E1 · Green battery
5 minRun the full battery.
Done when: node scripts/bateria.mjs; echo "saída: $?" → BATERIA: 24/24 ok and saída: 0.
Lesson 4 · 18 min
A frozen battery for your tools, and regression when the spec changes
When the code or the spec changes, a site with WebMCP fails silently: detection does not find the API, falls back to the form, and nobody notices. Only a battery catches it. In this lesson you freeze the contract and the cases for the tools, run them in all four modes, and watch the battery catch the old API name.
Start the lesson
In this lesson you will
- 1What freezing is and why the hash
- 2The four modes: sem, polyfill, native, and MCP
- 3The silent regression of the old name
- 4What to do when the expected result really changes
Goal: freeze the contract and cases for your tools, run them in four modes, and watch the battery catch the old API name regression.
01From the frozen battery to the verdict
Click each step.
Click each block in the diagram to see what it does.
The battery in course 2 tests the page; this one tests the page and the server against a contract that does not change without a decision.
02Rules for a battery that counts
Inherited from LOOP-R Engenharia (frozen tests) and applied to the tools.
1. Frozen
The expected result does not change just to make the test pass. Did the spec or the rule change? Re-freeze and explain it in the commit.
The hash makes the change visible: nobody edits casos.json without the battery refusing.
2. No model
The cases call the tools directly (executeTool, tools/call), as an agent would.
No model in the loop: the same code gives the same verdict.
3. Mode sem
The round with no API proves that the person books through the form.
It is the first line of the battery, before any agent.
4. Spec changed
New spec, new Chrome or new SDK: run the battery before anything else.
The watcher from lesson 6 tells you when it is time.
Order
watcher flags it → battery → read what changed → fix or re-freeze with a reason
03The full battery
Four modes, 24 checks, no model and no cost.
Terminal · frozen battery
cd ~/projetos/formacao-webmcp-expert-v72/kit node scripts/bateria.mjs; echo "saída: $?"
Example from the repository
== Chromium 156.0.8078.4 (Playwright) · bateria congelada (spec 2026-10-16, MCP 2026-07-28) ok [sem] fallback: página sem WebMCP e sem erro de JavaScript · sem ok [sem] fallback: a pessoa reserva pelo formulário ok [polyfill] contrato: 4 ferramentas com as anotações congeladas · 4 registradas ok [polyfill] c1 buscar_oficinas · of-301 ok [polyfill] c2 buscar_oficinas · erro ok [polyfill] c3 reservar_vaga (pessoa recusa) · 0 reserva(s) ok [polyfill] c4 reservar_vaga (pessoa confirma) · 1 reserva(s) ok [polyfill] c5 reservar_vaga (pessoa confirma) · erro, 1 reserva(s) ok [polyfill] c6 listar_comentarios · 2 comentário(s) ok [polyfill] nenhum erro de JavaScript ok [nativo] contrato: 4 ferramentas com as anotações congeladas · 4 registradas ok [nativo] c1 buscar_oficinas · of-301 ok [nativo] c2 buscar_oficinas · erro ok [nativo] c3 reservar_vaga (pessoa recusa) · 0 reserva(s) ok [nativo] c4 reservar_vaga (pessoa confirma) · 1 reserva(s) ok [nativo] c5 reservar_vaga (pessoa confirma) · erro, 1 reserva(s) ok [nativo] c6 listar_comentarios · 2 comentário(s) ok [nativo] nenhum erro de JavaScript ok [mcp] contrato: 2 ferramentas com as anotações congeladas · buscar_oficinas,reservar_vaga ok [mcp] c1 buscar_oficinas · of-301 ok [mcp] c2 buscar_oficinas · erro ok [mcp] c3 reservar_vaga (pessoa recusa) · 0 reserva(s) ok [mcp] c4 reservar_vaga (pessoa confirma) · 1 reserva(s) ok [mcp] c5 reservar_vaga (pessoa confirma) · erro, 1 reserva(s) BATERIA: 24/24 ok saída: 0
c5 asks the person and they confirm, but the rule refuses (of-102 has no spots left): an error and no new booking. In MCP, the question to the user comes before the rule's refusal.
BATERIA RECUSADA: casos.json ou contrato.json mudou depois de congelado. Se o esperado mudou de verdade (spec nova, regra nova), rode --recongelar e explique no commit. saída: 2
Someone changed "reservas": 0 to 1 in case c3 to "make the test pass". The battery does not even run: the hash does not match bateria/CONGELADA and the exit code is 2.
04The silent regression: the old name
navigator.modelContext is the old name (before PR #177, accepted on 05/19/2026). A copy of the page that uses it: the site keeps working, and only the battery notices.
