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Lesson 21 / 21 · Advanced

Capstone: trace a payment end to end

Build a technically honest verification map.

14 MIN WITH PRACTICEREAD → TRY → REFLECTNO WALLET NEEDED

By the end, you’ll be able to…

  • Connect transaction construction, validation and settlement.
  • Identify the assumptions at each layer.
Your learning map

Three questions to carry into this lesson.

01Connect transaction construction, validation and settlement.
02Identify the assumptions at each layer.
Use these goals to guide your reading. Try the paper exercise, then explain the result in your own words.

Choose a fictional transaction

Design a paper payment from an imaginary community treasury to an artist. Give it two inputs, a recipient output, a change output and an explicit fee. Describe the spending policy and whether a PSBT coordinator is involved. Use synthetic amounts and abstract keys; no real addresses, credentials or signing actions are required.

Trace the evidence

Explain what the signer checks, what the broadcasting node observes, what a miner includes and what the receiving wallet verifies. Add a possible replacement and a possible reorganization. If you choose a Lightning alternative, replace the on-chain-per-payment assumptions with channel, routing and settlement conditions. Do not pretend both paths provide identical observations or operational requirements.

Challenge your own account

Ask where an untrusted coordinator, stale node, missing policy backup or misleading interface could create an error. Identify what an independent reviewer can reproduce and what remains dependent on external behavior. The final artifact is a one-page explanation and a list of unresolved assumptions. A strong Bitcoin advocate should be able to explain both the system’s useful guarantees and the places where careful operations remain necessary.

Your turn / A paper experiment

Practice on paper

For inputs of 80,000 and 50,000 sats, a 90,000-sat artist payment and a 1,500-sat fee, compute change and name three independent checks in your map.

I’ve tried it — show the worked answer

Change is 38,500 sats. Useful independent checks include signer review of outputs and fee, node validation against consensus rules, and recipient reconciliation of the accepted transaction to the invoice. Each verifies a different property.

Want to explore with buttons and instant feedback? Try the practice lab ↗

Think it through

Make a choice. Discover why.

Choose an answer and check the explanation. You can retry as often as you like. These are practice questions, not a test of mastery; answers are not saved or sent.

1. Can one green checkmark summarize every trust boundary?
  • Yes
  • No
Read the explanation

No. Name the check and its scope.

2. Should a technical explainer hide uncertain implementation details?
  • Yes
  • No
Read the explanation

No. Label them and identify the evidence needed to resolve them.

One idea to take with you

Technical depth means being precise about both guarantees and assumptions.

Your learning, at your pace

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