mycelium-runtime 1.14.0

Runtime guards and YAML auto-instrumentation for reliable AI agent tools

Mycelium runtime

Current package: mycelium-runtime v1.14.0 (REPAIR gate + lease auto-renew + transition envelope).

One painful bug → a few lines of config

LangGraph Cloud redispatches a long tool call while the first is still running. Both complete. You pay twice. Side effects run twice. langgraph#7417

Mycelium’s answer is a transition envelope, not “idempotency key + cached result” alone: classify the tool (side-effect class), hold an execution lease while work is in flight (auto-extended for long @ledger tools), record terminal state (IN_FLIGHT / COMPLETED / UNKNOWN / …), and hard-block (or reconcile) when a mutating redispatch would be unsafe. Same key while in-flight → poll; completed → return stored; ambiguous payment-class → stop.

On LangGraph Cloud, long tool calls can be redispatched on the order of ~180s, aligned with the platform’s BG_JOB_HEARTBEAT sweep. Mycelium’s lease / auto-renew / poll / hard-block path is the operator-side guard for that window — see Resolution gates.

pip install 'mycelium-runtime[langgraph]'  # Python 3.10+; automatic runtime IDs
mycelium init                  # on-ramp scaffold (transition + one ledgered tool)
mycelium init --full           # reference scaffold (all guards; fill TODOs)
mycelium demo                  # see the bug and the fix (no LangGraph required)

tools:
  subagent_task:
    callable: my_agent.tools:subagent_task
    side_effect_class: non_idempotent_mutate

mycelium run --config mycelium.yaml -- python -m my_agent

In v1.11.0, the default mycelium init YAML enables integrations.langgraph. LangGraph's ToolNode / create_agent injects a hidden ToolRuntime, and Mycelium maps its tool_call_id, thread, run, and node into the transition key. No tool_call_id parameter is needed on your function. Explicit IDs still win; custom tool executors may continue passing them manually.

New in v1.12.0, mycelium run wraps all configured tool/task callable paths before application startup and then replaces itself with the child Python process. Existing @config.apply, @config.apply_task, and config.instrument flows remain supported for explicit code-level control.

What it does

Mycelium sits between your agent loop and your tools (after the LLM returns tool_calls):

	Problem	What Mycelium does
Core	Duplicate side effects on retry	Transition envelope: classify tools, durable transition key, lease (+ auto-renew while @ledger runs), terminal state, resolution gates (POLL / REPAIR / SOFT_BLOCK / HARD_BLOCK), external_operation_ref + Reconciler, ledgers, signed receipts
Core	Transition envelope fields	side_effect_class → spendability → side_effect_boundary → terminal_outcome → external_operation_ref → retry_permission — same system as above; payment/write needs the heavier set
Opt-in	Stale or broken context	TTL cache (@protect / Session); optional MessageValidator / HistoryGuard you call before the next LLM turn
Opt-in	Bad tool calls	@bounded input/output/scope checks; optional ToolRegistry allowlist — block before the tool runs

mycelium init / mycelium run center on the core path. Context and tool-boundary guards are available when you configure them.

Framework-agnostic. Raw message lists and plain Python functions (LangGraph, CrewAI, OpenAI tool loops, etc.).

Install

Requires Python 3.10+ (3.11+ recommended).

pip install mycelium-runtime
pip install 'mycelium-runtime[langgraph]'  # optional automatic LangGraph IDs
mycelium init              # on-ramp: duplicate-tool fix → ./mycelium.yaml
mycelium init --full       # reference: every guard section (not the default)
mycelium init --minimal    # smaller multi-guard scaffold
mycelium demo              # terminal demo of langgraph#7417

Quickstart: stale context & broken transcripts (opt-in)

from mycelium import protect, Session

@protect(entity_param="customer_id", ttl=60)
async def fetch_customer(customer_id: str) -> dict:
    return await db.get(customer_id)

async def handle_request(customer_id: str):
    async with Session():
        return await fetch_customer(customer_id=customer_id)

Sync tools (CrewAI, Smolagents):

from mycelium import protect_sync, Session

@protect_sync(entity_param="customer_id", ttl=60)
def fetch_customer(customer_id: str) -> dict:
    return db.get(customer_id)

with Session():
    customer = fetch_customer(customer_id="c1")

