Behavior-space diff for the P6 human gate. Review what changed in behavior, not 5,000-line diffs. Captures observable HTTP surface (status, content-type, body structure) before and after a change, then diffs in behavior-space: "3 endpoints changed shape, 1 removed, everything else identical."

The code diff is 5,000 lines; the behavior diff is six items. The human reviews intent vs behavior — the one judgment machines cannot make — while the manifest (C11) proves the machines already did the rest.

ADLC phase

P6 — Human Gate. Consumed directly by the human reviewer during the gate step. Pairs with gate-manifest (C11): manifest proves machine checks passed; behavior-diff proves observable behavior is within expected bounds.

Verbs

`capture`

Hit each route in a config file and record the observable HTTP behavior to a JSON snapshot.

behavior-diff capture --config behavior.json --out before.json

Flags:

Flag	Required	Description
`--config <path>`	yes	Path to the behavior config JSON file
`--out <path>`	yes	Output path for the snapshot JSON
`--json`	no	Print machine-readable result summary to stdout

Config format (behavior.json):

{
  "baseUrl": "http://localhost:3000",
  "routes": [
    { "method": "GET", "path": "/health" },
    { "method": "GET", "path": "/api/users" },
    { "method": "POST", "path": "/api/items", "body": { "name": "test" } },
    { "method": "GET", "path": "/api/data", "headers": { "Accept": "application/json" } }
  ]
}

Each route: { method, path, body?, headers? }. body is serialized as JSON with Content-Type: application/json automatically added.

Per-route errors are recorded (as { method, path, error }) without aborting the run. The snapshot always contains an entry for every route in the config.

Exit codes for capture:

Code	Meaning
`0`	All routes attempted, snapshot written (even if some routes errored)
`1`	Operational error: config file unreadable, invalid config, cannot write output

`compare`

Compare two snapshots and produce a human-readable behavior report.

behavior-diff compare before.json after.json [--json]

Flags:

Flag	Required	Description
`--json`	no	Print machine-readable diff result to stdout

Output (human-readable):

2 routes identical, 2 changed, 0 errored

  + GET /metrics: route added (absent in before, present in after)
  ~ GET /health:
      body (json): 2 changes:
        ~ version: "1.0.0" → "2.0.0"
        + newField: "added"
  ~ GET /users:
      status: 200 → 503
      body (json): 2 changes:
        - users: [...]
        + error: "Service Unavailable"

Diff rules:

Routes matched by METHOD path.
Status change: reported directly.
Content-type change: reported (parameters like ; charset=utf-8 stripped for comparison).
JSON bodies: structural recursive diff. Paths use dot notation (body.items[3].price). Arrays: length change reported + first divergent index drilled. Capped at 50 paths.
Text/binary bodies: compared by SHA-256 hash.
Route only in before → "removed". Route only in after → "added".

Exit codes for compare:

Code	Meaning
`0`	Gate passes — all routes identical
`1`	Operational error: snapshot file unreadable or invalid JSON
`2`	Gate fails — one or more routes changed, removed, or added

Typical workflow

# Before the change: capture baseline
behavior-diff capture --config behavior.json --out before.json

# Make your code change, restart the service, then:
behavior-diff capture --config behavior.json --out after.json

# Review behavior diff (exits 0 if clean, 2 if changes found)
behavior-diff compare before.json after.json

Snapshot format

{
  "baseUrl": "http://localhost:3000",
  "capturedAt": "2024-01-15T10:30:00.000Z",
  "routes": [
    {
      "method": "GET",
      "path": "/health",
      "status": 200,
      "contentType": "application/json",
      "body": { "status": "ok" }
    },
    {
      "method": "GET",
      "path": "/large-html",
      "status": 200,
      "contentType": "text/html",
      "body": { "textHash": "sha256hex...", "bytes": 4096 }
    },
    {
      "method": "GET",
      "path": "/broken",
      "error": "timeout after 10000ms"
    }
  ]
}

JSON responses: body is parsed and stored as a JSON value.
Non-JSON responses: body stored as { textHash, bytes }.
Errors: route stored as { method, path, error } (no status/body).

Future work (noted in spec)

Browser/headless mode — render routes via headless browser and diff rendered DOM or screenshots. Documented as future work; not yet implemented. HTTP fixture mode (this tool) is the current supported mode.

Core gaps

None. This tool is self-contained and uses only parseArgs, pass, gateFail, opError, and printJson from @adlc/core. Hashing is done locally via node:crypto rather than the core sha256 helper to avoid coupling the capture serialization format to a shared library.