fix(retrieval): return only units with body text, filling open slots from the next-ranked evidence

[hallelx2]

Every unit the engine returns now carries body text. Slots that a heading or a repeat used to take go to the next-ranked evidence instead.

The bug

On paged documents the walk returned its evidence pages plus every section whose page range overlapped them. Those sections include cover and part headings that span the whole filing, such as "THE BOEING COMPANY", "PART II" and "Introduction". These have no body text, and the rest are mostly text the pages already carry. Meanwhile the pages the walk had read and scored just below its evidence were discarded.

Change

retrieval.AssembleUnits (with UnitAssembler for the stream) builds the units for /v1/query, its stream, /v1/query/multi, both /v1/answer paths and the standalone server:

• Evidence pages lead the list, as before.
• Selected sections follow, but only those with body text:
  • A heading-only section takes the text beneath it when that is compact (at most 2 pages and 6,000 characters).
  • It is dropped when it is not compact, so a cover heading is never stood in for by the filing's opening pages.
  • A section whose word shingles the earlier units already hold (80% or more) is dropped as a repeat.
• Open slots under max_sections fill from the pages, then the leaves, ranked just below the evidence (Result.RunnerUpPages / RunnerUpIDs). These are marked runner_up.
• The page-based /v1/answer fills from the same runner-up pages when fewer than k pages pass the threshold. A two-statement question is then not answered from one page.
• unit_stats on the query response counts what was dropped and what was filled.

Measured on vlb-engine

The pilot's 50 FinanceBench questions (seed 0) were run against both builds. Answers come from the bench's shared answer step on gemini-2.5-flash-lite and are graded by Jev.

	ac02c29	this branch
questions with a unit under 150 chars	29	0
units under 150 chars	39	0
hit@5	0.86	0.90
F1@5	0.437	0.432
context tokens / question	2,807	3,340
graded correct	0.40	0.46
declined	0.34	0.30

Two caveats on these numbers:

• The answer change is 3 questions out of 50. Jev's verdicts are not deterministic, so read it as "no worse, likely better".
• Most declines come from the answer step, which HAL-3161 covers.

Verification

• go build ./..., go vet ./... and go test ./... pass on vlb-engine.
• New tests in pkg/retrieval/units_test.go cover:
  • a heading-only top hit;
  • a cover title that is dropped, not expanded;
  • a compact heading expanded to its note;
  • a section that repeats an evidence page;
  • runner-up fill under the cap and none without one;
  • the cap counting pages and sections together;
  • the incremental assembler;
  • runner-up ranking.
• The handler fixtures now carry realistic section text.

Closes HAL-3160

Summary by Sourcery

Return substantive, non-duplicative evidence units while filling unused result slots with the next-ranked evidence.

New Features:

• Return query and answer results as body-text-bearing evidence units, with runner-up evidence filling available capacity.

Bug Fixes:

• Prevent heading-only, empty, duplicate, and overly broad sections from displacing useful evidence.
• Ensure page-based answers include lower-ranked pages when too few evidence pages meet the threshold.

Enhancements:

• Expand compact heading-only sections into their body text and expose unit assembly statistics in query responses.
• Apply consistent unit assembly across query, streaming, multi-document query, answer, and standalone server paths.

Tests:

• Add coverage for heading filtering and expansion, duplicate detection, runner-up selection, shared caps, incremental assembly, and ranking.

Summary by CodeRabbit

• New Features
  • Search and answer results can now include relevant page evidence and runner-up results when space allows.
  • Compact sections may include supporting text from their subsections, and results include summary counts for assembled content.
• Improvements
  • Results avoid empty or substantially duplicated content, and section limits apply consistently across pages and sections.
  • Streaming results omit sections without available text.

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Reviewer's Guide

The PR replaces endpoint-specific section selection with shared unit assembly that prioritizes evidence, filters heading-only and duplicate sections, expands only compact headings, and fills available capacity from ranked runner-up evidence while reporting assembly statistics.