What @protect / protect_sync / Session do

• @protect / protect_sync: TTL cache with per-entity keys; auto-refetch when stale; clear on error
• Session: one cache per agent run; use in production to prevent cross-request leakage

MessageValidator

Run before each LLM call to catch broken transcripts:

from mycelium import MessageValidator

messages = MessageValidator().repair(messages)  # auto-fix what it can
# or
messages = MessageValidator().validate(messages)  # raise on first issue

Catches orphan tool results, duplicate tool-call IDs, invalid roles, and related serialization bugs.

HistoryGuard

Run before each LLM call to catch oversized or corrupted history:

from mycelium import HistoryGuard

guard = HistoryGuard(max_tokens=100_000)
messages = guard.validate(messages)
guard.check_for_drops(processed_messages)  # after framework trimming

Raises on token overflow, message count limits, duplicate turns, and silent message drops.

Quickstart: tool boundaries (opt-in)

from mycelium import bounded, ToolRegistry, ToolRunner

FETCH_CUSTOMER_SCHEMA = {
    "customer_id": {"type": "string", "required": True, "pattern": r"^c\d+$"},
}

CUSTOMER_RECORD_SCHEMA = {
    "customer_id": {"type": "string", "required": True},
    "name": {"type": "string", "required": True},
}

registry = ToolRegistry(allowed=["fetch_customer"])

@registry.register
@bounded(
    schema=FETCH_CUSTOMER_SCHEMA,
    output_schema=CUSTOMER_RECORD_SCHEMA,
    allowed_paths=["/workspace/src/"],
)
async def fetch_customer(customer_id: str) -> dict:
    return await db.get(customer_id)

runner = ToolRunner(registry=registry)
result = await runner.call(fetch_customer, customer_id="c1")

Sync tools:

from mycelium import bounded_sync

@bounded_sync(schema=FETCH_CUSTOMER_SCHEMA)
def fetch_customer(customer_id: str) -> dict:
    return db.get(customer_id)

Field spec keys: type (string, integer, number, boolean), required, pattern, min_length, max_length. You pass plain dicts; Mycelium validates internally; no Pydantic imports in your code.

What @bounded / bounded_sync do

• @bounded / bounded_sync: validate tool args against your field spec before the function runs
• output_schema: validate the return value after the function runs; bad results are not propagated
• allowed_paths / entity_pattern: user-defined scope gates (path prefixes, entity ID format)
• On failure, raises ToolBoundaryError with llm_message for the agent loop; does not retry by itself

ToolRegistry

Run before dispatch to enforce which tools this agent may call:

from mycelium import ToolRegistry

registry = ToolRegistry(allowed=["search_docs", "summarize"])
registry.validate_call("fetch_customer")  # raises ToolBoundaryError

Blocks calls to tools outside the developer-defined allowlist.

ToolRunner

Run around @bounded tools when you want automatic retries:

from mycelium import ToolRunner

runner = ToolRunner(registry=registry, max_llm_retries=2, max_tool_retries=3)

result, messages = await runner.run_with_llm_retry(
    fetch_customer,
    messages=messages,
    tool_call_id="call_1",
    kwargs={"customer_id": "c1"},
    invoke_llm=llm.ainvoke,
    parse_tool_kwargs=extract_tool_args,
)

• Input, allowlist, and scope failures → append tool error to messages → LLM retry
• Output failures → retry the tool up to max_tool_retries → then LLM retry
• Raises ToolBoundaryExhaustedError when retries are used up

Quickstart: idempotency & audit receipts (core — transition envelope)

Stop duplicate payments, emails, and API calls when the framework retries. Five effect-semantic side_effect_class values plus optional spendability (multi_use / single_use / non_replayable): reads poll in-flight duplicates; mutating tools hard-block ambiguous states instead of blind re-execute.

