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Sending-side chain direction: validators should gate final wire bytes, but SDKs disagree

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Assessment

Difficulty
4/5
Estimated time
3-5 days
Newbie friendliness
42/100
Issue type
Feature
Clarity
Mostly clear
Activity status
Quiet
Tech stack
python
Domain
backend-api-design, security, testing-qa

Research direction

Read the SEP context from #20 and the feature/python-sdk direction derivation, then inspect the chain-request-order-mutate-then-validate and redaction-defuses-relaybleed conformance fixtures. Confirm the sending-side order in the SEP text, align the Python SDK derivation, and verify the executable fixtures pass with the intended outcomes.

Written by the indexing model from the issue text.

Description

Found while matrix-testing SDKs against conformance fixtures (context: #20). Filing separately because this divergence changes security behavior, not just wire shape.

The divergence. For a request on the sending side, the SEP's client trust-boundary order runs mutators before validators, so validation covers the final bytes that will actually cross the boundary. feature/python-sdk derives validate-then-mutate for the same case (matching the Go SDK's receiving-side posture).

Why the order matters, the soundness argument. If validation runs before mutation, mutator output is never validated: a buggy or compromised mutator can reintroduce content after the validator approved the message. Validate-last is the only order where the validator's verdict applies to what is actually sent.

It also has a false-positive cost in the other direction. Concrete case from the behavior fixtures: a redaction mutator that strips credentials, composed with a cross-boundary exfiltration guard. In the SEP order, the redactor defuses the payload first and the guard correctly allows the now-safe write. In the derived validate-first order, the guard sees the verbatim credential before the redactor runs and denies a write that should have been allowed. Same interceptors, same message, opposite outcomes purely from chain direction.

Both behaviors are pinned as executable fixtures in the suite posted on #20 (chain-request-order-mutate-then-validate, redaction-defuses-relaybleed). Related in spirit to the C# divergences already tracked in #15 / #17: this class of cross-SDK drift is exactly what conformance should catch.

Proposal: pin the sending-side order (mutate, then validate) explicitly in the SEP text, and align the direction derivation in feature/python-sdk. Happy to PR the SDK fix with tests if the group agrees.

Dominant language
C#
Stars
23
Forks
15
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