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==0.4.0"]
# ///

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. Within one level AGENTKIT_DB_URL wins and that level's other keys are ignored, except AGENTKIT_DB_SCHEMA, which applies beside a URL. 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) 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.

A transaction-mode pooler is not supported: it keeps no session between transactions, so the schema binding, psycopg's prepared statements and LISTEN are lost. connect refuses Supabase's transaction pooler - a host under pooler.supabase.com on port 6543 - as transaction_pooler before anything is sent to it, and the hint names its session pooler, the same host on port 5432. Use a direct connection or a session-mode pooler.

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, 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 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.