Modernize a COBOL system with AI orchestration

Process study · proposed method

Capture the rules once. Challenge every replacement.

Explore how shared characterization fixtures, a bounded comparison and domain review can reduce repeated analysis while improving confidence in a COBOL migration.

Establish the baseline

COBOL source and copybooks
Job definitions and sample records
Approved reference outputs

Evidence to retain

Reusable reference fixtures
Bounded approach comparison
Reconciliation and domain decisions

Why work this way?

The process, and what it could improve.

Proposed method · no measured savings claim

Cost

Turn domain explanations into reusable fixtures.

Capture record layouts, decimal behavior and accepted outputs before translating code. Share those artifacts with implementation and review workers.

What to compare: Model charges and domain-expert effort per reconciled capability, including fixture creation and unexplained differences.
The tradeoff: Characterization costs effort upfront and may expose missing knowledge that expands the work.

Compare the same scope and acceptance criteria. Include setup, coordination, failed attempts and human review alongside the successful model call.

Design the work

One problem. A coordinated agent team.

Proposed task plan

A model reasons. A harness runs its agent session and tool loop. Orchard coordinates the work, context and handoffs across them.

Orchard orchestratorScopes work · delegates · synthesizes evidence

CONTROL

Implementation lead

Example session

Model familyClaudeHarnessClaude Code

Map copybooks, JCL, file layouts and callers; propose an API seam around the selected capability.

Bounded task assigned

Counterproposal agent

Example session

Model familyGPTHarnessCodex CLI

Assume the rules are incompletely documented. Challenge the replacement with precision, encoding and restart cases.

Opposing assumption assigned

Independent reviewer

Example session

Model familyGeminiHarnessGemini CLI

Reconcile old/new outputs and separate exact matches, explained differences and unresolved behavior.

Evidence criteria assigned

Shared project contract

Rules · skills · hooks · MCP configuration

Adapted to each harness

01 / 04 · Dispatch

Split the work. Preserve the question.

One objective, distinct responsibilities

Replace a bounded COBOL capability with a modern service while preserving its business semantics.

Approved legacy outputs are the comparison baseline. No production retirement follows from code translation alone.

Implementation leaddependency-map.md + service contract

Counterproposal agentgolden-master fixtures + semantic-differences.md

Independent reviewerreconciliation report + cutover questions

Illustrative execution planAssignment Evidence Review

Model families and pairings are illustrative. Assignments here are manual examples, not measured model comparisons or automatic cost optimization.

Trellis / shared knowledge

Legacy knowledge

Copybooks, job dependencies, record layouts and expert explanations.

Decision record

Reference fixtures, disputed semantics and accepted interface choices.

BEAM / coordination

Bounded tasks, ownership notices, findings and handoffs between sessions. A queued message and a processed receipt are distinct states.

Recall → work → retain decisions

Tools / execution

COBOL toolchain / sourceFixture comparison harnessTarget service + contract tests

Access follows the configured harness, credentials and approved scope.

Inspect the example work orderJSON

An illustrative work-order format for this scenario. Changing an example session updates this document; no agents run from this page.

{
  "example": "cobol-modernization",
  "objective": "Replace a bounded COBOL capability with a modern service while preserving its business semantics.",
  "boundary": "Approved legacy outputs are the comparison baseline. No production retirement follows from code translation alone.",
  "sessions": [
    {
      "role": "proposer",
      "model_family": "Claude",
      "harness": "Claude Code",
      "task": "Map copybooks, JCL, file layouts and callers; propose an API seam around the selected capability.",
      "expected_artifact": "dependency-map.md + service contract"
    },
    {
      "role": "challenger",
      "model_family": "GPT",
      "harness": "Codex CLI",
      "task": "Assume the rules are incompletely documented. Challenge the replacement with precision, encoding and restart cases.",
      "expected_artifact": "golden-master fixtures + semantic-differences.md"
    },
    {
      "role": "reviewer",
      "model_family": "Gemini",
      "harness": "Gemini CLI",
      "task": "Reconcile old/new outputs and separate exact matches, explained differences and unresolved behavior.",
      "expected_artifact": "reconciliation report + cutover questions"
    }
  ],
  "project_contract": [
    "Rules",
    "Skills",
    "Hooks",
    "Tool configuration"
  ],
  "shared_context": [
    "Legacy knowledge",
    "Decision record"
  ],
  "evidence_required": [
    {
      "label": "Money reconciles",
      "evidence": "Compare exact decimal values, signs, rounding and invalid-record handling."
    },
    {
      "label": "Operational behavior holds",
      "evidence": "Test job ordering, partial failure, restart and duplicate input."
    },
    {
      "label": "Differences are explained",
      "evidence": "Have a domain expert disposition every material discrepancy."
    }
  ],
  "release_owner_gate": "A domain expert and operations owner accept reconciliation and cutover readiness before production traffic moves."
}

The process principle

Judge the process by reconciled behavior.

Scope of this exampleRuntime and database choices follow discovery. Parallel comparisons support review; they do not establish universal business equivalence.

A proposed modernization approach. No production conversion or performance result is claimed. Incremental replacement pattern

Modernize a COBOL system

Different tools.
Shared project context.

Where the work changes.
How to judge the difference.

Outlook listener

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