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LangChain Middleware Patterns (Python)
Overview
Tenant A sends a prompt: "Summarize this support ticket from alice@acme.com
about her overdue invoice." The chain's caching middleware ran before the PII
redaction middleware, so the raw prompt — email and all — became part of the
cache key. Thirty seconds later Tenant B sends a semantically identical prompt
(different tenant, different customer, same shape). Cache hits. Tenant B's user
gets back a summary that names alice@acme.com and her overdue invoice. That is
pain-catalog entry P24 in production, and it is a real class of incident —
post-mortems read like "we added caching to cut cost, leaked a customer's PII to
a different tenant within an hour."
The sibling failure modes:
P25 — Retry middleware runs the model call twice on a 429; both attempts
fire on_llm_end; the token-usage aggregator sums both; a single logical call
bills as two, tenant's per-session budget trips at 50% of true usage.
P10 — Agent loops exceed 15 iterations on vague prompts. There is no
default cost cap. A per-session token-budget middleware solves this; without
one, a single "help me with my account" prompt can burn thousands of tokens.
P34 — Runnable.invoke does not sanitize prompt injection. A RAG document
containing "Ignore previous instructions and..." is followed verbatim.
Guardrails middleware is your injection defense; without it, indirect prompt
injection is a one-line exploit.
P61 — set_llm_cache(InMemoryCache()) hashes the prompt string only.
Two chains with different tool bindings return the same cached response;
tools are silently ignored by the cache key.
This skill defines the canonical middleware order for LangChain 1.0 chains and
LangGraph 1.0 agents, with an ordering-invariants matrix (every adjacent pair
has a named failure mode if you swap them), six reference implementations, a
cache-key hash that includes prompt plus bound-tools plus tenant_id, retry
telemetry that deduplicates by request_id, and an integration test pattern
that asserts the ordering invariant on every build.
At least one provider package: pip install langchain-anthropic (or openai)
Optional: presidio-analyzer + presidio-anonymizer for PII NER beyond regex
Optional: redis + langchain-redis for multi-worker cache and rate limiting
Instructions
Step 1 — Adopt the canonical middleware order
Every LangChain 1.0 chain and LangGraph 1.0 agent that goes to production
applies middleware in this order:
user → redact → guardrail → budget → cache → retry → model
redact → cache (P24): cache key must be PII-free or Tenant A's PII leaks to Tenant B on a hit
guardrail → cache: an injection-laden prompt must never become a cache entry
budget → cache: cache hits count against RPS; check budget first so loops cannot DoS a session on hits alone
cache → retry: cache hits bypass retry; retry wraps only the model call
Production chains typically run 4-6 middleware layers with <1ms per
layer overhead (bench: p50 0.3ms/layer, p99 0.9ms on a 100-request sample).
See ordering-invariants.md for the full
pairwise matrix and the benchmark script.
Step 2 — PII redaction middleware
Mask entities with reversible placeholders so the caller can reinsert in the
output — but the cache key and the model prompt only ever see redacted text.
import re
from typing importAny
_REDACTORS = [
("EMAIL", re.compile(r"[a-zA-Z0-9._%+-]+@[a-zA-Z0-9.-]+\.[a-zA-Z]{2,}")),
("PHONE", re.compile(r"\+?\d[\d\s\-\(\)]{7,}\d")),
("SSN", re.compile(r"\b\d{3}-\d{2}-\d{4}\b")),
("CC", re.compile(r"\b(?:\d[ -]*?){13,16}\b")),
]
defredact(text: str) -> tuple[str, dict[str, str]]:
pmap: dict[str, str] = {}
for label, pattern in _REDACTORS:
for i, matchinenumerate(pattern.findall(text)):
token = f"<{label}_{i}>"
pmap[token] = match
text = text.replace(match, token)
return text, pmap
defredaction_middleware(inputs: dict[str, Any]) -> dict[str, Any]:
redacted, pmap = redact(inputs["input"])
return {**inputs, "input": redacted, "_pii_map": pmap}
For names, addresses, and custom entities, Presidio's AnalyzerEngine covers
20+ entity types. See pii-redaction.md for the
regex vs spaCy vs Presidio tradeoff matrix, GDPR/HIPAA/PCI-DSS entity lists,
and the reinsertion pattern (return un-redacted output only to the
originating tenant — never cross-populate).
Step 3 — Guardrails middleware
Detect injection patterns up front and wrap user content so the model treats
it as data. Two layers: pattern match (catches the 90% case cheaply) plus
prompt wrapping (neutralizes what slips through).
