Firecracker MMDS currently honors Accept and may return application/json, while EC2 IMDS always returns IMDS-format plain text and ignores Accept. Guest code that cannot control libraries sending Accept: application/json therefore breaks on MMDS even though it works on EC2.
Add a way to configure MMDS so it always responds with IMDS-format plain text regardless of Accept, while leaving today’s behavior as the default so existing workloads keep working. Expose the setting through PUT /mmds/config, omit-means-false, and ensure the choice is preserved across snapshot save/restore (users who rely on it will need regenerated snapshots). When the compat mode is on, responses for a given path should match the existing IMDS-format path (including unsupported-type errors for non-string/object values); when it is off, Accept-based JSON vs IMDS selection must remain unchanged.
Interface contract — the change must expose exactly this surface:
PUT /mmds/config accepts optional boolean `imds_compat` (serde default false if omitted). GET full VM config / snapshot MMDS state must round-trip that field.
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