JSON Adapter

The JSON adapter provides four pure data manipulation primitives for composing structured outputs and declarative branching in orchestrations. All operations are pure functions with sub-millisecond execution — no IO, no external calls.

Operations

json:merge — Deep Merge

Merges a vector of maps using RFC 7396 JSON Merge Patch semantics. Later values win on key conflict; nested maps merge recursively; null values delete keys.

{
  "operation": "json:merge",
  "input": {
    "values": [
      { "decision": "ESCALATED", "tier": "manager" },
      { "invoice": { "vendor": "Acme Corp", "amount": 15600 } },
      { "policy_passed": true }
    ]
  }
}

Result:

{
  "decision": "ESCALATED",
  "tier": "manager",
  "invoice": { "vendor": "Acme Corp", "amount": 15600 },
  "policy_passed": true
}

Use merge to compose outputs from multiple pipeline steps (e.g., LLM decision + invoice data + policy rules).

json:cond — Conditional Selection

Returns the then value of the first case whose when is truthy. Only null and false are falsy — everything else (including 0, "", [], {}) is truthy.

{
  "operation": "json:cond",
  "input": {
    "cases": [
      { "when": false, "then": { "route": "auto-approve" } },
      { "when": true,  "then": { "route": "manager", "approver": "J. Martinez" } },
      { "when": false, "then": { "route": "vp" } }
    ],
    "default": { "route": "unknown" }
  }
}

Result: { "route": "manager", "approver": "J. Martinez" }

Use cond for declarative branching in orchestrations without invoking an LLM.

json:assoc — Set Value at Path

Sets a value at a path in a map, creating intermediate maps for missing keys. Equivalent to Clojure assoc-in or Lodash _.set.

{
  "operation": "json:assoc",
  "input": {
    "target": { "user": { "name": "alice" } },
    "path": ["user", "email"],
    "value": "alice@example.com"
  }
}

Result: { "user": { "name": "alice", "email": "alice@example.com" } }

The path can be a single string (top-level key) or an array of strings (nested path). Missing intermediate maps are created automatically.

json:select — Pick by Discriminator

Looks up a value in a map of cases by a discriminator key. Like a switch statement.

{
  "operation": "json:select",
  "input": {
    "key": "manager",
    "cases": {
      "auto": { "decision": "APPROVED" },
      "manager": { "decision": "ESCALATED", "target": "J. Martinez" },
      "vp": { "decision": "ESCALATED", "target": "VP Finance" }
    }
  }
}

Result: { "decision": "ESCALATED", "target": "J. Martinez" }

Returns default if the key is not found, or null if no default is provided.

Operations Reference

Operation	Input	Description
json:merge	values (array of maps)	RFC 7396 deep merge
json:cond	cases (array of {when, then}), default?	First truthy case
json:assoc	target?, path, value	Set value at nested path
json:select	key, cases (map), default?	Switch on discriminator

Use Cases

• Pipeline composition — merge combines outputs from multiple steps into a single result
• Declarative routing — cond and select branch without LLM calls, keeping orchestrations fast and deterministic
• Data enrichment — assoc adds fields to records before passing them downstream

Related

• Orchestrator — workflow coordination using these primitives
• Agents — agents can call JSON operations as tools