Tool-level idempotency

from mycelium import ledger_sync
from mycelium.transition import SideEffectClass, ToolTransitionBinding

binding = ToolTransitionBinding.for_tool(
    agent_id="payment-agent",
    policy_version="2026.07.1",
    side_effect_class=SideEffectClass.KEYED_MUTATE,
)

@ledger_sync(transition_binding=binding)
def send_payment(amount: float, recipient: str) -> dict:
    return gateway.charge(amount, recipient)

# Same logical call executes only once.
send_payment(amount=100.0, recipient="acct_123", tool_call_id="call_abc")
send_payment(amount=100.0, recipient="acct_123", tool_call_id="call_abc")

Or wire from YAML (recommended):

integrations:
  langgraph:
    enabled: true

transition:
  agent_id: payment-agent
  policy_version: "2026.07.1"
  lease_ttl: 3600
  # lease_renew_interval: 1200   # default = lease_ttl/3; 0 disables auto-renew

action_ledger:
  storage: file
  path: ./mycelium-ledger.json
  tools: [send_payment, search_docs]

tools:
  send_payment:
    callable: my_agent.tools:send_payment
    side_effect_class: keyed_mutate
    # spendability defaults to single_use for keyed_mutate
    retry_permission: manual_reconciliation_required
  search_docs:
    callable: my_agent.tools:search_docs
    side_effect_class: read
    # spendability defaults to multi_use for read

When enabled, command mode or @config.apply adds a hidden keyword-only runtime: ToolRuntime parameter. LangGraph treats it as a trusted injected argument (not an LLM-visible tool input), while the original function remains unchanged. Calls outside LangGraph still work. This requires mycelium-runtime[langgraph] and LangGraph's ToolNode or create_agent; custom executors must pass IDs themselves.

For zero-touch instrumentation, launch with:

mycelium run --config mycelium.yaml -- python -m my_agent

Every non-noop tool/task must declare a unique callable: module:function. Targets are imported and validated before the application entrypoint runs; missing/non-callable targets and partial Mycelium wrappers stop startup. A fully configured @config.apply or @config.apply_task target is skipped. Only the current Python interpreter is accepted, and -E, -I, and -S are rejected because they disable the startup hook. Keep target modules import-safe. Code that registers a function inside its own module before that import completes cannot be retroactively updated; move registration to the entrypoint or use explicit instrumentation for that target.

Async tools:

from mycelium import ledger

@ledger()
async def send_payment(amount: float, recipient: str) -> dict:
    return await gateway.charge(amount, recipient)

What @ledger / ledger_sync do

• Record every tool invocation in a durable ActionLedger
• Deduplicate retries and redispatches via a rich transition key (scope + tool + args + side_effect_class + policy), not only tool_call_id
• Resolve redispatches through gates (see Resolution gates) instead of re-running blindly
• Persist failed attempts with terminal outcomes (FAILED_BEFORE_EFFECT, FAILED_AFTER_EFFECT, UNKNOWN, EXPIRED, etc.) for audit and reconciliation

Resolution gates

Invariant: do not redispatch unless the previous transition is proven terminal (e.g. COMPLETED → return stored) or safely recoverable (poll in-flight, soft-block/retry a read UNKNOWN, or reconcile via external_operation_ref). Otherwise hard-block — never blind re-execute a side effect.

Each duplicate dispatch is classified to a gate. Read-only and side-effecting tools use different resolvers.

Gate	Typical trigger	What happens
ALLOW	no prior transition, or policy permits retry (e.g. FAILED_BEFORE_EFFECT + same provider key)	tool runs
RETURN	COMPLETED	return stored result — no re-execution
POLL	IN_FLIGHT with valid lease (LeaseValidity.HELD)	wait for the other worker
RECLAIM	read-only EXPIRED / FAILED_*	take over stale lease and run
REPAIR	incomplete durable key / boundary / terminal (healable)	fix record, re-resolve — no second side effect
SOFT_BLOCK	read-only UNKNOWN / BLOCKED only	retry by default (safe — reads don't spend); opt into deferral with defer_read_only_unknown=True → LedgerSoftBlockError
HARD_BLOCK	ambiguous mutating transition	stop; run Reconciler when external_operation_ref is present, else fail-closed

Public transition-sufficiency language: #7417-style discussions often use four words — ALLOW / REPAIR / SOFT_BLOCK / HARD_BLOCK (sometimes BLOCK). Mycelium implements that set and adds finer internals:

