diff --git a/packages/compiler/src/frontend/type-mapper.ts b/packages/compiler/src/frontend/type-mapper.ts
index 832857448..4c300d0d3 100644
--- a/packages/compiler/src/frontend/type-mapper.ts
+++ b/packages/compiler/src/frontend/type-mapper.ts
@@ -717,6 +717,12 @@ export class UnionRegistry {
     return id !== undefined && this.pendingRec.has(id);
   }
 
+  /** True when `unionId` is a recursive placeholder whose arms are not
+   * known yet (its outer frame is still mapping, or failed). */
+  isPendingPlaceholder(unionId: string): boolean {
+    return this.pendingRec.has(unionId);
+  }
+
   /** Completes a recursive placeholder with its canonical arm list and
    * registers the structural key (first writer wins, like shapes). */
   finalizeRecursive(t: ts.Type, arms: IrType[], discriminant?: IrUnionDiscriminant): string {
@@ -5335,8 +5341,19 @@ function mapCallSignature(sig: ts.Signature, ctx: TypeMapperCtx): IrType | null
     if (pt) pt = jsOpenObjectType(decl, pt, ctx.shapes, ctx.unions);
     // Belt and braces for non-strict type worlds: an optional param's
     // ABI slot is always the undefined-armed union (strictNullChecks
-    // already spells it that way; arm it here if the world didn't).
-    if (optional && pt && pt.kind !== "void" && pt.kind !== "dyn") {
+    // already spells it that way; arm it here if the world didn't). A
+    // back-reference to a union still being mapped (`options?: { then?:
+    // (ctx: Sub) => void }` where Sub extends the interface declaring the
+    // method) is a recursive placeholder with no arms yet; when the checker
+    // spelled the parameter `X | undefined`, the placeholder receives that
+    // undefined arm when its frame finishes.
+    const pendingOptional =
+      pt?.kind === "union" &&
+      ctx.unions.isPendingPlaceholder(pt.unionId) &&
+      ts
+        .constituentTypes(checker.getTypeOfSymbol(p))
+        .some((part) => (part.flags & ts.TypeFlags.Undefined) !== 0);
+    if (optional && pt && pt.kind !== "void" && pt.kind !== "dyn" && !pendingOptional) {
       const armed = withUndefinedArm(pt, ctx.unions);
       if (!armed) return null;
       pt = armed;
diff --git a/packages/compiler/test/ts7/baselines/order-parity/tests/corpus/recursive-optional-callback-params.ts.json b/packages/compiler/test/ts7/baselines/order-parity/tests/corpus/recursive-optional-callback-params.ts.json
new file mode 100644
index 000000000..abf783f85
--- /dev/null
+++ b/packages/compiler/test/ts7/baselines/order-parity/tests/corpus/recursive-optional-callback-params.ts.json
@@ -0,0 +1,6 @@
+{
+ "order": [
+  "<repo>/tests/corpus/recursive-optional-callback-params.ts"
+ ],
+ "diags": []
+}
diff --git a/tests/corpus/recursive-optional-callback-params.ts b/tests/corpus/recursive-optional-callback-params.ts
new file mode 100644
index 000000000..0c81235d6
--- /dev/null
+++ b/tests/corpus/recursive-optional-callback-params.ts
@@ -0,0 +1,107 @@
+// An optional parameter whose type reaches back to the interface that
+// declares the method keeps its whole union. A command context's
+// `newSession(options?)` takes callbacks typed by a subinterface of the
+// context, so mapping the context passes through the options union while
+// that union is still being mapped; the parameter still holds the options
+// record, not only `undefined`.
+
+interface CommandContext {
+  cwd: string;
+  newSession(options?: {
+    parent?: string;
+    withSession?: (ctx: ReplacedContext) => Promise<void>;
+  }): Promise<{ cancelled: boolean }>;
+  fork(
+    entryId: string,
+    options?: { position?: "before" | "at"; withSession?: (ctx: ReplacedContext) => Promise<void> },
+  ): Promise<{ cancelled: boolean }>;
+}
+
+interface ReplacedContext extends CommandContext {
+  depth: number;
+  send(content: string): Promise<void>;
+}
+
+interface Command {
+  description?: string;
+  handler: (args: string, ctx: CommandContext) => Promise<void>;
+}
+
+const log: string[] = [];
+const commands = new Map<string, Command>();
+
+function register(name: string, command: Command): void {
+  commands.set(name, command);
+}
+
+register("new", {
+  description: "start a session",
+  handler: async (args, ctx) => {
+    const first = await ctx.newSession({
+      parent: args,
+      withSession: async (next) => {
+        await next.send(`hello from ${next.cwd} at depth ${next.depth}`);
+        const nested = await next.newSession({
+          withSession: async (inner) => {
+            await inner.send(`nested at depth ${inner.depth}`);
+          },
+        });
+        log.push(`nested cancelled=${nested.cancelled}`);
+      },
+    });
+    log.push(`first cancelled=${first.cancelled}`);
+    const bare = await ctx.newSession();
+    log.push(`bare cancelled=${bare.cancelled}`);
+  },
+});
+
+register("fork", {
+  handler: async (args, ctx) => {
+    const r = await ctx.fork(args, {
+      position: "at",
+      withSession: async (next) => next.send(`forked ${args} at depth ${next.depth}`),
+    });
+    log.push(`fork cancelled=${r.cancelled}`);
+    const plain = await ctx.fork(args);
+    log.push(`plain fork cancelled=${plain.cancelled}`);
+  },
+});
+
+function makeContext(depth: number): ReplacedContext {
+  return {
+    cwd: `/work/${depth}`,
+    depth,
+    newSession: async (options) => {
+      log.push(`newSession depth=${depth} parent=${options?.parent ?? "-"}`);
+      if (options?.withSession) {
+        await options.withSession(makeContext(depth + 1));
+        return { cancelled: false };
+      }
+      return { cancelled: true };
+    },
+    fork: async (entryId, options) => {
+      log.push(`fork ${entryId} position=${options?.position ?? "-"}`);
+      const callback = options?.withSession;
+      if (callback === undefined) return { cancelled: true };
+      await callback(makeContext(depth + 1));
+      return { cancelled: false };
+    },
+    send: async (content) => {
+      log.push(`send: ${content}`);
+    },
+  };
+}
+
+async function run(): Promise<void> {
+  const root: CommandContext = makeContext(0);
+  for (const [name, command] of commands) {
+    log.push(`run ${name} (${command.description ?? "no description"})`);
+    await command.handler(`entry-${name}`, root);
+  }
+  for (const line of log) console.log(line);
+}
+
+run().catch((e: unknown) => {
+  console.log("failed", e instanceof Error ? e.message : String(e));
+  process.exitCode = 1;
+});
diff --git a/tests/harness/corpus-costs/recursive-optional-callback-params.ts.json b/tests/harness/corpus-costs/recursive-optional-callback-params.ts.json
new file mode 100644
index 000000000..c33a4b32c
--- /dev/null
+++ b/tests/harness/corpus-costs/recursive-optional-callback-params.ts.json
@@ -0,0 +1 @@
+{ "plain": 6000 }