neograph-engine 0.2.0

C++ graph agent engine — Python bindings (LangGraph-equivalent semantics, microsecond engine overhead)

NeoGraph

A C++ Graph Agent Engine — with Python bindings
Microsecond tail latency under 10k concurrent requests on 512 MB.
LangGraph's semantics, without the Python runtime tax — and now reachable from Python too.

Concepts · Quick Start · Python Binding · C++ Examples · Python Examples · Troubleshooting · API Reference · Doxygen · vs LangGraph · Benchmarks

---

What is NeoGraph?

NeoGraph is a C++17 graph-based agent orchestration engine that brings LangGraph-level capabilities to C++. Define agent workflows as JSON, execute them with parallel fan-out, checkpoint state for time-travel debugging, and integrate any LLM provider — all without Python.

#include <neograph/neograph.h>
#include <neograph/llm/openai_provider.h>
#include <neograph/graph/react_graph.h>

auto provider = neograph::llm::OpenAIProvider::create({
    .api_key = "sk-...", .default_model = "gpt-4o-mini"
});
auto engine = neograph::graph::create_react_graph(provider, std::move(tools));

neograph::graph::RunConfig config;
config.input = {{"messages", json::array({{{"role","user"},{"content","Hello!"}}})}};
auto result = engine->run(config);

Why NeoGraph?

Python + LangGraph	C++ + NeoGraph (measured)
~500 MB runtime (Python + deps)	1.1 MB static binary (stripped, example_plan_executor)
~300 MB steady RSS	2.9 MB peak RSS (Plan & Executor run)
2–8 s import / cold start	< 250 ms end-to-end (crash + resume cycle included)
GIL-limited parallelism	asio::thread_pool fan-out + lock-free RequestQueue
Cloud / server only	Raspberry Pi Zero 2W, Jetson, drones, IoT, edge

All figures are from example_plan_executor on x86_64 Linux built with CMAKE_BUILD_TYPE=MinSizeRel, -ffunction-sections -fdata-sections, -static-libstdc++ -static-libgcc -Wl,--gc-sections, then stripped. Only runtime dependency is libc.so.6. See the Benchmarks section below for the reproduction command.

NeoGraph is the only graph agent engine for C++. If you're building agents in robotics, embedded systems, games, high-frequency trading, or anywhere Python isn't an option — this is it.

Python Binding

NeoGraph also ships as a pip-installable Python package, so the same C++ engine can drive a LangGraph-style workflow from a Jupyter notebook, a Gradio app, or a FastAPI service:

pip install neograph-engine

Five-second demo (no API key)

The shortest thing that proves the install worked — one decorator-defined node, run it, read the output:

import neograph_engine as ng

@ng.node("greet")
def greet(state):
    return [ng.ChannelWrite("messages",
        [{"role": "assistant", "content": f"Hello, {state.get('name')}!"}])]

definition = {
    "name": "demo",
    "channels": {"name":     {"reducer": "overwrite"},
                 "messages": {"reducer": "append"}},
    "nodes":    {"greet": {"type": "greet"}},
    "edges":    [{"from": ng.START_NODE, "to": "greet"},
                 {"from": "greet",       "to": ng.END_NODE}],
}

engine = ng.GraphEngine.compile(definition, ng.NodeContext())
result = engine.run(ng.RunConfig(thread_id="t1", input={"name": "NeoGraph"}))

print(result.output["channels"]["messages"]["value"])
# [{'role': 'assistant', 'content': 'Hello, NeoGraph!'}]

ReAct agent with a real LLM

import neograph_engine as ng
from neograph_engine.llm import OpenAIProvider

class CalcTool(ng.Tool):
    def get_name(self):       return "calc"
    def get_definition(self): return ng.ChatTool(name="calc", description="multiply by 2",
        parameters={"type":"object","properties":{"x":{"type":"number"}}})
    def execute(self, args):  return str(args["x"] * 2)

ctx = ng.NodeContext(
    provider=OpenAIProvider(api_key="sk-..."),
    tools=[CalcTool()],
    instructions="Use `calc` for arithmetic.",
)

definition = {
    "name": "react",
    "channels": {"messages": {"reducer": "append"}},
    "nodes":    {"llm": {"type": "llm_call"}, "dispatch": {"type": "tool_dispatch"}},
    "edges":    [{"from": ng.START_NODE, "to": "llm"}, {"from": "dispatch", "to": "llm"}],
    "conditional_edges": [{"from": "llm", "condition": "has_tool_calls",
                           "routes": {"true": "dispatch", "false": ng.END_NODE}}],
}
engine = ng.GraphEngine.compile(definition, ctx)
result = engine.run(ng.RunConfig(thread_id="t1",
    input={"messages": [{"role": "user", "content": "What is 21 * 2?"}]},
    max_steps=10))

Reading the output

engine.run(...) returns a RunResult with these fields:

Field	Type	Meaning
output	dict	Final state — {"channels": {...}, "global_version": int}. Use output["channels"][name]["value"] to read a channel.
interrupted	bool	True if the run paused at an interrupt_before / interrupt_after / NodeInterrupt.
interrupt_node	str	Name of the node that triggered the interrupt (when interrupted).
interrupt_value	dict	Diagnostic payload — {"reason": ...} or {"message": ...}.
checkpoint_id	str	ID of the latest checkpoint saved during the run. Pass to engine.resume_async(checkpoint_id=...) to continue.
execution_trace	list[str]	Node names in the order they executed — useful for debugging routing.

RunConfig mirrors the LangGraph RunnableConfig idea:

Field	Default	Meaning
thread_id	required	Conversation / session identifier — keeps checkpoint streams separate.
input	{}	Initial channel values — keys must match the graph's channels definition.
max_steps	25	Super-step ceiling; ReAct loops typically need 10+.
stream_mode	StreamMode.OFF	Bitmask: EVENTS | TOKENS | DEBUG | VALUES | UPDATES | ALL. Only consulted by run_stream / run_stream_async.

