/wait"

For LangSmith Deployment specifically, the only path that fits the verify-work-settle lifecycle in one HTTP cycle is POST /threads/{thread_id}/runs/wait (or its stateless counterpart POST /runs/wait). Background runs (POST /runs) return immediately; streaming runs (POST /runs/stream) lose streaming due to body buffering (see Limitations).

`PaymentMiddleware` (direct use)

build_payment_app is the recommended entry point. If you need a custom Starlette or FastAPI app (e.g. additional middleware, custom routes), use PaymentMiddleware directly:

from fastapi import FastAPI
from starlette.middleware import Middleware
from payments_py.langsmith import PaymentMiddleware, RouteConfig

app = FastAPI(
    middleware=[
        Middleware(
            PaymentMiddleware,
            payments=payments,
            routes={"POST /runs/wait": RouteConfig(plan_id="...", credits=1)},
        ),
    ]
)

Observability

When LANGSMITH_TRACING=true is set, the middleware opens a top-level nvm:x402-request trace per gated request, with nvm:verify and nvm:settlement child spans nested under it. Both child spans carry the same nvm.* metadata the decorator emits (plan_ids, scheme, network, payer, credits_redeemed, balance.after, tx_hash, payment_token abbreviated, verify/settle durations) — see the Observability section in LangChain Integration. The same metadata is also attached to the parent trace. The graph’s own LangGraph-emitted trace appears as a sibling top-level trace, not a child of nvm:x402-request — langgraph-api initiates the graph trace at the graph-invocation boundary, independent of our middleware’s trace context. Verification failures (missing token, invalid signature, insufficient credits) raise PaymentRequiredError inside the verify_span so LangSmith marks the parent + child as failed via the canonical context-manager exit path. Settle failures after a successful 2xx mark the settle span as failed but leave the parent trace successful (matching the buyer-visible outcome).

Limitations

Streaming responses are buffered. The middleware reads the downstream response body in full before attaching the payment-response settlement header. SSE / /runs/stream endpoints become blocking-then-bulk. Gate /runs/wait only, or accept the trade-off.
Python only. LangSmith Deployment’s custom-app surface is documented as Python-only by LangChain. TypeScript variant tracked in the LangChain integration epic (TS-3).
Sync I/O in async dispatch. The four sync SDK calls (resolve_scheme, resolve_network, verify_permissions, settle_permissions) are wrapped in asyncio.to_thread(...) so they do not block the event loop. langgraph dev’s blocking-call detector treats unwrapped sync HTTP calls as fatal warnings; the wrapping is load-bearing.

Documentation Index

​Installation

​Exports

​build_payment_app

​Signature

​Why FastAPI and not plain Starlette

​Example

​Lifecycle

​RouteConfig

​Fields

​Route matching

​PaymentMiddleware (direct use)

​Observability

​Limitations

​See also