diff --git a/packages/compiler/ambient/scriptc-node-fallback.d.ts b/packages/compiler/ambient/scriptc-node-fallback.d.ts
index 2a3f11be9..c5fce2423 100644
--- a/packages/compiler/ambient/scriptc-node-fallback.d.ts
+++ b/packages/compiler/ambient/scriptc-node-fallback.d.ts
@@ -3512,7 +3512,8 @@ declare module "child_process" {
    * killSignal at the deadline and the result carries error: ETIMEDOUT +
    * the signal, never a throw), killSignal (a signal NAME literal),
    * stdio ("ignore"/"inherit"/"pipe" or a 3-tuple of those — captures
-   * read "" for non-piped streams), and windowsHide (a POSIX no-op). */
+   * read "" for non-piped streams), env (replaces the child's
+   * environment), and windowsHide (a POSIX no-op). */
   export function spawnSync(
     command: string,
     args?: string[],
@@ -3521,6 +3522,7 @@ declare module "child_process" {
       timeout?: number;
       killSignal?: string;
       stdio?: "pipe" | "ignore" | "inherit" | ("pipe" | "ignore" | "inherit")[];
+      env?: { [k: string]: string | undefined };
       windowsHide?: boolean;
     },
   ): SpawnSyncReturns;
diff --git a/packages/compiler/src/backend/llvm/lib-shared.ts b/packages/compiler/src/backend/llvm/lib-shared.ts
index 9a65d9a24..f18e4a7d5 100644
--- a/packages/compiler/src/backend/llvm/lib-shared.ts
+++ b/packages/compiler/src/backend/llvm/lib-shared.ts
@@ -754,6 +754,7 @@ export const LIB_FN_SYMS: Record<string, string> = {
   "cp.spawnSync": "scr_spawn_sync",
   "cp.spawnSyncOpts": "scr_spawn_sync_opts",
   "cp.spawnSyncStdioStr": "scr_spawn_sync_stdio_str",
+  "cp.spawnSyncEnv": "scr_spawn_sync_env",
   "cp.execSync": "scr_exec_sync",
   "cp.execCapture": "scr_exec_capture",
   "spawnRes.stdout": "scr_spawn_res_stdout",
diff --git a/packages/compiler/src/backend/llvm/reference-writes.ts b/packages/compiler/src/backend/llvm/reference-writes.ts
index d31f14d12..6d1593a55 100644
--- a/packages/compiler/src/backend/llvm/reference-writes.ts
+++ b/packages/compiler/src/backend/llvm/reference-writes.ts
@@ -118,6 +118,7 @@ const INERT_LIB_CALLS = new Set([
   "worker.parentPort",
   "worker.root",
   "cp.spawnSyncOpts",
+  "cp.spawnSyncEnv",
 ]);
 
 /** jsval conversions (ir/jsval.ts): no user code, no stores. */
diff --git a/packages/compiler/src/frontend/lowering/builtins/child-process.ts b/packages/compiler/src/frontend/lowering/builtins/child-process.ts
index ea2dd379d..9096316a3 100644
--- a/packages/compiler/src/frontend/lowering/builtins/child-process.ts
+++ b/packages/compiler/src/frontend/lowering/builtins/child-process.ts
@@ -115,6 +115,8 @@ export function lowerSpawnSyncCall(lowerer: Lowerer, expr: ts.CallExpression, lo
   // "pipe"`) — accepted when its TYPE proves every arm is a supported
   // literal; the runtime maps the value to the modes at the call.
   let stdioStr: IrExpr | null = null;
+  // The `env` option's [k,v,...] pairs, which replace the child's environment.
+  let envPairs: IrExpr | null = null;
   let plain = true; // no behavior-changing option: the historical libCall
 
