feat(libsy): let classifier routes judge the latest user turn as the task

[himorishige]

What

Add task_anchor to capability and custom llm_classifier routes. It says which user message the judge treats as the task.

task_anchor	without recent_turn_window	with recent_turn_window = N
opening_task (default)	opening task + latest user follow-up when they differ (unchanged)	opening task + the last N messages after it (unchanged)
latest_user_turn	the newest ordinary user message alone	the N messages before that message, then the message itself last

"Ordinary user message" is the existing rule: a user-role message with at least one block that is not a tool call, tool result, or reasoning, so an Anthropic-style tool continuation never becomes the task. In the windowed form the window keeps tool call and result pairs whole, the same way the trailing window does today, and the trailing routing instruction is still appended after the task.

Changes:

• crates/libsy/src/algorithms/llm_class.rs: TaskAnchor enum (serde snake_case, default opening_task), latest_user_turn / latest_task_message / trim_messages_before_latest, task_anchor on TaskClassifierConfig and CustomClassifierConfig, and TaskInput dispatching on (task_anchor, recent_turn_window). task_messages now shares is_task_content and task_only instead of a local closure; its behavior is unchanged.
• crates/switchyard-runner/src/algorithm.rs: optional task_anchor on the TOML route for capability and custom modes; escalation mode rejects it (also in the implicit escalation-table form, since the key is new and no existing configuration relies on it being ignored); 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 (capability and custom tables), docs/routing_algorithms/llm_classifier_routing.md (tuning table and the user_turn section), crates/switchyard-server/README.md.

Why

Closes #848.

classify_trigger = "user_turn" re-decides the tier on every new user message, but the judge's task stayed anchored to the conversation's first user message, with the new message shown as a follow-up. In an interactive coding session that assumption stops holding after the first job: a session that opens with "add caching to the fetcher" and later asks "now write the migration" was judged with the caching request as its task. recent_turn_window only widened the tail after that stale anchor, and 0 left the judge with the opening task alone.

The client cannot fix this from outside, because coding agents send the whole transcript on every request. The judge's copy is the one place the task can be re-anchored without touching what the answer model sees. The default is unchanged, so existing user_turn routes that want the opening task as context keep it.

Tests

• latest_user_turn_anchor_sends_the_newest_user_message_alone: the newest ordinary user message is the only judge message; a trailing tool-result-only user message does not displace it.
• latest_user_turn_anchor_window_precedes_the_task_and_keeps_tool_pairs_whole: window 0 is the task plus the routing instruction; window 1 is the message before the task, then the task; window 2 would open on a tool result and widens to include its call, and the opening task stays out.
• task_anchor_parses_and_defaults_to_the_opening_task: serde default and latest_user_turn parsing.
• task_anchor_is_a_capability_setting (runner): accepted on a capability route, unknown values fail at parse time, rejected on an escalation route including the implicit form.
• Existing window and default-path tests are unchanged and still pass.
• cargo fmt --all --check clean, cargo clippy --locked --all-targets -- -D warnings clean, and cargo test --locked green for switchyard-libsy (324), switchyard-llm-client (75 + 14), switchyard-runner (63 + 3), and switchyard-server (27 + 3 + 1 + 56), 0 failures, Rust 1.96.1 on linux aarch64.

Notes for reviewers

Start at TaskInput::build_messages; the four arms map directly onto the table above. trim_messages_before_latest reuses window_start, so tool-pair integrity in the preceding window follows the same rule as the trailing window. The rest is plumbing for one enum field.

Summary by CodeRabbit

• New Features
  • Classifier routes can now base decisions on either the opening user task or the latest user turn. An optional context window can include surrounding conversation.
• Documentation
  • Added guidance on selecting the task anchor and configuring the context window. The default remains the opening task.

[coderabbitai]

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Walkthrough

Classifier task selection now supports the opening ordinary user message or the latest ordinary user message. The setting is available on capability and custom routes, defaults to opening-task mode, and is documented with its interaction with recent_turn_window.

Changes

Task Anchor Selection

Layer / File(s)	Summary
Task selection and context window crates/libsy/src/algorithms/llm_class.rs	TaskInput supports opening-task and latest-user-turn anchors. It filters tool calls, tool results, and reasoning from task content. Windowed selection preserves complete tool-call/result pairs. Tests cover selection, windowing, parsing, and defaults.
Route configuration and runtime propagation crates/switchyard-runner/src/algorithm.rs, crates/switchyard-runner/src/config.rs, crates/libsy/src/lib.rs, crates/switchyard-py/src/libsy_bindings.rs, crates/switchyard-server/README.md, docs/reference/toml_schema.md, docs/routing_algorithms/llm_classifier_routing.md	Capability and custom routes resolve and pass task_anchor to runtime classifiers. Escalation routes reject the setting. The public export and Python default initialization are updated. Configuration tests and documentation cover accepted values, defaults, and window behavior.

Priority: ➖ Normal

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

Severity of issue fixed: Medium

Merge Risk: 🟡 Moderate · up to f5dd6

Latest-turn classification can fail for a user message containing both text and a tool result. Filter the selected task before merging.

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 70.00% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 30 functions across 5 files. (3 skipped: …	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 summarizes the main change: classifier routes can use the latest user turn as the task. It is concise and specific.
Linked Issues check	✅ Passed	The PR satisfies the coding requirements in [#848]. TaskAnchor supports opening_task as the default and latest_user_turn for capability and custom classifier routes. The latest-turn logic select…
Out of Scope Changes check	✅ Passed	The changes stay within [#848]. They update classifier logic, route configuration, validation, bindings, tests, and documentation for task_anchor. The public enum export and default initialization s…

Full details: Docstring Coverage

Explanation

Docstring coverage is 70.00% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 30 functions across 5 files. (3 skipped: 3 unsupported.)

• Fix all pre-merge checks with AI

---

_{A rabbit reads the latest turn,
And checks which task to learn.
Tool pairs stay together in the trail,
While routing notes arrive without fail.
“Opening or latest?” asks the hare,
Then bounds along with context there.}

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

[coderabbitai]

Actionable comments posted: 1

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Inline comments:
In `@crates/libsy/src/algorithms/llm_class.rs`:
- Line 218: Update trim_messages_before_latest to pass the selected task message
through task_only before appending it, so tool results without their calls are
excluded when recent_turn_window is zero. Preserve complete tool pairs in the
surrounding context.

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📥 Commits

Reviewing files that changed from the base of the PR and between 9cf6fad and f5dd6c8.

📒 Files selected for processing (8)

• 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-runner/src/config.rs
• crates/switchyard-server/README.md
• docs/reference/toml_schema.md
• docs/routing_algorithms/llm_classifier_routing.md

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