MCP tools reference

Parameter specs and usage examples for every tool the liel MCP server exposes.

All tools return a JSON string. On error they always return the same shape:

{ "error": { "code": "slug", "message": "human-readable text" } }

---

Read tools

liel_overview

Return the overall shape of the memory graph. This is the best first call after connecting.

Parameters: none

Returns:

{
  "node_count": 42,
  "edge_count": 87,
  "node_labels": { "Person": 15, "Module": 10 },
  "edge_labels": { "DEPENDS_ON": 40, "ABOUT": 12 },
  "sample_nodes": [
    { "id": 7, "labels": ["Module"], "name": "auth" }
  ],
  "db_path": "/path/to/project.liel",
  "liel_format": "1.0",
  "file_size": 49152
}

liel_format and file_size match the CLI liel stats --format json fields (version and file_size from db.info()). There is no separate liel_stats MCP tool.

liel_find

Find nodes by label and exact property match. Use it to narrow the search space before exploring.

Parameters:

Name	Type	Default	Description
label	string	""	Node label filter. Empty means any label.
where	string (JSON)	""	JSON object of exact-match filters.
limit	integer	20	Max results per page (cap 100).
cursor	integer	0	Pagination offset.

liel_explore

Explore the neighbourhood of one node with BFS. Returns nodes, edges, and a Mermaid diagram.

Parameters:

Name	Type	Default	Description
node_id	integer	required	Starting node ID.
max_depth	integer	2	BFS depth cap (max 4).
edge_label	string	""	Traverse only this edge label when set.

liel_trace

Trace the shortest path between two nodes.

Parameters:

Name	Type	Default	Description
from_node	integer	required	Start node ID.
to_node	integer	required	End node ID.
edge_label	string	""	Restrict traversal to one edge label.

liel_map

Render a chosen subgraph as Mermaid.

Parameters:

Name	Type	Default	Description
node_ids	string	""	Comma-separated IDs like "1,3,5".
limit	integer	30	Auto-sampling cap when node_ids is omitted.

liel_diff

Compare two .liel files on disk. The JSON payload matches CLI liel diff --format json / liel diff — same shape as the reference pages for diff.

Parameters:

Name	Type	Default	Description
left	string	required	Path to the left/base .liel file.
right	string	required	Path to the right/other .liel file.
node_key_json	string	""	Optional JSON array of property names for key-aware diff, e.g. "[\"path\"]". Empty means ID-based diff.
identity_rules_path	string	""	Optional path to identity-rules JSON (same format as CLI --identity-rules). Mutually exclusive with node_key_json.

liel_merge_preview

Preview merging two .liel files (merge_from semantics) without writing an output file. Same JSON as CLI liel merge --dry-run --format json.

Not the same as liel_merge below (atomic merge into the already-open graph).

Parameters:

Name	Type	Default	Description
left	string	required	Base file path (copied conceptually first).
right	string	required	Incoming file path merged into the preview.
output_path	string	""	Optional planned output path string for the report only.
node_key_json	string	""	Optional JSON array of property names (same as CLI --node-key).
identity_rules_path	string	""	Optional identity-rules JSON path (same as CLI --identity-rules).
edge_strategy	string	"append"	append or idempotent.
on_node_conflict	string	"keep_dst"	keep_dst, overwrite_from_src, or merge_props.

liel_manifest

Return the deterministic manifest object for one .liel file (sorted nodes and edges, manifest_version, counts). Same content as CLI liel manifest stdout JSON — suitable for signing workflows.

Parameters:

Name	Type	Default	Description
source	string	""	Path to a .liel file. If empty, uses the MCP server’s connected file (--path).

---

Write tools

The MCP write tools do not currently add standard metadata properties such as creation timestamps, update timestamps, or session IDs. A reserved metadata convention may be added in a future release, but the initial MCP behavior keeps created graph records exactly to the user-supplied labels, properties, and edges.

liel_append

Append new nodes and edges in one atomic commit.

Use this when the AI knows it is recording new durable knowledge and does not need reuse or dedupe behavior.