   if (optsNode) {
@@ -227,11 +229,16 @@ export function lowerSpawnSyncCall(lowerer: Lowerer, expr: ts.CallExpression, lo
         case "windowsHide":
           lowerer.lowerExpr(m.value); // Node no-op on POSIX
           break;
+        case "env":
+          // A REPLACEMENT environment, as for spawn and execFileSync.
+          envPairs = recordToEnvPairs(lowerer, m.value);
+          plain = false;
+          break;
         default:
           lowerer.noLowering(
             `spawnSync option '${m.name}'`,
             p,
-            "encoding, timeout, killSignal, stdio, and windowsHide are the supported options",
+            "encoding, timeout, killSignal, stdio, env, and windowsHide are the supported options",
           );
       }
     }
@@ -239,6 +246,13 @@ export function lowerSpawnSyncCall(lowerer: Lowerer, expr: ts.CallExpression, lo
   if (plain) {
     return { kind: "libCall", fn: "cp.spawnSync", args: [cmd, argv], type: SPAWNRES_T, loc };
   }
+  if (stdioStr !== null && envPairs !== null) {
+    lowerer.noLowering(
+      "spawnSync with a runtime stdio value and env",
+      optsNode!,
+      'pass stdio as a "pipe"/"ignore"/"inherit" literal when env is set',
+    );
+  }
   if (stdioStr !== null) {
     return {
       kind: "libCall",
@@ -250,7 +264,7 @@ export function lowerSpawnSyncCall(lowerer: Lowerer, expr: ts.CallExpression, lo
   }
   return {
     kind: "libCall",
-    fn: "cp.spawnSyncOpts",
+    fn: envPairs !== null ? "cp.spawnSyncEnv" : "cp.spawnSyncOpts",
     args: [
       cmd,
       argv,
@@ -259,6 +273,7 @@ export function lowerSpawnSyncCall(lowerer: Lowerer, expr: ts.CallExpression, lo
       numLit(inMode, loc),
       numLit(outMode, loc),
       numLit(errMode, loc),
+      ...(envPairs !== null ? [envPairs] : []),
     ],
     type: SPAWNRES_T,
     loc,
diff --git a/packages/compiler/src/ir/builtin-signatures.ts b/packages/compiler/src/ir/builtin-signatures.ts
index fcdbe8694..6b7dc1741 100644
--- a/packages/compiler/src/ir/builtin-signatures.ts
+++ b/packages/compiler/src/ir/builtin-signatures.ts
@@ -1318,6 +1318,12 @@ export const LIB_FN_SIGS = defineLibFnSignatures({
     argTypes: [STRING, arrayOf(STRING), F64, STRING, F64, F64, F64],
     result: SPAWNRES_T,
   },
+  /** cp.spawnSyncOpts with the `env` option: the trailing [k,v,...]
+   * pairs replace the child's environment (recordToEnvPairs). */
+  "cp.spawnSyncEnv": {
+    argTypes: [STRING, arrayOf(STRING), F64, STRING, F64, F64, F64, arrayOf(STRING)],
+    result: SPAWNRES_T,
+  },
   /** cp.spawnSyncOpts with the stdio carried as a RUNTIME string —
    * "pipe" | "ignore" | "inherit", proven by the call site's TYPE (the
    * defaultRunner idiom `stdio: options?.stdio ?? "pipe"`); the runtime
diff --git a/packages/compiler/test/ts7/baselines/order-parity/tests/corpus/7030-spawn-sync-env.ts.json b/packages/compiler/test/ts7/baselines/order-parity/tests/corpus/7030-spawn-sync-env.ts.json
new file mode 100644
index 000000000..f8afbe00b
--- /dev/null
+++ b/packages/compiler/test/ts7/baselines/order-parity/tests/corpus/7030-spawn-sync-env.ts.json
@@ -0,0 +1,6 @@
+{
+ "order": [
+  "<repo>/tests/corpus/7030-spawn-sync-env.ts"
+ ],
+ "diags": []
+}
diff --git a/packages/runtime/src/scr_child.c b/packages/runtime/src/scr_child.c
index ec685656d..387885c42 100644
--- a/packages/runtime/src/scr_child.c
+++ b/packages/runtime/src/scr_child.c
@@ -1105,9 +1105,9 @@ char **scr_child_argv(ScrStr *cmd, ScrArr *args) {
 
 static ScrSpawnRes *scr_spawn_sync_core(ScrStr *cmd, ScrArr *args, double timeout_ms,
                                         int killsig, int in_mode, int out_mode,
-                                        int err_mode) {
+                                        int err_mode, ScrArr *env_pairs) {
   ScrWinSyncRes w;
-  scr_win_run_sync(cmd, args, NULL, in_mode, out_mode, err_mode, NULL, NULL,
+  scr_win_run_sync(cmd, args, NULL, in_mode, out_mode, err_mode, NULL, env_pairs,
                    timeout_ms, &w);
   if (w.spawn_errname != NULL) {
     free(w.out.data);
@@ -1136,19 +1136,26 @@ static ScrSpawnRes *scr_spawn_sync_core(ScrStr *cmd, ScrArr *args, double timeou
 }
 