Built-in reducers

Channels need a reducer — how new writes combine with existing values. Two built-ins ship today:

Reducer	Behavior	Typical use
"overwrite"	New value replaces old.	Single-value channels: name, current_question, intermediate scratch.
"append"	New list concatenated to existing list.	Conversation history, intermediate results, anything you want to accumulate across nodes.

Custom reducers register from Python (since v0.1.9):

ng.ReducerRegistry.register_reducer("sum",
    lambda current, incoming: (current or 0) + incoming)

# Now `"reducer": "sum"` works in your channel definitions.

Same pattern for conditional routing — ng.ConditionRegistry.register_condition("name", fn) where fn(state) -> str returns one of the route keys.

What's covered by the binding

• Engine surface — GraphEngine.compile / run / run_stream / run_async / run_stream_async / resume_async / get_state / update_state / fork, RunConfig, RunResult, set_worker_count, set_checkpoint_store, set_node_cache_enabled.
• Custom Python nodes — subclass neograph_engine.GraphNode, register via NodeFactory.register_type or the @neograph_engine.node decorator. Engine dispatches under proper GIL handling, including from fan-out worker threads.
• Custom Python tools — subclass neograph_engine.Tool, pass into NodeContext(tools=[...]). Engine takes ownership at compile time.
• Async — every *_async binding returns an asyncio.Future bound to the calling thread's running loop. Stream callbacks are hopped to the loop thread via loop.call_soon_threadsafe so callbacks run where asyncio expects.
• Checkpoints — InMemoryCheckpointStore always; PostgresCheckpointStore when the binding is built from source with -DNEOGRAPH_BUILD_POSTGRES=ON (libpq bundling for the PyPI wheel is pending).
• OpenAI Responses over WebSocket — SchemaProvider(schema="openai_responses", use_websocket=True).

Wheels: Linux x86_64 (manylinux_2_34), Linux aarch64 (manylinux_2_34), macOS arm64 (14+), Windows x64 (MSVC), for Python 3.9 → 3.13. **20 wheels

• sdist per release** via cibuildwheel.

See bindings/python/examples/ for the full example index — minimal graph, ReAct, HITL, intent routing, async, multi-agent debate, JSON graph round-trip, and a Gradio chat with a deep-research subgraph (Crawl4AI + Postgres optional).

The agent runtime that fits in L3 cache

NeoGraph's hot code path is small enough that N concurrent agents share one L3-resident working set. We measured this with Valgrind cachegrind on a Ryzen 7 5800X (Zen 3: 32 KB L1i/d 8-way, 32 MB L3 16-way), sweeping N = 1 → 10,000 concurrent requests through benchmarks/concurrent/bench_concurrent_neograph:

N	I refs	L3 instruction misses	L3i miss rate	Native p50
1	5.3 M	4,313	0.08%	17 µs
10	5.9 M	4,304	0.07%	16 µs
100	11.8 M	4,320	0.04%	6 µs
1,000	69.7 M	4,327	0.01%	6 µs
10,000	648 M	4,329	0.00%	5 µs

L3 instruction misses stay flat at ~4,320 across four orders of magnitude of N. The unique hot code working set is roughly 4,330 × 64 B = 277 KB — 0.85 % of the 32 MB L3. At N = 10,000 we processed 648 million instructions and only 4,329 of them reached DRAM (≈ 1 miss per 150,000 instructions).

Native per-request latency drops from 17 µs (cold) to 5 µs (warm) as N grows — the 3.4× improvement is pure I-cache warming. Throughput at N = 10,000 is ~1.1 M req/s on the single thread pool, with 5.2 MB peak RSS (≈ 100 B / agent marginal cost).

Why this matters: DRAM access on Zen 3 is ~250 cycles vs ~46 for an L3 hit — roughly 5.5× slower per access. If NeoGraph's working set had overflowed L3 (as Python interpreters + dict-heavy state typically do), the same N = 10,000 sweep would have paid +420 to +840 ms in memory stalls instead of the measured 9 ms total wall time — 47–94× slower depending on how much of the miss chain reaches DRAM. The whole L3 stays available for your workload (conversation history, embeddings, tool responses): the engine itself is a rounding error.

Reproduce:

g++ -std=c++20 -O2 -DNDEBUG -Iinclude -Ideps -Ideps/yyjson -Ideps/asio/include \
    -DASIO_STANDALONE benchmarks/concurrent/bench_concurrent_neograph.cpp \
    build-release/libneograph_core.a build-release/libyyjson.a -pthread -o bench_ng

valgrind --tool=cachegrind --cache-sim=yes \
    --I1=32768,8,64 --D1=32768,8,64 --LL=33554432,16,64 ./bench_ng 10000

Holds end-to-end with a real LLM in the loop

The L3 story survives full-stack production: we point NeoGraph at a locally-hosted Gemma-4 E2B (Q4_K_M, 4.65 B params, 2.9 GB GGUF) served by TransformerCPP's OpenAI-compatible HTTP endpoint — zero NeoGraph code changes, just OpenAIProvider::Config::base_url = "http://localhost:8090". See examples/31_local_transformer.cpp.

