capabilities-contract

The capabilities contract as a Python library. Its first part, capabilities_contract.db, is the one way the capabilities manager, its capabilities and ContextKit reach the shared PostgreSQL database: resolve which database a project uses, connect bound to its schema, migrate each owner's tables once under a ledger, and report where they stand. It does nothing else: no SQLite, no project registry, no records.

Install

pip install capabilities-contract

Its one dependency is psycopg[binary]>=3.2,<4, imported lazily: import capabilities_contract.db does not import psycopg, only connect, migrate and status do. In a PEP 723 script, pin the exact version:

# /// script
# dependencies = ["capabilities-contract==0.5.1"]
# ///

Which database

resolve_setting(project_root) returns the database setting in force for a process standing in that project, as a Setting, reading and never writing. It asks three levels in order, and the first that answers is used whole, never completed from a lower level:

1. project - the AGENTKIT_DB_* keys in the project's .env.local and .env, .env.local winning key by key;
2. environment - the same keys in the process environment;
3. machine - the machine's store setting file, $XDG_CONFIG_HOME/agentkit/store.json, or ~/.config/agentkit/store.json when XDG_CONFIG_HOME is unset, an agentkit.store.v1 document whose format is the capabilities package's SHEBANG.md, "The store setting". setting_path() gives its absolute path.

A level answers when it sets any AGENTKIT_DB_* key to a non-empty value, or, for the machine level, when the file exists. project_root=None means the process stands in no project, and the project level is not asked.

The keys (KEYS): AGENTKIT_DB_URL, AGENTKIT_DB_HOST, AGENTKIT_DB_PORT (default 5432), AGENTKIT_DB_NAME, AGENTKIT_DB_USER, AGENTKIT_DB_PASSWORD, AGENTKIT_DB_SCHEMA (default agentkit), AGENTKIT_DB_SSLMODE (default require), AGENTKIT_DB_SSLROOTCERT, AGENTKIT_DB_TRANSACTION_HOST, AGENTKIT_DB_TRANSACTION_PORT. Within one level AGENTKIT_DB_URL wins and that level's other keys are ignored, except AGENTKIT_DB_SCHEMA and the two transaction keys, which apply beside a URL.

The store has one address unless a level names a transaction address as well: AGENTKIT_DB_TRANSACTION_HOST and AGENTKIT_DB_TRANSACTION_PORT, or transaction_host and transaction_port in the machine file. It is the same store reached at another endpoint, typically a transaction-mode pooler such as Supabase's on port 6543, and it takes the level's own database, user, password, sslmode and schema; a transaction address naming only a port keeps the session address's host, and one naming only a host keeps its port. The address the level names otherwise is the session address. Setting.has_transaction_address says whether a level named one, Setting.transaction_kwargs() gives the driver's arguments for it (the session address's when none is named), and the report shows it under transaction. A transaction address alone does not make a level answer whole: a level setting only a transaction key is refused as incomplete, never completed from a lower level. A transaction port is 1-65535, a transaction host has no whitespace, and sslmode disable reaches only a local transaction host. A URL is a postgresql:// or postgres:// URL; one that names no sslmode is connected with require. A project file setting a key with the prefix that is not one of these is refused.

The checks are the same at every level: host, database and user with no whitespace, port 1-65535, sslmode one of require, verify-ca, verify-full, or disable for a local host (localhost, a loopback address or a Unix socket directory, which is also what a URL naming no host reaches); disable elsewhere, allow and prefer are refused as sslmode_too_weak; a schema is a lowercase identifier and never public, information_schema or pg_*. A refusal from the project or environment level names the level and the files or variables it read. A machine file of a newer agentkit.store.* version is refused as store_setting_too_new. With no level answering it raises DbError("store_not_configured", ...) with a hint naming the keys and capabilities store set.

Setting.level names the level that answered (LEVELS), and Setting.sources every file, as an absolute path, or variable that supplied a value in force. Setting.report() gives the whole setting for a status or doctor surface with every secret replaced by ***: the password field, or for a URL the parameters libpq reads from it (every parameter libpq's own option table marks as a password, such as password, sslpassword and oauth_client_secret, and the SCRAM keys hidden, sslmode defaulting to require), never the URL's text; a URL libpq cannot parse reports as {"unparseable": true}. The repr never shows the password or the URL.