Public	Mycelium	Notes
ALLOW	ALLOW	run / safe retry
REPAIR	REPAIR	heal durable context; owner auto-renew / renew_lease() for a live lease
SOFT_BLOCK	SOFT_BLOCK	read-only defer / safe retry
HARD_BLOCK / BLOCK	HARD_BLOCK	stop; reconcile if ref present
(must not run again)	RETURN / POLL	already done, or wait on a held lease
(read reclaim)	RECLAIM	take over an expired read lease and run

Public BLOCK ≈ Mycelium HARD_BLOCK. RETURN and POLL are also “do not execute again” under the richer internal taxonomy — use the four public words with platforms; use the full table when implementing or debugging.

Lease validity (v1.10.0) / auto-renew (v1.14.0): lease_until is resolution metadata — not part of transition_key (so renewals do not fork identity). Before reclaim/retry, resolution classifies the window via LeaseValidity (HELD → poll, EXPIRED → reclaim or hard-block by class, UNBOUNDED → no TTL). While a @ledger / @ledger_sync tool body runs, Mycelium auto-extends the lease (default every lease_ttl / 3). Set lease_renew_interval: 0 to disable; call renew_lease() for an extra manual bump or when claiming outside the decorator.

Cloud-style proof (v1.13.4): mycelium demo --redis (or prove_two_worker_redis_redispatch()) runs two OS processes against a real Redis ledger. Worker A claims and runs; worker B redispatches the same request_id while A is IN_FLIGHT. B polls and returns A's result — the side effect runs once. Set MYCELIUM_TEST_REDIS_URL or use redis://127.0.0.1:6379/15. This is the partner-facing #7417 proof beyond an in-process double call.

REPAIR (v1.13.0): when the durable record is incomplete but healable (missing idempotency_key, invalid/missing side_effect_boundary or terminal_outcome, or status/terminal drift), claim loops call repair_transition() then re-resolve. A held in-flight lease is still POLL for peers; the owner keeps it held via auto-renew (or renew_lease()), not a second execute.

Read-only (side_effect_class: read): poll, reclaim, retry failed-before-effect, soft-block on ambiguous UNKNOWN/BLOCKED.

Mutating (payment, email, subagent, irreversible, …): return completed, poll in-flight, hard-block ambiguity. For EXPIRED + not_crossed, the gate is HARD_BLOCK until a reconciler proves the effect never ran — see Stale lease + reconcile.

Transition envelope fields

Six fields decide whether an unresolved prior execution is merely wasteful (safe to retry/poll) or unsafe (must not re-run). Priority order:

#	Field	Role
1	side_effect_class	What kind of effect (read, keyed_mutate, non_idempotent_mutate, …)
2	spendability	How many times the same intent may spend (multi_use / single_use / non_replayable)
3	side_effect_boundary	Whether the external call was crossed (not_crossed / maybe_crossed / crossed)
4	terminal_outcome	Where the prior attempt ended (IN_FLIGHT, COMPLETED, UNKNOWN, EXPIRED, …)
5	external_operation_ref	Provider handle for read-only reconcile (id or idempotency key)
6	retry_permission	Whether automatic retry is allowed (and same-key enforcement when opted in)

Invariant: for a given tool class, the fields that class requires must already be supported and recorded on the transition before a redispatch is treated as a safe retry. Reads need a lighter set (class + terminal + lease). Payment / write / email / subagent need spendability, boundary, terminal outcome, and usually an external receipt/ref — without them, a second dispatch is an unsupported second transition, not a retry.

Also on the durable record: transition_key, idempotency_key, owner, lease_until, receipt_ref.

Side-effect classes

Class	Typical use	Duplicate handling
read	search, fetch	poll / reclaim / retry; SOFT_BLOCK on UNKNOWN
idempotent_mutate	upsert / set status	retry if boundary not crossed
keyed_mutate	Stripe-style create/charge	retry only with same provider key
non_idempotent_mutate	send email, spawn subagent	hard-block on ambiguity
irreversible	wire / on-chain burn	hard-block → human

Legacy aliases (read_only, payment, subagent, …) still parse. Set per tool in YAML with side_effect_class. Required when transition: is configured and the tool is ledgered.

