What causes "MCH file failed to be generated at: %s" in dotnet/runtime?

Re-run with -log_level debug to capture the mcs command output and return code.

dotnet/runtime · error · RuntimeError

MCH file failed to be generated at

Error message

MCH file failed to be generated at: %s

What it means

Raised by SuperPMICollect.__merge_mc_files__ in superpmi.py after invoking `mcs -merge ... *.mc`. It checks os.path.isfile(self.base_mch_file) (the 'base.mch' output of mcs); if mcs failed to produce it, the merge did not succeed and downstream steps cannot run.

Solutions

Re-run with -log_level debug to capture the mcs command output and return code.
Verify mcs_path resolves to a working mcs built for this JIT/EE interface version.
Free disk space / check write permissions in temp_location; avoid reusing a temp_dir from a failed run without --clean.
If a specific .mc is corrupt, isolate and exclude it, or re-collect.

Example fix

// before
python superpmi.py collect 'cmd'   # merge produced no base.mch -> 'MCH file failed to be generated at: .../base.mch'
// after
python superpmi.py collect 'cmd' -log_level debug   # inspect mcs failure, fix mcs/disk, then re-run

Defensive patterns

Strategy: validation

Validate before calling

import os
if not os.path.isfile(base_mch_file):
    raise SystemExit(f"mcs -merge did not produce {base_mch_file}; check mcs tool and inputs")

Prevention

Ensure mcs is built for the same JIT/EE interface version as the collected .mc files.
Run with -log_level debug to capture the mcs command and its output for diagnosis.

When it happens

Trigger: The `mcs -merge -recursive -dedup -thin base.mch *.mc` command failed (non-zero, crashed, or wrote no output), so base.mch does not exist after the run.

Common situations: mcs tool missing or wrong version; mcs hit an internal error on malformed .mc files; out of disk space in temp; permissions issue; a stale temp_dir reused incorrectly.

Related errors

AI-assisted analysis of dotnet/runtime@60108ba66e (2026-08-10). Data as JSON: /api/errors/0e7387e52bca558d. Report an issue: GitHub.

Appendix: source

Thrown at src/coreclr/scripts/superpmi.py:1368

            raise RuntimeError("No .mc files generated.")

    def __merge_mc_files__(self):
        """ Merge the mc files that were generated

        Notes:
            mcs -merge <s_baseMchFile> <s_tempDir>\\*.mc -recursive -dedup -thin

        """

        logging.info("Merging MC files")

        pattern = os.path.join(self.temp_location, "*.mc")

        command = [self.mcs_path, "-merge", self.base_mch_file, pattern, "-recursive", "-dedup", "-thin"]
        run_and_log(command)

        if not os.path.isfile(self.base_mch_file):
            raise RuntimeError("MCH file failed to be generated at: %s" % self.base_mch_file)

        # All the individual MC files are no longer necessary, now that we have
        # merged them into the base.mch. Delete them.
        if not self.coreclr_args.skip_cleanup:
            mc_files = [os.path.join(self.temp_location, item) for item in os.listdir(self.temp_location) if item.endswith(".mc")]
            for item in mc_files:
                os.remove(item)

    def __merge_mch_files__(self):
        """ Merge MCH files in the mch_files list. This is only used with the `--merge_mch_files` argument.

        Notes:
            mcs -concat <s_baseMchFile> [self.coreclr_args.mch_files]

        """

        logging.info("Merging MCH files")

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