port-fidelity - SKILL.md Agent Skill

name: port-fidelity description: Audit, diff, and port changes between TypeScript and Python codebases to keep them in sync during the migration.

Port Fidelity

Systematic comparison of TypeScript and Python implementations to detect drift and port changes faithfully.

When to Use

Before shipping a Python change that touches ported logic
After a TS change to a file listed in packages/opal-backend/PROVENANCE.md
When asked to audit or sync the two codebases
During a /daily-dig that targets opal-backend

Workflow

1. Identify the pair

Open packages/opal-backend/PROVENANCE.md and find the row for the module you're investigating. Note the TS source path.

2. Read both sides

Open the Python module and its TS counterpart side by side. Focus on:

Public API shape: function names, parameter lists, return types
Control flow: early returns, error branches, fallback logic
Constants/config: model names, endpoint paths, schema shapes
Wire format: to_dict() / JSON output must match both sides exactly

3. Classify differences

Category	Action
Intentional (architectural, e.g., Protocol vs inline)	Document in PROVENANCE.md notes column
Drift (TS was updated, Python wasn't)	Port the change
Bug (Python diverges from TS behavior)	File as finding, write a test
Enhancement (Python is ahead of TS)	Confirm intent with maintainer

4. Port changes

When porting a TS change to Python:

Copy the TS diff (logical, not textual)
Translate idioms:
- Promise<T> → async def ... -> T
- Record<K,V> → dict[K, V]
- interface → Protocol or @dataclass
- ?. → explicit if x is not None guards
- Array.map/filter → list comprehensions
- throw new Error → raise ValueError
Update the Python module's docstring provenance line if the TS source changed
Run tests: npm run test -w packages/opal-backend

5. Report

Produce a structured summary:

## Port Fidelity Report: <module>

**Python**: `opal_backend/<module>.py` **TypeScript**:
`visual-editor/src/a2/agent/<ts-file>.ts`

### Status

- [ ] API shape matches
- [ ] Control flow matches
- [ ] Constants/config matches
- [ ] Wire format matches

### Differences Found

| #   | Category | Description                  | Action         |
| --- | -------- | ---------------------------- | -------------- |
| 1   | drift    | TS added retry logic in v2.3 | Port to Python |

### Changes Made

- Ported retry logic from TS commit abc123
- Added test for retry behavior

Common Gotchas

Wire format is sacred. Both sides emit JSON that the client parses. A field name mismatch (camelCase vs snake_case) breaks the protocol.
Shared schemas. In TS, schemas are spread inline in each function file. In Python, they're centralized in shared_schemas.py. When a TS schema changes, update it there, not in individual function files.
Model names. Hardcoded in both codebases. When TS updates a model name, grep both sides.
Protocol modules have no TS pair. http_client.py, backend_client.py, and interaction_store.py are new for Python. Don't look for a TS counterpart — but do verify they're consistent with how TS uses the same backend APIs.