Invalid block parameters syntax: invalid identifier name \`$

Error message

Invalid block parameters syntax: invalid identifier name \`${state.name}\`

What it means

A collected block param identifier is invalid: it is either `this` (which cannot be a block param name) or matches characters not allowed in Handlebars identifiers (checked via ID_INVERSE_PATTERN). The parser validates each identifier when it hits a pipe or space that terminates it.

Source

Thrown at packages/@glimmer/syntax/lib/parser/tokenizer-event-handlers.ts:471

          this.pendingError = {
            mustache(loc: src.SourceSpan) {
              throw generateSyntaxError(
                `Invalid block parameters syntax: mustaches cannot be used inside parameters list`,
                loc
              );
            },
            eof(loc: src.SourceOffset) {
              throw generateSyntaxError(
                `Invalid block parameters syntax: expecting the tag to be closed with ">" or "/>" after parameters list`,
                as.start.until(loc)
              );
            },
          };
        } else if (next === '|' || isSpace(next)) {
          let loc = state.start.until(this.offset());

          if (state.name === 'this' || ID_INVERSE_PATTERN.test(state.name)) {
            throw generateSyntaxError(
              `Invalid block parameters syntax: invalid identifier name \`${state.name}\``,
              loc
            );
          }

          element.params.push(b.var({ name: state.name, loc }));

          state = next === '|' ? { state: 'AfterEndPipe' } : { state: 'BeforeBlockParamName' };
          this.tokenizer.consume();
        } else if (next === '>' || next === '/') {
          throw generateSyntaxError(
            `Invalid block parameters syntax: expecting "|" but the tag was closed prematurely`,
            as.start.until(this.offset().move(1))
          );
        } else {
          // slurp up anything else into the name, validate later
          state.name += next;
          this.tokenizer.consume();

View on GitHub (pinned to 26f97246a8)

Solutions

Rename the param to a valid Handlebars identifier (letters, digits, underscore, non-leading dash per handlebars ID rules)
Never use `this` as a block param name; pick a distinct name

Example fix

// before
<Foo as |this|>{{this}}</Foo>
// after
<Foo as |value|>{{value}}</Foo>

Defensive patterns

Strategy: validation

Validate before calling

// Validate each block param name against Handlebars ID rules
const ID_INVERSE = /[!"#%&'()*+,./;<=>@[\]^`{|}~]/;
for (const m of template.matchAll(/as\s+\|([^|]*)\|/g)) {
  for (const name of m[1].trim().split(/\s+/)) {
    if (name === 'this' || ID_INVERSE.test(name)) {
      throw new Error(`Invalid block param name: ${name}`);
    }
  }
}

Prevention

Never use `this` as a block param name
Stick to simple identifiers (letters, digits, underscore) for params

When it happens

Common situations: Trying to shadow `this`; using dashes-plus-invalid characters, unicode, or punctuation in param names; typos like `as |my param!|`.

Related errors

AI-assisted analysis of emberjs/ember.js@26f97246a8 (2026-09-01). Data as JSON: /api/errors/7a8e0252a0b34a7d. Report an issue: GitHub.