Spendability

Orthogonal to side_effect_class — how many times the same intent may produce an effect:

Value	Meaning	Default for
multi_use	may produce effects again	read, idempotent_mutate
single_use	one effect; COMPLETED returns stored result	keyed_mutate, non_idempotent_mutate
non_replayable	ambiguity → hard-block / reconcile	irreversible

Override with spendability: only when the class default is wrong for your tool. Same transition key always returns the COMPLETED result; a deliberate re-spend needs a new key.

Marking the side-effect boundary (side_effect())

By default a failing tool is recorded as FAILED_BEFORE_EFFECT — safe to retry. But if the external call already happened (e.g. the charge succeeded and then response parsing threw), that classification is wrong. Wrap the external operation in side_effect() so the ledger knows where the point of no return is:

from mycelium import ledger_sync, side_effect

@ledger_sync(transition_binding=binding)
def send_payment(amount: float, recipient: str) -> dict:
    validate(amount, recipient)          # boundary: not_crossed
    with side_effect():                  # -> maybe_crossed before the call
        resp = gateway.charge(amount, recipient)   # -> crossed on clean exit
    return parse(resp)

The boundary drives failure classification and only ever moves forward (not_crossed → maybe_crossed → crossed):

Boundary when it fails/crashes	Terminal outcome	Redispatch
not_crossed (before the block)	FAILED_BEFORE_EFFECT	retry if policy allows
maybe_crossed (inside the block / crash)	UNKNOWN	hard-block → reconcile
crossed (clean exit, or mark_crossed())	FAILED_AFTER_EFFECT	hard-block

Because maybe_crossed is written durably before the call, a process crash mid-call leaves the entry ambiguous and a redispatch hard-blocks instead of double-spending. For finer control use mark_maybe_crossed() / mark_crossed() directly. Works the same inside async tools.

Read-only SOFT_BLOCK (v1.9.0)

When a read-only tool ends in UNKNOWN or BLOCKED, the resolver returns SOFT_BLOCK — not a terminal stop. Re-running a read is always safe, so the default is retry (reset to a fresh in-flight claim and run once more). For expensive reads, opt into deferral:

from mycelium import ledger_sync, LedgerSoftBlockError

@ledger_sync(transition_binding=read_binding, defer_read_only_unknown=True)
def search_docs(query: str) -> dict:
    ...

With defer_read_only_unknown=True, ambiguous read-only states raise LedgerSoftBlockError so the caller can retry later (cost-dependent). Side-effecting tools never use SOFT_BLOCK; they use HARD_BLOCK / reconcile.

Recording the provider handle (record_external_operation())

When a side-effecting tool talks to a provider, record the provider's operation handle — its returned id (Stripe pi_..., a message id, a run id) or the idempotency key you sent — so an ambiguous transition can later be reconciled against the provider instead of parked for a human:

from mycelium import ledger_sync, side_effect, record_external_operation

@ledger_sync(transition_binding=binding)
def send_payment(amount, recipient):
    with side_effect():
        intent = gateway.charge(amount, recipient, idempotency_key=key)
        record_external_operation(intent.id)   # durable on the ledger entry
    return intent

The ref is stored on the entry (external_operation_ref) across all backends and shown in the hard-block message. Prefer recording the idempotency key before the call for keyed providers — it survives a crash mid-call, unlike a returned id.

external_operation_ref is the handle for provider lookup; it is not proof by itself. Proof comes from the reconciler's read-only query (below).

Reconciling automatically (Reconciler)

Instead of parking an ambiguous transition for a human, give the ledger a read-only Reconciler that asks the provider "did operation X actually complete?" using the recorded ref. It runs only when a side-effecting transition would otherwise hard-block and a ref is present:

from mycelium import ledger_sync, Reconciler, ReconcileResult

class StripeReconciler:  # read-only: never charges, never retries
    def reconcile(self, entry) -> ReconcileResult:
        pi = stripe.PaymentIntent.retrieve(entry.external_operation_ref)
        if pi.status == "succeeded":
            return ReconcileResult.completed(pi)
        if pi.status in ("canceled", "requires_payment_method"):
            return ReconcileResult.not_executed()
        return ReconcileResult.unknown()

