mermaid-js/mermaid · error

Invalid direction given for left hand side of edge ${lhsId}-

Error message

Invalid direction given for left hand side of edge ${lhsId}--${rhsId}. Expected (L,R,T,B) got ${String(lhsDir)}

What it means

Thrown by ArchitectureDB.addEdge when lhsDir fails the isArchitectureDirection type guard. Valid directions are exactly the single uppercase characters 'L', 'R', 'T', 'B' (left, right, top, bottom). Any other value — lowercase, multi-character, undefined, or a number — is rejected before the edge is created.

Source

Thrown at packages/mermaid/src/diagrams/architecture/architectureDb.ts:204

      in: parent,
    });
  }
  public getGroups(): ArchitectureGroup[] {
    return [...this.groups.values()];
  }
  public addEdge({
    lhsId,
    rhsId,
    lhsDir,
    rhsDir,
    lhsInto,
    rhsInto,
    lhsGroup,
    rhsGroup,
    title,
  }: ArchitectureEdge): void {
    if (!isArchitectureDirection(lhsDir)) {
      throw new Error(
        `Invalid direction given for left hand side of edge ${lhsId}--${rhsId}. Expected (L,R,T,B) got ${String(lhsDir)}`
      );
    }
    if (!isArchitectureDirection(rhsDir)) {
      throw new Error(
        `Invalid direction given for right hand side of edge ${lhsId}--${rhsId}. Expected (L,R,T,B) got ${String(rhsDir)}`
      );
    }

    if (!this.nodes.has(lhsId) && !this.groups.has(lhsId)) {
      throw new Error(
        `The left-hand id [${lhsId}] does not yet exist. Please create the service/group before declaring an edge to it.`
      );
    }
    if (!this.nodes.has(rhsId) && !this.groups.has(rhsId)) {
      throw new Error(
        `The right-hand id [${rhsId}] does not yet exist. Please create the service/group before declaring an edge to it.`
      );

View on GitHub (pinned to d93e9c88c0)

Solutions

1. Use one of the four uppercase chars: 'L', 'R', 'T', 'B' for lhsDir.
2. If directions come from external input, validate with isArchitectureDirection before calling addEdge.

Example fix

// before
db.addEdge({ lhsId: 'a', rhsId: 'b', lhsDir: 'left', rhsDir: 'R' });
// after
db.addEdge({ lhsId: 'a', rhsId: 'b', lhsDir: 'L', rhsDir: 'R' });

import { isArchitectureDirection } from './architectureTypes.js';
function addEdgeSafe(db, e) {
  if (!isArchitectureDirection(e.lhsDir)) {
    throw new Error(`lhsDir must be one of L,R,T,B; got ${String(e.lhsDir)}`);
  }
  db.addEdge(e);
}

import { isArchitectureDirection } from './architectureTypes.js';
const isValidDirection = (d: unknown): d is 'L'|'R'|'T'|'B' =>
  d === 'L' || d === 'R' || d === 'T' || d === 'B';

When it happens

Trigger: Calling db.addEdge({ lhsId, rhsId, lhsDir: 'left', rhsDir: 'R' }) or lhsDir: 'x', or lhsDir: undefined. The first direction check that fails is always the LHS one.

Common situations: Programmatic API misuse passing full words or lowercase; spreads/merges that leave lhsDir undefined; DSL grammar changes that emit a new token not understood by older code.

Related errors

• Invalid direction given for right hand side of edge ${lhsId}
• The left-hand id [${lhsId}] is modified to traverse the grou
• The right-hand id [${rhsId}] is modified to traverse the gro
• The service id [${id}] is already in use by another ${this.r
• The service [${id}] cannot be placed within itself

AI-assisted analysis of mermaid-js/mermaid@d93e9c88c0 (2026-08-12). Data as JSON: /api/errors/ccc5a5d509680ad5. Report an issue: GitHub.