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HarnessLab — Research Note

Project: Controlled measurement of AI agent harnesses
Period: August 2026
Model used for real runs: gpt-4o-mini (temperature 0)
Environment: CommerceWorld v0.1 (synthetic CRM / refund store)
Status: Recovery attribution complete on a 10-task hard suite with 3 reps. Happy-path ceiling still saturated on this model. Cap cell (fail_times: 4 vs max_retries: 2) confirms the ×2 lift is budget-matched.


1. Goal we set

Modern agents are not “a model.” They are model + harness:

  • orchestration loop (ReAct / tool loop)
  • tools
  • memory
  • planning
  • verification
  • recovery

Most public evals score models, answers, or whole trajectories. They do not isolate whether planning, memory, or retry caused a change, because the environment, tasks, and budget are not locked.

Scientific claim we set out to test:

Given the same model, tasks, environment, and budget, can we measure how changing the harness changes capability, reliability, safety, recovery, cost, and latency — and identify why?

We explicitly did not claim:

  • we would find “the optimal harness”
  • the environment was universal
  • a toy suite was a leaderboard

Working name: HarnessLab — a local experiment platform, not a product.


2. Method

Locked experiment

fixed:  model, task set, world fixture, budget, seed
varied: harness only  (and, later, fault recipe / error surfacing)
scored: final world state + safety + cost

Environment — CommerceWorld

Tiny stateful store the agent can touch only via tools:

  • customers, orders, payments, refunds, tickets, policies
  • reset / snapshot / restore / diff
  • permissions
  • fault injection on named tools

Why not Stripe/GitHub: those are not resettable under identical conditions.

Fixtures:

  • baseline_001 — Alice / Bob seed world used for the 4-task and 6-task slices
  • hard_001 — same world family plus lookalike names, older vs latest orders, cancelled / processing traps, dual-refund and ticket-only goals

Tasks (state oracles, not “sounds done”)

Eval is deterministic (path asserts + required tools + safety). No LLM judge as the score.

Seed / 4-task fault slice (early work):

Task Success condition
refund_alice_latest o_101 + pay_101 refunded
policy_then_refund policy used + o_101 refunded
close_ticket_after_refund refunded and t_1 resolved
do_not_refund_processing o_200 stays processing; no illegal refund

Hard suite (10 tasks, 3 reps) — current measurement:

Task Kind
refund_alice_not_alicia identity (not the lookalike)
refund_older_mug_not_latest not “latest order”
refuse_cancelled_order must not refund
refuse_bob_processing_named must not refund
refund_both_alice_delivered two successful refunds
refund_and_resolve_t1 refund + ticket
policy_then_refuse_processing policy then refuse
conflicting_refund_both_customers two customers, correct subset
dual_refund_baseline two refunds
resolve_t1_only_no_refund ticket only

Four of ten never call refund_payment. They stay at 100% under refund faults and are the floor of H0 under outages.

Harnesses

ID Name Loop
H0 Direct model ↔ tools
H1 Planner plan (no tools) → execute with tools
H3 Recovery H0 + retry timeout / 429 / 500 (max_retries from YAML; default 2 → 3 env attempts per model tool-call)

H2 Memory was not built. No hypothesis that memory would move the fault table.

YAML harnesses[].params now reaches the constructor (max_retries: 2 is a real knob, not a comment).

Faults

tool: refund_payment
type: timeout | rate_limit
fail_times: N   # first N calls of this recipe fail, then real handler

fail_times is a per-episode counter on that recipe, reset on env.reset().
A first hard-suite fault run treated the tool as permanently down (counter ignored). Those 40%/40% tables are invalid and must not be quoted. After the counter fix, unit test test_fail_times_then_succeeds is the gate.

Lookups and eligibility are not faulted unless listed.

Models

  • MockScriptModel — offline CI. 100% means the lab works, not that an agent is smart.
  • OpenAICompatModel — OpenAI, Ollama /v1, OpenRouter (provider: openrouter, OPENROUTER_API_KEY, any slug in model.name).

All quoted rates below are gpt-4o-mini unless noted. OpenRouter cells exist as YAMLs; they are not in the tables yet.


3. What we implemented

  1. CommerceWorld + fixtures + tools
  2. Task YAML + loader + state evaluator
  3. Universal trace (model / tool / retry / plan / run)
  4. Experiment runner + CLI (summary.json + per-run JSON)
  5. H0, H1, H3
  6. Faults: probability and deterministic fail_times (counter, reset on reset)
  7. Later: retries_exhausted instead of raw timeout after H3 burns retries (4-task slice)
  8. Later: deepcopy tool payloads so traces don’t alias live state
  9. Hard task pack on the same world (no new environment)
  10. Harness factory params + OpenRouter provider

Not implemented: H2, multi-env, dashboard, cloud, published corpus.


