feat(libsy): make the unmatched verdict threshold step configurable

[himorishige]

What

Add unmatched_steps (0, 1, or 2; default 1) to capability-mode llm_classifier routes. It sets how many threshold_steps an unmatched verdict adds on top of base_threshold. The default keeps today's behavior, where unmatched shares the single step of uncertain; 2 makes an unmatched verdict clear the same bar as unsupported.

verdict	threshold
supported	base_threshold
uncertain	base_threshold + threshold_step
unmatched	base_threshold + unmatched_steps * threshold_step
unsupported	base_threshold + 2 * threshold_step

Validation rejects values above 2, so base_threshold + 2 * threshold_step <= 1 still bounds every threshold. Setting the key on an escalation or custom mode route is rejected with the same error as the other capability-only keys. Stage, composite, and the Python bindings keep the default; only the TOML capability route exposes the knob.

Changes:

• crates/libsy/src/algorithms/llm_class.rs: unmatched_steps on TaskClassifierConfig (serde default 1), boundary_steps takes it, validate rejects > 2, DEFAULT_UNMATCHED_STEPS exported.
• crates/switchyard-runner/src/algorithm.rs: optional unmatched_steps on the TOML route, included in the capability-only key check; stage and composite pass the default.
• crates/switchyard-py/src/libsy_bindings.rs: default wired through, no Python API change.
• Docs: docs/reference/toml_schema.md, docs/routing_algorithms/llm_classifier_routing.md, crates/switchyard-server/README.md.

Why

Closes #845.

An unmatched verdict means no capability rule in the Card applied to the request. The judge still emits a p_solve, but there is no rule behind it, so the number carries less evidence than an uncertain verdict, which does name a rule. Today both boundaries use one step, so an operator who wants unmatched requests to prove more before they go to the efficient tier has no setting for it short of raising base_threshold for every verdict.

We run a fine-tuned judge in front of a coding agent with base_threshold = 0.75 and threshold_step = 0.1. Ten design-discussion requests (no code, no tools) came back unmatched from every judge variant we tried, and three of them carried p_solve = 0.85, exactly the one-step threshold, so they went to the efficient tier, where a blind review found technical errors in two answers. Adding a rule to the Card would change what the rules mean, retraining did not move this distribution, and raising base_threshold would cut the supported-tier sends we want to keep. With unmatched_steps = 2 all ten routed to the capable tier, and an 87-request regression path stayed at 85/87 across three runs.

Tests

• unmatched_steps_raise_only_the_unmatched_threshold: with threshold_step = 0.1 and unmatched_steps = 2, p_solve 0.85 unmatched routes capable and 0.95 routes efficient, while uncertain 0.85 and supported 0.75 keep their targets.
• unmatched_steps_default_to_one_and_reject_values_above_two: a config without the key parses to 1; 3 fails validation.
• cargo fmt --all --check and cargo clippy --locked --all-targets -- -D warnings clean; cargo test --locked green for switchyard-libsy (323), switchyard-runner (62 + 3), and switchyard-server (27 + 3 + 1 + 56), 0 failures, Rust 1.96.1 on linux aarch64.

Notes for reviewers

Start at boundary_steps in llm_class.rs; everything else is plumbing for the one extra u8. The route-level validation lives next to threshold_step in switchyard-runner, so the capability-only rule reads the same for both keys. About 45 of the 125 changed lines are tests and docs.

Summary by CodeRabbit

• New Features
  • Capability routing now lets you configure the confidence threshold adjustment for unmatched verdicts. Choose 0, 1, or 2 increments; the default is 1. A setting of 2 applies the same adjustment as for unsupported verdicts.
  • Windowed classifier inputs now include a routing instruction after the conversation.
• Documentation
  • Updated configuration and routing guidance to explain unmatched verdict thresholds.

[coderabbitai]

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Walkthrough

Capability-mode classifier routes can configure the threshold increment for unmatched verdicts independently. The setting defaults to 1, accepts values from 0 to 2, and is rejected when explicitly set in escalation or custom mode.

Changes

Unmatched threshold configuration

Layer / File(s)	Summary
Classifier threshold policy crates/libsy/src/algorithms/llm_class.rs, crates/libsy/src/lib.rs, crates/switchyard-server/README.md, docs/reference/toml_schema.md, docs/routing_algorithms/llm_classifier_routing.md	The public classifier configuration adds unmatched_steps, defaulting to 1. The threshold calculation uses this value for unmatched verdicts, validates the range from 0 to 2, and tests the default and configured behavior. The re-export and documentation describe the setting.
Route configuration and defaults crates/switchyard-runner/src/algorithm.rs, crates/switchyard-py/src/libsy_bindings.rs	Capability routes pass the configured value, or its default, to the classifier. Escalation and custom modes reject an explicitly supplied value. Stage and Python configurations use the default.

Priority: ➖ Normal

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

Severity of issue fixed: Medium

Merge Risk: 🔵 Low · up to fea11

Implicit escalation routes can silently accept an ineffective setting. This is a bounded configuration risk that should be fixed or explicitly accepted before merging.

🚥 Pre-merge checks | ✅ 3 | ❌ 2

❌ Failed checks (2 warnings)

Check name	Status	Explanation	Resolution
Out of Scope Changes check	⚠️ Warning	The PR also changes windowed classifier inputs by appending a routing instruction after the conversation and adds a test for that behavior. Issue #845 concerns only configurable thresholds for `unmatc…	Remove the windowed routing-instruction change and its test, or link a directly applicable issue that requires this behavior.
Docstring Coverage	⚠️ Warning	Docstring coverage is 52.94% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 17 functions across 4 files. (3 skipped: …	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (3 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: making the unmatched verdict threshold step configurable in libsy.
Linked Issues check	✅ Passed	Issue #845 requires a route-level unmatched_steps setting for capability-mode llm_classifier routes. The PR adds the setting to LlmClassifierRouteConfig and TaskClassifierConfig, applies `base…

Full details: Out of Scope Changes check

Explanation

The PR also changes windowed classifier inputs by appending a routing instruction after the conversation and adds a test for that behavior. Issue #845 concerns only configurable thresholds for unmatched verdicts. The routing-instruction behavior has no demonstrated connection to that objective and changes classifier input behavior outside the linked issue scope.

Full details: Docstring Coverage

Explanation

Docstring coverage is 52.94% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 17 functions across 4 files. (3 skipped: 3 unsupported.)

• Fix all pre-merge checks with AI

---

_{A rabbit adjusts the threshold dial,
One step, two steps, measured in style.
Unmatched hops to its chosen place,
While supported keeps its steady pace.
The default rests at one, neat and small.}

_{Comment @coderabbitai help to get the list of available commands.}

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Actionable comments posted: 1

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Inline comments:
In `@crates/switchyard-runner/src/algorithm.rs`:
- Line 969: Update the validation guard for implicitly selected escalation
routes so it rejects any configuration with unmatched_steps, even when mode is
omitted. Keep the existing mode-dependent rejection behavior for the other
legacy settings unchanged.

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📥 Commits

Reviewing files that changed from the base of the PR and between 9cf6fad and fea116e.

📒 Files selected for processing (7)

• crates/libsy/src/algorithms/llm_class.rs
• crates/libsy/src/lib.rs
• crates/switchyard-py/src/libsy_bindings.rs
• crates/switchyard-runner/src/algorithm.rs
• crates/switchyard-server/README.md
• docs/reference/toml_schema.md
• docs/routing_algorithms/llm_classifier_routing.md

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