@ledger_sync(transition_binding=binding, reconciler=StripeReconciler())
def send_payment(amount, recipient):
    with side_effect():
        intent = gateway.charge(amount, recipient, idempotency_key=key)
        record_external_operation(intent.id)
    return intent

Reconcile result	What happens on redispatch
COMPLETED	returns the reconciled result — the tool body does not run again
NOT_EXECUTED	the tool is allowed to run exactly once more
UNKNOWN	hard-blocks, exactly as if no reconciler were set

Reconciliation is fail-closed: no ref, no reconciler, or a reconciler that raises/times out all resolve to a hard-block — an exception in the reconciler never propagates. Async tools can implement reconcile_async; the async claim path prefers it and falls back to reconcile. Wire a reconciler via @ledger / @ledger_sync or ActionLedger(reconciler=...).

Stale lease + reconcile (EXPIRED + not_crossed)

When a worker dies or a lease expires while a side-effecting tool is still IN_FLIGHT, the transition becomes EXPIRED. Resolution depends on boundary and class:

Situation	Gate	Reclaim?
EXPIRED + maybe_crossed / crossed	HARD_BLOCK	no — effect may have happened
EXPIRED + not_crossed, strict class, no external_operation_ref	HARD_BLOCK	no — not provable
EXPIRED + not_crossed + ref + reconciler → NOT_EXECUTED	reconcile → fresh claim	yes — provider proves effect never ran
EXPIRED + not_crossed + ref + reconciler → COMPLETED	RETURN	no — return stored/reconciled result
EXPIRED + not_crossed, multi_use + SAFE_RETRY (e.g. idempotent read/write)	ALLOW	yes — reclaim without reconcile

If a duplicate worker is polling an in-flight transition and the lease expires mid-poll, the poll loop returns (v1.9.2) so the claim path can reconcile instead of hard-blocking immediately.

Record external_operation_ref early (ideally the idempotency key before the provider call) so stale-lease and UNKNOWN cases can be resolved automatically instead of parking for a human.

Enforcing the same provider idempotency key (provider_idempotency_key_param)

retry_only_with_same_provider_idempotency_key (the default for keyed_mutate) means "a retry is safe only if it reuses the same provider idempotency key so the provider dedupes." By default Mycelium trusts you to reuse it. To have Mycelium enforce it, declare which kwarg carries the key:

tools:
  send_payment:
    side_effect_class: keyed_mutate          # retry_only_with_same_provider_idempotency_key
    provider_idempotency_key_param: idempotency_key

or in code: ToolTransitionBinding.for_tool(..., provider_idempotency_key_param="idempotency_key").

With it declared, on a retry of a transition that failed before the effect:

Incoming key vs stored key	Gate
same key	ALLOW (retry proceeds; provider dedupes)
different key	HARD_BLOCK (would risk a second, undeduped effect)
missing on either side	HARD_BLOCK

The declared key is excluded from the transition-key fingerprint, so a retry that swaps the key still resolves to the same transition and is caught rather than silently starting a new one. This is opt-in: tools that don't declare the param keep the previous cooperative behavior.

Storage backends:

Backend	Use case	YAML storage
memory	Single process, tests	memory (default)
file	Local dev, single host (fcntl lock)	file + path
redis	Multi-worker, in-flight TTL	redis + url or url_env
postgres	Audit/compliance, durable SQL	postgres + dsn or dsn_env

from mycelium import ActionLedger, FileLedgerStorage, InMemoryLedgerStorage
from mycelium import RedisLedgerStorage, PostgresLedgerStorage

ledger = ActionLedger(storage=InMemoryLedgerStorage())
ledger = ActionLedger(storage=FileLedgerStorage("./mycelium-ledger.json"))
ledger = ActionLedger(storage=RedisLedgerStorage("redis://localhost:6379/0"))
ledger = ActionLedger(storage=PostgresLedgerStorage("postgresql://localhost/mycelium"))

Optional extras: pip install 'mycelium-runtime[redis]' or pip install 'mycelium-runtime[postgres]'.