Parameters:

Name	Type	Default	Description
nodes	string (JSON)	"[]"	JSON array of new node objects.
edges	string (JSON)	"[]"	JSON array of edge objects.
session	string	""	Reserved for possible future metadata support; currently no properties are added automatically.

Each node object supports:

{
  "ref": "decision",
  "labels": ["Decision"],
  "props": { "title": "Keep MCP first" }
}

Each edge object supports:

{
  "from": "decision",
  "to": 12,
  "label": "ABOUT",
  "props": { "weight": 1 }
}

from and to may reference:

• an existing node ID
• a ref created earlier in the same request

Returns:

{
  "created_nodes": [
    { "id": 43, "labels": ["Decision"], "title": "Keep MCP first" }
  ],
  "created_edges": [
    { "id": 88, "label": "ABOUT", "from": 43, "to": 12 }
  ],
  "ref_map": { "decision": 43 }
}

liel_merge

Merge nodes and edges in one atomic commit.

Use this when the AI wants to reuse or update existing nodes and create edges idempotently.

Parameters:

Name	Type	Default	Description
nodes	string (JSON)	"[]"	JSON array of merge node objects.
edges	string (JSON)	"[]"	JSON array of edge objects; edges are added with idempotent semantics.
session	string	""	Reserved for possible future metadata support; currently no properties are added automatically.

Each node object may use one of these patterns:

1. Update a known node directly:

{
  "id": 7,
  "props": { "owner": "platform" }
}

1. Reuse one existing node by exact property match:

{
  "ref": "auth",
  "labels": ["Module"],
  "match": { "path": "src/auth.rs" },
  "props": { "name": "auth", "owner": "platform" }
}

If match finds exactly one node, that node is reused and its properties are updated by overlaying props. If no node matches, a new node is created from labels and props. If multiple nodes match, the tool returns ambiguous_match.

Match semantics

liel_merge is intentionally client-neutral. It does not depend on CLAUDE.md, prompt files, or any single AI tool. The merge contract is:

• id updates a known node directly.
• match performs exact property matching to find one existing node.
• zero matches means "create a new node".
• more than one match returns ambiguous_match.

This means match works as an upsert-like operation only when the caller uses a property set that is stable enough to identify one node.

Choosing stable keys for match

Prefer properties that are naturally unique or close to unique for a label:

• Module -> path
• Document -> url
• Issue -> issue_id
• Person -> email or external_id
• Task -> project-specific task ID

Avoid weak keys such as name alone unless the dataset really guarantees uniqueness. If no stable key exists yet, call liel_find first and then merge by id.

Returns:

{
  "created_nodes": [
    { "id": 44, "labels": ["Decision"], "title": "Auth owned by platform" }
  ],
  "merged_nodes": [
    { "id": 7, "labels": ["Module"], "path": "src/auth.rs", "owner": "platform" }
  ],
  "merged_edges": [
    { "id": 89, "label": "ABOUT", "from": 44, "to": 7 }
  ],
  "ref_map": { "auth": 7 }
}

---

Read-side guidance

Use the read tools in this order unless you already know the graph well:

1. liel_overview for the broad picture
2. liel_find to narrow candidates
3. liel_explore to inspect local structure
4. liel_trace when the question is about impact or dependency paths
5. liel_map when Mermaid will help explain the result

---

Write-side guidance

• Use liel_append when you want guaranteed new records.
• Use liel_merge when you want to reuse existing nodes, update known nodes, or create edges without accidental duplication.
• When using liel_merge with match, prefer stable label-specific keys such as path, url, or external_id.
• Batch several related nodes and edges into one call instead of committing one tiny fact at a time.

---

Error handling

This section documents the MCP tool-layer JSON error shape. For Python API exceptions such as GraphDBError, AlreadyOpenError, and MergeError, see the Python guide.

Typical stable error codes include:

• invalid_json
• invalid_nodes
• invalid_edges
• invalid_labels
• invalid_node_id
• node_not_found
• unknown_ref
• ambiguous_match

The code field is the stable branch point; the message is explanatory text.