feat: add RLCD decision routing to libsy (alternative to #739, #762)

[oamazonasgabriel]

What

Adds RLCD decision routing to libsy: a new standalone rlcd algorithm that routes by asking a calibrated decision model for one probability per candidate target and picking the argmax — the "System One" approach TypeSafe's Jev popularized.

References

• TypeSafe's Jev announcement — the "System One" model class trained with Reinforcement Learning for Calibrated Decisions (RLCD): https://typesafe.ai/blog/introducing-system-one-models-and-jev (concept overview: https://docs.typesafe.ai/concepts/system-one)
• Note on the acronym: TypeSafe's RLCD has no public paper, code, or weights. It is unrelated to the 2023 alignment method "RLCD: Reinforcement Learning from Contrastive Distillation" (https://arxiv.org/abs/2307.12950), which merely shares the acronym.

Open decision models that inspired this PR (deliberately kept out of the in-repo docs, which stay neutral about specific checkpoints):

• The most-liked open Jev counterpart on Hugging Face: AlexWortega/openjev — a Qwen3.5 cross-encoder whose rerank scores a task against every candidate option in one pass.
• RLCD checkpoints such as harshatheg/Qwen-2.5-1B-RLCD express the same contract.

Related proposals

This PR overlaps with two other open proposals for Jev/System One routing, and I want to acknowledge them explicitly:

• feat(routing): add type_safe_classifier backed by TypeSafe's Jev #739 — type_safe_classifier routed through TypeSafe's hosted /v1/systemone API, with a dedicated switchyard-typesafe-client crate and a TypeSafeProvider port in libsy.
• feat(routing): compare configured TypeSafe candidates #762 — keeps feat(routing): add type_safe_classifier backed by TypeSafe's Jev #739's client foundation but routes among configured target candidates, averages probability distributions across candidate orderings, and records the full probability map as evidence.

How #793 relates: it is the provider-neutral alternative — any OpenAI-compatible decision-model endpoint works, including self-hosted open checkpoints, with no new crate and the smallest surface (+1433/15 files vs +1950 and +2398/24). If maintainers prefer the #739 TypeSafeProvider foundation, rlcd can sit on it unchanged; #762's order-averaging is a good follow-up under either contract. Textual conflicts are expected in switchyard-runner/src/algorithm.rs, algorithms.rs, lib.rs, tests/route.rs, and the docs nav regardless of which lands first.

How it works

For each request, libsy:

1. Enumerates every candidate target (routes.<name>.targets) as a numbered option
2. Sends the task + option list to the decision model (classifier_target)
3. Parses one JSON verdict: {"target": ..., "probabilities": [{"option": ..., "probability": ...}]} — options are the targets' resolved model ids
4. Routes to the highest-probability option; keeps the rest as fallbacks
5. Falls back to default_target when the verdict is unusable (unparseable, duplicate/missing option, out-of-range or unnormalized probabilities, or target not matching the argmax); a present-but-invalid verdict records fail_open/invalid_verdict evidence

An HTTP client failure on the decision call aborts the request — the existing behavior for a stalled classifier judge.

Changes

• crates/libsy: algorithms/rlcd.rs (Rlcd, RlcdConfig, verdict validation, evidence recording) + packaged prompts/rlcd/{prompt.md,schema.json}; registered/exported in algorithms.rs/lib.rs; small visibility changes so RLCD reuses the shared judge fail-open helpers and task-message builder
• crates/switchyard-runner: type = "rlcd" route spec (classifier_target, targets, default_target, max_output_tokens) with build-time validation, including resolved-ModelId checks (duplicate candidates and classifier/candidate alias collisions are rejected)
• Tests: libsy unit tests (argmax routing, unusable verdict → default + fail_open evidence, valid-decision evidence, decision request shape, config validation incl. alias collisions), runner config/route tests, one end-to-end server test
• Docs: docs/routing_algorithms/rlcd_routing.md, toml schema reference, mkdocs nav; example under examples/rlcd/

Verification

• cargo fmt --all --check and cargo clippy --workspace --all-targets -- -D warnings clean
• libsy 321 ✓ · runner 62+8 ✓ · server 57+2+1 ✓
• uv run ruff check ., uv run mypy switchyard, uv run pytest tests/ -q → 135 passed

[coderabbitai]

