feat(evals): add offline TypeSafe replay harness

[cpakkamisaac-sae]

What

• Add a standard-library-only evaluator for replaying recorded TypeSafe/Jev probability distributions without provider calls.
• Add a versioned synthetic fixture and deterministic JSON reports for routing selection, fallback, order sensitivity, quality, cost, and fixed-target regret.
• Reject unsupported fixture versions, malformed distributions, incomplete candidate sets, duplicate cases, and repeated candidate orders.
• Document the fixture contract and its sanitization boundary.

Why

The TypeSafe routing work has useful benchmark evidence, but no checked-in way to reproduce the decision calculations or compare a policy without credentials and additional model spend. This provides an offline evaluation boundary while remaining independent of the runtime implementation in #762.

Closes #882

Notes for reviewers

Start with benchmark/typesafe_replay.py and tests/test_typesafe_replay.py. The checked-in fixture is synthetic and deliberately excludes request text, credentials, provider response bodies, and error details.

Validation:

• focused replay tests
• full hermetic Python suite
• Ruff and mypy
• Rust formatting, workspace Clippy, and workspace tests

No live provider calls were made.

Summary by CodeRabbit

• New Features
  • Added an offline replay tool that evaluates recorded routing evidence without credentials or provider calls.
  • Replay reports include routing decisions, fallback behavior, order sensitivity, quality and cost metrics, and comparisons with the best fixed target.
  • Added a synthetic fixture and usage guidance for running replays and interpreting their JSON output.

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Walkthrough

Adds an offline TypeSafe/Jev routing replay tool. The tool validates versioned JSON fixtures, computes routing and baseline metrics, and writes stable JSON output without provider calls.

Changes

Offline TypeSafe/Jev replay

Layer / File(s)	Summary
Fixture contract and validation benchmark/typesafe/fixtures/*, benchmark/typesafe/README.md, benchmark/typesafe_replay.py, tests/test_typesafe_replay.py, benchmark/README.md	Adds a versioned fixture schema, a synthetic two-candidate fixture, input validation, usage documentation, and tests for unsupported versions, invalid probability totals, and duplicate orders.
Routing replay and metrics benchmark/typesafe_replay.py, tests/test_typesafe_replay.py	Averages and normalizes candidate probabilities, applies confidence-based fallback, and reports routing outcomes, order sensitivity, quality, cost, and comparisons with the best fixed target. Tests check replay results and metrics.
CLI output benchmark/typesafe_replay.py, tests/test_typesafe_replay.py	Adds fixture loading and CLI output to stdout or an output file. A test checks that two runs in an empty environment produce identical output without stderr.

Priority: ➖ Normal

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

Merge Risk: 🟡 Moderate · up to 77b59

The evaluator rejects fixtures without optional cost measurements, preventing quality-only replay. Support missing costs before merging and reject Boolean schema versions.

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Linked Issues check	⚠️ Warning	The implementation satisfies the main #882 objectives: it validates a versioned synthetic fixture, rejects invalid distributions and repeated orders, performs deterministic offline replay, reports rou…	Support fixtures that omit per-target cost. Define the report behavior when cost data is unavailable, and add focused tests for a cost-free fixture and its deterministic output.

✅ 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 and concisely describes the main change: adding an offline TypeSafe replay harness for evaluations.
Out of Scope Changes check	✅ Passed	The changes stay within #882. They add the offline replay tool, synthetic fixture, focused tests, and usage documentation. They do not modify the runtime TypeSafe/Jev router, add provider calls, or in…
Docstring Coverage	✅ Passed	Docstring coverage is 100.00% which is sufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 18 functions across 2 files. (3 skipped: 3…

Full details: Linked Issues check

Explanation

The implementation satisfies the main #882 objectives: it validates a versioned synthetic fixture, rejects invalid distributions and repeated orders, performs deterministic offline replay, reports routing and fixed-target metrics, documents sanitization, and includes focused tests. However, #882 specifies optional per-target cost. replay() always requires outcome["cost"], and the README requires cost for every candidate outcome. Fixtures without cost data therefore fail validation.

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_{A rabbit checks the routes by moonlit light,
Two candidates hop through probabilities right.
When confidence dips, the fallback takes the lead,
Stable JSON records each measured deed.
No provider calls disturb the burrow’s night.}

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

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Actionable comments posted: 2

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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:
Review comments at @benchmark/typesafe_replay.py:
- Line 193: Update the outcome parsing around number_value so missing cost
measurements remain valid while quality evaluation proceeds. Represent
unavailable cost totals and deltas as null, and define tie-breaking behavior for
incomplete cost measurements; update the typesafe README to document these
behaviors.
- Line 89: Update the schema version validation around SCHEMA_VERSION to reject
Boolean values explicitly, since Python treats True as equal to 1. Preserve
acceptance of integer and floating-point versions equal to SCHEMA_VERSION,
including integral floats.

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ℹ️ Review info

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📥 Commits

Reviewing files that changed from the base of the PR and between 0266133 and 77b5997.

📒 Files selected for processing (5)

• benchmark/README.md
• benchmark/typesafe/README.md
• benchmark/typesafe/fixtures/synthetic-routing.json
• benchmark/typesafe_replay.py
• tests/test_typesafe_replay.py

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[cpakkamisaac-sae]

Addressed both review comments in d858a3e. Boolean schema versions are now rejected, and fixtures can omit cost measurements while retaining quality evaluation and deterministic tie-breaking.