A Peg’s First Stress Test: Can the Loop Absorb a Shock?

A fixed exchange-rate arrangement becomes easier to judge once its anchor commitment, balance sheet, reserve flows, rates, and market frictions are viewed as one operating loop. This checkpoint connects those already-established relationships and applies them to a practical question: when a shock arrives, is the peg absorbing it through a functioning feedback mechanism, or merely moving the pressure into reserves, interest rates, and banks? The answer supports a more useful judgment than observing whether the quoted exchange rate moved.

One shock, one operating loop

A shock can enter the system in several places. Foreign investors can move funds across borders. A terms-of-trade change can alter the supply of foreign currency. The public can change its demand for cash. The government can collect taxes or spend from its account at the central bank. Banks can suddenly want more precautionary reserves because they trust counterparties less. These events have different immediate causes, but they become relevant to the peg through a common set of channels.

First, the shock changes either the supply of foreign currency, the central bank’s foreign assets, the quantity of bank reserves, or the demand for those reserves. Second, the reserve position changes the conditions in the domestic money market. Third, the resulting rate movement changes the incentives for capital flows and FX transactions. Fourth, the flow response may restore the reserve position—or may arrive too late, prove too small, or fail to appear. At every stage, transaction costs, settlement timing, country risk, counterparty risk, and market depth can determine whether the response is stabilizing.

The exchange-rate commitment is the constraint that holds the loop together. The central bank stands ready, within the design of the regime, to exchange local and foreign currency at the committed rate. That action keeps the external price from doing the adjustment that a floating exchange rate would normally perform. The adjustment instead appears in the central bank’s foreign assets, bank reserves, and domestic interest rate. Capital flows then determine how much of that adjustment is reversed or prolonged.

flowchart LR S["Shock: FX flow or autonomous factor"] --> B["Central bank balance sheet"] B --> R["Bank reserves"] R --> I["Domestic short-term rate"] I --> F["Capital-flow and FX response"] F --> B R --> M["Bank liquidity and market depth"] I --> M M --> W["Friction, risk, or timing wedge"] W --> O["Stress absorbed, delayed, or displaced"] F --> O

This loop is useful because it separates a shock’s source from its visible symptom. A higher overnight rate may be the consequence of an FX outflow, a government payment, a rise in precautionary reserve demand, or a market that no longer redistributes funds smoothly. Those causes can look similar in a rate series while requiring different interpretations. The first question is therefore not “why did rates rise?” in isolation.