diff --git a/src/code/providers/interactive-api-provider.test.ts b/src/code/providers/interactive-api-provider.test.ts
index d4d61e66..c14cc486 100644
--- a/src/code/providers/interactive-api-provider.test.ts
+++ b/src/code/providers/interactive-api-provider.test.ts
@@ -730,6 +730,13 @@ describe('InteractiveApiProvider', () => {
     expect(shouldSaveAsAttachment(contentAtThreshold)).toBe(true)
     expect(shouldSaveAsAttachment(contentAboveThreshold)).toBe(true)
 
+    // CFM-18 / spec R6: the size check measures UTF-8 bytes, not UTF-16 code units.
+    // A run of multi-byte characters can stay below the threshold by String length
+    // yet exceed it - and Firestore's byte-based 1 MiB limit - once UTF-8 encoded.
+    const multiByteContent = "€".repeat(kDynamicAttachmentSizeThreshold / 2)  // "€" is 1 code unit, 3 UTF-8 bytes
+    expect(JSON.stringify(multiByteContent).length).toBeLessThan(kDynamicAttachmentSizeThreshold)
+    expect(shouldSaveAsAttachment(multiByteContent)).toBe(true)
+
     // with an explicit kAttachmentUrlParameter value attachments are always used
     setQueryParams(`interactiveApi=${kAttachmentUrlParameter}`)
     expect(shouldSaveAsAttachment(contentBelowThreshold)).toBe(true)
diff --git a/src/code/providers/interactive-api-provider.ts b/src/code/providers/interactive-api-provider.ts
index 57402b81..5322b56f 100644
--- a/src/code/providers/interactive-api-provider.ts
+++ b/src/code/providers/interactive-api-provider.ts
@@ -20,7 +20,11 @@ export const shouldSaveAsAttachment = (content: any) => {
     return true
   }
 
-  const aboveDynamicThreshold = JSON.stringify(content).length >= kDynamicAttachmentSizeThreshold
+  // Measure the UTF-8 byte length, not String.prototype.length: the latter counts
+  // UTF-16 code units and undercounts non-ASCII content, whereas Firestore's 1 MiB
+  // document limit — and kDynamicAttachmentSizeThreshold — are byte-based. (spec R6)
+  const serializedBytes = new TextEncoder().encode(JSON.stringify(content)).byteLength
+  const aboveDynamicThreshold = serializedBytes >= kDynamicAttachmentSizeThreshold
   if (aboveDynamicThreshold) {
     return true
   }
@@ -61,8 +65,19 @@ interface InteractiveApiProviderParams {
 // pass `interactiveApi=attachment` as url parameter to always save state as an attachment
 export const kAttachmentUrlParameter = "attachment"
 
-// can save it twice with room to spare in 1MB Firestore limit
-export const kDynamicAttachmentSizeThreshold = 480 * 1024
+// Interactive state at or above this size is offloaded to an S3 attachment;
+// smaller state is saved directly into the Firestore answer document.
+//
+// CFM-18: this was 480 KiB, justified as "can save it twice with room to spare
+// in the 1MB Firestore limit" (480 * 2 = 960 KiB). That math was wrong. Activity
+// Player stores the interactive state twice in one answer doc (`answer` +
+// `report_state`), but it embeds each copy as an escaped JSON *string* inside a
+// `report` wrapper alongside `attachments` and other fields. Measured expansion
+// is ~2.2 bytes of Firestore document per char of interactive state, so a state
+// near 480 KiB produced a ~1.06 MB document that Firestore rejected.
+// The 1 MiB (1,048,576 byte) limit / 2.2 ≈ 465 KiB is the hard ceiling;
+// 400 KiB leaves margin for escaping-density variation and wrapper overhead.
+export const kDynamicAttachmentSizeThreshold = 400 * 1024
 
 // in solidarity with legacy DocumentStore implementation and S3 sharing implementation
 export const kLegacyAttachmentFilename = "file.json"
diff --git a/src/test/setupTests.ts b/src/test/setupTests.ts
index 298e39a1..a6e32d76 100644
--- a/src/test/setupTests.ts
+++ b/src/test/setupTests.ts
@@ -16,6 +16,10 @@ g.createReactClassFactory = (classDef: any) => g.createReactFactory(g.createReac
 // providers use window.alert() (which isn't implemented in JSDom) to signal unimplemented methods
 window.alert = jest.fn(msg => console.error(msg))
 
+// TextEncoder is a standard browser global but isn't exposed by the jsdom test
+// environment; polyfill it from Node's util module so code under test can use it.
+g.TextEncoder = g.TextEncoder ?? require('util').TextEncoder
+
 declare global {
   function jestSpyConsole(method: ConsoleMethod, fn: JestSpyConsoleFn,
                           options?: IJestSpyConsoleOptions): Promise<jest.SpyInstance>