	Pure NeoGraph	NeoGraph + local Gemma (HTTP)
L3 instruction misses	4,320	7,262
Hot code working set	277 KB	465 KB (1.42% of L3)
Per-request TTFT	—	25–27 ms (curl baseline 9–10 ms → ~15 ms NeoGraph overhead)
Per-request total	—	146–213 ms @ 19–27 tokens (~130 tok/s)
NeoGraph agent RSS	5.2 MB	7.6 MB (+2.4 MB for httplib + JSON streaming)
Gemma server RSS	n/a	2.45 GB (mmap GGUF)
VRAM (RTX 4070 Ti)	n/a	3.06 GB

The inference process lives in a separate address space, so its 2.5 GB of model weights never touch NeoGraph's L3 cache lines. The agent's 465 KB working set stays L3-resident regardless of how large the model is. That's the architectural payoff of the two-process split: you can swap in a 70 B model without inflating the agent.

Burst-tested with 5 concurrent NeoGraph agents against the same server: aggregate wall 1.58 s / 5 requests (2.65× speedup from coroutine overlap). Per-agent throughput drops under queue pressure because the Gemma server doesn't implement continuous batching — that's a TransformerCPP concern, not an agent one. NeoGraph dispatched all 5 cleanly with no resource pressure and the RSS stayed flat at ~7 MB.

Quick Start

Requirements

• C++20 compiler — coroutines are on the public API surface as of 2.0.0. Verified toolchains:
  • GCC 13.3 — core + all tests green. The OpenAI Responses built-in-tools demo (example_openai_responses_ws_tools) is skipped because GCC 13 trips a coroutine-cleanup ICE (build_special_member_call at cp/call.cc:11096); the rest of the project is unaffected and the skip is automatic.
  • GCC 14.2+ — everything including the tools demo.
  • Clang 18+ — everything including the tools demo.
  • MSVC 2022 — core builds + non-Postgres tests in CI; runtime not yet load-tested.
• CMake 3.16+.
• OpenSSL (HTTPS), libpq (optional, Postgres checkpoint), SQLite3 (optional, SQLite checkpoint).

Platform support

Platform	Tier	Notes
Linux x86_64 (Ubuntu 24.04, GCC 13)	GA	Reference — 356/356 ctest green, 310/310 ASan+UBSan, Valgrind clean on coroutine subset
macOS (Apple Silicon, Clang)	beta	CI builds + non-Postgres tests; runtime differences (coroutine scheduling, SIGPIPE) not yet exercised in production
Linux ARM64 (Ubuntu 24.04, GCC 13)	alpha	306/306 ctest green via docker buildx --platform linux/arm64 under QEMU emulation — see Dockerfile.arm64-smoke. Native ARM64 hardware validation pending (Raspberry Pi, Graviton, Apple Silicon Linux). Stripped binary 0.81–0.88 MB.
Windows (MSVC 2022, x64)	alpha	CI builds + non-Postgres tests; MCP stdio (named-pipe overlapped) + PG async socket wrap written against MSDN spec but unvalidated under load

CI matrix (GitHub Actions): build-and-test (Ubuntu, full with PG service), build-macos, build-windows, bench-regression (3 committed floors). See CHANGELOG.md for the full stability rationale per platform.

Build

git clone https://github.com/fox1245/NeoGraph.git
cd NeoGraph
mkdir build && cd build
cmake ..
make -j$(nproc)

Run an example (no API key needed)

./example_custom_graph      # Mock ReAct agent
./example_parallel_fanout   # Parallel fan-out/fan-in (3 researchers run concurrently)
./example_send_command      # Dynamic Send + Command routing

Integration

FetchContent (recommended):

include(FetchContent)
FetchContent_Declare(neograph
  GIT_REPOSITORY https://github.com/fox1245/NeoGraph.git
  GIT_TAG main)
FetchContent_MakeAvailable(neograph)

target_link_libraries(my_app PRIVATE neograph::core neograph::llm)

add_subdirectory:

add_subdirectory(deps/neograph)
target_link_libraries(my_app PRIVATE neograph::core neograph::llm)

Features

Core Engine (neograph::core)

• JSON-defined graphs — No recompilation to change agent workflows
• Super-step execution — Pregel BSP model with cycle support
• Parallel fan-out/fan-in — asio::experimental::make_parallel_group on the engine's executor; opt-in asio::thread_pool for CPU-bound branches via set_worker_count(N)
• Send (dynamic fan-out) — Nodes spawn N parallel tasks at runtime
• Command (routing override) — Nodes control routing + state in one return
• Checkpointing — Full state snapshots at every super-step
• HITL (Human-in-the-Loop) — interrupt_before / interrupt_after + resume()
• State management — get_state(), update_state(), fork(), time-travel
• Dynamic breakpoints — throw NodeInterrupt("reason") from any node
• Retry policies — Per-node exponential backoff with configurable limits
• Stream modes — EVENTS | TOKENS | VALUES | UPDATES | DEBUG bitflags
• Subgraphs — Hierarchical composition via JSON (Supervisor pattern)
• Intent routing — LLM-based classification + dynamic routing
• Cross-thread Store — Namespace-based shared memory across threads
• Custom nodes — Register via NodeFactory with zero framework changes

LLM Providers (neograph::llm)

• OpenAIProvider — OpenAI, Groq, Together, vLLM, Ollama (any OpenAI-compatible API)
• SchemaProvider — Claude, Gemini, and any custom provider via JSON schema
• Built-in schemas — "openai", "claude", "gemini" embedded at build time
• Agent — ReAct loop with streaming support

MCP Client (neograph::mcp)

• HTTP transport — JSON-RPC 2.0 over Streamable HTTP, session-aware
• stdio transport — MCPClient({"python", "server.py"}) spawns the MCP server as a child subprocess and exchanges newline-delimited JSON-RPC over its stdin / stdout; subprocess lifetime is tied to the last MCPTool that references it
• Tool discovery — get_tools() auto-discovers tools from either transport; returned MCPTools plug straight into Agent / GraphEngine

Utilities (neograph::util)

• RequestQueue — Lock-free worker pool with backpressure (moodycamel::ConcurrentQueue)

