[rluders]

Closes #20

API

• ParseOptions.Precognition bool — per-endpoint opt-in. When false, the header is ignored (request processed normally).
• Client sends Precognition: true. IsPrecognitionRequest(r) exposes the check.
• ErrPrecognitionHandled sentinel + IsPrecognitionHandled(err).

Behavior (opted-in + header)

Same pipeline as today (decode → bind path params → validate), then stop:

Outcome	Response	err
Valid	204 No Content, Precognition-Success: true	ErrPrecognitionHandled
Validation failed	422, validator's ProblemDetails (extensions kept)	validation error
Malformed / too large / bad path param	unchanged 400 / 413	parse error

Opted-in endpoints also add Vary: Precognition.

Business logic can't run: a successful precognitive request returns a non-nil error, so the existing if err != nil { return } pattern stops the handler. No change for current callers.

Also

• README section, and examples/restapi POST /users opts in.
• Tests cover every row above, opt-out, ordinary request on opted-in endpoint, Vary header, header parsing, and that the validator's problem is not mutated.

go vet, go test -race ./..., validation/playground and examples tests pass locally.

---

Generated by Claude Code

Summary by CodeRabbit

• New Features
  • Endpoints can opt in to request pre-validation. Matching requests receive a success response when validation passes, or a validation error when it fails, without continuing through normal request handling.
  • Added helpers to identify pre-validation requests and handled responses.
• Documentation
  • Documented response behavior, request-inspection helpers, and the limits of pre-validation. Opted-in endpoints include the relevant Vary header; requests without the option continue through normal handling.

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Reviewing files that changed from the base of the PR and between 8afdf5a and 32c60ae.

📒 Files selected for processing: 7

• README.md
• examples/restapi/main.go
• request.go
• request_internal.go
• request_precognition.go
• request_precognition_test.go
• request_types.go

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Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 87.39%. Comparing base (8afdf5a) to head (32c60ae).
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Additional details and impacted files

@@            Coverage Diff             @@
##             main      #26      +/-   ##
==========================================
+ Coverage   86.27%   87.39%   +1.11%     
==========================================
  Files          14       15       +1     
  Lines         583      603      +20     
==========================================
+ Hits          503      527      +24     
+ Misses         62       58       -4     
  Partials       18       18              

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