mermaid-js/mermaid · error

An align directive requires at least two members; got ${hint

Error message

An align directive requires at least two members; got ${hint.members.length}

What it means

Thrown by ArchitectureDB.addLayoutHint when hint.members has fewer than 2 entries. An align directive aligns two or more nodes along a row or column, so a single-member or empty alignment is meaningless. Note the Langium grammar already requires `(members+=ID)+`, so this guard primarily protects non-grammar (programmatic) callers.

Source

Thrown at packages/mermaid/src/diagrams/architecture/architectureDb.ts:265

      title,
    };

    this.edges.push(edge);
    const lhsNode = this.nodes.get(lhsId);
    const rhsNode = this.nodes.get(rhsId);
    if (lhsNode && rhsNode) {
      lhsNode.edges.push(this.edges[this.edges.length - 1]);
      rhsNode.edges.push(this.edges[this.edges.length - 1]);
    }
  }

  public getEdges(): ArchitectureEdge[] {
    return this.edges;
  }

  public addLayoutHint(hint: ArchitectureLayoutHint): void {
    if (hint.members.length < 2) {
      throw new Error(
        `An align directive requires at least two members; got ${hint.members.length}`
      );
    }
    const seen = new Set<string>();
    hint.members.forEach((id) => {
      if (this.registeredIds.get(id) !== 'node') {
        throw new Error(
          `align ${hint.direction} references [${id}], which is not a service or junction`
        );
      }
      if (seen.has(id)) {
        throw new Error(`align ${hint.direction} lists [${id}] more than once`);
      }
      seen.add(id);
    });
    this.layoutHints.push(hint);
  }

View on GitHub (pinned to d93e9c88c0)

Solutions

1. Ensure members contains at least 2 ids.
2. Skip emitting the hint entirely when fewer than 2 members would be present.

Example fix

// before
db.addLayoutHint({ direction: 'row', members: ['a'] });
// after
db.addLayoutHint({ direction: 'row', members: ['a', 'b'] });

function addLayoutHintSafe(db, hint) {
  if (hint.members.length < 2) {
    throw new Error(`align needs >=2 members; got ${hint.members.length}`);
  }
  db.addLayoutHint(hint);
}

When it happens

Trigger: Calling db.addLayoutHint({ direction: 'row', members: [] }) or members: ['only_one']. In DSL this is hard to hit because the grammar enforces >=1, but a single-member align can still slip through some code paths.

Common situations: Programmatic generation of layout hints that produces empty or singleton arrays; DSL edge cases where a rule reduces members to one; tests exercising the DB API directly.

Related errors

• align ${hint.direction} references [${id}], which is not a s
• align ${hint.direction} lists [${id}] more than once
• No nodes found in layout data
• Layout data is required
• Configuration is required in layout data

AI-assisted analysis of mermaid-js/mermaid@d93e9c88c0 (2026-08-12). Data as JSON: /api/errors/55f42f91d9b38ff3. Report an issue: GitHub.