Examples

#	Example	Description	API Key
01	react_agent	Basic ReAct agent with calculator tool	Required
02	custom_graph	JSON-defined graph with mock provider	No
03	mcp_agent	Real MCP server tool integration	Required
04	checkpoint_hitl	Checkpointing + Human-in-the-Loop (interrupt/resume)	No
05	parallel_fanout	Parallel fan-out/fan-in via make_parallel_group (3 workers)	No
06	subgraph	Hierarchical graph composition (Supervisor pattern)	No
07	intent_routing	Intent classification + expert routing	No
08	state_management	get_state / update_state / fork / time-travel	No
09	all_features	All 6 advanced features in one demo	No
10	send_command	Dynamic Send fan-out + Command routing override	No
11	clay_chatbot	Multi-turn chatbot UI (Clay + Raylib)	Optional
12	rag_agent	RAG agent with in-memory vector search (CLI)	Required (OpenAI)
13	openai_responses	ReAct via OpenAI /v1/responses through SchemaProvider	Required (OpenAI)
14	plan_executor	Plan & Executor: 5-way Send + crash/resume via pending_writes	No
15	reflexion	Self-critique loop until acceptance (Anthropic)	Required (Anthropic)
16	tree_of_thoughts	BFS over LLM thought branches, top-k pruning	Required (Anthropic)
17	self_ask	Follow-up decomposition across multiple hops	Required (Anthropic)
18	multi_agent_debate	Proponent / opponent / judge pattern	Required (Anthropic)
19	rewoo	Reasoning WithOut Observation — plan once, fan out, synthesize	Required (Anthropic)
20	mcp_hitl	MCP + checkpoint HITL (interrupt_before tool dispatch, resume after approval)	Required (OpenAI)
21	mcp_fanout	Parallel MCP tool calls via Send fan-out inside one super-step	No
22	mcp_stdio	MCP over stdio transport — subprocess MCP server spawned by the client	Required (OpenAI)
23	mcp_multi	One agent routing tools across two MCP servers (HTTP + stdio)	Required (OpenAI)
24	mcp_feedback	Human-feedback loop — draft answer, operator pushes back, agent revises	Required (OpenAI)
25	deep_research	open_deep_research-style multi-step web research loop (Crawl4AI + Anthropic)	Required (Anthropic)
26	postgres_react_hitl	ReAct + Postgres-backed checkpoint HITL — survives process restart	Required (Anthropic + Postgres)
27	async_concurrent_runs	Hosting many concurrent agent runs on one shared asio::io_context	No
28	corrective_rag	Corrective RAG (arXiv:2401.15884) — retrieve → evaluator routes to refine / web / both → generate, all over /v1/responses	Required (OpenAI)
29	responses_envelope	Wire-level dump of /v1/responses's output[] envelope — debug/pedagogy aid for understanding tool-calling shape before SchemaProvider flattens it	Required (OpenAI)
30	reasoning_effort	Same prompt at reasoning.effort ∈ {none, low, medium, high} on a reasoning model — compares wall, hidden-CoT tokens, and answer	Required (OpenAI, reasoning model)

Every API-using example above auto-loads .env from the cwd or any parent directory via the bundled cppdotenv, so the recipe is just echo 'OPENAI_API_KEY=...' > .env && ./example_*. Process-environment values still take precedence if both are set.

Run with a real LLM

# Set your API key (auto-loaded by every API-using example via cppdotenv)
echo "OPENAI_API_KEY=sk-..." > .env

# ReAct agent with OpenAI
./example_react_agent

# MCP agent (start demo server first: python examples/demo_mcp_server.py)
./example_mcp_agent http://localhost:8000 "What time is it?"

# Visual chatbot
cmake .. -DNEOGRAPH_BUILD_CLAY_EXAMPLE=ON && make example_clay_chatbot
./example_clay_chatbot --live

Architecture

GraphEngine is a thin super-step orchestrator that delegates to four purpose-built classes extracted in the 0.1 refactor:

• GraphCompiler — pure JSON → CompiledGraph parser.
• Scheduler — signal-dispatch routing plus barrier accumulation.
• NodeExecutor — retry loop (async-native with timer-based backoff), parallel fan-out via asio::experimental::make_parallel_group, Send dispatch.
• CheckpointCoordinator — save / resume / pending-writes lifecycle behind a (store, thread_id) façade.

Each class has a dedicated unit-test suite so engine behaviour is verifiable without spinning up a full run. See docs/reference-en.md §7b for the full API surface.

Dependency Isolation

Link target	What gets pulled in
neograph::core	yyjson (compiled, bundled), asio (header-only, standalone)
neograph::core + llm	+ OpenSSL (httplib stays PRIVATE)
neograph::core + mcp	+ OpenSSL (httplib stays PRIVATE)
neograph::util	+ moodycamel::ConcurrentQueue (header-only)

httplib is never exposed to your code. core has zero network dependencies. Taskflow was removed in 3.0 — parallel fan-out now runs on asio's coroutine primitives (see Features).

Concurrency & Async

NeoGraph supports two concurrency models out of the box — pick the one that fits your hosting pattern:

• Thread-per-agent (sync) — run() / run_stream() / resume() dispatched onto any executor you already use. Safe up to roughly a thousand concurrent agents; ~5 µs engine overhead per call on a Release -O3 -DNDEBUG build (the super-step loop routes through run_sync(execute_graph_async) so both entry points share one coroutine path). Detailed below.
• Coroutine-based async — run_async() / run_stream_async() / resume_async() returning asio::awaitable<RunResult>. One asio::io_context hosts thousands of concurrent agents without a thread per run; all Provider / MCP / checkpoint I/O points are non-blocking co_await under the hood. Short intro below; full migration guide in docs/ASYNC_GUIDE.md.

