Agent Observability Auto Experiment

auto-experiment — local hill-climb improvement loop

This is the local, Claude-Code-driven version of the auto_experiments Temporal/Atlas worker (domains/ml_observability/apps/apis/auto_experiments/). There, a remote Bits/Code-Gen agent runs the loop; here YOU (Claude Code) are the agent and run it directly on the current git checkout. No Temporal, no Code-Gen API — just git commits, a local eval harness, and Datadog LLM-Obs MCP tools for the data.

Read references/rubrics.md in full before iteration 1 and keep it in mind every iteration. It holds the non-negotiable rules (never invent a score; what to score; where the data lives; the harness spec; the metric schema). This file is the control loop; that file is the law.

Inputs (the experiment config)

Collect these from the user (ask only for what is missing — infer the rest). Repo = current working directory.

Persist the config to .auto_experiment/config.json and update it as the run progresses (it is the run's state + audit trail):

{
  "repo_url": "...", "base_branch": "...", "files_to_optimize": [...],
  "goal": "...", "evaluators": "...", "ml_app": "...",
  "dataset_id": "...", "trace_ids": [...],
  "max_iterations": 2,
  "reps": 3,
  "min_delta": 0.02,
  "iteration_results": [],
  "final_result": {}
}

Scope — optimize the whole selected surface, not just the prompt

files_to_optimize is a scope, not a prompt pointer. It may be a set of files, a directory, or globs — expand a directory to its editable files (e.g. every *.py under it) and treat all of them as the code under test. Within that scope you may change anything that moves the metric: retrieval/tool code, request logic, filtering, output shape, ranking, config, or prompts. Let the failure census decide which file the lever lives in — do not default to rewording a prompt. In practice the biggest wins are often in tool/retrieval code (what the model can fetch), not prompt phrasing; a prompt-only search finds nothing when the headroom is in the tools.

Hard scope guard: never edit a file outside files_to_optimize. If the census's dominant lever is out of scope, say so (that's a finding) — do not silently tweak in-scope-but-irrelevant files.

Setup

1. Confirm a clean-ish working tree (stash or warn on unrelated changes). Note the starting SHA. If files_to_optimize names a folder/globs, resolve it to the concrete editable file list and record that list in config.json (it is the scope for every iteration + the restore boundary).
2. Create a scratch branch off base_branch for the experiment (e.g. auto-experiment/<short-goal>). All iteration commits land here; the user reviews/keeps the best commit at the end.
3. Write .auto_experiment/config.json. Add .auto_experiment/ output files to nothing special — they are committed on purpose (they are the audit trail).
4. This run reports one score per iteration to the LLM-Obs experiment identified by the DD_AUTO_EXPERIMENT_ID environment variable (set in the environment before this skill is invoked — read it, don't ask the user). See Report each iteration's score to LLM-Obs.
5. Record the run context on the experiment before iterations start. Call update_llmobs_experiment once with experiment_id = $DD_AUTO_EXPERIMENT_ID (skip if unset) and metadata set to a JSON struct containing the repo name, the scratch branch name, and the model running this skill, e.g. {"repo": "<repo>", "branch": "<scratch-branch>", "model": "<model>"}. Derive repo from the git remote (basename -s .git $(git remote get-url origin), or owner/repo), branch from the branch created in step 2, and model = the provider/model-id of the model/agent driving this session (e.g. openai/gpt-4-turbo, anthropic/claude-opus-4-8). metadata replaces existing metadata, so include all three keys in the one call. Do this in Setup, before Step 1.

Execution model — orchestrator + fresh per-iteration sub-agents

Split the two roles so context stays clean and iterations don't anchor on each other:

