feat(answer): do an answer's arithmetic in code and trace each operand to its page

[hallelx2]

/v1/answer now does an answer's arithmetic in code and traces every operand to the page that states it.

Why

• Wrong arithmetic. A model that sets a calculation up correctly still gets the arithmetic wrong: gemini-2.5-flash-lite wrote 365 * 1,380.5 / 7,890 = 67.75 for Corning's FY2020 DPO, which is 63.86.
• Declined derived figures. The answer prompt declined any figure the pages did not state outright.
• Audit trail. A computed figure with its inputs cited to pages is the audit trail we sell.

Change

• pkg/calc evaluates <<expression>> set-ups:
  • numbers read the way a filing writes them ($1,174, 12.5%), with + - * /, parentheses and signs;
  • nothing else is evaluated, and anything else is reported as not computable rather than guessed;
  • each set-up is replaced in the answer by value (= expression).
• calc.Trace finds each operand on the evidence block that states it:
  • it matches as written or in the other common form (2763 / 2,763), as a whole number only (12 is not found in 312 or 1,234);
  • a figure on no page is found: false, which marks formula constants and anything the model brought itself.
• All answer paths. Both /v1/answer paths (pages and sections) and the standalone server include calc.Instructions in the prompt and return calculations (expression, value, display, operands with evidence index and page). The array is empty when the answer has no calculation.

Measured

The product's own /v1/answer on vlb-engine, for the pilot's 50 FinanceBench questions, graded by Jev. Both builds include #92 and #93 (retry); the only difference is this PR.

	before	this PR
graded correct	0.64	0.72
incorrect	8	5
declined	0.08	0.08
answers with a calculation	0	13
cost per answer	$0.0032	$0.0030

On the 15 questions the pilot's Vectorless row declined:

• 11 are now answered (6 correct, 3 partly, 2 wrong).
• 11 of the 105 operands appear on no page. Most are formula constants (365, 2, 100).
• Of the two wrong, Corning's DPO was worked out in prose instead of in the notation. Both are tracked on HAL-3211, which checks arithmetic a model writes out itself.

Verification

• go build ./..., go vet ./... and go test ./... pass on vlb-engine.
• New: pkg/calc/calc_test.go (evaluation, refusals, resolve, trace, whole-number matching, formatting) and internal/handler/answer_calc_test.go (PepsiCo's EBITDA margin computed with each operand traced to pages 62 and 64; an answer without a calculation returns an empty array).

Closes HAL-3161

Summary by Sourcery

Compute supported answer arithmetic in code and expose each operand's supporting evidence page for auditable derived figures.

New Features:

• Add arithmetic evaluation for derived figures in answer responses, preserving the original expression alongside the computed value.
• Return calculation metadata with operand values and evidence-page traceability across all answer paths.

Bug Fixes:

• Prevent models from returning incorrect arithmetic by computing supported expressions deterministically and marking unsupported expressions as non-computable.
• Allow answers to use derived figures when their inputs are present in the evidence rather than requiring the derived figure itself to be stated.

Enhancements:

• Standardize calculation prompting, formatting, operand matching, and evidence tracing for page- and section-based answers.

Tests:

• Add calculator coverage for expression evaluation, rejected inputs, formatting, operand resolution, whole-number matching, and evidence tracing.
• Add answer-handler coverage for computed calculations, page-level operand citations, and empty calculation arrays.

Summary by CodeRabbit

• New Features
  • Answers can now include evaluated arithmetic calculations, showing the setup and computed result.
  • Calculation inputs are linked to matching evidence pages when available.
  • Responses consistently include a calculations list, including when no calculations are present.
• Improvements
  • Answer generation is instructed to show calculation expressions and avoid guessing when evidence is insufficient.

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Reviewer's Guide

Introduces a constrained, code-backed arithmetic workflow for all answer paths: models express derivations, the engine evaluates and displays them, traces every operand to supporting evidence pages, and returns structured calculation audit data.

Sequence diagram for code-backed answer calculations

sequenceDiagram
    participant Client
    participant AnswerHandler
    participant Model
    participant Calc
    participant Evidence

    Client->>AnswerHandler: POST /v1/answer
    AnswerHandler->>Model: synthesiseAnswer(...)
    Model-->>AnswerHandler: Answer with <<expression>>
    AnswerHandler->>Calc: Resolve(answerText)
    Calc-->>AnswerHandler: Answer with value (= expression), calculations
    AnswerHandler->>Calc: Trace(calculations, sources)
    Calc->>Evidence: containsNumber(operand)
    Evidence-->>Calc: Evidence index and page
    Calc-->>AnswerHandler: Operand trace results
    AnswerHandler-->>Client: answer, calculations, citations

Loading

Flow diagram for constrained arithmetic evaluation

flowchart LR
    A["Model receives calc.Instructions"] --> B["Writes derivation as <<expression>>"]
    B --> C{"Evaluate(expression)"}
    C -->|valid arithmetic| D["Replace with value (= expression)"]
    C -->|unsupported or invalid| E["Mark as could not be computed"]
    D --> F["Trace operands against evidence pages"]
    E --> F
    F --> G["Return calculations audit data"]