 ScrSpawnRes *scr_spawn_sync(ScrStr *cmd, ScrArr *args) {
-  return scr_spawn_sync_core(cmd, args, 0, SIGTERM, 0, 0, 0);
+  return scr_spawn_sync_core(cmd, args, 0, SIGTERM, 0, 0, 0, NULL);
 }
 
 ScrSpawnRes *scr_spawn_sync_opts(ScrStr *cmd, ScrArr *args, double timeout_ms,
                                  ScrStr *killsignal, double in_mode,
                                  double out_mode, double err_mode) {
+  return scr_spawn_sync_env(cmd, args, timeout_ms, killsignal, in_mode, out_mode, err_mode,
+                            NULL);
+}
+
+ScrSpawnRes *scr_spawn_sync_env(ScrStr *cmd, ScrArr *args, double timeout_ms,
+                                ScrStr *killsignal, double in_mode, double out_mode,
+                                double err_mode, ScrArr *env_pairs) {
   int killsig = SIGTERM;
   if (killsignal->len > 0) {
     int resolved = scr_signal_from_name(killsignal);
     if (resolved > 0) killsig = resolved;
   }
   return scr_spawn_sync_core(cmd, args, timeout_ms, killsig, (int)in_mode,
-                             (int)out_mode, (int)err_mode);
+                             (int)out_mode, (int)err_mode, env_pairs);
 }
 
 ScrSpawnRes *scr_spawn_sync_stdio_str(ScrStr *cmd, ScrArr *args, double timeout_ms,
@@ -3101,9 +3108,14 @@ static ScrSpawnRes *scr_spawn_res_new(bool has_status, double status,
  * documented divergence). timeout_ms > 0 sends `killsig` at the deadline
  * and the result carries error: ETIMEDOUT + the signal — never a throw,
  * Node's spawnSync shape. */
+static char **scr_exec_envp(ScrArr *pairs);
+static void scr_exec_envp_free(char **envp);
+
+/* env_pairs: NULL inherits the parent's environment; else [k,v,...]
+ * replaces it, as Node's `env` option does. */
 static ScrSpawnRes *scr_spawn_sync_core(ScrStr *cmd, ScrArr *args, double timeout_ms,
                                          int killsig, int in_mode, int out_mode,
-                                         int err_mode) {
+                                         int err_mode, ScrArr *env_pairs) {
   if (out_mode == 2 || err_mode == 2) {
     /* Inherited output fds: flush the parent's buffers first so earlier
      * logs precede the child's writes (Node's practical ordering). */
@@ -3138,10 +3150,13 @@ static ScrSpawnRes *scr_spawn_sync_core(ScrStr *cmd, ScrArr *args, double timeou
   }
 
   char **argv = scr_child_argv(cmd, args);
+  char **envp = env_pairs != NULL ? scr_exec_envp(env_pairs) : NULL;
   pid_t pid = -1;
-  int spawn_err = posix_spawnp(&pid, cmd->data, &fa, NULL, argv, environ);
+  int spawn_err = posix_spawnp(&pid, cmd->data, &fa, NULL, argv,
+                               envp != NULL ? envp : environ);
   posix_spawn_file_actions_destroy(&fa);
   free(argv);
+  scr_exec_envp_free(envp);
   if (cap_out) close(outfd[1]);
   if (cap_err) close(errfd[1]);
 
@@ -3242,7 +3257,7 @@ static ScrSpawnRes *scr_spawn_sync_core(ScrStr *cmd, ScrArr *args, double timeou
 }
 
 ScrSpawnRes *scr_spawn_sync(ScrStr *cmd, ScrArr *args) {
-  return scr_spawn_sync_core(cmd, args, 0, SIGTERM, 0, 0, 0);
+  return scr_spawn_sync_core(cmd, args, 0, SIGTERM, 0, 0, 0, NULL);
 }
 
