"The validity of the public debt of the United States, authorized by law, including debts incurred for payment of pensions and bounties for services in suppressing insurrection or rebellion, shall not be questioned."

It is strangely worded, but I think it could be interpreted to also mean that the government shall not do anything that would call the validity of the debt into question.

It doesn't really make sense to pass an amendment that tries to mandate the perception of the validity of the debt. if the government stops honoring it, then the invisible hand of the market will consider it invalid.

That would be a reach. The non-reaching interpretation is the face value one: the validity of the public debt ... shall not be questioned. It's hardly the most anti-market law around - the US has farm subsidies, and some countries even have price controls.

What would that even mean though? Bonds are sold at auctions, and already sell at a price different from face value. The difference determines the yield.

You could have "no one questioning the validity" but people are still going to either not participate in the auction, or bid only for high yields.

One of the factors in evaluating a case is whether the proposed remedy is feasible or even possible. I'm not sure what forcing everyone else to "consider valid" a government bond that is not being honored.

Do you arrest and imprison anyone who reveals that they are not getting paid on their bond at maturity? Do you force buyers to buy it at face value, even though they know it will not be honored?

I don't think that's a reach. There's a credible argument that it compels the President to continue to service debts

I think it’s even stronger than that. It doesn’t just compel the president, which is a political office, but it compels every civil servant in the treasury as it’s a constitutional mandate. So in theory even if the president declares no debt payment, the treasury would still make the payments.

On my latest re-reading of it in your comment, it sounds more like it's saying, "shut up, you lost, the measures taken to suppress your rebellion are valid, move the frak on".

The context at the time was doubt that the treasury would pay its debts, because it was widely viewed that it would not post the civil war. The language is not clear, but the intent as I understand it from a historical perspective is pretty well understood to mean the treasury must always pay its debts. The language continues to clearly delineate debts accrued by the confederacy (insurrection) as NOT payable, which further enforces the topic is payment of debt.

You have to recall the context in which those provisions were passed: coming out of the Civil War, you had both Union and Confederate bonds and other public debts used to finance the war. Amongst the other conditions upon which the rebellious states were re-admitted to political authority were the cancellation of Confederate debt (the Union was not going to pay for the wages of rebellion) and the guarantee of Union debt (so that Southern and Western states could not collude in Congress to prevent repayment of that debt), both of which were written into the Public Debt Clause. So the clause was absolutely meant to restrict the government's ability to repudiate debts other than those explicitly excluded, because you had a Republican Congress working to tie the hands of a potential future Democratic one. (N.B., I'm not exactly sure what you mean by "required to accept T-bills at face value" -- you can buy T-bills and TIPS on the secondary market at whatever price the market sets. If the US were to start defaulting on its debt, then that price ... would not be as stable as it has traditionally been.)