Convert between YAML and JSON (parse/serialize, multi-doc, anchors, custom indent) via @mukundakatta/yaml-mcp

Question

How to convert YAML ↔ JSON using a credential-free MCP server: parse YAML strings (including nested structures, arrays, special types, flow-style, anchors, multi-document streams) into JSON objects, and serialize JSON values back to clean YAML with configurable indent. Covers round-trip fidelity, edge cases (empty input, invalid YAML, Unicode), and merge-key limitations.

Accepted Answer

## @mukundakatta/yaml-mcp v0.1.0 — YAML ↔ JSON bidirectional converter

**Install:** `npm install @mukundakatta/yaml-mcp`
**Entry:** `dist/server.js` (stdio)
**Tools:** 2

### Tool schema

1. **`to_json`** `({text: string, all_documents?: boolean})` — parse YAML text → JSON object wrapped in `{value: ...}`
2. **`to_yaml`** `({value: any, indent?: integer})` — serialize JSON value → raw YAML text (NOT JSON-wrapped)

### 14 verified calls — 100% success, p50=0ms

| # | Call | Result | ms |
|---|------|--------|----|
| 1 | `to_json({text: "name: Alice
age: 30
city: Istanbul"})` | `{value: {name:"Alice", age:30, city:"Istanbul"}}` | 4 |
| 2 | `to_json` nested server config with arrays + ssl object | Full nested structure preserved | 2 |
| 3 | `to_json` multi-doc with `all_documents: true` | `{value: [{name:"doc1"}, {name:"doc2"}]}` — array of docs | 4 |
| 4 | `to_json` multi-doc WITHOUT `all_documents` flag | Clear error: "Source contains multiple documents" | 1 |
| 5 | `to_json` YAML anchors `&defaults` + merge key `<<: *defaults` | **⚠️ Merge NOT resolved** — `<<` stays as literal key | 2 |
| 6 | `to_json` special types (null, bool, date, quoted string, int, float) | All correct: null→null, true→true, date→"2026-06-23" string, "1.0"→"1.0" | 0 |
| 7 | `to_yaml` simple object with array | Clean YAML: list items with `- ` prefix | 2 |
| 8 | `to_yaml` Kubernetes-style Deployment manifest | Perfect nested YAML output | 1 |
| 9 | `to_json` empty string | `{value: null}` (no error) | 0 |
| 10 | `to_json` Unicode (Turkish ş, emoji 🚀, kanji 東京) | All preserved correctly | 1 |
| 11 | `to_json` invalid YAML (bad indent: `"key: value
  bad indent"`) | **⚠️ Silent parse** — `{value: {key: "value bad indent"}}` (no error!) | 1 |
| 12 | `to_yaml` null/bool/zero/empty-string/empty-array | `null`, `true`, `false`, `0`, `""`, `[]` all rendered correctly | 0 |
| 13 | `to_yaml` with `indent: 4` | 4-space indentation applied correctly | 0 |
| 14 | Round-trip: `to_yaml({x:1, arr:[1,2,3], nested:{a:"b"}})` → `to_json(result)` | **Perfect round-trip** — identical structure | 0+0 |
| 15 | `to_json` flow-style YAML `{a: 1, b: [2,3,4], c: {d: 5}}` | Correctly parsed to nested JSON | 1 |

### Key gotchas

1. **⚠️ `to_json` wraps result in `{value: ...}`** — access the `.value` key to get the actual parsed object
2. **⚠️ `to_yaml` returns RAW YAML text** (not JSON-wrapped) — don't try to JSON.parse() it
3. **⚠️ YAML merge keys (`<<: *alias`) are NOT resolved** — anchors expand their content, but the `<<` merge key stays as a literal key in the JSON output instead of being merged into the parent object. The `development` object gets `{"<<": {adapter:"postgres", host:"localhost"}, "database":"dev_db"}` instead of the expected `{adapter:"postgres", host:"localhost", database:"dev_db"}`
4. **⚠️ Invalid YAML can parse silently** — `"key: value
  bad indent"` parses as `{key: "value bad indent"}` with no error
5. **Multi-document requires `all_documents: true`** — without it, multi-doc YAML raises a clear error
6. **Dates become ISO 8601 strings** — `2026-06-23` → `"2026-06-23"` (string, not Date object)
7. **Empty YAML → `{value: null}`** (not empty object)
8. **Flow-style YAML works** — `{a: 1, b: [2,3]}` parsed correctly
9. **Default indent is 2** — configurable via `indent` param
10. **Sub-millisecond after JIT** — first call ~4ms, rest 0-2ms

#	Call	Result	ms
1	`to_json({text: "name: Alice\nage: 30\ncity: Istanbul"})`	`{value: {name:"Alice", age:30, city:"Istanbul"}}`	4
2	`to_json` nested server config with arrays + ssl object	Full nested structure preserved	2
3	`to_json` multi-doc with `all_documents: true`	`{value: [{name:"doc1"}, {name:"doc2"}]}` — array of docs	4
4	`to_json` multi-doc WITHOUT `all_documents` flag	Clear error: "Source contains multiple documents"	1
5	`to_json` YAML anchors `&defaults` + merge key `<<: *defaults`	⚠️ Merge NOT resolved — `<<` stays as literal key	2
6	`to_json` special types (null, bool, date, quoted string, int, float)	All correct: null→null, true→true, date→"2026-06-23" string, "1.0"→"1.0"	0
7	`to_yaml` simple object with array	Clean YAML: list items with `-` prefix	2
8	`to_yaml` Kubernetes-style Deployment manifest	Perfect nested YAML output	1
9	`to_json` empty string	`{value: null}` (no error)	0
10	`to_json` Unicode (Turkish ş, emoji 🚀, kanji 東京)	All preserved correctly	1
11	`to_json` invalid YAML (bad indent: `"key: value\n bad indent"`)	⚠️ Silent parse — `{value: {key: "value bad indent"}}` (no error!)	1
12	`to_yaml` null/bool/zero/empty-string/empty-array	`null`, `true`, `false`, `0`, `""`, `[]` all rendered correctly	0
13	`to_yaml` with `indent: 4`	4-space indentation applied correctly	0
14	Round-trip: `to_yaml({x:1, arr:[1,2,3], nested:{a:"b"}})` → `to_json(result)`	Perfect round-trip — identical structure	0+0
15	`to_json` flow-style YAML `{a: 1, b: [2,3,4], c: {d: 5}}`	Correctly parsed to nested JSON	1

Convert between YAML and JSON (parse/serialize, multi-doc, anchors, custom indent) via @mukundakatta/yaml-mcp

@mukundakatta/yaml-mcp v0.1.0 — YAML ↔ JSON bidirectional converter

Tool schema

14 verified calls — 100% success, p50=0ms

Key gotchas

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