Quickstart: task-level idempotency

Stop entire tasks from re-running on framework-level retries:

from mycelium import task_ledger_sync

@task_ledger_sync()
def process_invoice(invoice_id: str) -> dict:
    customer = fetch_customer(customer_id=...)
    payment = send_payment(...)
    return {"invoice_id": invoice_id, "status": "paid"}

# Framework retries the task with the same task_id
process_invoice(invoice_id="inv-42", task_id="invoice-42")  # executes
process_invoice(invoice_id="inv-42", task_id="invoice-42")  # returns stored result

Use id_from to derive the task id from business keys automatically:

@task_ledger_sync(id_from=["invoice_id"])
def process_invoice(invoice_id: str, amount: float) -> dict:
    ...

# Both calls map to the same task id because invoice_id is the same.
process_invoice(invoice_id="inv-42", amount=100.0)
process_invoice(invoice_id="inv-42", amount=200.0)  # returns first result

Correction retries

If a completed task produced a bad result and the LLM/agent needs to re-attempt it, use a new task id. The framework will normally generate fresh tool call ids for the new attempt, so the task re-executes cleanly.

r1 = process_invoice(invoice_id="inv-42", task_id="invoice-42-attempt-1")  # bad result
r2 = process_invoice(invoice_id="inv-42", task_id="invoice-42-attempt-2")  # fresh attempt

YAML configuration

Separate YAML sections per guard type. Global ledger settings inherit into tools/tasks so you do not repeat storage paths on every function.

Minimum integration (3 steps):

# mycelium.yaml: global sections (configure once)
transition:
  agent_id: payment-agent
  policy_version: "2026.07.1"
  lease_ttl: 3600
  # lease_renew_interval: 1200   # default = lease_ttl/3; 0 disables auto-renew

action_ledger:
  storage: file
  path: ./mycelium-ledger.json
  tools: [send_payment, search_docs]

task_ledger:
  storage: file
  path: ./mycelium-task-ledger.json
  tasks: [process_invoice]

state_flush:
  storage: file
  path: ./mycelium-state.json

audit_receipt:
  signing_key_env: MYCELIUM_SIGNING_KEY
  storage: file
  path: ./mycelium-receipts.jsonl

# Per-tool: side_effect_class + schemas
tools:
  fetch_customer:
    callable: my_agent.tools:fetch_customer
    side_effect_class: read
    protect: {entity_param: customer_id, ttl: 60}
    bounded:
      schema:
        customer_id: {type: string, required: true, pattern: "^c\\d+$"}

  send_payment:
    callable: my_agent.tools:send_payment
    side_effect_class: keyed_mutate
    bounded:
      schema:
        amount: {type: number, required: true}
        recipient: {type: string, required: true}

  search_docs:
    callable: my_agent.tools:search_docs
    side_effect_class: read

tasks:
  process_invoice:
    callable: my_agent.tasks:process_invoice
    ledger: true
    id_from: [invoice_id]

registry:
  auto: true                     # allowlist = all configured tools

history_guard:
  max_tokens: 100000

message_validator:
  enabled: true

# Zero-touch mode: callable paths above select the functions.
mycelium run --config mycelium.yaml -- python -m my_agent

Or instrument explicitly in Python:

from mycelium import load_config
import my_tools

config = load_config("mycelium.yaml")
tools = config.instrument(my_tools)   # one call wraps tools + tasks

with config.run(thread_id):
    messages = config.prepare_messages(messages)  # message validation + state flush
    ...

ledger: true inherits from action_ledger / task_ledger. When audit_receipt is configured with auto: true (default), all ledgered tools/tasks get signed receipts automatically. Set transition.agent_id for receipt identity (replaces audit_receipt.agent_id from v1.2).

Configs without transition: keep v1.2 ledger behavior. See CHANGELOG for breaking changes.

Legacy per-tool style still works. Start with mycelium init; use mycelium init --full for the all-guards reference template.

---

For contributors (repo layout)

Clone the GitHub repo to run proofs and tests. PyPI installs only the mycelium package.

git clone https://github.com/mycelium-labs/mycelium.git
cd mycelium/sdk && pip install -e ".[dev]"
pytest tests/ -v