• You are the orchestrator. You own the durable state (config.json, census.json, best_sha, the branch), the harness, and every keep/discard decision. You do NOT accumulate the raw work of each attempt in your own context.
• Each improvement iteration runs in a FRESH sub-agent (spawn via the Agent tool). Hand it a compact briefing — not your whole transcript: the goal/evaluators, the full editable scope (files_to_optimize expanded — it may change ANY file in scope, not just a prompt), the ranked census.json buckets (+ the bucket to target this iteration), the current best_sha, and one-line summaries of prior attempts (what was tried → kept/discarded, from iteration_results) so it won't repeat them. Its job: make ONE change + return a short summary (what it changed, which bucket, feasibility-probe result). You (orchestrator) run the harness, apply the noise gate + mechanism audit, commit/keep/discard, and update state.
• Why: a fresh bounded context per iteration avoids anchoring on dead ideas and stops the orchestrator's context from bloating over a long run — the same reason the production loop spawns a new claude --print per iteration instead of one long-lived agent. If sub-agents are unavailable, emulate it: before each iteration, re-read only the briefing above and deliberately ignore the narrative of previous attempts beyond their one-line outcomes.

Iteration 1 — baseline + first improvement

Mirrors build_initial_prompt. Four steps, in order.

Step 1 — Load the evaluation data

Pick the data source in this priority order and materialize it to .auto_experiment/data.jsonl (one scoreable datapoint per line: the input, plus expected/reference output if present):

1. dataset_id present → get_llmobs_dataset_records + get_llmobs_full_dataset_records.
2. else non-empty trace_ids → get_llmobs_trace (full tree), get_llmobs_span_details, get_llmobs_span_content.
3. else → fetch the last ~30 LLM traces for ml_app (search LLM-Obs spans), and record the trace IDs you used back into config.json trace_ids so later iterations reuse the SAME corpus.

Extract input/output per the messages-source guidance in references/rubrics.md (score the messages field on the child LLM span, not the thin root input.value). Apply the data-selection guidance: keep only traces with a scoreable target span; exclude infra/setup spans from the set entirely.

Then split once, deterministically (hash of datapoint id, ~70/30) into .auto_experiment/data.val.jsonl (the hill-climb gate) and .auto_experiment/data.test.jsonl (held out) — see the rubric's Held-out split. Every iteration scores on val (AUTO_EXP_DATA=.auto_experiment/data.val.jsonl); test is run only in the final report.

Step 2 — Build the harness and compute BEFORE (baseline)

Copy references/eval_harness_template.py to .auto_experiment/eval_harness.py and fill in generate_output (run the REAL code under test from files_to_optimize) and judge. Prefer a deterministic ground-truth metric (reference output / programmatic checker / pipeline count) and use an LLM-as-judge only when no ground truth exists — see the rubric's Metric selection. No score literals anywhere.

Run it against the original, unmodified code with AUTO_EXP_REPS (= config reps, default 3): the harness re-runs the whole eval R times and prints {mean, stdev, rep_means, ...}. before_score = the printed mean; also record stdev (the noise floor). Both computed numbers, never literals — obey the scoring policy and the Noise & keep/discard policy in the rubric.

Commit eval_harness.py, data.jsonl, data.val.jsonl, data.test.jsonl, eval_results.jsonl.

Then report the baseline to LLM-Obs as iteration 0 — before you start the first iteration. Submit exactly one eval-metric datapoint with score_value = before_score and tags ["iteration:0", "git.commit.sha:<baseline_commit_sha>", "decision:baseline"] (the sha is the full 40-character hash of the commit you just made — git rev-parse HEAD, not a short hash). Same call shape and rules as Report each iteration's score to LLM-Obs; this is the only submission with iteration:0 and decision:baseline.

Step 2.5 — Census the baseline failures

Before changing anything, decompose where the baseline loses per the rubric's Baseline failure census: bucket every failing datapoint by root cause, write .auto_experiment/census.json, commit it, and surface the ranked buckets. This tells you which lever is worth pulling — and whether the dominant failure mode is even reachable by editing files_to_optimize.

Step 3 — Improve

Read the whole scope (files_to_optimize, expanded). Make ONE focused change toward goal, aimed at the largest census bucket you can plausibly move (name that bucket in the iteration's reasoning), in whichever in-scope file holds the lever — edit the tool/retrieval code if the census says the misses are retrieval, the output/format code if they're formatting, and so on. Do not default to rewording a prompt when the lever is elsewhere. Commit it on the scratch branch with a message explaining what changed and why.

Before the (expensive) full eval, run a feasibility probe per the rubric's Feasibility probe: the cheapest offline check that this change could move a failing census bucket. If the probe reaches 0 failing datapoints, record the iteration no_change with the probe result and skip to the next hypothesis — do not spend a full eval on a dead lever.

