feat(runner): let a route declare image input so Codex stops omitting images

[mrPronin]

What

Adds a vision route capability, so GET /v1/models can advertise image input instead of always declaring input_modalities: ["text"].

Why

The hardcoded declaration is not cosmetic metadata. Codex reads input_modalities from the model card and, when it reads text-only, replaces an attached image with the literal text

image content omitted because you do not support image input

before it sends. So routing a vision-capable model through Switchyard loses the image in the client, and the proxy never receives one to forward. The response is 200, nothing is logged, and the only other signal is a smaller prompt-token count — the model simply answers that it was given no image.

Measured at the wire against a stand-in upstream that logs the request body, driving real codex exec -i <file> through a one-route passthrough:

Run	Outbound body	Last user-message content
Codex → upstream, no proxy (control)	759,745 B	input_image (543,102-char data URI)
Codex → Switchyard, ["text"]	248,385 B	60-char placeholder
Codex → Switchyard, ["text","image"]	739,155 B	input_image, same as control

Closes #563

How

vision: Option<bool> beside the existing tool_calling and reasoning, with the same rationale — a serving surface cannot probe it, so a route opts in via config:

[routes.sees]
id = "sees"
type = "passthrough"
target = "hosted-vision-model"
vision = true

⭐ Failing closed matters more here than for the other two capabilities. A route may resolve to a target with no vision at all, and declaring image support for such a target sends an image the backend cannot read. So an undeclared route stays text-only, and the documentation says to declare vision = true only when every target the route can select accepts images.

The OpenAI data entry reports the raw Option, so an undeclared route stays distinguishable from one that declared false.

How tested

• cargo test --workspace green (33 suites, 0 failures)
• cargo fmt --check clean
• Added models_endpoint_advertises_image_input_only_for_vision_routes: a vision = true route advertises ["text","image"], an undeclared route stays ["text"], and the data entry reports true vs null.
• Manual smoke: real codex exec -i <png> through a locally built switchyard-server into a request-logging upstream, before and after declaring vision = true — the table above.
• uv run ruff check . / mypy / pytest — n/a, no Python touched

Notes for reviewers

• ⚠ DCO sign-off is not yet on the commit. Happy to git rebase --signoff and force-push on request; I did not want to add the attestation line unasked.
• This does not address the neighbouring hardcoded fields on the same model card. supports_image_detail_original is still false, and base_instructions / default_reasoning_level / truncation_policy are still constants — the base_instructions one has a larger consequence and is filed separately as [bug] base_instructions stub replaces Codex's own system prompt on every routed session #565.
• The existing codex_model_entry_json comment already flags this class of problem, with a TODO about sourcing capabilities from the backend rather than route config. This change follows the current convention (declare in config, fail closed) rather than pre-empting that refactor; if you would rather input_modalities be derived from a backend probe where one exists, I am glad to rework it that way.
• Found together with [bug] Responses codec emits adjacently-tagged ImageSource/FileSource into image_url/file #564 (Responses image encode shape, PR fix: encode Responses image and file content as wire-shaped parts #566). They are independent defects: [bug] Responses codec emits adjacently-tagged ImageSource/FileSource into image_url/file #564 is unreachable on a same-format route, and fixing it does not make images work, which is why both exist.

🤖 Generated with Claude Code

https://claude.ai/code/session_018yveJruskBHpt3EXehSuwo

Summary by CodeRabbit

• New Features

  • Added optional per-route vision capability configuration.
  • Model listings now indicate whether routes support image inputs.
  • Vision-capable routes advertise both text and image input support.

• Documentation

  • Documented the new vision route option and its behavior when omitted.

• Bug Fixes

  • Improved image-input handling by accurately reporting route capabilities.

[coderabbitai]

Walkthrough

The change adds an optional route-level vision capability, propagates it into model capabilities, and uses it to advertise image input in model listings. Tests and TOML schema documentation cover the behavior.

