Diff v1 → v2

  # Losing Control: When the Agent Goes Off-Plan
  
  You asked for a bug fix. You got a refactor.
  You asked for one endpoint. The agent rewrote three.
  You said "confirm before running the migration." It ran the migration.
  
  Three modes — drift, hallucination, premature execution — all produce code that looks plausible, runs, and does the wrong thing.
  
  ## Three modes
  
  ```mermaid
  flowchart LR
      Intent[User intent] --> Agent
      Agent --> S1[Scope creep]
      Agent --> S2[Hallucination]
      Agent --> S3[Premature execution]
      S1 & S2 & S3 --> Ship[Code that ships]
      Ship -.->|surprise| Intent
  ```
  
  **Scope creep.** Charitable interpretation of your prompt → "improvements" you didn't ask for. "Fix the timezone bug" becomes "fix it + refactor the date utility + rename three functions + extract a new package."
  
  **Hallucination.** Invented APIs, packages, file paths. The dangerous variant is _plausible_ — a function name that matches the naming convention so closely that a reviewer skims past it.
  
  **Premature execution.** Tool calls that have side effects: migrations, force-pushes, destructive deletes, emails. By the time you notice, the artefact exists.
  
  ## What drift feels like
  
  > Agent: "I've updated the auth middleware to use the new token format. I also took the opportunity to centralise error logging across the package, and I noticed the old `legacy_auth` file was unused so I removed it."
  > User: "wait — what?"
  
  Three steps from intent before you can pause.
  
  ## Countermeasures
  
  | Failure | Countermeasure | Where |
  |---------|----------------|-------|
  | Scope creep | Reviewed plan _before_ code is written | [[Spec-Driven Workflow]] plan phase |
+ | Over-building | Decision ladder before any construct is written | [[The Restraint Principle: YAGNI for Agents]] |
  | Hallucination | Pre-fetched context; subagent can't grep outside the list | [[Subagents and Context Injection]] |
  | Hallucinated facts | Quote-the-source rule | All phases |
  | Silent gaps | Adversarial verify on the diff | [[Trust but Verify]] |
  | Premature execution | Confirm-before-irreversible | Tool policy in system prompt |
  
  The plan becomes the contract. Anything not in the plan is out of scope until the plan is revised.
  
  ## Diagnosing drift after the fact
  
  1. **Spec says it?** If not, you didn't tell it — update spec, re-run from `plan`.
  2. **Plan says it?** If not, tighten task scope and the implementer's leash.
  3. **Verify caught it?** If no, sharpen the verify dimensions.
  4. **Happened before?** Add to `known-failures.md`. 3+ → constitution (see [[The Compounding Layer]]).
  
  Drift in implementation is almost always a symptom of a leakier phase upstream.
  
  > Untyped trust is the same as trust in a hallucination.