Step 4 — Compute AFTER (re-run the SAME harness)

Re-run .auto_experiment/eval_harness.py (same evaluate_line, same data) against the changed code. after_score = the new printed mean. Re-write eval_results.jsonl. Write the metric object (schema in the rubric) to .auto_experiment/result.json and commit it in the same commit as the change. delta = after_score - before_score.

Decide is_best per the optimization direction in goal and the Noise & keep/discard policy: keep only if |after_score − before_score| > max(pooled_stdev, min_delta). A within-noise gain is is_best: false (discarded), not kept. If the band is cleared, run the Mechanism audit (rubric) — diff this iteration's eval_results.jsonl against the baseline's (same-count denominator; the gain comes from datapoints the change touched) before keeping. If iteration 1 clears the band AND passes the audit, it becomes the best (best_sha = this commit, best_score = after_score). Append the row to config.json iteration_results.

Then report this iteration's score to LLM-Obs (tag iteration:1) — see Report each iteration's score to LLM-Obs.

Iterations 2+ — hill climb

Mirrors build_followup_prompt. Baseline is already known — do not recompute it.

1. Restore to the best-so-far, so a discarded attempt cannot contaminate this one:
   • if a commit was kept → git reset --hard <best_sha> (stays on the scratch branch; the committed harness + data live in that commit, so they are preserved — do not recreate them).
   • if nothing has been kept yet → git checkout <base_branch> -- <files_to_optimize> (restore only the target files; the harness/data live only in the previous commit on this branch, so a hard reset to base would delete them).
2. before_score = the current best score (from iteration_results; iteration-1 baseline if nothing kept yet). Do NOT re-run the baseline.
3. Reuse the data from data.jsonl and the committed eval_harness.py — do not reload or rebuild.
4. Make ONE new change, different from every previous attempt (you can see prior attempts in iteration_results), aimed at a named census.json bucket, in whichever in-scope file holds the lever (tool/retrieval/pipeline/config/prompt — not prompt-only). Commit it.
5. Feasibility probe first (rubric): cheap offline check the change can move its target bucket; if it reaches 0 failing datapoints, record no_change and skip the full eval. Otherwise re-run the SAME harness on val → after_score. Re-write eval_results.jsonl + result.json, commit.
6. Keep or discard: keep only if the delta clears the noise band (`|after_score − before_score|

   max(pooled_stdev, min_delta), per the Noise policy) in the goal's direction **and passes the Mechanism audit** (rubric) — diff eval_results.jsonl vs the best commit's (git show :.auto_experiment/eval_results.jsonl); same denominator, gain from datapoints the change touched. Then → update best_sha/best_score, decision kept. A within-noise gain, a denominator artifact, or a regression → discarded`. Append the row.

7. Report this iteration's score to LLM-Obs (tag iteration:<n>) — see Report each iteration's score to LLM-Obs.

Report each iteration's score to LLM-Obs (every scored iteration)

Once you have a computed score for an iteration, submit exactly one eval-metric datapoint to LLM-Obs with the submit_llmobs_experiment_events MCP tool. Do this once per iteration, right after the score is computed and the iteration's commit / result.json is written — including iteration 1 and the iteration-0 baseline (see Step 2; there score_value = before_score and the decision tag is decision:baseline).

Call submit_llmobs_experiment_events with a single metric shaped exactly like this:

• experiment_id: the value of the DD_AUTO_EXPERIMENT_ID environment variable (read it from the environment; do not ask the user and do not invent one). If it is unset or empty, skip the submission and note that in the iteration reasoning.
• metrics: an array containing exactly one object with these fields and no others:
  • label: always the literal string auto_experiment_score.
  • metric_type: score.
  • score_value: the score this iteration produced (after_score) — the number computed by the harness, never a literal or a rounded-for-display value.
  • timestamp_ms: the current wall-clock time as an epoch timestamp in milliseconds.
  • tags: ["iteration:<n>", "git.commit.sha:<sha>", "decision:<decision>"], where <n> is this iteration's number (1 for the first improvement, 2 for the next, and so on), <sha> is the full 40-character Git commit SHA of the commit this iteration created for its change — the complete hash from git rev-parse HEAD after committing the iteration (e.g. fd0fbab7c1232e125df7b22d9df856a2ef73ab65), never the abbreviated 7/8-char short hash — and <decision> is this iteration's keep/discard decision recorded in iteration_results (kept or discarded; baseline for iteration 0).
  • reasoning: this iteration's reasoning string from iteration_results — what was tried, which census bucket it targeted, and why it was kept or discarded (for iteration 0, that it is the baseline). Use the same text recorded in result.json; do not fabricate.
  • Do not include span_id, categorical_value, or boolean_value.