Connect

from capabilities_contract.db import DbError, Step, connect, migrate

conn = connect(application_name="automations", project_root=root)

connect(*, application_name, project_root=None, setting=None, connect_timeout=10, transaction=False, local_settings=None) returns a psycopg 3 connection to the database resolve_setting(project_root) names, or to setting when one is passed, whose search_path is that setting's schema alone, never public, so unqualified names land in that schema. The schema need not exist yet: migrate creates it, in the schema the connection is bound to. A store that does not answer within connect_timeout seconds (CONNECT_TIMEOUT_SECONDS) is refused as store_unreachable. Both ends of the connection send TCP keepalives - after 30 seconds idle, every 10 seconds, giving up after 3 unanswered probes - so the store drops a client that died without closing and the client learns of a store that went away.

That is a session connection, to the session address: it binds the schema for its session and may hold session state, as a process holding LISTEN must. With transaction=True, connect returns a transaction connection, to the transaction address, or to the one address when the setting names none. It carries no session state, so a transaction-mode pooler, which hands the server session to another client after every transaction, serves it correctly: psycopg prepares no statement on it (prepare_threshold=None), the schema is bound with SET LOCAL search_path at the start of every transaction rather than once for the session, also around each statement in autocommit mode, and a statement that would leave state in the session - LISTEN, UNLISTEN, PREPARE, a session SET or RESET, set_config(..., false), a session advisory lock, a cursor WITH HOLD - is refused as session_state before it is sent. Transaction-level state is fine: SET LOCAL, set_config(..., true), pg_advisory_xact_lock. local_settings, a mapping of run-time parameters such as {"statement_timeout": "30000"}, is set beside the schema at the start of every transaction of a transaction connection, and once for the session of a session connection. A transaction connection sends no startup option, which a transaction-mode pooler refuses: each run-time parameter the setting's options or PGOPTIONS names (-c name=value, --name=value) is set for each of its transactions instead, and default_transaction_read_only=on makes each of them read-only; any other startup option is refused as session_state. A transaction connection at a transaction address of its own makes one round trip as it connects, severed when it is not back within connect_timeout, so a pooler that holds the client while the store behind it does not answer is refused as store_unreachable in that time rather than after the pooler's own wait. Client-side TCP keepalives apply to both kinds; only a session connection also asks the server to probe it. A server-side (named) cursor is not bound; use client cursors on a transaction connection. migrate and status work on either kind.

A transaction connection is the process's working connection, since it carries nothing from one transaction to the next: the first connect(..., transaction=True) opens it, close() or leaving with ends the loan - rolling back what was not committed and resetting what the borrower changed, such as autocommit or row_factory - and every later connect for the same address is lent that same connection, its schema, local_settings and application_name bound for each of its transactions as that call names them. A short command therefore connects once, and a long-running process keeps one connection for all its work. A loan taken while another is still out gets a working connection of its own; each beyond the first is closed once it has sat idle for 30 seconds (SPARE_IDLE_SECONDS), and every one is closed when the process exits. disconnect() closes the idle ones at once, for a process that is about to wait a long time without the store; disconnect(every=True) closes the lent ones too. A child of fork never uses or closes its parent's. After close() the connection reads as closed and refuses statements, as any closed connection does. A session connection is never lent: each connect without transaction opens one, and its close() closes it.

A working connection the store or the network lost is replaced: as it is lent, one the store closed is noticed without a round trip, and one that sat idle for 10 seconds (PING_AFTER_SECONDS) makes one round trip, cut at the connect timeout, so a connection that stopped answering is not waited on. A statement that finds its connection gone before anything of its transaction reached the store - the statement that opens a transaction, a statement in autocommit before its commit is sent, or the start of a transaction() block - runs once more on a new connection in place of the old one. So does a plain read - SELECT, WITH, VALUES, TABLE or SHOW, naming no write, lock, sequence or setting - in a transaction at READ COMMITTED that has run only plain reads: each such read sees the store as it is when it starts, so it runs on the new connection in a transaction opened again there, with every transaction() block it is inside opened again too, and the commit of a transaction that has only read - commit(), the end of a block, or an autocommit statement's own - is not refused when its connection is gone, since it has nothing to commit. A store that ends the session itself (an administrator's or a shutdown's error) counts as a lost connection. A commit whose answer was lost is never sent again, and a transaction that loses its connection after earlier statements fails as before, since what it did is gone; its rollback() is then not refused, since the store rolled the transaction back with the connection, and the next statement runs on a new connection. A connect that fails before the store answers - refused, reset, timed out - is tried again after a short pause, at most 3 times (CONNECT_ATTEMPTS) while connect_timeout is not spent, each try waiting at most half of it; a store that answers with a refusal of its own, such as a wrong password, is not tried again.