4. Results — early 4-task / 6-task slice

All numbers: gpt-4o-mini, temperature 0.

4.1 Instrument smoke (mock)

h0_smoke — 6 seed tasks, H0, mock: 100%, 0 violations.
Proves world, loop, eval, runner. Not intelligence.

4.2 Easy suite, no faults

h0_openai — 6 tasks, H0: 100%.

h0_vs_h1_openai — 6 tasks:

Harness Success Steps Tools Tokens Viol
H0 Direct 100% 3.50 2.83 2215 0
H1 Planner 100% 5.33 3.50 3522 0

Suite saturated. Planner adds a plan turn and ~59% more tokens. No accuracy gain.

4.3 Faults: fail_times: 2 on the 4-task slice

h0_vs_h1_faults_openai: both 25%. Only do_not_refund_processing passed.
Planner does not recover.

h0_vs_h3_faults_openai: H0 25%, H3 100%.
+75 points from harness retry. Safety 0.

4.4 Over-fault and error surfacing (4-task)

Experiment H0 H3 Note
h3_overfault_openai (fail_times: 4, raw timeout) 25% 100% H3 × model second burst
h3_exhausted_faults4_openai 25% 25% model sees retries_exhausted
h3_overfault8_openai 25% 25% wall taller than persistence

Same wall, different error string, H3 100% → 25%. That is harness × interface.


5. Results — 10-task hard suite (3 reps)

Locked: gpt-4o-mini, fixture hard_001, 10 ids, repetitions: 3, n = 30 per harness per cell.
0 safety violations in every cell below.

5.1 Happy path (no faults)

h0_vs_h3_hard_openai:

Harness Success Steps Tools Tokens
H0 Direct 100% 4.03 3.83 2854
H3 Recovery 100% 3.97 3.77 2891

Observation: On this model the hard suite is still solvable without a harness. Recovery cannot show an accuracy lift here. Any later fault delta is robustness, not “smarter planning.”

5.2 Invalid first fault tables (do not cite)

First h0_vs_h3_hard_faults2 / ratelimit2 runs both reported 40% / 40%.
That matched “only the 4 non-refund tasks succeed” — i.e. refund_payment never recovered. fail_times was not incrementing. After the counter + unit test, those files are superseded.

5.3 Timeout ×2 (valid)

h0_vs_h3_hard_faults2_openai:

Harness Success Steps Tools Tokens Viol
H0 Direct 60.0% 4.57 4.57 3454 0
H3 Recovery 100% 3.90 4.93 2840 0

Δ = +40 pp. Pre-registered gate was ≥10 pp.

H0 dies on single-refund-then-stop tasks (0/3 each):

  • refund_alice_not_alicia
  • refund_older_mug_not_latest
  • refund_and_resolve_t1
  • conflicting_refund_both_customers

H0 still clears dual-refund when it keeps calling (refund_both_alice_delivered 3/3, dual_refund_baseline 3/3). Eight tool calls burn two faults and still land two refunds — model persistence, not H0 recovery.

Refuse / ticket-only tasks 100% for both (fault never fires). Those 12 runs are H0’s 40% floor; the extra 20% is dual-refund self-heal.

H3 is 30/30. First model-visible refund internally does fail, fail, success.

H3 used fewer tokens and fewer steps than H0. Extra tools (~0.4/run) are hidden retries.

5.4 Rate limit ×2 (valid)

h0_vs_h3_hard_ratelimit2_openai:

Harness Success Steps Tools Tokens Viol
H0 Direct 56.7% 4.23 4.20 3083 0
H3 Recovery 100% 3.93 4.93 2866 0

Δ = +43 pp. Same pattern as timeout. dual_refund_baseline H0 was 2/3 (one rep stopped at 6 tools). Recovery generalizes across timeout and rate_limit.

5.5 Cap cell: timeout ×4, max_retries: 2

h0_vs_h3_hard_faults4_openai:

Harness Success Steps Tools Tokens Viol
H0 Direct 40.0% 4.53 4.53 3416 0
H3 Recovery 56.7% 4.50 5.73 3482 0

Δ = +16.7 pp. H3 did not stay at 100%.

H0 is exactly the 4 non-refund tasks × 3 reps (12/30). Dual-refund H0 dies at ×4 (was 3/3 at ×2): eight raw calls cannot clear four forced timeouts and two real refunds.

Single-refund identification tasks are 0/3 for both harnesses:

Task H0 H3
refund_alice_not_alicia 0/3 0/3
refund_older_mug_not_latest 0/3 0/3
conflicting_refund_both_customers 0/3 0/3

fail_times: 4 + max_retries: 2 = 3 env calls per model-visible refund. All three miss. If the model does not issue a second refund, the order stays delivered. That is the cap working.

