Failed ensuring public API type output is valid. {:#} You

Error message

Failed ensuring public API type output is valid.

{:#}

You may have discovered a bug in Deno. Please open an issue at: https://github.com/denoland/deno/issues/

What it means

As a final quality gate, publish type-checks the public API type output that fast-check generates for your package (unused-parameter diagnostics excluded, because stripped bodies drop them). If checking the generated output produces diagnostics, the `.d.ts`-equivalent Deno would upload is itself invalid — which the message interprets as a probable Deno bug and asks you to report.

Source

Thrown at cli/tools/publish/mod.rs:497

        // fast check passed, type check the output as a temporary measure
        // until we know that it's reliable and stable
        let mut diagnostics_by_folder = self.type_checker.check_diagnostics(
          graph,
          CheckOptions {
            build_fast_check_graph: false, // already built
            lib: self.cli_options.ts_type_lib_window(),
            reload: self.cli_options.reload_flag(),
            type_check_mode: self.cli_options.type_check_mode(),
          },
        )?;
        // ignore unused (type) parameter diagnostics that may occur due to fast
        // check not having function body implementations
        for result in diagnostics_by_folder.by_ref() {
          let check_diagnostics = result?;
          let check_diagnostics =
            check_diagnostics.filter(|d| d.include_when_remote());
          if check_diagnostics.has_diagnostic() {
            bail!(
              concat!(
                "Failed ensuring public API type output is valid.\n\n",
                "{:#}\n\n",
                "You may have discovered a bug in Deno. Please open an issue at: ",
                "https://github.com/denoland/deno/issues/"
              ),
              check_diagnostics
            );
          }
        }
        Ok(diagnostics_by_folder.into_graph())
      }
    }
  }

  async fn prepare_publish(
    &self,
    package: &JsrPackageConfig,

View on GitHub (pinned to f7822238ca)

Solutions

Upgrade Deno (`deno upgrade`) or pin back to the previous version — fast-check regressions are typically fixed in a patch release.
Adjust the exported constructs named in the attached diagnostics until the generated output type-checks.
If it persists, open an issue at https://github.com/denoland/deno/issues including the diagnostics and a minimal reproduction.

Defensive patterns

Strategy: try-catch

Try / catch

#!/usr/bin/env bash
out="$(deno publish --dry-run 2>&1)" || {
  if printf '%s' "$out" | grep -q 'You may have discovered a bug in Deno'; then
    echo "Deno fast-check bug suspected — pin the previous Deno version and report upstream" >&2
    exit 75  # dedicated code so pipelines can route it to an issue tracker
  fi
  printf '%s\n' "$out" >&2
  exit 1
}

Prevention

Pin the Deno version in CI (`denoland/setup-deno` with a fixed version) so a new fast-check regression cannot break releases unattended.
Keep exported entrypoints' TypeScript simple; re-test publishes with `--dry-run` when adopting complex exported types.
Capture full diagnostics in CI logs — they are the required content of the upstream issue.

When it happens

Trigger: The post-fast-check `TypeChecker` run yields diagnostics with `include_when_remote()` — e.g. exported constructs (complex overloads, declaration merging, inference-heavy generics) that Deno's public-API stripping renders invalid. `--allow-slow-types` does not bypass this check.

Common situations: Publishing packages with advanced TypeScript in exported entrypoints right after a Deno upgrade changed fast-check behavior; regressions introduced by a new Deno version.

Related errors

AI-assisted analysis of denoland/deno@f7822238ca (2026-08-20). Data as JSON: /api/errors/dc6a6a87dc18f8b3. Report an issue: GitHub.