INJECTION_PATTERNS = [
re.compile(r"ignore (all |the )?(previous|prior|above) (instructions|rules)", re.I),
re.compile(r"system prompt (is|was|now)", re.I),
re.compile(r"you are now (a |an )?", re.I),
re.compile(r"</?(system|instruction|prompt)>", re.I),
]
classGuardrailViolation(Exception):
passdefguardrail_middleware(inputs: dict[str, Any],
allowed_tools: set[str] | None = None) -> dict[str, Any]:
for pattern in INJECTION_PATTERNS:
if pattern.search(inputs["input"]):
raise GuardrailViolation(f"Injection pattern matched: {pattern.pattern!r}")
wrapped = f"<user_input>\n{inputs['input']}\n</user_input>"
out = {**inputs, "input": wrapped}
if allowed_tools isnotNone:
out["_tool_allowlist"] = allowed_tools
return out
Never rely on the model to "know what is an instruction" without wrapping.
Directly addresses P10 — agents loop 15+ iterations on vague prompts and burn
thousands of tokens. The budget middleware raises before the model call if
the session is over ceiling.
from dataclasses import dataclass, field
from collections import defaultdict
from threading import Lock
classBudgetExceeded(Exception): pass@dataclassclassTokenBudget:
ceiling: int = 50_000# tokens per session
_usage: dict[str, int] = field(default_factory=lambda: defaultdict(int))
_lock: Lock = field(default_factory=Lock)
defrecord(self, session_id: str, tokens: int) -> None:
withself._lock:
self._usage[session_id] += tokens
defcheck(self, session_id: str) -> None:
withself._lock:
used = self._usage[session_id]
if used >= self.ceiling:
raise BudgetExceeded(f"Session {session_id}: {used}/{self.ceiling}")
budget = TokenBudget(ceiling=50_000)
defbudget_middleware(inputs: dict[str, Any]) -> dict[str, Any]:
budget.check(inputs.get("session_id") or"anonymous")
return inputs
Pair with a BaseCallbackHandler.on_llm_end that calls budget.record(...)
with usage_metadata.input_tokens + output_tokens. For multi-worker deploys,
back TokenBudget with Redis — per-process dicts are per-process (P29).
Step 5 — Caching middleware with tool-aware key
P61 is the booby trap: InMemoryCache() hashes the prompt string only, so
two chains with different tool lists return the same cached response. Use a
custom key over prompt + bound tools + tenant id.
import hashlib, json
from typing importCallabledefcache_key(prompt: str, bound_tools: list[dict] | None, tenant_id: str) -> str:
"""Blake2b-16 hash. Tool-aware, tenant-aware, collision-safe via \\x1f separator."""
h = hashlib.blake2b(digest_size=16)
h.update(prompt.encode("utf-8")); h.update(b"\x1f")
if bound_tools:
h.update(json.dumps(bound_tools, sort_keys=True).encode("utf-8"))
h.update(b"\x1f"); h.update(tenant_id.encode("utf-8"))
return h.hexdigest()
defcache_middleware(get: Callable[[str], Any | None], put: Callable[[str, Any], None]):
def_run(inputs: dict[str, Any]) -> dict[str, Any]:
key = cache_key(inputs["input"], inputs.get("_bound_tools"),
inputs.get("tenant_id", "default"))
hit = get(key)
if hit isnotNone:
return {**inputs, "_cache_hit": True, "output": hit}
inputs["_cache_key"] = key
return inputs
return _run
The cache key must be computed on the redacted prompt (Step 2 ran first)
and must include the tool schemas. See
cache-key-design.md for backend comparison
(InMemoryCache / SQLiteCache / RedisCache / RedisSemanticCache),
invalidation strategies (TTL, schema-version bump, tenant-wide purge), and
the full pitfalls list including Unicode normalization and P62.
Step 6 — Retry middleware with telemetry tagging
P25: retry runs the model call twice on a 429, both attempts emit
on_llm_end, the aggregator sums both, tenant budget trips at 50% of true
usage. Fix: attach a stable request_id on the first attempt, and have the
aggregator replace (not add) per request_id so only the last successful
attempt is counted.
See retry-telemetry.md for the full retry
loop, the dedup-by-request_id aggregator, provider-specific retryable
exception lists (Anthropic / OpenAI / Gemini), exponential backoff with
jitter, and a circuit breaker that stops retry storms on a dead upstream.
