How must an instance record version constraints before closing a session?

**Date of Record:** 2026 October 25 **Scope:** Applies when an agentic session relies on external libraries, API contracts, CLI tools, or language runtimes to derive its conclusions, and is preparing to terminate. Does not apply to immutable, fully sandboxed environments (such as pinned Docker images or Nix flakes) where the environment cannot drift out-of-band.

Core Distinction

The separation between **transient execution state** (the specific memory addresses, process IDs, and temporary variables active during the session) and **version constraints** (the exact semantic versions, API schemas, and toolchain dependencies under which the conclusions were validated). Transient state evaporates on teardown; version constraints define the immutable baseline required for a successor to reproduce the logic. A conclusion persisted without its version constraints assumes the successor will operate in a perfectly identical environment, which is rarely true in mutable distributed systems.

What Was Observed

In multi-session agent architectures, failing to record version constraints before session termination produces three recurring failure modes:

  1. **Silent API Upgrades:** A successor instance queries an external API assuming the v1 contract, but the endpoint has silently upgraded to v2, causing schema mismatches or changed behavior that invalidates the original conclusion.
  2. **Library Incompatibility:** The session's Python code relied on a specific minor version of a package (e.g., `v2.1.3`), but the successor's environment installs the latest major version (`v3.0.0`), breaking backward compatibility and inducing syntax or runtime errors.
  3. **Tooling Mismatch:** A CLI tool's flags or output format changed between versions, causing the successor's verification predicate to fail or misinterpret the environment state.

Recorded Elements

  1. **`package_versions`:** The exact names and semantic versions (or lockfile hashes) of installed libraries and dependencies.
  2. **`api_contracts`:** The specific endpoint URIs, API version headers, and schema fingerprints required for external communication.
  3. **`tooling_constraints`:** The required versions or compatibility bounds of CLI tools, operating system kernels, or language runtimes.
  4. **`drift_policy`:** The fail-closed rule dictating whether a minor patch bump permits action, or whether any version deviation forces full re-verification.

Condition of Validity

* **Applies when:** The terminating session was executed in a mutable environment where packages, APIs, or tools could be updated out-of-band, and a successor instance will rely on the session's conclusions. * **Does not apply when:** The environment is strictly immutable (e.g., content-addressed storage, pinned container images), or the session produced no conclusions that depend on external tooling.

Decision Boundary

  1. **Did the session execute code or queries that relied on an external package, API, or tool?** If no, no version constraints are necessary. If yes, proceed to log them.
  2. **Is the exact version identifier captured (not just a major version)?** If a conclusion was validated against a specific API schema or library behavior, the exact semver or hash must be recorded. A loose constraint (e.g., `>=1.0`) is insufficient for critical operational invariants.
  3. **Does the successor environment have an enforcement mechanism to verify these constraints before resuming?** If no, the instance must persist the `drift_policy` explicitly, instructing the successor to fail closed and re-probe if the current environment does not match the recorded constraints.
  4. **Conversely, when can a successor bypass the version check?** Only if the record explicitly declares the conclusion as environment-agnostic, or if the constraint is marked as a soft dependency where minor drift is permitted.

Operational Effect

  1. **Reproducibility Guarantee:** Ensures that any conclusion inherited by a successor can be evaluated against the exact software baseline it was originally tested on.
  2. **Fail-Fast Environments:** Prevents the successor from executing costly or non-idempotent operations in an environment where the underlying dependencies have silently mutated.
  3. **Prevention of Silent Drift:** Surfaces API or library upgrades as explicit boundary violations rather than subtle runtime errors or corrupted data.
from dataclasses import dataclass
from enum import Enum, auto
import hashlib
import json


class DependencyType(Enum):
    LIBRARY = auto()
    API_CONTRACT = auto()
    CLI_TOOL = auto()


class DriftPolicy(Enum):
    FAIL_CLOSED_EXACT_MATCH = auto()
    ALLOW_MINOR_PATCH = auto()
    ENVIRONMENT_AGNOSTIC = auto()


@dataclass(frozen=True)
class VersionConstraint:
    dependency_name: str
    dependency_type: DependencyType
    required_version: str
    drift_policy: DriftPolicy


@dataclass(frozen=True)
class SessionVersionManifest:
    constraints: tuple[VersionConstraint, ...]
    manifest_digest: str


@dataclass(frozen=True)
class CurrentEnvironment:
    active_versions: tuple[tuple[str, str], ...]


class VersionConstraintRecorder:
    """
    Extracts and seals version constraints required to safely resume a session,
    ensuring successors validate the environment before acting on conclusions.
    """

    @staticmethod
    def _hash(val: str) -> str:
        return hashlib.sha256(val.encode("utf-8")).hexdigest()

    @classmethod
    def seal_constraints(
        cls, constraints: tuple[VersionConstraint, ...]
    ) -> SessionVersionManifest:
        if not constraints:
            raise ValueError("At least one version constraint must be recorded.")

        for c in constraints:
            if not c.required_version.strip():
                raise ValueError(
                    f"Dependency {c.dependency_name!r} lacks a required version."
                )

        digest_payload = {
            "dependencies": tuple(
                sorted(f"{c.dependency_name}@{c.required_version}" for c in constraints)
            ),
            "policies": tuple(
                sorted(c.drift_policy.name for c in constraints)
            ),
        }
        digest = cls._hash(
            json.dumps(digest_payload, sort_keys=True, separators=(",", ":"))
        )

        return SessionVersionManifest(
            constraints=constraints,
            manifest_digest=digest,
        )

    @classmethod
    def verify_environment(
        cls,
        manifest: SessionVersionManifest,
        current_env: CurrentEnvironment,
    ) -> tuple[str, ...]:
        """
        Fetching-time check: returns a tuple of violated dependencies.
        An empty tuple means the environment is compatible.
        """
        active = dict(current_env.active_versions)
        violations: list[str] = []

        for constraint in manifest.constraints:
            if constraint.drift_policy is DriftPolicy.ENVIRONMENT_AGNOSTIC:
                continue

            active_version = active.get(constraint.dependency_name)
            if active_version is None:
                violations.append(
                    f"MISSING: {constraint.dependency_name} requires {constraint.required_version}"
                )
                continue

            if constraint.drift_policy is DriftPolicy.FAIL_CLOSED_EXACT_MATCH:
                if active_version != constraint.required_version:
                    violations.append(
                        f"MISMATCH: {constraint.dependency_name} expected {constraint.required_version}, found {active_version}"
                    )
            
            # Note: A full semantic version parser would be required here to implement
            # ALLOW_MINOR_PATCH logic correctly. For this reference pattern, we strictly
            # demonstrate the FAIL_CLOSED and AGNOSTIC cases.

        return tuple(violations)