fix(translation): Responses wire compatibility for strict backends (system/developer roles, list input)

[RedEyeNinja-BKK]

Strict Responses backends (the chatgpt.com Codex endpoint) reject two shapes that the Responses codec currently emits:

1. system-role input items - 400 "System messages are not allowed"; the backend requires the developer role.
2. Top-level input STRING shorthand - 400 "Input must be a list" when encode_responses_input emits a bare string for a single-user-text-message request.

This series makes every Responses-encoded request wire-legal for strict backends:

• role_to_responses maps Role::System to developer (mirroring the Codex wire rules for chat -> Responses conversions). developer is a first-class role in the Responses API, so non-strict backends are unaffected.
• Exact-request passthrough replays a preserved body verbatim; system-role input items are normalized to developer at replay time (typed {"type":"message","role":"system"} items and untyped role-keyed items alike), so preserved same-format requests stay wire-legal too.
• encode_responses_input always emits canonical message-item lists. Message-item lists are valid for every Responses backend, so this is wire-shape hygiene rather than a behavior fork.

The inline-instruction decode architecture is unaffected: system/developer items decode to instruction blocks and re-encode into the instructions field; the role mapping additionally protects any system-role IR message reaching the generic encoder. Regression tests cover typed/untyped normalization, wire-level absence of system roles, the exact-replay and embed-preservation boundaries, and single-message list encoding.

DCO signed.

Summary by CodeRabbit

• Bug Fixes
  • OpenAI Responses requests using system roles are now consistently translated to developer roles.
  • Preserved and replayed requests now produce canonical input arrays without unsupported system roles.
  • Improved compatibility for both structured and unstructured input items when processing role-based instructions.
• Tests
  • Added coverage for role normalization across direct translation and preserved-request replay scenarios.

[coderabbitai]

Walkthrough

The Responses buffered codec now normalizes preserved system input roles to developer. Tests cover typed, untyped, and embedded-preservation request flows.

Changes

Responses role normalization

Layer / File(s)	Summary
Normalize preserved and encoded roles crates/switchyard-translation/src/codecs/responses/buffered.rs	Preserved system input items are rewritten to developer. Normalized System roles encode as developer.
Validate preservation and replay behavior crates/switchyard-translation/tests/request_translation.rs	Tests cover typed and untyped items, preserved user roles, serialized-role rejection, and embedded-preservation replay.

Estimated code review effort: 3 (Moderate) | ~20 minutes

Merge Risk: 🟡 Moderate · up to e8ca6

This change normalizes system roles, but single-message requests can still be sent as scalar input rather than the required message-item list, which may cause strict Responses backends to reject requests. Resolve the canonical input encoding gap before merge.

Poem

A rabbit taps the codec door
System hops to developer’s floor
Typed and untyped trails align
Replay returns a canonical line
No system roles remain in sight

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 42.86% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 7 functions across 2 files.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 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 change: Responses translation now supports strict backends by normalizing system and developer roles. It is specific and related to the changeset.
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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Actionable comments posted: 1

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Inline comments:
In `@crates/switchyard-translation/src/codecs/responses/buffered.rs`:
- Line 132: Update encode_responses_input and normalize_system_input_roles so
scalar input values are converted to the canonical single user message-item
array before returning, for both preserved and non-preserved requests. Remove
the scalar fast path, and add regression coverage for both request paths.

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• crates/switchyard-translation/src/codecs/responses/buffered.rs
• crates/switchyard-translation/tests/request_translation.rs

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[afourniernv]

I pulled on this a bit more. The compatibility issue looks real.

The normal Responses API accepts scalar input and system messages, but the ChatGPT Codex endpoint has been observed rejecting both. OpenAI’s Codex client sends list input and developer messages instead.

Since both endpoints use the Responses format, we need something else to tell them apart. I couldn’t find an earlier Switchyard issue or maintainer decision covering these Codex restrictions.

Where I’m uncomfortable is the boundary. #619 makes the Codex restrictions part of the shared Responses codec, even though those request shapes are valid on the normal OpenAI API.

#621 then adds Backend::is_codex() as public API and changes shared URL handling to identify the endpoint. This feels like a larger API and behavior decision than the original compatibility bug needs.

I don’t think we should merge this as-is. The outbound client looks like the more natural place to isolate backend-specific handling, but I don’t want to prescribe whether that should use the URL, an explicit backend type, or something else.

@grahamking could you weigh in on where you’d want this boundary before we ask for a rework?