Async (Stage 3)

#include <asio/co_spawn.hpp>
#include <asio/detached.hpp>
#include <asio/io_context.hpp>

asio::io_context io;
for (const auto& user : users) {
    asio::co_spawn(
        io,
        [&, user]() -> asio::awaitable<void> {
            RunConfig cfg;
            cfg.thread_id = user.session_id;
            cfg.input     = {{"messages", user.history}};
            auto result = co_await engine->run_async(cfg);
            handle(result);
        },
        asio::detached);
}
io.run();  // drives all agents on this thread

Stage 4 reality: engine->run_async() stays on the caller's executor end-to-end — every super-step suspension point (node dispatch, checkpoint I/O, parallel fan-out, retry backoff) is a real co_await. The three 50 ms steps above therefore overlap on one io_context thread and the wall time lands at ~50 ms, not 3 × 50 ms. One thread, N concurrent agents. For CPU-bound fan-out across cores, switch the driver to a shared asio::thread_pool — that's the pattern in benchmarks/concurrent/CONCURRENT.md where N = 10,000 finishes in 52 ms. Within a single run, the make_parallel_group fan-out overlaps too: three parallel-fanout researchers collapse from 370 ms sequential to 150 ms.

Custom nodes join the async path by overriding execute_async instead of execute:

class FetchNode : public GraphNode {
  public:
    asio::awaitable<std::vector<ChannelWrite>>
    execute_async(const GraphState& state) override {
        auto ex = co_await asio::this_coro::executor;
        auto res = co_await neograph::async::async_post(ex, /*...*/);
        co_return std::vector<ChannelWrite>{/*...*/};
    }
    std::string get_name() const override { return "fetch"; }
};

Async-shaped tools derive from AsyncTool:

class FetchTool : public neograph::AsyncTool {
  public:
    asio::awaitable<std::string>
    execute_async(const json& args) override { /* co_await HTTP */ }
    // sync execute() is final, routes through run_sync automatically.
};

See examples/27_async_concurrent_runs.cpp for the multi-agent pattern and examples/05_parallel_fanout.cpp for fan-out within one run.

Sync (thread-per-agent)

NeoGraph does not ship its own async runtime — it exposes synchronous run() / run_stream() / resume() and lets you pick the executor. A single compiled GraphEngine is safe to share across threads that invoke run() concurrently with distinct thread_ids, so hosting multi-tenant agent workloads is a matter of dispatching onto whatever executor you already use.

// One engine, many concurrent sessions — no external runtime required.
auto engine = GraphEngine::compile(def, ctx, std::make_shared<InMemoryCheckpointStore>());

std::vector<std::future<RunResult>> sessions;
for (const auto& user : users) {
    sessions.push_back(std::async(std::launch::async, [&engine, user]() {
        RunConfig cfg;
        cfg.thread_id = user.session_id;
        cfg.input = {{"messages", user.history}};
        return engine->run(cfg);
    }));
}
for (auto& f : sessions) handle(f.get());

Works the same way with an asio::thread_pool, a std::async-backed task system, or your web framework's worker pool — NeoGraph stays out of the executor decision. If you need CPU-parallel fan-out inside a single sync run() call (rather than N sync run()s on N threads), call engine->set_worker_count(N) once after compile() to install an engine-owned asio::thread_pool that run_parallel_async and the multi-Send branch dispatch onto.

Using the bundled RequestQueue

For multi-tenant servers that want a fixed worker pool with backpressure (rejecting new sessions when the queue is saturated instead of unbounded memory growth), link neograph::util and use the built-in lock-free queue — no external executor needed:

#include <neograph/util/request_queue.h>
using namespace neograph::util;

RequestQueue pool(16, 1000);           // 16 workers, max 1000 pending sessions
auto engine = GraphEngine::compile(def, ctx,
                                   std::make_shared<InMemoryCheckpointStore>());

std::vector<RunResult>          results(users.size());
std::vector<std::future<void>>  futs;

for (size_t i = 0; i < users.size(); ++i) {
    auto [accepted, fut] = pool.submit([&, i]() {
        RunConfig cfg;
        cfg.thread_id = users[i].session_id;
        cfg.input     = {{"messages", users[i].history}};
        results[i]    = engine->run(cfg);
    });
    if (!accepted) {
        // Backpressure: queue is full — shed load, return 503, retry later, …
        reject(users[i]);
        continue;
    }
    futs.push_back(std::move(fut));
}

for (auto& f : futs) f.get();           // propagates exceptions from run()

auto s = pool.stats();
log("pending={} active={} completed={} rejected={}",
    s.pending, s.active, s.completed, s.rejected);

submit() returns {accepted, std::future<void>}: capture the RunResult via a shared output slot (as above) or a per-task std::promise<RunResult>. The queue is backed by moodycamel::ConcurrentQueue (lock-free) and workers park on a condvar when idle — no busy-spin.

Rules for safe concurrent use:

• Configuration mutators (set_retry_policy, set_checkpoint_store, set_store, own_tools, …) must be called before any concurrent run(). Treat the engine as frozen after the first dispatch.
• Concurrent run() calls sharing the same thread_id do not crash but produce unspecified checkpoint interleaving. Serialize per-session access yourself if you need deterministic history.
• Custom GraphNode subclasses must be stateless or self-synchronized. Node instances are owned by the engine and reused across every run on every thread — per-run scratch data belongs in graph channels, not in node member variables.
• User-supplied CheckpointStore, Store, Provider, and Tool implementations must be thread-safe. The bundled InMemoryCheckpointStore and InMemoryStore already are.