Changes

Vision capability propagation

Layer / File(s)	Summary
Route capability propagation crates/switchyard-runner/src/config.rs, crates/switchyard-runner/src/route.rs	Route deserialization preserves the optional vision setting and exposes it through ModelCapabilities.
Model discovery metadata and validation crates/switchyard-server/src/lib.rs, crates/switchyard-server/tests/server.rs, docs/reference/toml_schema.md	Model listings advertise ["text", "image"] only for routes with vision = true. Integration tests and schema documentation describe the behavior.

Estimated code review effort: 2 (Simple) | ~10 minutes

Merge Risk: ⚪ Minimal · up to b01a9

The PR changes route capability metadata for image input; the only remaining issue is missing documentation on a public type. No actionable merge-blocking risk remains, though the documentation follow-up should be completed.

Poem

A rabbit reads the route with care
And finds a vision flag there
Text stays text when images hide
Enabled paths let pictures glide
Tests and docs hop side by side

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 33.33% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 6 functions across 4 files. (1 skipped: 1…	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 and concisely describes the main change: enabling routes to declare image-input support so Codex preserves images.
Linked Issues check	✅ Passed	The PR satisfies issue #563 by adding an opt-in, fail-closed route vision capability and using it to advertise ["text", "image"] only for vision-enabled routes. Undeclared routes remain text-only, and…
Out of Scope Changes check	✅ Passed	The changes remain within scope. They update route configuration, capability metadata, model listing behavior, integration tests, and schema documentation required to resolve issue #563.

Full details: Linked Issues check

Explanation

The PR satisfies issue #563 by adding an opt-in, fail-closed route vision capability and using it to advertise ["text", "image"] only for vision-enabled routes. Undeclared routes remain text-only, and tests cover both cases.

Full details: Docstring Coverage

Explanation

Docstring coverage is 33.33% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 6 functions across 4 files. (1 skipped: 1 unsupported.)

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Actionable comments posted: 1

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Inline comments:
In `@crates/switchyard-runner/src/route.rs`:
- Around line 29-38: Add a concise Rust doc comment immediately above the public
ModelCapabilities struct describing its role and capability fields, while
leaving the existing field-level comments unchanged.

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📥 Commits

Reviewing files that changed from the base of the PR and between 1ce5420 and b01a902.

📒 Files selected for processing (5)

• crates/switchyard-runner/src/config.rs
• crates/switchyard-runner/src/route.rs
• crates/switchyard-server/src/lib.rs
• crates/switchyard-server/tests/server.rs
• docs/reference/toml_schema.md

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[afourniernv]

Looks good to me. One note: adding vision to public ModelCapabilities breaks struct literals for consumers tracking main, but the type has not shipped in a release yet. DCO still needs fixing.

[mrPronin]

Thanks for the review.

Struct literals — good catch, and I've taken the stronger fix: ModelCapabilities is now #[non_exhaustive] (6cd2803f), so adding a capability won't break a downstream literal again.

You're right that the window is open, and that's why it seemed worth doing now rather than later. v0.2.0 shipped the struct private inside switchyard-server; it only became public when #517 moved it here on 2026-08-24. So there are no released consumers to break today — after the next release, there would be.

What it means for callers: build with ModelCapabilities::default() and assign fields. Fields stay pub, so reading is unaffected. Worth knowing that #[non_exhaustive] also rejects ..Default::default(), not just the plain literal — I checked both fail with E0639 from an external crate, and that default() + assignment still works. In-tree the only literal is RouteConfig::capabilities, inside the defining crate; the three external sites already call default().

Happy to drop that commit if you'd rather it were its own PR, or if you'd prefer the type stay freely constructible.

DCO — fixed, both commits signed. I also rebased onto 035d7023 to pick up the seven commits since, including #547's rework of the same file. The original patch is untouched (range-diff shows only the added trailer), and the suite is green on the new base: 678 tests, 0 failures.

One ask: your note is recorded as a comment rather than an approving review, so the PR still shows none — and that's the only thing blocking it now. Could you add one if you're happy with it?