fix(translation): stop same-format replay from dropping router additions

[sabhatinas]

Summary

A same-format hop replays the exact inbound body, which is what keeps it lossless. That replay was unconditional, so anything a routing algorithm added to the request IR after decoding never reached the wire — the codec returned the body captured before the addition. stage_router's efficient_system_prompt and capable_system_prompt were silently dropped whenever the inbound request and the selected target shared a format, and handoff notes went the same way.

This is not OpenAI-specific. All three buffered codecs had the identical short-circuit — anthropic/buffered.rs:165, openai_chat/buffered.rs:180, responses/buffered.rs:122. An Anthropic-inbound request routed to an Anthropic target dropped its prompt too, and Responses likewise. Cross-format hops escaped only by accident: the preserved body is keyed by inbound format, so the target codec found nothing to replay and re-encoded from normalized fields.

Fix

PreservationMetadata now carries a seal recorded when the body is captured. Codecs seal after decoding; exact_preserved_request refuses to replay once the IR no longer matches. Callers that mutate the IR need do nothing and cannot forget — the previous contract asked them to clear the entry by hand, which is what libsy was not doing.

model is excluded from the seal: routing rewrites it on every hop and send_encoded stamps the resolved name onto the encoded body afterwards, so a replayed body is never wrong about it.

Verification

Reproduced the ticket end-to-end with a stub upstream that echoes the system message it received:

┌───────────────┬──────────────────────────────────────────────────────────────────┐
│ build │ efficient tier sees │
├───────────────┼──────────────────────────────────────────────────────────────────┤
│ upstream main │ │
├───────────────┼──────────────────────────────────────────────────────────────────┤
│ this branch │ Respond with exactly EFFICIENT_SYSTEM_SENTINEL and nothing else. │
└───────────────┴──────────────────────────────────────────────────────────────────┘

Tests added:

• both tier prompts invalidate exact replay; an unprompted target keeps it
• handoff notes likewise
• same-format encoding carries the instruction for all three formats
• a router-added instruction reaches the wire across all nine source/target format pairs

The two mutation-shape tests were confirmed to fail on unpatched code. Full workspace green, clippy clean, cargo fmt --check clean.

Breaking change

exact_preserved_request now takes &LlmRequest rather than &PreservationMetadata. Out-of-tree codecs implementing BufferedCodec need that one-line change, plus a seal_preservation() call after decoding if they want exact replay.

Fixes SWITCH-1224

Summary by CodeRabbit

• Bug Fixes

  • Prevented stale preserved request bodies from being replayed after request content changes.
  • Ensured added instructions are retained during same-format and cross-format request encoding.
  • Improved exact request replay behavior across supported Anthropic, OpenAI Chat, and Responses formats.

• Tests

  • Added regression coverage for modified requests, configured prompts, one-off notes, and instruction propagation.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Path: .coderabbit.yaml

Review profile: CHILL

Plan: Enterprise

Run ID: 46f35135-236f-44e9-8c02-a355bb63778a

📥 Commits

Reviewing files that changed from the base of the PR and between 759658e and a845afc.

📒 Files selected for processing (8)

• crates/libsy/src/algorithms/util/prompts.rs
• crates/protocol/src/llm.rs
• crates/switchyard-translation/src/codecs/anthropic/buffered.rs
• crates/switchyard-translation/src/codecs/openai_chat/buffered.rs
• crates/switchyard-translation/src/codecs/responses/buffered.rs
• crates/switchyard-translation/src/util.rs
• crates/switchyard-translation/tests/extension_points.rs
• crates/switchyard-translation/tests/request_translation.rs

---

Walkthrough

The change adds request fingerprints to preservation metadata, seals decoded requests, and rejects exact replay when the request IR changes. Tests cover configured prompts, notes, same-format replay, and cross-format instruction propagation.

Changes

Preserved Request Validity

Layer / File(s)	Summary
Request fingerprint contract crates/protocol/src/llm.rs	PreservationMetadata stores a request seal. LlmRequest computes the normalized request fingerprint and checks whether preserved data remains current.
Translation replay integration crates/switchyard-translation/src/codecs/*/buffered.rs, crates/switchyard-translation/src/util.rs, crates/switchyard-translation/tests/extension_points.rs	Provider decoders seal preserved requests. exact_preserved_request receives the full request and rejects stale preserved bodies.
Preservation regression coverage crates/libsy/src/algorithms/util/prompts.rs, crates/switchyard-translation/tests/request_translation.rs	Tests cover prompt and note mutations, exact same-format replay, and instruction propagation across wire formats.

Estimated code review effort: 3 (Moderate) | ~25 minutes

Poem

I’m a rabbit with a sealed request,
Keeping old bytes at their best.
When instructions hop inside,
Exact replay must step aside.
Fresh fingerprints guide the way—
New encoded words now stay.

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly describes the main fix: preventing same-format replay from dropping router-added request fields.
Docstring Coverage	✅ Passed	Docstring coverage is 100.00% which is sufficient. The required threshold is 80.00%.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

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