llmsessioncontract 0.2.0

Runtime monitor for LLM agent interaction protocols based on session type theory

llmcontract

A runtime monitor for LLM agent interaction protocols based on session type theory.

llmcontract lets you define communication protocols using a concise DSL inspired by session types, then monitor agent interactions at runtime to catch protocol violations the moment they happen.

Installation

pip install -e .

Protocol DSL

Protocols are written as strings using this syntax:

Syntax	Meaning
!label	Send action
?label	Receive action
!{a, b}	Internal choice (sender chooses)
?{a, b}	External choice (receiver chooses)
.	Sequence
rec X. ...X...	Recursion
end	Terminal state

Examples

A flight booking protocol — a strict linear sequence:

!SearchFlights.?FlightResults.!PresentOptions.?UserApproval.!BookFlight.?BookingConfirmation.end

A card payment protocol — with branching and recursion:

!CreateCard.?{CardCreated.rec X.!Transaction.?{TransactionOK.X, SessionEnd}, CardError}.end

Usage

from llmcontract import Monitor, Ok, Violation, Blocked

protocol = "!SearchFlights.?FlightResults.!BookFlight.?BookingConfirmation.end"
m = Monitor(protocol)

m.send("SearchFlights")    # Ok()
m.receive("FlightResults") # Ok()
m.send("BookFlight")       # Ok()
m.receive("BookingConfirmation") # Ok()
assert m.is_terminal

Catching violations

m = Monitor("!Ping.?Pong.end")
m.send("Ping")       # Ok()
m.send("Pong")       # Violation(expected=['?Pong'], got='!Pong')
m.send("Anything")   # Blocked('monitor halted after a previous violation')

Working with choices

protocol = "!CreateCard.?{CardCreated.!Done.end, CardError.end}"
m = Monitor(protocol)
m.send("CreateCard")       # Ok()
m.receive("CardError")     # Ok() — the receiver chose this branch
assert m.is_terminal

Recursion

protocol = "rec X.!Ping.?Pong.X"
m = Monitor(protocol)
for _ in range(100):
    m.send("Ping")     # Ok()
    m.receive("Pong")  # Ok()

Integration Layer

For real agent loops, llmcontract provides a client wrapper and tool middleware that share a single monitor — so the full interaction is tracked automatically.

Client Wrapper

Wraps any LLM client call. Checks !Send before calling the LLM and ?Receive after getting the response. SDK-agnostic — you provide a small adapter function.

from llmcontract import Monitor, MonitoredClient, LLMResponse, ToolCall

monitor = Monitor(
    "rec Loop.!Request.?{ToolCall.!ToolResult.Loop, FinalAnswer.end}"
)

# Adapt your SDK's response to LLMResponse
def adapt(raw):
    if raw.tool_calls:
        return LLMResponse(tool_calls=[
            ToolCall(name=tc.function.name, arguments=tc.arguments, id=tc.id)
            for tc in raw.tool_calls
        ])
    return LLMResponse(content=raw.content)

client = MonitoredClient(
    llm_call=openai.chat.completions.create,
    response_adapter=adapt,
    monitor=monitor,
    send_label="Request",
    receive_label=lambda r: "ToolCall" if r.has_tool_calls else "FinalAnswer",
)

response = client.call(model="gpt-4", messages=[...])
# Automatically fires !Request then ?ToolCall or ?FinalAnswer

Tool Middleware

Wraps tool execution. When the LLM requests a tool, the middleware checks ?Receive (tool requested) and !Send (result returned) against the protocol.

from llmcontract import ToolMiddleware

middleware = ToolMiddleware(
    monitor=monitor,  # same monitor as the client
    tools={
        "search": search_fn,
        "book": book_fn,
    },
)

# Process all tool calls from a response
results = middleware.process(response)
# Each tool call checks ?receive and !send against the protocol

Combined Agent Loop

from llmcontract import (
    Monitor, MonitoredClient, ToolMiddleware,
    LLMResponse, ToolCall, ProtocolViolationError,
)

protocol = "rec Loop.!Request.?{ToolCall.!ToolResult.Loop, FinalAnswer.end}"
monitor = Monitor(protocol)

client = MonitoredClient(
    llm_call=my_llm_fn,
    response_adapter=my_adapter,
    monitor=monitor,
    send_label="Request",
    receive_label=lambda r: "ToolCall" if r.has_tool_calls else "FinalAnswer",
)

while True:
    try:
        response = client.call(messages=messages)
    except ProtocolViolationError as e:
        print(f"Protocol violated: {e}")
        break

    if not response.has_tool_calls:
        break  # FinalAnswer — protocol complete

    # Execute tools, send results back
    for tc in response.tool_calls:
        result = tools[tc.name](**tc.arguments)
        monitor.send("ToolResult")  # record the send
        messages.append(tool_result_msg(tc.id, result))

Langfuse Integration

Track protocol compliance in Langfuse — every send/receive is recorded as a guardrail observation with a pass/fail score.

pip install llmsessioncontract[langfuse]

from langfuse import get_client
from llmcontract.integration.langfuse import LangfuseMonitor

langfuse = get_client()

with langfuse.start_as_current_observation(name="agent-run") as trace:
    monitor = LangfuseMonitor(
        protocol="!Request.?{ToolCall.!ToolResult.end, FinalAnswer.end}",
        langfuse=langfuse,
    )

    monitor.send("Request")       # guardrail: ok ✓
    monitor.receive("ToolCall")   # guardrail: ok ✓
    monitor.send("ToolResult")    # guardrail: ok ✓
    monitor.send("ExtraCall")     # guardrail: VIOLATION ✗

langfuse.flush()

Each step appears as a guardrail observation in your Langfuse trace with:

• Input: the action attempted, direction, label, protocol
• Output: passed: true/false, violation details if applicable
• Score: protocol_compliance (boolean) for filtering and analytics

Research

This work is based on the theory developed in:

Christian Bartolo Burlò, Adrian Francalanza, Alceste Scalas. "On the Monitorability of Session Types, in Theory and in Practice". 35th European Conference on Object-Oriented Programming (ECOOP 2021), pp. 20:1–20:30, Schloss Dagstuhl, 2021. [PDF] [Google Scholar]

Architecture

DSL string ──▶ Parser ──▶ AST ──▶ FSM Compiler ──▶ Automaton ──▶ Monitor

• Parser (llmcontract.dsl.parser) — hand-written recursive descent parser that produces an AST
• AST (llmcontract.dsl.ast) — frozen dataclasses: Send, Receive, InternalChoice, ExternalChoice, Sequence, Recursion, RecVar, End
• FSM Compiler (llmcontract.monitor.automaton) — compiles the AST into a finite state automaton with transitions keyed by (direction, label)
• Monitor (llmcontract.monitor.monitor) — steps through the automaton on each send/receive call, returning Ok, Violation, or Blocked
• MonitoredClient (llmcontract.integration.client) — wraps any LLM client call with automatic protocol checks
• ToolMiddleware (llmcontract.integration.middleware) — intercepts tool execution with protocol checks
• LangfuseMonitor (llmcontract.integration.langfuse) — records protocol events as Langfuse guardrail observations

Tests

pip install -e ".[dev]"
pytest llmcontract/tests/ -v

License

MIT