Sequence diagram for query unit assembly and runner-up filling

sequenceDiagram
    participant Client
    participant QueryHandler
    participant Strategy
    participant UnitAssembler
    participant Storage

    Client->>QueryHandler: HandleQuery
    QueryHandler->>Strategy: SelectWithCost
    Strategy-->>QueryHandler: Result with evidence and runner-ups
    QueryHandler->>UnitAssembler: AssembleUnits
    UnitAssembler->>UnitAssembler: AddPage
    loop selected sections
        UnitAssembler->>Storage: loadContent
        Storage-->>UnitAssembler: section body text
        UnitAssembler->>UnitAssembler: AddSection
    end
    opt open slots remain
        UnitAssembler->>UnitAssembler: AddPage or AddSection for runner-up
    end
    UnitAssembler-->>QueryHandler: units and UnitStats
    QueryHandler-->>Client: sections with content and unit_stats

Loading

Sequence diagram for streaming unit assembly

sequenceDiagram
    participant Client
    participant QueryStreamHandler
    participant StreamStrategy
    participant UnitAssembler
    participant Storage

    Client->>QueryStreamHandler: HandleQueryStream
    QueryStreamHandler->>StreamStrategy: SelectStream
    StreamStrategy-->>QueryStreamHandler: EventSectionSelected
    QueryStreamHandler->>UnitAssembler: AddSection
    UnitAssembler->>Storage: loadContent
    Storage-->>UnitAssembler: section body text
    alt accepted unit
        UnitAssembler-->>QueryStreamHandler: Unit with content
        QueryStreamHandler-->>Client: section event
    else empty or duplicate
        UnitAssembler-->>QueryStreamHandler: rejected unit
    end

Loading

Flow diagram for shared retrieval unit assembly

flowchart TD
    A[Retrieval Result] --> B[Evidence pages]
    B --> C[Selected sections]
    C --> D{Body text available?}
    D -->|No| E[Expand compact heading]
    E --> F{Within 2 pages and 6000 characters?}
    F -->|No| G[Drop empty unit]
    F -->|Yes| H[Check duplicate shingles]
    D -->|Yes| H
    H -->|80 percent or more repeated| I[Drop duplicate]
    H -->|Distinct| J[Append section]
    B --> K{Open slots under max_sections?}
    J --> K
    K -->|Yes| L[Add ranked runner-up pages or leaves]
    K -->|No| M[Return units and unit_stats]
    L --> M

Loading

File-Level Changes

Change	Details	Files
Centralize response-unit assembly so returned units always contain substantive body text and share one cap across evidence pages, sections, and fallback evidence.	Add AssembleUnits and incremental UnitAssembler with minimum-content validation, compact heading expansion, duplicate-shingle filtering, page deduplication, and unit statistics. Load section content through reusable storage callbacks and mark expanded or runner-up units in response representations. Replace endpoint-specific section truncation and content loading across query, streaming query, multi-query, answer, and standalone server paths.	pkg/retrieval/units.go pkg/retrieval/units_test.go internal/api/server.go internal/handler/query.go internal/handler/query_stream.go internal/handler/query_multi.go internal/handler/answer.go internal/handler/answer_pages.go
Preserve and use below-threshold retrieval candidates to fill capacity after selected evidence is rejected or omitted.	Collect ranked runner-up pages and section leaf IDs from judge navigation while excluding evidence pages and preserving rank order. Use runner-up pages for page-based answers when fewer than k evidence pages pass the threshold. Append runner-up units only when capped slots remain and expose their status with runner_up.	pkg/retrieval/judgewalk.go pkg/retrieval/strategy.go internal/handler/answer_pages.go internal/api/server.go internal/handler/query.go
Expose and verify the new assembly behavior and response accounting.	Add unit_stats counters for empty, duplicate, expanded, and runner-up units to query responses. Expand handler fixtures with realistic section bodies and add coverage for heading drops, compact expansion, repeats, caps, incremental assembly, and runner-up ordering.	internal/api/server.go internal/handler/query.go pkg/retrieval/units.go pkg/retrieval/units_test.go internal/handler/query_strategy_test.go

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Review configuration:

⚙️ Run configuration

• Configuration used: defaults
• Review profile: CHILL
• Plan: Advanced
• Run ID: 02dd8aa5-a458-4467-842d-32b754ab46a6

📥 Commits

Reviewing files that changed from the base of the PR and between 4d186e5 and 5ac7ef3.

