denoland/deno · warning

Failed to extract with ${propagator.constructor.name}.

Error message

Failed to extract with ${propagator.constructor.name}.

What it means

The extract twin of the inject warning in ext/telemetry/telemetry.ts: CompositePropagator.extract reduces over registered propagators, and any propagator that throws while parsing incoming headers is reduced to this warning; the reduce then returns the previous context (line 1825), so extraction silently degrades to no propagated trace. Requests continue, but spans will not be correlated with the upstream.

Source

Thrown at ext/telemetry/telemetry.ts:1843

      try {
        propagator.inject(context, carrier, setter);
      } catch (err) {
        // deno-lint-ignore no-console
        console.warn(
          `Failed to inject with ${propagator.constructor.name}.`,
          err,
        );
      }
    }
  }

  extract(context: Context, carrier: unknown, getter: TextMapGetter): Context {
    return ArrayPrototypeReduce(this.#propagators, (ctx, propagator) => {
      try {
        return propagator.extract(ctx, carrier, getter);
      } catch (err) {
        // deno-lint-ignore no-console
        console.warn(
          `Failed to extract with ${propagator.constructor.name}.`,
          err,
        );
      }
      return ctx;
    }, context);
  }

  fields(): string[] {
    return ArrayPrototypeSlice(this.#fields);
  }
}

let builtinTracerCache: Tracer;

function builtinTracer(): Tracer {
  if (!builtinTracerCache) {
    builtinTracerCache = new Tracer(OtelTracer.builtin());

View on GitHub (pinned to a961cdec3b)

Solutions

1. Log and inspect the exact incoming traceparent/b3 headers when the warning appears; wrong length or non-hex characters are the usual culprit
2. Validate/strip malformed tracing headers at your edge middleware before otel extraction sees them
3. If a custom propagator is registered, harden its extract to return the input context instead of throwing on bad input
4. Upgrade Deno if the headers are well-formed and the vendored propagator still throws

Example fix

// before: malformed header reaches extraction, warning logged, trace context lost
const ctx = propagator.extract(ROOT_CONTEXT, request.headers);

// after: gate extraction on a format check
const tp = request.headers.get("traceparent") ?? "";
const ctx = /^\d{2}-[\da-f]{32}-[\da-f]{16}-[\da-f]{2}$/.test(tp.toLowerCase())
  ? propagator.extract(ROOT_CONTEXT, request.headers)
  : ROOT_CONTEXT;

// drop malformed tracing headers before otel ever parses them
const TRACEPARENT = /^[\da-f]{2}-[\da-f]{32}-[\da-f]{16}-[\da-f]{2}$/;
export function extractContext(ctx: Context, headers: Headers): Context {
  const tp = headers.get("traceparent");
  if (tp && !TRACEPARENT.test(tp.toLowerCase())) return ctx; // malformed -> start fresh trace
  return propagator.extract(ctx, headers);
}

function isTraceparent(h: string | null): boolean {
  return !!h && /^[\da-f]{2}-[\da-f]{32}-[\da-f]{16}-[\da-f]{2}$/.test(h.toLowerCase());
}

When it happens

Trigger: An incoming carrier (typically request headers) carries a malformed traceparent, tracestate, or b3/b3-multi header that makes a propagator's extract throw; or a custom registered propagator throws on the getter/carrier combination. Note the W3C propagator usually ignores unparseable headers, so a throw typically implicates custom propagators or SDK bugs.

Common situations: Upstream services or load balancers generating non-compliant traceparent values (wrong lengths, non-hex chars, extra spaces); hand-set tracing headers in test fixtures; proxies that truncate or mangle distributed-tracing headers; mixing b3 and W3C formats across services.

Related errors

• Failed to inject with ${propagator.constructor.name}.
• TracerProvider can not be constructed
• startActiveSpan requires a function argument
• vsock is not supported on this platform
• invalid vsock addr

AI-assisted analysis of denoland/deno@a961cdec3b (2026-08-20). Data as JSON: /api/errors/2d99c0e5f4248cb5. Report an issue: GitHub.