Persistent checkpointing with PostgreSQL

For multi-process deployments or when checkpoints must survive a restart, link neograph::postgres and swap InMemoryCheckpointStore for PostgresCheckpointStore:

#include <neograph/graph/postgres_checkpoint.h>

auto store = std::make_shared<PostgresCheckpointStore>(
    "postgresql://user:pass@host:5432/dbname");
auto engine = GraphEngine::compile(def, ctx, store);

The schema mirrors LangGraph's PostgresSaver (three tables prefixed neograph_* to coexist with LangGraph state in the same database) and deduplicates channel values by (thread_id, channel, version). A 1000-step session that touches one channel per super-step costs roughly O(steps + channels) blob rows instead of O(steps × channels).

Build flag: -DNEOGRAPH_BUILD_POSTGRES=ON (default). Requires libpqxx-dev (apt) / libpqxx-devel (rpm). Set the flag OFF to skip the dependency entirely.

Running the integration tests: spin up a throwaway local PG and point the test binary at it:

docker run -d --rm --name neograph-pg-test \
    -e POSTGRES_PASSWORD=test -e POSTGRES_DB=neograph_test \
    -p 55432:5432 postgres:16-alpine

NEOGRAPH_TEST_POSTGRES_URL='postgresql://postgres:test@localhost:55432/neograph_test' \
    ctest --test-dir build -R PostgresCheckpoint --output-on-failure

Without the env var the 19 PG tests are GTEST_SKIP'd so the rest of the suite stays green on machines without a Postgres handy.

Coverage: tests/test_graph_engine.cpp contains ConcurrentRunDifferentThreadIds (16 threads × 25 runs = 400 parallel executions, validates per-session output + checkpoint isolation) and ConcurrentRunSameThreadIdNoCrash (8 threads × 50 runs on one shared thread_id, validates crash-free behavior).

JSON Graph Definition

{
  "name": "research_agent",
  "channels": {
    "messages": {"reducer": "append"},
    "findings": {"reducer": "append"},
    "__route__": {"reducer": "overwrite"}
  },
  "nodes": {
    "planner":    {"type": "llm_call"},
    "researcher": {"type": "tool_dispatch"},
    "classifier": {
      "type": "intent_classifier",
      "routes": ["deep_dive", "summarize"]
    },
    "inner_agent": {
      "type": "subgraph",
      "definition": { "...nested graph..." }
    }
  },
  "edges": [
    {"from": "__start__", "to": "planner"},
    {"from": "planner", "condition": "has_tool_calls",
     "routes": {"true": "researcher", "false": "classifier"}},
    {"from": "researcher", "to": "planner"},
    {"from": "classifier", "condition": "route_channel",
     "routes": {"deep_dive": "inner_agent", "summarize": "__end__"}}
  ],
  "interrupt_before": ["researcher"]
}

Comparison with LangGraph

Feature	LangGraph (Python)	NeoGraph (C++)
Graph engine	StateGraph	GraphEngine
Checkpointing	MemorySaver + Postgres/SQLite/Redis	CheckpointStore (interface) + InMemory + Postgres
HITL	interrupt_before/after	interrupt_before/after + NodeInterrupt
get_state / update_state	Yes	Yes
Fork	Yes	Yes
Time travel	get_state_history	get_state_history
Subgraphs	CompiledGraph as node	SubgraphNode (JSON inline)
Parallel fan-out	Static	make_parallel_group (+ opt-in asio::thread_pool)
Send (dynamic fan-out)	Send()	NodeResult::sends → parallel_group fan-out
Command (routing+state)	Command(goto, update)	NodeResult::command
Retry policy	RetryPolicy	RetryPolicy + exponential backoff
Stream modes	values/updates/messages	EVENTS/TOKENS/VALUES/UPDATES/DEBUG
Cross-thread Store	Store (Postgres)	Store (interface) + InMemory
Multi-LLM	LangChain required	SchemaProvider built-in (3 vendors)
MCP support	None (separate impl)	MCPClient built-in
Performance	Python (GIL)	C++20 coroutines + asio
Memory footprint	~300MB+	~10MB
Edge/embedded	Not possible	Raspberry Pi, Jetson, IoT

Project Structure

NeoGraph/
├── include/neograph/
│   ├── neograph.h              # Convenience header
│   ├── types.h                 # ChatMessage, ToolCall, ChatCompletion
│   ├── provider.h              # Provider interface (abstract)
│   ├── tool.h                  # Tool interface (abstract)
│   ├── graph/
│   │   ├── types.h             # Channel, Edge, NodeContext, GraphEvent,
│   │   │                       # NodeInterrupt, Send, Command, RetryPolicy, StreamMode
│   │   ├── state.h             # GraphState (thread-safe channels)
│   │   ├── node.h              # GraphNode, LLMCallNode, ToolDispatchNode,
│   │   │                       # IntentClassifierNode, SubgraphNode
│   │   ├── engine.h            # GraphEngine, RunConfig, RunResult
│   │   ├── checkpoint.h        # CheckpointStore, InMemoryCheckpointStore
│   │   ├── store.h             # Store, InMemoryStore (cross-thread memory)
│   │   ├── loader.h            # NodeFactory, ReducerRegistry, ConditionRegistry
│   │   └── react_graph.h       # create_react_graph() convenience
│   ├── llm/
│   │   ├── openai_provider.h   # OpenAI-compatible provider
│   │   ├── schema_provider.h   # Multi-vendor LLM (JSON schema driven)
│   │   ├── agent.h             # ReAct agent loop
│   │   └── json_path.h         # JSON dot-path utilities
│   ├── mcp/
│   │   └── client.h            # MCP client + tool wrapper
│   └── util/
│       └── request_queue.h     # Lock-free worker pool
├── src/
│   ├── core/                   # 13 source files (engine + compiler/scheduler/executor/coordinator split)
│   ├── llm/                    # 3 source files
│   └── mcp/                    # 1 source file
├── schemas/                    # Built-in LLM provider schemas
│   ├── openai.json
│   ├── claude.json
│   └── gemini.json
├── deps/                       # Vendored dependencies
│   ├── yyjson/                 # Compiled C JSON library (yyjson.c + yyjson.h)
│   ├── asio/                   # Standalone asio (header-only, C++20 coroutines)
│   ├── httplib.h               # cpp-httplib (PRIVATE to llm/mcp)
│   ├── concurrentqueue.h       # moodycamel lock-free queue
│   ├── cppdotenv/              # .env loader (example 13)
│   ├── clay.h                  # Clay UI layout
│   └── clay_renderer_raylib.c  # Clay + raylib renderer glue (example 11)
├── benchmarks/                 # NeoGraph vs LangGraph engine-overhead bench
├── examples/                   # 18 runnable examples + Clay chatbot
└── scripts/
    └── embed_schemas.py        # Build-time schema embedding

