Diff engine

Every write is preceded by a diff in Claude Code. The diff engine renders the before/after so nothing reaches Postman by surprise — and so you can see, at a glance, which requests came from a typed spec versus heuristic code parsing.

Module: diff/render.py.

A new request

SYNC PREVIEW — POST /payments  →  collection / payments   [NEW] [openapi]

+ Request    POST {{base_url}}/payments
+ Auth       Bearer {{token}}              (from require_auth middleware)
+ Body       { "amount": 4200, "currency": "USD", "method": "card" }
+ Responses  201 Created, 400, 401, 422, 500
+ Tests      status(201) · schema(PaymentResponse) · business(amount > 0)
+ Examples   1 success, 4 error

Write? [y / n]

A modified request

Modified requests show field-level ~ changes and explicitly list what is preserved (human-owned scripts and examples):

SYNC PREVIEW — PUT /orders/{id}  →  collection / orders   [MODIFIED] [code]

~ Body       + include_items (boolean)
~ Responses  + 409 Conflict
= Preserved  2 test scripts, 1 manual example (human-owned)

Write? [y / n]

Source labels

Each request is tagged with where its model came from:

• [openapi] — derived from a typed OpenAPI spec. High confidence.
• [code] — derived from code parsing. Lower confidence, especially for untyped bodies (e.g. Express). Worth a closer look.

This makes per-route mixing visible: in one diff you might see most routes [openapi] and a manually mounted one [code].

The two-phase contract

The diff is produced by calling the MCP tool without confirm — it returns the preview and writes nothing. Only after you answer yes does Claude Code call the tool again with confirm: true to perform the write. There is no skip flag: you cannot write without first generating a diff. On n, nothing is written.