Example arguments for iteration 5 whose harness computed a score of 0.72:

{
  "experiment_id": "<value of $DD_AUTO_EXPERIMENT_ID>",
  "metrics": [
    {
      "label": "auto_experiment_score",
      "metric_type": "score",
      "score_value": 0.72,
      "reasoning": "Rewrote the retrieval query builder to include entity synonyms (targeting the 'missed-retrieval' census bucket); cleared the noise band and passed the mechanism audit, so kept.",
      "timestamp_ms": 1752430000000,
      "tags": ["iteration:5", "git.commit.sha:33ec6e0959bd46b0ea9c337cf6a28a763d3eeb0a", "decision:kept"]
    }
  ]
}

Rules:

• Exactly one metric per iteration. Never submit more than one metric for the same iteration and never batch several iterations into one call.
• Only for scored iterations. A no_change iteration has no computed score, so there is no score_value to send — skip the submission for it (emitting a score metric without a real score would violate the scoring policy). Record the skip in reasoning.
• The value you submit is the same computed after_score recorded in result.json; the two must always agree.

Stop conditions & guards

• Stop when iteration == max_iterations.
• Plateau within noise — stop early. If the last 3 iterations all landed discarded within the noise band (no delta cleared max(pooled_stdev, min_delta)), stop and report the current best with stop_reason: "plateau (deltas within noise)". Continuing past a noise plateau just burns budget generating within-noise wiggle; escalate instead (a new census bucket, a different dimension, or accept the ceiling). Distinguish this from a real regression streak.
• A change with no computable score is no_change, never a fabricated number (harness won't run / no new commit / judge unreachable / feasibility probe reached 0). Record the blocker in reasoning.
• Track consecutive no_change iterations; after 5 in a row, stop early and report the best result so far with a stop reason (do not keep burning iterations).

Final report

1. Ask yourself the run-level wrap-up and write final_result into config.json: { "baseline_score", "best_score", "best_iteration", "best_sha", "iterations_run", "stop_reason", "reasoning" } (reasoning = what was tried across all iterations, what worked, what didn't, why the winner won).
2. Held-out test comparison (the real headline). Run the harness once on the baseline commit and once on the best commit against .auto_experiment/data.test.jsonl (AUTO_EXP_DATA=.auto_experiment/data.test.jsonl), both at reps reps. Report the baseline-vs-best test delta with its noise band as the run's result — the val hill-climb gain is not the headline. If test did not clear the noise band even though val did, say so explicitly (the win did not generalize) and treat baseline as best.
3. Print a per-iteration table (iteration, val delta, decision, sha) and name the best commit.
4. If nothing beat the baseline on test: report the baseline as the best result and leave the original code in place (best_sha empty). Do not fabricate an improvement.
5. Tell the user the scratch branch + best commit so they can open a PR from it if they want.
6. Mark the experiment finished in LLM-Obs. Call update_llmobs_experiment with experiment_id = $DD_AUTO_EXPERIMENT_ID (skip if unset) exactly once at the very end — after the last iteration, or immediately whenever you give up early. Set status: "completed" for any run that reached the final report (including one where baseline stayed best — a run that finished cleanly is completed, not failed). Set status: "failed" with a short error when the run could not finish — the harness never ran, setup was blocked, or you abandoned before any scored iteration. This status update is separate from the per-iteration metric submissions; make it once, last.

Notes

• Every score is computed by running code. If you ever find yourself about to type a score number, stop — run the harness instead.
• Keep .auto_experiment/ committed; it is the reproducible record of the run.