CMake Targets

Target	Description	Dependencies
neograph::core	Graph engine + types	yyjson (bundled), asio (header-only), Threads
neograph::llm	LLM providers + Agent	core + OpenSSL (httplib PRIVATE)
neograph::mcp	MCP client	core + OpenSSL (httplib PRIVATE)
neograph::util	RequestQueue	core + concurrentqueue
neograph::async	asio HTTP/SSE helpers	core + OpenSSL

Build Options

Option	Default	Description
NEOGRAPH_BUILD_LLM	ON	Build LLM provider module
NEOGRAPH_BUILD_MCP	ON	Build MCP client module
NEOGRAPH_BUILD_UTIL	ON	Build utility module
NEOGRAPH_BUILD_POSTGRES	ON	Build PostgresCheckpointStore (libpq)
NEOGRAPH_BUILD_SQLITE	ON	Build SqliteCheckpointStore (libsqlite3)
NEOGRAPH_BUILD_EXAMPLES	ON	Build example programs
NEOGRAPH_BUILD_CLAY_EXAMPLE	OFF	Build Clay+Raylib chatbot (fetches Raylib)
BUILD_SHARED_LIBS	OFF	Build neograph_* as .so/.dylib instead of .a (Linux/macOS — Windows DLL exports not yet wired)

Shared library mode

Pass -DBUILD_SHARED_LIBS=ON at configure time to ship libneograph_core.so, libneograph_llm.so, libneograph_mcp.so, libneograph_async.so, and libneograph_sqlite.so instead of static archives. Build-tree binaries get an $ORIGIN-relative RPATH so they find the libraries beside themselves with no LD_LIBRARY_PATH gymnastics.

Trade-offs (Linux, stripped, measured 2026-04-25):

Configuration	Single agent binary	N agents on same host
Static (default)	~2.2 MB per agent	N × 2.2 MB
Shared	~0.25 MB per agent	N × 0.25 MB + 13.1 MB shared .so set (one-time)

Crossover at N≈7 agents. For deployments shipping multiple NeoGraph agents on the same host (or for staged-rollout scenarios where one subsystem like the LLM provider is patched independently of the rest) shared mode is strictly better. For a single-agent embedded edge deployment, static keeps everything in one self-contained binary.

Patch-update size example: replacing libneograph_llm.so (one subsystem, ~4 MB) updates every agent on the host without rebuilding or redeploying any of them.

Windows: BUILD_SHARED_LIBS=ON will warn at configure time and fail at link with undefined symbols — the public-symbol __declspec annotations have not yet been added to the headers. Use STATIC on Windows until that work lands.

Benchmarks

Engine overhead vs. Python graph/pipeline frameworks

Matched-topology, zero-I/O workloads: graph compiled once, invoked in a hot loop. Measures what the engine itself costs (dispatch, state writes, reducer calls) — no LLM, no sleep, no network.

Per-iteration engine overhead (µs, lower is better). All rows measured 2026-04-22 on the same x86_64 Linux host. NeoGraph built with Release -O3 -DNDEBUG (10-run median); Python rows are 3-run median through CPython 3.12.3.

Framework	seq (3-node chain)	par (fan-out 5 + join)	seq vs. NeoGraph
NeoGraph 3.0	5.0 µs	11.8 µs	1×
Haystack 2.28.0	144.1 µs	290.0 µs	28.8×
pydantic-graph 1.85.1	235.9 µs	286.1 µs¹	47.2×
LangGraph 1.1.9	656.7 µs	2,348.7 µs	131.3×
LlamaIndex Workflow 0.14.21	1,780.3 µs	4,683.5 µs	356.1×
AutoGen GraphFlow 0.7.5	3,209.2 µs	7,292.7 µs	641.8×

¹ pydantic-graph is a single-next-node state machine and cannot fan out; par is a serial 6-node emulation.

Whole-process metrics (warm-up + both workloads, 10k seq + 5k par iters):

	NeoGraph 3.0	best Python (Haystack)	worst (AutoGen)
Total elapsed	~0.16 s	2.91 s	68.29 s
Peak RSS	4.8 MB	80.3 MB	52.4 MB²
Parallel fan-out executor	asio::experimental::make_parallel_group	single-thread asyncio (GIL)	single-thread asyncio (GIL)

² AutoGen has a smaller RSS than LlamaIndex but its per-iter cost is 64× higher — different tradeoff axes. Full matrix in benchmarks/README.md.

Engine overhead disappears under LLM latency. A 500 ms OpenAI round trip swamps every engine; the per-iter gap only shows up in non-LLM nodes (data transforms, routing decisions, pure-compute tool calls) and in dense agent orchestration. Where it does show up, it shows up big: on a Raspberry Pi 4 / Jetson Nano / any SBC-class target, a 10–20× RAM delta is the difference between "fits" and "swap thrash."