Terminal · page with the old name
cd ~/projetos/formacao-webmcp-expert-v72/kit
E=$(bash scripts/copia.sh a4)
# a regressão: alguém cola um exemplo antigo que usa o nome de antes de 19/05/2026
sed -i 's/const mc = document.modelContext; /const mc = navigator.modelContext; /' $E/pagina/ferramentas.mjs
cd $E && node scripts/bateria.mjs sem nativo | grep -E "FALHA|ok \[sem\]|BATERIA"; echo "saída: ${PIPESTATUS[0]}"Example from the repository
ok [sem] fallback: página sem WebMCP e sem erro de JavaScript · sem ok [sem] fallback: a pessoa reserva pelo formulário FALHA [nativo] contrato: 4 ferramentas com as anotações congeladas · 0 registradas FALHA [nativo] c1 buscar_oficinas · ferramenta ausente FALHA [nativo] c2 buscar_oficinas · ferramenta ausente FALHA [nativo] c3 reservar_vaga · ferramenta ausente FALHA [nativo] c4 reservar_vaga · ferramenta ausente FALHA [nativo] c5 reservar_vaga · ferramenta ausente FALHA [nativo] c6 listar_comentarios · ferramenta ausente BATERIA: 3/10 ok saída: 1
The sem mode passes: the person books normally. In nativo, the detection does not find navigator.modelContext in Chromium 156, the page falls back, and the four tools disappear. No JavaScript error: without the battery, nobody would see it.
05Rehearsal: the expected result changed
On a throwaway copy of the kit.
Terminal · casos.json changed
cd ~/projetos/formacao-webmcp-expert-v72/kit E=$(bash scripts/copia.sh a4c) # o erro comum: mudar o esperado para o teste passar sed -i 's/"reservas": 0/"reservas": 1/' $E/bateria/casos.json cd $E && node scripts/bateria.mjs mcp; echo "saída: $?"
06Practice
The battery goes green, the regression gets caught, and a case of your own. Each exercise ends in a done when.
E2 · Catch the old name
5 minRun the rehearsal for the page that uses navigator.modelContext.
Done when: The block prints FALHA [nativo] contrato: 4 ferramentas com as anotações congeladas · 0 registradas, BATERIA: 3/10 ok and saída: 1.
E3 · The battery refuses to run
3 minRun the rehearsal with the changed casos.json.
Done when: The block prints BATERIA RECUSADA: casos.json ou contrato.json mudou depois de congelado. and saída: 2.
E4 · A case of your own
15 minIn a copy of the kit (bash scripts/copia.sh e4), add a case c7 to bateria/casos.json (e.g. buscar_oficinas with tema mcp, expecting of-302), re-freeze it with --recongelar, and run the battery on the copy.
Done when: In the copy, node scripts/bateria.mjs; echo "saída: $?" → BATERIA: 27/27 ok and saída: 0 (c7 in polyfill, nativo and mcp).
07Lesson quiz
The explanation appears after the answer.
Question 1 of 4
The page uses navigator.modelContext. What does the visitor see in Chromium 156?
Correct
Detection doesn't find the API and the page falls back: nothing breaks on screen. It's a silent regression: only the battery (contract with 0 registered) shows it.Not this one
Detection doesn't find the API and the page falls back: nothing breaks on screen. It's a silent regression: only the battery (contract with 0 registered) shows it.Review: The full batteryQuestion 2 of 4
What is the bateria/CONGELADA file for?
Correct
The sha256 of casos.json and contrato.json. If the expected result changes without --recongelar → BATERIA RECUSADA (exit 2). Re-freezing is a decision, with a reason in the commit.Not this one
The sha256 of casos.json and contrato.json. If the expected result changes without --recongelar → BATERIA RECUSADA (exit 2). Re-freezing is a decision, with a reason in the commit.Review: From the frozen battery to the verdictQuestion 3 of 4
Why does the battery run the sem mode before the others?
Correct
WebMCP is a draft: without an origin trial or flag, the visitor doesn't have the API. The fallback is the main path, and the battery proves that first.Not this one
WebMCP is a draft: without an origin trial or flag, the visitor doesn't have the API. The fallback is the main path, and the battery proves that first.Review: Rules for a battery that countsQuestion 4 of 4
The spec changed and a tool's expected annotation is now different. The right move is:
Correct
The expected result changes when the source changes, with a recorded decision. Editing without re-freezing is refused; deleting a case or mode hides exactly what the battery exists to show.Not this one
The expected result changes when the source changes, with a recorded decision. Editing without re-freezing is refused; deleting a case or mode hides exactly what the battery exists to show.Review: Rules for a battery that countsQuiz complete
You got {a} of {n} right. If you get them all right, the lesson is marked as completed.Choose an option. The explanation appears after your attempt.
08In three sentences
FrozenContract and cases with a hash: the expected result only changes with a decision.
Four modesNo API, polyfill, native and MCP server: the fallback first, then each door.
SilenceThe old API name doesn't break the screen; the battery is what sees it.