A session address that is Supabase's transaction pooler - a host under pooler.supabase.com on port 6543 - is refused as transaction_pooler before anything is sent to it, whichever kind of connection asks, because session connections cannot work through it; the hint names its session pooler, the same host on port 5432, as the address, and port 6543 as the transaction port. As the transaction address it is accepted.

Migrate

result = migrate(conn, "automations", [
    Step("0001-runs", "CREATE TABLE automations_runs (id bigint PRIMARY KEY)"),
    ("0002-runs-host", "ALTER TABLE automations_runs ADD COLUMN host text"),
], major=1, minor=1)
for warning in result.warnings:
    print(warning)

migrate(conn, owner, steps, *, major, minor) applies each step that is not yet applied, in order, once. The owner is the tool that owns the tables, a lowercase identifier without underscores. The first call creates the schema and two platform tables in it: schema_ledger(owner, step, checksum, applied_at, library_version) and schema_version(owner, major, minor, updated_at). Each step runs in its own transaction under a per-owner advisory lock together with its ledger row, so concurrent processes apply it exactly once. When every step is already applied and the store records the caller's version, it reads that without taking any lock, so a session holding one stops no tool whose tables are in place. Otherwise a lock is tried rather than queued for, so a process that dies while waiting leaves nothing in the queue: after 10 seconds held elsewhere (LOCK_WAIT_SECONDS) migrate refuses with store_busy, and a session that holds a lock and then issues no statement for 5 seconds (LOCK_IDLE_SECONDS) is ended by the server, which frees the lock. It returns a MigrateResult with applied, skipped, warnings, major and minor.

It refuses with a DbError, rolling the step back: an applied step whose SQL changed (checksum_mismatch); a step that creates a relation, index, sequence, type, function, collation, statistics object or schema not named <owner> or <owner>_* inside the configured schema, or named schema_ledger or schema_version (naming_law); a step that fails or ends its own transaction (step_failed). A step cannot use statements that refuse a transaction, such as CREATE INDEX CONCURRENTLY.

The version rule lets tools at different releases share one database. When the store records a newer major for the owner than the caller's major, migrate refuses with schema_too_new before applying anything, and the hint says to update. When it records the same major with a newer minor, it proceeds and adds a warning to result.warnings, keeping the stored version. Otherwise it records the caller's version.

Status

from capabilities_contract.db import status

report = status(conn, "automations", steps, major=1, minor=1)
print(report.state, report.pending)

status(conn, owner, steps, *, major, minor) reports what migrate would do with the same arguments, changing nothing and taking no lock, so it answers while another session holds one. It returns a MigrateStatus with state, major and minor (the caller's), stored_major and stored_minor (the store's, or None), and the step ids under applied, pending and changed, plus warnings. state is current when every step is applied and the store records the caller's version or a newer minor (which adds a warning), pending when migrate has something to apply or record, and checksum_mismatch or schema_too_new when migrate would refuse.

Errors

Every failure is a DbError with slug (stable, for the caller to map to an exit code), message and hint (possibly None). Slugs: store_not_configured, store_not_postgres, store_setting_unreadable, store_setting_too_new, bad_store_setting, sslmode_too_weak, bad_schema_name, store_unreachable, transaction_pooler, session_state, driver_missing, bad_application_name, bad_owner, bad_version, bad_step, connection_busy, checksum_mismatch, naming_law, step_failed, schema_too_new, store_busy.

Development

uv venv && uv pip install -e ".[dev]"
uv run pytest

The suite starts a throwaway PostgreSQL cluster with TLS under a temporary directory (initdb and pg_ctl on PATH, or in $PG_BIN) on a random port. Set CAPABILITIES_CONTRACT_TEST_URL to a TLS-enabled server's admin URL to run it against that server instead, as CI does. The transaction-pooler tests run when CAPABILITIES_CONTRACT_TEST_PGBOUNCER names a pgbouncer executable: they put it in front of the test cluster in pool_mode=transaction with one server connection. The tests that compare the machine file reader with the manager's own run only when CAPABILITIES_STORE_TIER names the manager's contract/store.py; otherwise they skip.