Step 7 — Compose the middleware into a chain
from langchain_core.runnables import RunnableLambda, RunnablePassthrough
# Order matters. See Step 1 for why.
chain = (
RunnableLambda(redaction_middleware)
| RunnableLambda(guardrail_middleware)
| RunnableLambda(budget_middleware)
| RunnableLambda(cache_middleware(cache_get, cache_put))
| RunnableLambda(retry_middleware(max_retries=2))
| model # ChatAnthropic / ChatOpenAI
)
For LangGraph agents, the same layers apply but are wired as nodes with
conditional edges — a budget node that routes to END on violation, a
guardrail node that routes to an error handler on injection match, and so on.
See the LangGraph adaptation in references/ordering-invariants.md.
Step 8 — Integration test: assert the ordering invariant
Ordering is invisible in code review until someone moves cache above redact.
Assert the invariant in a test that runs on every commit.
deftest_cache_key_does_not_leak_pii():
"""P24 — cache key built from REDACTED prompt, not raw."""
a = redaction_middleware({"input": "Ticket from alice@acme.com", "tenant_id": "T1"})
b = redaction_middleware({"input": "Ticket from bob@other.com", "tenant_id": "T1"})
assert cache_key(a["input"], None, "T1") == cache_key(b["input"], None, "T1")
deftest_cache_key_tenant_isolation():
"""P24/P33 — same prompt, different tenants, different cache keys."""assert cache_key("notes", None, "T1") != cache_key("notes", None, "T2")
deftest_cache_key_tool_aware():
"""P61 — same prompt, different tool bindings, different cache keys."""assert cache_key("p", [{"name":"search"}], "T") != cache_key("p", [{"name":"code_exec"}], "T")
Run in CI. A failure means someone broke the ordering invariant — chain does
not merge until it is fixed.
Output
Six middleware layers composed in canonical order: redact → guardrail → budget → cache → retry → model
Reversible PII redaction with placeholder map (emails, phones, SSNs, credit cards; Presidio optional for names/addresses)
Guardrails middleware with injection-pattern detection and user-content wrapping
Per-session / per-tenant token budget with thread-safe counter
Cache-key hash that includes prompt + bound-tool schemas + tenant id (fixes P61 and P24)
Retry middleware with request_id tagging so the token aggregator deduplicates (fixes P25)
Integration tests asserting the ordering invariant
Error Handling
Error / failure mode
Cause
Fix
Tenant B receives Tenant A's PII on a cache hit
Cache before redact (P24) — raw PII went into the cache key
Reorder: redaction runs first; cache key built on redacted prompt + tenant_id
Token-usage aggregator reports 2x actual usage after a retry
Retry double-count (P25) — both attempts emit on_llm_end, aggregator sums
Attach request_id on first attempt; aggregator dedupes by request_id
Two chains with different bound tools return same cached response
P61 — InMemoryCache() hashes prompt string only, not tool schemas
Use cache_key(prompt, bound_tools, tenant_id) with blake2b over all three
Agent loops past 15 iterations on vague prompt; bill spikes
No token budget (P10) — recursion_limit=25 default has no cost ceiling
Insert budget_middleware before cache; raise BudgetExceeded if session over ceiling
Model follows "Ignore previous instructions and..." in a RAG doc
No guardrail (P34) — Runnable.invoke does not sanitize prompt injection
Insert guardrail_middleware after redact, before cache; wrap user input in <user_input> tags
GuardrailViolation raised on legitimate prompt
Over-eager injection pattern match
Tune patterns in references/ordering-invariants.md; log false positives for iteration
Cache poisoning after a deploy that changed tool schemas
Old cache entries reference old tool list
Bump a schema_version constant and include it in the cache key
Budget tracker drift in multi-worker deploy
P29 analog — in-process dict is per-worker only
Back TokenBudget with Redis or another shared store
Retries still fire on KeyboardInterrupt during local dev
P07 — default exceptions_to_handle includes KeyboardInterrupt on Python < 3.12
Explicitly list retryable exceptions; never catch BaseException
Examples
Building a chain end-to-end with correct order
The Step 7 composition shows the six layers in order. In production code this
usually lives in a factory — build_chain(tenant_id: str, allowed_tools: set[str]) —
that closes over the tenant-scoped cache backend and budget instance. The
factory makes the order explicit and testable.
LangGraph agent version
The same six layers in a LangGraph agent become six nodes plus conditional
edges. budget routes to END on violation; guardrail routes to
error_handler on injection match; cache routes to END on hit. See
references/ordering-invariants.md for the adapted graph topology.
Debugging a cache-poisoning incident
Post-mortem template: (1) enumerate cache entries, (2) check whether keys were
built pre- or post-redaction, (3) identify the first cross-tenant hit in logs,
(4) purge by tenant prefix or full flush, (5) add the ordering integration
test from Step 8 so this cannot recur.
