/**
 * `${ENV_VAR}` substitution for MCP header values.
 *
 * SDK1 ergonomic — header values like `"Bearer ${HANDBOOK_API_TOKEN}"` are
 * resolved against `process.env` at connect time so consumers can author
 * static `mcpServers` configs (typically loaded from disk or embedded in
 * source) without committing secrets. Preserved verbatim to keep SDK1 →
 * SDK2 migration ergonomically equivalent for typical MCP-header
 * substitution use cases.
 *
 * Semantics:
 *   - `${VAR}` references `process.env.VAR`. Unset variables resolve to the
 *     empty string; this matches shell expansion (`echo "$UNSET"` → `""`)
 *     and SDK1's behavior.
 *   - `$VAR` (no braces) is NOT substituted — we keep substitution explicit
 *     so a literal `$200` in a header isn't accidentally interpreted as a
 *     reference to `process.env[200]`.
 *   - `\${VAR}` is an escape — emits the literal `${VAR}` without
 *     substitution. Useful for the rare case where the header value
 *     genuinely contains `${...}` literals.
 *
 * The implementation is platform-neutral: `process.env` is the only OS
 * touchpoint. Browsers don't run the MCP bridge (subprocess + arbitrary
 * fetch authority are server-side concerns), so a Node-only assumption is
 * fine; the browser shim falls through to identity behavior.
 */
/**
 * Substitute `${VAR}` references in a single string against `process.env`.
 * Returns the resolved string; unset references collapse to empty.
 *
 * `\${VAR}` is the escape form — it emits the literal `${VAR}` so callers
 * can carry through a value that contains the syntax verbatim.
 */
export declare function substituteEnvInString(value: string, env?: NodeJS.ProcessEnv): string;
/**
 * Substitute `${VAR}` references across every value in a string-keyed map.
 * Returns a fresh object; the input is not mutated. Keys are passed through
 * unchanged (header names aren't subject to env substitution).
 */
export declare function substituteEnvInHeaders(headers: {
    readonly [key: string]: string;
} | undefined, env?: NodeJS.ProcessEnv): {
    [key: string]: string;
} | undefined;
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