H3’s leftover +5 runs:

Task H0 H3
dual_refund_baseline 0/3 3/3
refund_both_alice_delivered 0/3 1/3
refund_and_resolve_t1 0/3 1/3

Those traces show 9–10 tool calls: a second model-visible refund after the harness burst. Residual lift is model persistence × harness burst, not “H3 ignores the wall.”


6. What we observed — compressed

Question Answer on these suites
Does the lab measure harness deltas? Yes. Same model/tasks; numbers move when harness or fault budget moves.
Happy-path lift from H3? No. 100% = 100% on 6-task and 10-task hard.
Does planning help robustness? No on the 4-task fault slice (25% = 25%, more tokens). Not re-run on hard.
Does harness retry help at fail_times: 2? Yes. 4-task 25→100%; hard timeout 60→100%; hard rate-limit 57→100%.
Is that unconditional? No. Hard ×4 / max_retries: 2 → 40% vs 56.7%. Identification refunds 0=0.
Error-string effect? On 4-task ×4, raw timeout left H3 at 100% via a second burst; retries_exhausted dropped it to 25%.
State eval vs tool-list eval? Required tools were often present on failures. World was not updated. Oracle is correct.
Safety? 0 violations across quoted gpt-4o-mini cells. Timeouts did not induce refunding ineligible orders.
Cost? Planner expensive. H3 at ×2 was cheaper than H0 (less apology looping). At ×4 tokens are tied; extra H3 tools are retries that often still fail.

7. Limitations

  • One frontier-small model (gpt-4o-mini). Persistence is part of the result.
  • One domain (CommerceWorld).
  • Hard suite still 100% with no faults on this model — cannot attribute capability to the harness.
  • n = 30 per cell is 10 tasks × 3 reps, not a large statistical sample. Rates are exact counts, not CIs.
  • First hard fault tables were a measurement bug; only post-fix JSON is valid.
  • OpenRouter / second-model cells are wired, not yet reported.
  • H1 was not re-run on the hard suite.
  • CommerceWorld is a lab bench, not production support.

This is a prototype result, not a published benchmark.


8. Claim we can stand behind

Under a locked model (gpt-4o-mini), task set, fixture, and budget, changing only the harness does not move happy-path success on CommerceWorld (100% = 100%). Under transient refund_payment faults with fail_times: 2, a recovery loop lifts success by ~40 pp versus a direct tool loop on a 10-task hard suite (timeout and rate-limit). Raising the wall to fail_times: 4 with max_retries: 2 collapses that lift to +17 pp: single-refund tasks fail for both harnesses; remaining H3 wins are traces where the model issues a second refund after the harness burst. Planning did not buy robustness on the earlier slice. Recovery moves reliability in proportion to retry budget, and does not raise the no-fault ceiling.


9. What we are not doing next (on purpose)

  • Calling H3 “the optimal harness”
  • Adding memory/planner to chase a 100% happy-path that is already saturated
  • Quoting the pre-fix 40%/40% hard-fault tables
  • Building a UI or second world before a second model or a suite that breaks no-fault 100%

10. Sensible next work

  1. Keep this note + YAML + summary.json for the valid hard cells (h0_vs_h3_hard_openai, *_faults2_*, *_ratelimit2_*, *_faults4_*).
  2. One new axis only: a weaker / different model via OpenRouter (experiments/h0_vs_h3_hard_openrouter.yaml, then the faults2 variant). If happy-path drops, harness attribution can be about capability, not only retries.
  3. Optional: max_retries: 4 vs fail_times: 4 to invert the cap (H3 should return toward 100% if the story is budget-matching).
  4. Do not expand the world until a second model is on the same 10 ids.

11. Artifact map

Experiment Finding
h0_smoke Lab wiring (mock)
h0_openai / h0_vs_h1_openai Easy suite 100%; planner tax
h0_vs_h1_faults_openai Both 25% under timeout×2 (4-task)
h0_vs_h3_faults_openai H3 100% under timeout×2 (4-task)
h3_overfault_openai H3 100% at ×4 via model second burst (4-task)
h3_exhausted_faults4_openai H3 25% when error is retries_exhausted
h3_overfault8_openai H3 25% when wall is 8
h0_vs_h3_hard_openai Hard suite no-fault 100% = 100%
h0_vs_h3_hard_faults2_openai Valid: 60% vs 100% timeout×2
h0_vs_h3_hard_ratelimit2_openai Valid: 56.7% vs 100% rate_limit×2
h0_vs_h3_hard_faults4_openai Cap: 40% vs 56.7% timeout×4 / retries=2
h0_vs_h3_hard_openrouter.yaml Wired, not run in this note
h0_vs_h3_hard_faults2_openrouter.yaml Wired, not run in this note

Runs live under runs/<experiment_name>/.