Eval Banana

Aspect-based evaluation framework - deterministic checks + harness judges. Score anything (agentic outputs, workflows, banana!) with simple YAML check definitions.

_{The name was inspired by this song (my kids love it)}

What it does

Eval Banana discovers YAML check definitions from eval_checks/ directories, runs them, and produces a report. Every check scores 0 or 1 with equal weight.

Two check types:

The harness judge uses one of the following: codex, gemini, claude, openhands, opencode, pi

Writing checks

Create a directory called eval_checks/ anywhere in your project. Add YAML files -- one per check.

Deterministic check

schema_version: 1
id: output_file_exists
type: deterministic
description: Verify that output.json was generated.
script: |
  import json, sys
  from pathlib import Path
  ctx = json.loads(Path(sys.argv[1]).read_text())
  output = Path(ctx["project_root"]) / "output.json"
  assert output.exists(), "output.json not found"

Harness judge check

schema_version: 1
id: summary_is_accurate
type: harness_judge
description: The generated summary accurately reflects source data.
instructions: |
  Read summary.txt and source_data.json.
  Compare the summary against the source data.
  Score 1 if accurate, 0 if it contains fabricated claims.

Requires a configured harness agent. Set [harness] agent in config or pass --harness-agent.

Inspiration

Eval Banana's binary 0/1 scoring philosophy draws directly on two earlier bodies of work:

• Hamel Husain's Creating LLM-as-a-Judge that drives business results — argues that binary pass/fail judgments produce more reliable, actionable evals than Likert-style 1-5 scales.
• RAGAS's Aspect Critic metric — evaluates outputs against a natural-language aspect definition and returns a binary verdict.

The harness_judge check type is essentially an Aspect Critic: you describe what "good" looks like in plain language, and the judge returns {"score": 0|1}.

Skills

eval-banana ships agent skills in the skills/ directory of the repository, including:

• eval-banana: guidance for writing and debugging eval-banana checks
• gemini_media_use: Gemini media upload and analysis helpers
• voxtral-transcribe: Mistral Voxtral transcription patterns for generated audio evals

Install repo-local skills into your project with the npx skills CLI:

npx skills add https://github.com/writeitai/eval-banana

The CLI auto-detects installed agents and copies skills into their native directories (.claude/skills/, .codex/skills/, .agents/skills/, .gemini/skills/, etc.).

Quick start

# Install
uv sync

# Initialize project config
eb init

# Run all discovered checks
eb run

# List discovered checks without running
eb list

# Validate YAML definitions without running
eb validate

Installation

# Using uv (recommended)
uv add eval-banana

# Using pip
pip install eval-banana

# From source (development)
git clone https://github.com/writeitai/eval-banana.git
cd eval-banana
uv sync --extra dev

After installation the CLI is available as eb.

Harness configuration

harness_judge checks require a configured harness agent. Configure it via TOML or CLI flags.

TOML

# .eval-banana/config.toml
[harness]
agent = "codex"
# codex GPT-5.6 tiers: gpt-5.6-sol (flagship), gpt-5.6-terra, gpt-5.6-luna
model = "gpt-5.6-sol"
# reasoning_effort = "high"
# timeout_seconds = 300

Running in CI / cloud

The harness subprocess inherits the parent shell environment, so provide API keys the same way you would when running the agent locally:

Example GitHub Actions step:

- name: Run evals
  env:
    ANTHROPIC_API_KEY: ${{ secrets.ANTHROPIC_API_KEY }}
  run: eb run

You can also inject extra env vars via [harness.env] in your config:

[harness.env]
MY_CUSTOM_VAR = "value"

Custom agent templates

Add [agents.<name>] sections to override built-in templates or define new ones:

[agents.myagent]
command = ["my-cli", "run"]
shared_flags = ["--headless"]
prompt_flag = "--prompt"
model_flag = "--model"

Configuration

Eval Banana uses a single project-level TOML config at .eval-banana/config.toml.

Create it with eb init.