📒 Files selected for processing: 2

• pkg/retrieval/units.go
• pkg/retrieval/units_test.go

📝 Walkthrough

Retrieval results now carry runner-up pages and sections. A shared assembler builds capped evidence units, expands compact sections, and filters empty or duplicate content. Query and answer paths use the assembled units in responses and synthesis.

Changes: Retrieval unit assembly

Layer / File(s)	Summary
Runner-up evidence in retrieval results pkg/retrieval/strategy.go, pkg/retrieval/judgewalk.go	Result adds runner-up page and section fields. Retrieval selection populates them from ranked pages that are not evidence pages.
Assemble and validate retrieval units pkg/retrieval/units.go, pkg/retrieval/units_test.go	The assembler combines evidence pages and sections, loads content, expands compact sections, filters empty and duplicate units, and applies caps. Tests cover assembly, expansion, filtering, and runner-up ordering.
Use assembled units in query and answer paths internal/api/server.go, internal/handler/answer.go, internal/handler/answer_pages.go, internal/handler/query.go, internal/handler/query_multi.go, internal/handler/query_stream.go, internal/handler/query_strategy_test.go	Query and answer paths use assembled units for response sections and synthesis evidence. Query responses add unit statistics and runner-up pages. Streaming skips sections when assembly produces no unit.

Priority: ➖ Normal

Estimated code review effort: 3 (Moderate) | ~30 minutes

Change: Bug fix

Sequence Diagram(s)

sequenceDiagram
  participant HandleQuery
  participant AssembleUnits
  participant readStored
  participant QueryResponse
  HandleQuery->>AssembleUnits: Assemble ranked evidence units
  AssembleUnits->>readStored: Load referenced section content
  readStored-->>AssembleUnits: Return content or error
  AssembleUnits-->>HandleQuery: Return units and statistics
  HandleQuery->>QueryResponse: Render sections and runner-up pages

Loading

Merge Risk: 🔵 Low · up to 4d186

Retrieval now returns only units that contain body text. One edge case remains: a section title with unusual Unicode characters can crash a query or answer request. This is rare and the fix is small, so the change is mergeable once it is addressed or accepted.

Pre-merge checks: 4 · 1

Failed checks: 1 warning

Check name	Status	Explanation	Resolution
Docstring Coverage		Docstring coverage is 40.00% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 25 functions across 11 files.	Write docstrings for the functions missing them to satisfy the coverage threshold.

Passed checks: 4 passed

Check name	Status	Explanation
Description Check		Check skipped - CodeRabbit’s high-level summary is enabled.
Title check		The title clearly describes the main retrieval change: exclude units without body text and fill available slots with next-ranked evidence.
Linked Issues check		Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check		Check skipped because no linked issues were found for this pull request.

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Actionable comments posted: 1

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• 🪄 Fix comments: 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
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Inline comments:
Review comments at @pkg/retrieval/units.go:
- Around line 257-260: Update the title-prefix handling in substantive to avoid
slicing s at len(title) after a case-insensitive match. Determine the matching
prefix boundary without assuming title and s use the same byte length, then trim
s from that valid boundary.

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• Configuration used: defaults
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• Run ID: 36c32285-b0cd-4404-a450-0cd448c6891c

📥 Commits

Reviewing files that changed from the base of the PR and between ac02c29 and 4d186e5.

📒 Files selected for processing: 11

• internal/api/server.go
• internal/handler/answer.go
• internal/handler/answer_pages.go
• internal/handler/query.go
• internal/handler/query_multi.go
• internal/handler/query_strategy_test.go
• internal/handler/query_stream.go
• pkg/retrieval/judgewalk.go
• pkg/retrieval/strategy.go
• pkg/retrieval/units.go
• pkg/retrieval/units_test.go

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