Loading

File-Level Changes

Change	Details	Files
Adds a restricted arithmetic engine that evaluates model-provided calculation set-ups and preserves unsupported expressions as explicitly non-computable.	Parses numbers, percentages, signs, parentheses, and basic arithmetic operators with division-by-zero and invalid-input checks. Replaces valid set-ups with formatted values alongside the original expression, while recording errors for refused calculations. Extracts operands for later evidence tracing and formats results without floating-point noise.	pkg/calc/calc.go pkg/calc/calc_test.go
Traces calculation operands back to the evidence pages that state them and exposes an auditable calculations payload.	Matches numbers as written or with/without thousands separators while enforcing whole-number boundaries. Marks unstated operands, including formula constants, as not found. Returns calculation expression, value, display text, operand evidence indexes, pages, and found status; guarantees an empty array when no calculation exists.	internal/api/server.go internal/handler/answer.go internal/handler/answer_pages.go pkg/calc/calc.go
Enables calculation notation across every answer-generation path and validates end-to-end behavior.	Adds shared instructions directing the model to write derivations as <> without evaluating them itself. Applies resolution and tracing to page-based, section-based, and standalone server answers. Tests computed PepsiCo EBITDA margin operands across pages and verifies answers without calculations return an empty array.	internal/api/server.go internal/handler/answer.go internal/handler/answer_pages.go internal/handler/answer_calc_test.go

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Review configuration:

⚙️ Run configuration

• Configuration used: defaults
• Review profile: CHILL
• Plan: Advanced
• Run ID: ac504e59-c80e-47ed-bb6c-9a78dd2f7c65

📥 Commits

Reviewing files that changed from the base of the PR and between 9717705 and 0a86ed1.

📒 Files selected for processing: 2

• pkg/calc/calc.go
• pkg/calc/calc_test.go

📝 Walkthrough

The change adds arithmetic expression resolution, result formatting, and operand-to-evidence tracing. Both answer endpoints include calculation results and calculation guidance in synthesis prompts, and return an empty calculations array when no expressions are present.

Changes: Answer calculations

Layer / File(s)	Summary
Calculation evaluation and tracing pkg/calc/calc.go, pkg/calc/calc_test.go	The new calc package evaluates supported arithmetic expressions, formats results, resolves expressions in text, and traces operands to evidence sources. Tests cover expression handling, result formatting, and evidence matching.
Calculation support in answer flows internal/api/server.go, internal/handler/answer.go, internal/handler/answer_pages.go, internal/handler/answer_calc_test.go	Both answer paths add calculation instructions to synthesis prompts, resolve calculations, trace operands against evidence, and include calculations in responses. Handler tests cover answers with and without calculations.

Priority: ➖ Normal

Estimated code review effort: 3 (Moderate) | ~25 minutes

Change: Feature

Sequence Diagram(s)

sequenceDiagram
  participant answerFromPages
  participant Synthesis
  participant calc.Resolve
  participant EvidencePages
  participant calc.Trace
  participant AnswerResponse
  answerFromPages->>Synthesis: Request answer with calc.Instructions
  Synthesis-->>answerFromPages: Return answer text with calculation expressions
  answerFromPages->>calc.Resolve: Resolve expressions in answer text
  calc.Resolve-->>answerFromPages: Return resolved text and calculation details
  answerFromPages->>EvidencePages: Build sources from page text
  answerFromPages->>calc.Trace: Trace operands against sources
  calc.Trace-->>answerFromPages: Return evidence locations for operands
  answerFromPages->>AnswerResponse: Include answer and calculations array

Loading

Merge Risk: 🔵 Low · up to 97177

Calculations work, but some operand citations can point to the wrong value or page. Correct signed-operand tracing before merging and account for the page-attribution limitation.

Pre-merge checks: 4 · 1

Failed checks: 1 warning

Check name	Status	Explanation	Resolution
Docstring Coverage		Docstring coverage is 48.00% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 25 functions across 6 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 and concisely describes the main change: performing answer arithmetic in code and tracing each operand to its evidence page.
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/calc/calc.go:
- Around line 356-405: Update operand extraction in operands to preserve leading
unary signs in each Operand’s Text and Value, recognizing signs at the start of
an expression or after an operator or opening parenthesis. Then update
containsNumber and containsToken to require evidence to match the operand’s
sign, including the minus sign, while preserving existing number-boundary and
formatting checks.

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ℹ️ Review info

⚙️ Run configuration

• Configuration used: defaults
• Review profile: CHILL
• Plan: Advanced
• Run ID: 187c86e4-fb28-44e3-9f0e-068e68327a81

📥 Commits

Reviewing files that changed from the base of the PR and between 0cd393e and 9717705.

📒 Files selected for processing: 6

• internal/api/server.go
• internal/handler/answer.go
• internal/handler/answer_calc_test.go
• internal/handler/answer_pages.go
• pkg/calc/calc.go
• pkg/calc/calc_test.go

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