Config precedence (highest to lowest)

1. CLI arguments (--output-dir, --harness-model, etc.)
2. Environment variables (EVAL_BANANA_*)
3. Project config (.eval-banana/config.toml)
4. Built-in defaults

Key settings

CLI reference

eb init [--force]                Create project config
eb run [OPTIONS]                  Run all discovered checks
eb list [OPTIONS]                 List discovered checks
eb validate [OPTIONS]             Validate YAML without running

Options for run/list/validate:
  --check-dir PATH              Scan only this directory
  --check-id TEXT               Run only this check ID
  --output-dir TEXT             Override output directory
  --flat-output                 Write directly to an explicit empty output dir
  --pass-threshold FLOAT        Minimum pass ratio (0.0-1.0)
  --no-project-config           Ignore .eval-banana/config.toml (run/validate)
  --verbose                     Enable debug logging
  --cwd TEXT                    Working directory

Harness options:
  --harness-agent TEXT          Agent used by harness_judge checks (run/validate)
  --harness-model TEXT          Model override for the agent
  --harness-reasoning-effort TEXT  Reasoning effort level
  --harness-timeout-seconds INTEGER  Per-agent wall-clock limit (default: 300)

Output

Each run creates a timestamped directory under the configured output_dir:

.eval-banana/results/<run_id>/
  report.json       # Machine-readable full report
  report.md         # Human-readable Markdown report
  checks/
    <safe_check_id_stem>.json       # Per-check result
    <safe_check_id_stem>.prompt.txt # Exact harness-judge input (harness only)
    <safe_check_id_stem>.stdout.txt # Captured stdout (if any)
    <safe_check_id_stem>.stderr.txt # Captured stderr (if any)

An orchestration system that already owns an attempt-unique directory can use an exact, non-nested layout:

eval-banana run \
  --flat-output \
  --output-dir /absolute/path/to/attempt/eval

--flat-output requires an explicit --output-dir. The exact path must be absent or an empty, non-symlink directory; eval-banana refuses conflicting contents instead of overwriting them. Without this flag, the timestamped <output_dir>/<run_id>/ layout is unchanged.

For every harness process that is invoked, eval-banana writes the exact prompt argument first as checks/<safe_check_id_stem>.prompt.txt. The prompt and its result JSON use the same deterministic filesystem-safe stem. The stem is a normalized human-readable label of at most 40 characters plus the full SHA-256 of the exact check ID. The digest keeps case-only, underscore-only, and other distinct valid IDs separate even on case-insensitive filesystems, while the bounded label keeps arbitrarily long IDs below filename limits. The prompt remains available when the agent fails, times out, or returns an invalid verdict, so an attempt-level trace naturally inventories both the judge input and its outputs.

report.json declares "schema_version": 1; readers can therefore reject a future incompatible report shape explicitly. Every check result in that report and in checks/<safe_check_id_stem>.json includes check_definition_sha256, formatted as sha256:<64 lowercase hex>. The digest binds every byte that defines the check: the exact YAML bytes and, for script_path deterministic checks, the referenced script bytes frozen before execution. The runner executes that same frozen script snapshot, so a concurrent file change cannot produce a verdict under the older digest. Its private launcher preserves the original resolved script path as __file__ and sys.argv[0], plus its parent as the first import path. Inline scripts are already part of the YAML component.

The canonical digest input is versioned and length-framed. It starts with the ASCII domain eval-banana/check-definition-sha256/v1 followed by a NUL byte. Each component is u64be(name_length) || name || u64be(content_length) || content. The first component is always definition.yaml; a readable referenced script adds referenced-script. A missing or unreadable referenced script adds an empty referenced-script-unavailable component and produces an error result. The reported value is SHA-256 over the complete framed input.

Orchestrators must not substitute a raw YAML file hash for this canonical digest. Use the producer-owned API to validate a report without duplicating the framing algorithm:

from pathlib import Path

from eval_banana.runner import compute_check_definition_sha256

definition_sha256 = compute_check_definition_sha256(
    source_path=Path("eval_checks/goal.yaml")
)

The function reads one exact YAML snapshot and applies the same referenced script snapshot rules as the runner. It raises OSError when the YAML cannot be read and ValueError when the bytes are not a valid check definition.

Harness-judge details also record the resolved agent_type, model, nullable reasoning_effort, and positive timeout_seconds. Configure the per-agent wall-clock limit with [harness] timeout_seconds, EVAL_BANANA_HARNESS_TIMEOUT_SECONDS, or --harness-timeout-seconds; it defaults to 300 seconds. A judge result is accepted only when its process exits 0; a non-zero exit is an error with score 0, even when stdout contains valid passing JSON. If the selected template has no model flag/environment variable or no reasoning-effort flag containing {effort}, an attempted override becomes an error before launch and the corresponding effective detail remains null.

Development

uv sync --extra dev
make test         # Run tests
make fix          # Auto-fix lint + format
make pyright      # Type check
make all-check    # Lint + format + types + tests (matches CI)

Contributing

Issues and pull requests are welcome. Please run make all-check before opening a PR.

Changelog

See CHANGELOG.md for release notes.

License

Apache License 2.0 — see LICENSE for details.

Copyright 2026 WriteIt.ai s.r.o.