Behavioral verification · COBOL → Java

Prove an AI migration didn't change what your software does.

Strust runs the legacy system and its AI-written replacement on the same inputs, pinpoints the first behavior that diverged, and hands the evidence to both the coding agent and human reviewer.

See Strust isolate the change Become a design partner

Evidence run 0841,215 trace pairs

$128.005Input amount

$128.005Equivalent

−$4.10Fee subtract

−$4.10Equivalent

HALF_EVENRounding mode

HALF_UPFirst divergence

$123.90Ledger write

$123.91Downstream effect

First changed decisionMidpoint rounding rule

Same input. Different branch. One-cent ledger change.

Reference

Executable source

Comparison

Same-input replay

Decision

Human-owned review

Starting lane

COBOL → Java

Interactive v1 demonstrator

Agent + reviewer views

One change. The agent fixes it; the reviewer decides it.

Agent view isolates the first changed decision and the next patch. Reviewer view translates that evidence into impact and a human decision. This controlled example illustrates the intended product loop; the published paper documents the current evidence and limits.

COBOL to Java

Agent receives

Trace-local cause, the first divergent decision, a suggested patch, and the exact rerun command.

Agent diagnosis / F-02

12 divergent traces

First divergence: Branch action

Batch branch B-17 / null-account path · Trace 417 / 1,215

Cause isolated. Patch the changed branch and rerun.

Reference system

COBOL

IF ACCOUNT-ID = SPACES

NEXT SENTENCE

ELSE PERFORM WRITE-CLAIM

END-IF

Candidate system

Java

if (accountId == null) {

writeFallback(claim);

} else { writeClaim(claim); }

batch.advance();

Execution checkpoints

Source value / target value

NULL
Account input
NULL
Equivalent
TRUE
Guard B-17
TRUE
Equivalent
SKIP_WRITE
Branch action
WRITE_FALLBACK
Origin
0
Rows emitted
1
Changed
4,808
Batch count
4,809
Changed

First divergence

The first changed decision is Branch action: the source returns SKIP_WRITE while Java calls writeFallback.

Capture → Compare → Isolate → Patch → Rerun

Migration was the first proving ground.

First product laneCOBOL → Java

COBOL — observed

01 CUSTOMER-CODE PIC X(8).

IF CUSTOMER-CODE = 'VIP'
  MOVE 'PRIORITY' TO QUEUE-NAME

Java — converted

String code = record.customerCode();
if (code.equals("VIP")) { // ← flagged
  queueName = "PRIORITY";
}

A legacy system is already an executable reference. Strust turns representative source behavior into evals for the system replacing it. Here, COBOL pads the shorter fixed-width value; Java treats VIP····· and VIP as different strings. Both compile; the behavior changes.

✓Capture the source system as executable behavior

✓Review each source-to-target divergence

✓Keep approved cases for every later change

Read the original migration research

A benchmark for whether migration agents preserve behavior.

SemaMig-Bench · Benchmark v1

Run the source. Run the migration. Score the behavior.

SemaMig-Bench turns COBOL-to-Java migration into a paired, held-out evaluation. The same workload runs against the executable source and native Java candidate; a task passes only when the contract-relevant observations agree.

Open benchmark paper

Protocol

Published

Five-page design and preregistered evaluation protocol

Task corpus

In construction

50 tasks planned across diagnostic, workflow, and system tiers

Leaderboard

Results forthcoming

Baseline scores publish after the v1 corpus and verifiers are frozen

Seeking design partners

Bring one real migration into the loop.

Strust is looking for engineering teams, modernization firms, and coding-agent builders with an important software change to verify. Bring the workflow; we will map the behavior that matters and show where the candidate diverges.

1.Define the behaviors and side effects that matter

2.Run the reference and candidate under one contract

3.Review retained evidence and the first divergence

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