Diff v3 → v4

  # Trust but Verify
  
  The defining hazard of generative coding is **plausible-but-wrong** output. Diff looks right, names match the convention, tests pass — and a spec requirement is missing, or a contract is silently broken.
  
  Trust-but-verify is the workflow's answer: an independent adversarial reviewer between implement and ship.
  
  ## The principle
  
  - **Hallucinations don't announce themselves.** You can't ask the same agent "did you do this correctly?" and trust the answer.
  - **An independent reviewer is cheap.** One Opus invocation against a finished diff costs a fraction of a re-implementation.
  
  Every workflow that ships AI-written code needs a verify phase. Not "we should code-review more" — a phase, in the pipeline, with a structured output and a pass/fail gate.
  
  ## The phase
  
  ```mermaid
  flowchart LR
      Impl["/speckit.implement/"] --> V["/speckit.agentic.verify/"]
      Spec & Plan & Tasks & Diff --> V
      V --> Findings[BLOCKER / WARN / NOTE]
      Findings --> Refute{Survives refutation?}
      Refute -->|no| Drop[Dropped]
      Refute -->|yes| Verdict{PASS / FAIL?}
      Verdict -->|FAIL| Fix[Back to implement]
      Verdict -->|PASS| Retro["/speckit.agentic.retrospective/"]
  ```
  
  Inputs: spec, plan, tasks, diff. Output: structured findings + verdict.
  
  ## Seven dimensions
  
  | # | Dimension | What it catches |
  |---|-----------|-----------------|
  | 1 | Spec divergence | Requirements not implemented |
  | 2 | Missing tasks | Tasks marked `[X]` without matching diff content |
  | 3 | Security | OWASP Top 10:2025 — auth, injection, secrets, scopes, supply chain (A03), failing open (A10) |
  | 4 | Correctness | Off-by-one, races, edge cases, swallowed errors, false idempotency claims |
  | 5 | Contract breaks | Schema, struct, signature, response-shape changes that break callers |
  | 6 | Necessity | Abstraction with one caller, a dependency replacing three lines, unrequested config |
  | 7 | Velocity illusions | Unread "working" code, edge-blind tests, PR text that doesn't match the diff |
  
  Spec divergence catches the most common failure. Contract breaks catch the most expensive. Necessity catches the one nobody else is looking for — code that works and shouldn't exist is still a defect, and no test will ever flag it.
  
  Dimension 7 is `NOTE`-only. It rarely blocks anything; it predicts where the next three bugs land.
  
  ## Hunt until dry, then try to kill it
  
  One pass finds the obvious problems and stops. Two protocols wrap the dimensions:
  
  **Loop until dry.** Sweep, then sweep again on whatever produced findings. Stop when two consecutive rounds surface nothing new. Deduplicate against every finding seen — *including refuted ones*, or a rejected candidate resurfaces each round and the loop never terminates. A fixed target ("find 10 bugs") either stops early or pads to reach itself; convergence does neither.
  
  **Refute before reporting.** For each finding above `NOTE`, try to kill it. Default to killed when uncertain — the burden of proof sits on the finding. Ask whether it reproduces, whether the path is reachable, whether a caller already guards it. Where independent subagents are available, run several refutation attempts and keep the finding only if a majority fail.
  
  Findings that are plausible but wrong cost more than findings you miss, because they get acted on.
  
  ## Severity
  
  | Severity | Effect |
  |----------|--------|
  | `BLOCKER` | Must fix. Verdict = FAIL. |
  | `WARN` | Real but non-blocking. Verdict still PASS if no BLOCKERs. |
  | `NOTE` | Observation. Informational. |
  
  Verdict math: `BLOCKER == 0` → PASS.
  
  ## The reviewer's prompt
  
  The single most important property: **look for what's missing, not validate what's present.**
  
  > "You are an adversarial reviewer. Find missing items, contract breaks, security regressions, correctness bugs. Quote line ranges for every finding. Do not praise correctness — that is not your role."
  
  Without this framing, the reviewer falls into the same trap as the implementer: reads code, sees plausible patterns, approves. Same model, same diff, different prompt → different catch-rate.
  
  ## Example finding
  
  ```markdown
  ### Spec divergence — FR-005 (hard delete on file removal)
  **[BLOCKER]** Spec FR-005 requires that when a feature folder disappears,
  all rows for that slug are removed. The diff implements file-level deletion
  in `ScanFeature`, but `ScanAll` does not call `DeleteAEFeature` for slugs
  that have disappeared. Quote: importer.go:50-60.
  ```
  
  ## Composition
  
  Verify is the last line. Earlier gates do the heavy lifting:
  
  - Plan review catches design-level errors before code is written.
  - [[Subagents and Context Injection|Pre-fetched context]] prevents drift into unrelated files.
  - [[The Restraint Principle: YAGNI for Agents|The decision ladder]] keeps unnecessary code from being written at all — necessity is a verify dimension, not just correctness.
  - [[The Compounding Layer|Constitution rules]] block recurring mistakes by category.
  
  Verify catches the residual.
  
+ All seven dimensions read the *diff*. For anything with a UI, that leaves a gap
+ nothing above closes: the artifact itself is never observed running. A build can
+ satisfy every requirement in the spec, break no contract, and still throw on
+ first click or take six seconds to paint — none of which is visible in a patch.
+ Driving a real browser against the built thing is the complement, not a
+ substitute: see [[QA and QoE Testing with Playwright MCP]] for the mechanics and
+ [[QA and QoE]] for why the experience half needs measuring separately from the
+ correctness half.
+ 
  ## Anti-patterns
  
  - **"We have tests, we don't need verify."** Tests check the test's claims. Verify checks the spec's claims. Different.
  - **Skip when in a hurry.** Same trap as [[The Illusion of Speed]]. Verify is faster than the bug-hunt cycle it replaces.
  - **Same model as implement.** Lower yield. Opus for verify even when implement ran Sonnet.
+ - **Verified the diff, never opened the page.** Seven green dimensions and nobody loaded the app. For UI work the browser pass is part of verification, not a follow-up.
  
  > Hallucinations don't announce themselves. The verifier is the announcement.