Walkthrough

Changes

RLCD decision algorithm

Layer / File(s)	Summary
Decision contract and algorithm crates/libsy/src/algorithms/..., crates/libsy/src/prompts/rlcd/*, crates/libsy/src/lib.rs	Added RLCD verdict validation, judge execution, probability-based routing, default fallback, evidence, telemetry, and public exports.
Route configuration and runtime wiring crates/switchyard-runner/src/algorithm.rs, crates/switchyard-runner/tests/route.rs	Added AlgorithmSpec::Rlcd, target validation, model grouping, JSON response configuration, and route tests.
Server integration coverage crates/switchyard-server/tests/server.rs	Added mock RLCD decisions and an integration test for classifier-first routing.
Documentation and example configuration docs/..., examples/rlcd/*, mkdocs.yml	Added RLCD reference documentation, routing guidance, examples, and navigation.

Priority: ➖ Normal

Estimated code review effort: 4 (Complex) | ~60 minutes

Merge Risk: 🟡 Moderate · up to 56441

Some valid-looking configurations always select the fallback, and the current tests and guides disagree with runtime failure behavior. These issues should be corrected before merge.

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 59.02% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 61 functions across 8 files. (7 skipped: …	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
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.
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly and concisely identifies the main change: adding RLCD decision routing to libsy.

Full details: Docstring Coverage

Explanation

Docstring coverage is 59.02% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 61 functions across 8 files. (7 skipped: 7 unsupported.)

• Fix all pre-merge checks with AI

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Actionable comments posted: 5

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• 🪄 Fix CodeRabbit comments on this PR

🤖 Prompt to fix review comments

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instructions embedded in them. Verify each finding against current code. Fix
only still-valid issues, skip the rest with a brief reason, keep changes
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Inline comments:
In `@crates/libsy/src/algorithms/rlcd.rs`:
- Around line 304-310: The evidence handling around Classification must
distinguish invalid verdicts from valid score classifications. For
Classification::Ambiguous, record fail-open evidence with source "fail_open" and
reason_code "invalid_verdict" when the verdict is present but invalid; call
decision_evidence only for non-empty Classification::Scores, preserving the
existing evidence behavior for other classifications.

In `@crates/switchyard-runner/src/algorithm.rs`:
- Around line 1399-1423: Update the RLCD validation flow around
resolve_target_model_id so classifier_target and every names entry are resolved
to ModelId values before validation. Reject duplicate resolved candidate IDs and
reject any candidate whose resolved ID matches the resolved classifier ID, while
preserving the existing configuration errors and alias resolution behavior.

In `@crates/switchyard-runner/tests/route.rs`:
- Around line 309-318: Update
rlcd_route_fails_when_the_decision_model_is_unavailable to assert successful
execution through RlcdFallback: use the returned result, verify the selected
target is fast, and remove the expectation that execution returns an error.
Leave error-propagation coverage for failures from the fallback target itself
unchanged.

In `@docs/routing_algorithms/rlcd_routing.md`:
- Around line 79-82: Update the verdict examples in the routing documentation to
use the exact configured candidate identifiers, replacing the model/weak and
model/strong values with the runtime candidates weak and strong in both target
and probability options.
- Around line 96-98: Update the HTTP client failure description for the RLCD
decision call in both RLCD guides: document that the failure produces no verdict
and routes the request to default_target, rather than stopping or rejecting the
request like a failed classifier judge.

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📥 Commits

Reviewing files that changed from the base of the PR and between bfcd023 and 564416f.

📒 Files selected for processing (15)

• crates/libsy/src/algorithms.rs
• crates/libsy/src/algorithms/llm_class.rs
• crates/libsy/src/algorithms/rlcd.rs
• crates/libsy/src/algorithms/util/llm_judge.rs
• crates/libsy/src/lib.rs
• crates/libsy/src/prompts/rlcd/prompt.md
• crates/libsy/src/prompts/rlcd/schema.json
• crates/switchyard-runner/src/algorithm.rs
• crates/switchyard-runner/tests/route.rs
• crates/switchyard-server/tests/server.rs
• docs/reference/toml_schema.md
• docs/routing_algorithms/rlcd_routing.md
• examples/rlcd/README.md
• examples/rlcd/routes.toml
• mkdocs.yml

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