The Operating-System Test: When Is a Peg Credible and Resilient?

The subject of this series has been the implementation of a currency peg: not just the exchange-rate number, but the central bank balance sheet, bank reserves, money-market rates, capital flows, liquidity operations, and reserve incentives that make the number plausible. The final question is therefore broader than whether the rate stayed fixed. A peg is credible and resilient when its operating system can trace ordinary pressure, absorb shocks with tools matched to their timing and persistence, preserve a functioning market, and make clear what the available evidence does—and does not—establish.

Credibility is a property of the whole system

A stable exchange-rate quote is an outcome. It is not, by itself, a diagnosis of how that outcome was achieved or what cost the banking system paid to achieve it.

Under an open capital account, a fixed exchange rate links the local currency to an anchor currency and limits the central bank’s freedom to choose domestic monetary conditions independently. A currency board makes the link especially explicit by tying local-currency issuance to foreign-currency reserves. A conventional peg may allow more discretion, but it still leaves the central bank managing a balance between the external commitment, local liquidity, and the incentives created by interest-rate differences.

The first credibility test is therefore institutional: can market participants understand what the authority is committed to exchange, at what rate, and under what operating rules? The rule must be more than a public promise. Banks need to know how the central bank’s balance sheet will respond when foreign currency enters or leaves, how reserve accounts settle, and what happens when a bank is temporarily short of liquidity. Investors need to know whether a rate difference reflects a deliberate response to capital flows, a temporary reserve shortage, or a country-risk premium. A fixed quote with opaque or inconsistent operating rules can look stable until the first large shock makes the hidden rules visible.

The second test is balance-sheet visibility. Foreign-exchange transactions change foreign assets and bank reserves. Currency demand changes reserves through currency in circulation. Tax receipts and government spending move balances through the government account. Banks may demand more reserves even when reserve supply has not changed. A central bank that cannot distinguish these channels cannot know whether it is facing an FX outflow, a domestic liquidity injection, a forecast error, or a confidence-driven demand for safety.

The third test is the quality of the feedback loop. A reserve shortage should not be judged only by whether it raises the domestic rate. The question is whether the rate change reaches capital flows and FX transactions in time, and whether those flows restore a workable reserve position without forcing banks into destabilizing behavior. A reserve surplus deserves the same scrutiny. Low rates and abundant balances may be comfortable, or they may be weakening interbank trading and leaving the system less prepared for a reversal.

flowchart LR A["Anchor commitment"] --> B["Balance sheet: FX reserves, bank reserves, autonomous factors"] B --> C["Shock loop: rates, FX flows, capital flows"] C --> D["Control stack: policy rate, OMOs, averaging, facilities, collateral"] D --> E["Reserve incentives: marginal cost and quantity"] E --> F["Market function and bank liquidity"] F --> G["Credibility and resilience judgment"] C --> F G --> H["Evidence boundary: normal conditions, samples, institutions"]

This is why credibility is not the same as an absence of movement. A peg can be credible while allowing a domestic rate to adjust, foreign reserves to change, or banks to pay a temporary premium for liquidity. What matters is whether the adjustment is consistent with the rules and whether the system can return toward a workable state. Conversely, a peg can appear calm because the central bank is absorbing pressure through reserves, because banks are hoarding liquidity, or because a structural surplus has suppressed the money-market rate. Calm is informative only when its mechanism is understood.