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S4 Checkpoint Protocol

At phase boundaries, the orchestrator performs an S4 checkpoint to assess whether the current approach remains valid.

Temporal Horizon: S4 operates at a days horizon—asking questions about the current milestone or sprint, not minute-level implementation details. See the pact-orchestrator agent body §S3/S4 Operational Modes for the full horizon model.

Trigger Points

  • After PREPARE phase completes
  • After ARCHITECT phase completes
  • After CODE phase completes (before TEST)
  • When any agent reports unexpected complexity
  • On user-initiated "pause and assess"

Checkpoint Questions

  1. Environment Change: Has external context shifted?

    • New requirements discovered?
    • Constraints invalidated?
    • Dependencies changed?

  2. Model Divergence: Does our understanding match reality?

    • Assumptions proven wrong?
    • Estimates significantly off?
    • Risks materialized or emerged?

  3. Plan Viability: Is current approach still optimal?

    • Should we continue as planned?
    • Adapt the approach?
    • Escalate to user for direction?

  4. Shared Understanding (CT): Do we and the completing specialist agree?

    • Orchestrator's understanding matches specialist's handoff?
    • Key decisions interpreted consistently?
    • No misunderstandings disguised as agreement?

    Verification: At final gates (TEST→PR, comPACT, plan-mode), SendMessage to the completing specialist to confirm your understanding. At intermediate boundaries, the downstream agent's teachback verifies shared understanding. Background: pact-ct-teachback.md.

Checkpoint Outcomes

Finding Action
All clear Continue to next phase
Minor drift Note in handoff, continue
Significant change Pause, assess, may re-run prior phase
Fundamental shift Escalate to user (S5)

Checkpoint Format (Brief)

S4 Checkpoint [Phase→Phase]:

  • Environment: [stable / shifted: {what}]
  • Model: [aligned / diverged: {what}]
  • Plan: [viable / adapt: {how} / escalate: {why}]
  • Agreement: [verified / corrected: {what}]

Output Behavior

Default: Silent-unless-issue — checkpoint runs internally; only surfaces to user when drift or issues detected.

Examples:

Silent (all clear):

(Internal) S4 Checkpoint Post-PREPARE: Environment stable, model aligned, plan viable, agreement verified → continue

Surfaces to user (issue detected):

S4 Checkpoint [PREPARE→ARCHITECT]:

  • Environment: Shifted — API v2 deprecated, v3 has breaking changes
  • Model: Diverged — Assumed backwards compatibility, now false
  • Plan: Adapt — Need PREPARE extension to research v3 migration path
  • Agreement: Corrected — Preparer assumed v2 compatibility; confirmed v3 migration needed

Relationship to Variety Checkpoints

S4 Checkpoints complement Variety Checkpoints (see Variety Management):

  • Variety Checkpoints: "Do we have enough response capacity for this complexity?"
  • S4 Checkpoints: "Is our understanding of the situation still valid?"

Both occur at phase transitions but ask different questions.