Reproduction and methodology: benchmarks/README.md.

Burst concurrency (1 CPU / 512 MB sandbox)

What happens under thousands of simultaneous requests? Burst test: N requests submitted at t=0 to each engine, all-in / all-wait, inside a Docker cgroup limited to 1 CPU and 512 MB RAM — roughly a Raspberry Pi 4 process budget.

At N=10,000 concurrent requests in asyncio mode (the default deployment shape for every Python framework):

Engine	Wall	P99 latency	Peak RSS	Status
NeoGraph 3.0	52 ms	7 µs	5.5 MB	✅ 10000 / 0
pydantic-graph	886 ms	158 µs	42.6 MB	✅ 10000 / 0
Haystack	3.1 s	2.9 s	130.7 MB	✅ 10000 / 0
LangGraph	23.4 s	23.0 s	416.2 MB	✅ 10000 / 0
LlamaIndex	—	—	—	❌ OOM killed
AutoGen	—	—	—	❌ OOM killed

Two frameworks don't complete — LlamaIndex Workflow and AutoGen GraphFlow exhaust the 512 MB cgroup and get OOM-killed before 10k concurrent coroutines can drain. The remaining Python frameworks degrade rather than die, but their P99 latency grows linearly with N because the CPython GIL serializes every coroutine's CPU work. This is not a LangGraph-specific pathology — it shows up in every Python asyncio runtime.

NeoGraph 3.0 beats every Python asyncio runtime on throughput, tail latency, and RSS: 7 µs P99 at N=10k, ~76× lower RSS than LangGraph at the same load, and 3 orders of magnitude ahead of the GIL-serialized Python curves. Even pydantic-graph — the leanest Python state-machine — sits at 158 µs P99 and ~8× NeoGraph's RSS.

multiprocessing.Pool mode bypasses the GIL across worker processes but saturates at pool size and pays fork + pickle overhead; full numbers and the mp-mode story are in benchmarks/concurrent/CONCURRENT.md.

Size & cold-start footprint (Plan & Executor demo)

All numbers below were measured on x86_64 Linux (GCC 13) using example_plan_executor — a self-contained Plan & Executor demo that runs a 5-way Send fan-out, crashes sub-topic #2 on the first run, and resumes with the failure cleared. No LLM calls, no API keys, no network.

Binary size (MinSizeRel + static libstdc++ + strip)

Build configuration	Size
MinSizeRel -Os, static libstdc++, --gc-sections, stripped	1,203 KB (1.2 MB)

The MinSizeRel binary's only dynamic dependency is libc.so.6 — libstdc++ and libgcc_s are linked in statically. Drop it onto any Linux host with a matching libc and it runs. 3.0 is ~80 KB larger than 2.0 because asio's coroutine machinery (steady_timer, make_parallel_group, use_future) is pulled into the engine path; Taskflow was header-only and --gc-sections stripped most of it anyway, so its removal doesn't offset the coroutine growth.

Runtime footprint

Metric	Value
Peak RSS (full Plan & Executor run, crash + resume included)	2.9 MB
Wall-clock (cold start → both phases complete)	~720 ms
Dynamic dependencies	libc.so.6 only

example_plan_executor sleeps 120 ms per Send target to simulate an LLM call; the 5-way fan-out runs serially on the default single-threaded super-step loop (5 × 120 ms × 2 phases ≈ wall time). Call engine->set_worker_count(N) after compile() to get the 2.x-style multi-threaded fan-out (cuts this demo's wall time roughly in half on a 2-core host). Steady-state footprint (RSS) is unchanged between 2.0 and 3.0.

Reproduction

git clone https://github.com/fox1245/NeoGraph.git
cd NeoGraph

cmake -B build-minsize -S . \
    -DCMAKE_BUILD_TYPE=MinSizeRel \
    -DNEOGRAPH_BUILD_MCP=OFF \
    -DNEOGRAPH_BUILD_TESTS=OFF \
    -DCMAKE_CXX_FLAGS="-ffunction-sections -fdata-sections" \
    -DCMAKE_EXE_LINKER_FLAGS="-Wl,--gc-sections -static-libstdc++ -static-libgcc"

cmake --build build-minsize --target example_plan_executor -j$(nproc)

strip --strip-all build-minsize/example_plan_executor
ls -la    build-minsize/example_plan_executor        # binary size
ldd       build-minsize/example_plan_executor        # dynamic deps (libc only)
/usr/bin/time -v build-minsize/example_plan_executor  # peak RSS + wall time

What the numbers mean for embedded / robotics

• 1.1 MB static binary fits a Docker scratch image at ~1 MB, fits on-board flash of a Pixhawk companion computer, fits comfortably in a Jetson Orin boot partition. Python + LangGraph does not.
• 2.9 MB RSS means you can host 100+ concurrent agent sessions on an RPi Zero 2W (512 MB RAM) by sharing one compiled engine across threads — the Concurrency & Async section covers the pattern.
• < 250 ms cold start fits inside a drone watchdog reset window; a Python LangGraph process still hasn't finished import by then.
• libc.so.6 only makes cross-compilation trivial: pick glibc or musl and link — no transitive dependency hell.

Acknowledgments

Inspired by:

• LangGraph — Graph agent orchestration for Python
• agent.cpp — Local LLM agent framework for C++
• asio — Cross-platform C++ networking and coroutine primitives (the 3.0 engine runtime)
• Clay — High-performance UI layout library

Previously (2.x): also built on Taskflow for parallel fan-out. 3.0 replaced that path with asio::experimental::make_parallel_group to unify sync and async execution on one coroutine runtime.

License

MIT License. See LICENSE for details.

Third-party licenses: THIRD_PARTY_LICENSES.md