 /* The options entry (cp.spawnSyncOpts): killSignal as a NAME ("" = the
@@ -3252,13 +3267,20 @@ ScrSpawnRes *scr_spawn_sync(ScrStr *cmd, ScrArr *args) {
 ScrSpawnRes *scr_spawn_sync_opts(ScrStr *cmd, ScrArr *args, double timeout_ms,
                                   ScrStr *killsignal, double in_mode,
                                   double out_mode, double err_mode) {
+  return scr_spawn_sync_env(cmd, args, timeout_ms, killsignal, in_mode, out_mode, err_mode,
+                            NULL);
+}
+
+ScrSpawnRes *scr_spawn_sync_env(ScrStr *cmd, ScrArr *args, double timeout_ms,
+                                ScrStr *killsignal, double in_mode, double out_mode,
+                                double err_mode, ScrArr *env_pairs) {
   int killsig = SIGTERM;
   if (killsignal->len > 0) {
     int resolved = scr_signal_from_name(killsignal);
     if (resolved > 0) killsig = resolved;
   }
   return scr_spawn_sync_core(cmd, args, timeout_ms, killsig, (int)in_mode,
-                              (int)out_mode, (int)err_mode);
+                             (int)out_mode, (int)err_mode, env_pairs);
 }
 
 /* The runtime-string stdio entry (cp.spawnSyncStdioStr): the compiler
diff --git a/packages/runtime/src/scr_child.h b/packages/runtime/src/scr_child.h
index e899130d8..a5853ad59 100644
--- a/packages/runtime/src/scr_child.h
+++ b/packages/runtime/src/scr_child.h
@@ -23,6 +23,11 @@ ScrSpawnRes *scr_spawn_sync(ScrStr *cmd, ScrArr *args); /* +1, never throws */
 ScrSpawnRes *scr_spawn_sync_opts(ScrStr *cmd, ScrArr *args, double timeout_ms,
                                   ScrStr *killsignal, double in_mode,
                                   double out_mode, double err_mode);
+/* scr_spawn_sync_opts with an `env` option: [k,v,...] pairs replace the
+ * child's environment (NULL inherits the parent's). */
+ScrSpawnRes *scr_spawn_sync_env(ScrStr *cmd, ScrArr *args, double timeout_ms,
+                                ScrStr *killsignal, double in_mode, double out_mode,
+                                double err_mode, ScrArr *env_pairs);
 /* The runtime-string stdio entry: the compiler proved the value is
  * "pipe" | "ignore" | "inherit" by type; the mode mapping happens here. */
 ScrSpawnRes *scr_spawn_sync_stdio_str(ScrStr *cmd, ScrArr *args, double timeout_ms,
diff --git a/tests/corpus/7030-spawn-sync-env.ts b/tests/corpus/7030-spawn-sync-env.ts
new file mode 100644
index 000000000..e613c81f0
--- /dev/null
+++ b/tests/corpus/7030-spawn-sync-env.ts
@@ -0,0 +1,28 @@
+// spawnSync's env option replaces the child's environment, alone or with
+// the other options.
+import { spawnSync } from "node:child_process";
+
+const merged = spawnSync("sh", ["-c", 'echo "$GREETING $HOME_SET"'], {
+  env: { ...process.env, GREETING: "hello", HOME_SET: process.env.HOME ? "yes" : "no" },
+  encoding: "utf8",
+});
+console.log(merged.stdout.trim(), merged.status);
+
+const only = spawnSync("/bin/sh", ["-c", 'echo "${GREETING:-unset} ${HOME:-nohome}"'], {
+  env: { GREETING: "only" },
+  encoding: "utf8",
+});
+console.log(only.stdout.trim());
+
+const empty = spawnSync("/bin/sh", ["-c", "env | grep -c ."], { env: {}, encoding: "utf8" });
+console.log(Number(empty.stdout.trim()) <= 3, empty.status);
+
+const quiet = spawnSync("sh", ["-c", 'echo "$MODE" >&2; exit 4'], {
+  env: { PATH: process.env.PATH ?? "/usr/bin:/bin", MODE: "loud" },
+  stdio: ["ignore", "pipe", "pipe"],
+  timeout: 5000,
+});
+console.log(quiet.status, quiet.stderr.toString().trim(), quiet.stdout.toString() === "");
+
+const missing = spawnSync("/nonexistent/tool", [], { env: { A: "1" } });
+console.log(missing.status, missing.error?.message);
diff --git a/tests/harness/corpus-costs/7030-spawn-sync-env.ts.json b/tests/harness/corpus-costs/7030-spawn-sync-env.ts.json
new file mode 100644
index 000000000..cf8ef4d26
--- /dev/null
+++ b/tests/harness/corpus-costs/7030-spawn-sync-env.ts.json
@@ -0,0 +1 @@
+{ "plain": 2460, "san": 4060 }