diff --git a/src/codec.ts b/src/codec.ts
index 6e8fbe1..b7833ae 100644
--- a/src/codec.ts
+++ b/src/codec.ts
@@ -53,5 +53,8 @@ export interface GeneratedRangeStartItem {
   flags: number;
   line?: number;
   column: number;
-  definitionIdx?: number;
+  definition?: {
+    sourceIdx: number;
+    scopeIdx: number;
+  };
 }
diff --git a/src/decode/decode.test.ts b/src/decode/decode.test.ts
index a2a8421..a819cb1 100644
--- a/src/decode/decode.test.ts
+++ b/src/decode/decode.test.ts
@@ -410,14 +410,78 @@ describe("decode", () => {
     assertEquals(info.ranges[0]?.values, [""]);
   });
 
+  it("handles trailing VLQs in GENERATED_RANGE_START items", () => {
+    const info = new ScopeInfoBuilder().startRange(0, 0).endRange(10, 0)
+      .build();
+    const map = encode(info);
+
+    assertExists(map.scopes);
+
+    const parts = map.scopes.split(",");
+    parts[0] += encodeUnsigned(42);
+    parts[0] += encodeSigned(-16);
+    map.scopes = parts.join(",");
+
+    assertEquals(decode(map), { ...info, hasVariableAndBindingInfo: false });
+  });
+
+  it("handles trailing VLQs in GENERATED_RANGE_CALL_SITE items", () => {
+    const info = new ScopeInfoBuilder().startSource().startScope(0, 0, {
+      key: "fn",
+    }).endScope(10, 0).endSource().startRange(0, 0, {
+      scopeKey: "fn",
+      callSite: { sourceIndex: 0, line: 2, column: 4 },
+    }).endRange(10, 0).build();
+    const map = encode(info);
+
+    assertExists(map.scopes);
+
+    const parts = map.scopes.split(",");
+    parts[3] += encodeUnsigned(42);
+    parts[3] += encodeSigned(-16);
+    map.scopes = parts.join(",");
+
+    assertEquals(decode(map), { ...info, hasVariableAndBindingInfo: false });
+  });
+
+  it("throws if GENERATED_RANGE_START.definition has an invalid source index in strict mode", () => {
+    const encoder = new ItemEncoder();
+    encoder.addUnsignedVLQs(
+      Tag.GENERATED_RANGE_START,
+      GeneratedRangeFlags.HAS_DEFINITION,
+      0,
+    ).addSignedVLQs(1).addUnsignedVLQs(0).finishItem();
+    encoder.addUnsignedVLQs(Tag.GENERATED_RANGE_END, 2).finishItem();
+    const map = createMap(encoder.encode(), []);
+
+    assertThrows(() => decode(map, { mode: DecodeMode.STRICT }));
+  });
+
+  it("ignores if GENERATED_RANGE_START.definition has an invalid source index in lax mode", () => {
+    const encoder = new ItemEncoder();
+    encoder.addUnsignedVLQs(
+      Tag.GENERATED_RANGE_START,
+      GeneratedRangeFlags.HAS_DEFINITION,
+      0,
+    ).addSignedVLQs(1).addUnsignedVLQs(0).finishItem();
+    encoder.addUnsignedVLQs(Tag.GENERATED_RANGE_END, 2).finishItem();
+    const map = createMap(encoder.encode(), []);
+
+    const info = decode(map, { mode: DecodeMode.LAX });
+
+    assertExists(info.ranges[0]);
+    assertStrictEquals(info.ranges[0].originalScope, undefined);
+  });
+
   it("throws if GENERATED_RANGE_START.definition is not a valid original scope in strict mode", () => {
     const encoder = new ItemEncoder();
+    encoder.addUnsignedVLQs(Tag.ORIGINAL_SCOPE_START, 0, 0, 0).finishItem();
+    encoder.addUnsignedVLQs(Tag.ORIGINAL_SCOPE_END, 1, 0).finishItem();
     encoder.addUnsignedVLQs(
       Tag.GENERATED_RANGE_START,
       GeneratedRangeFlags.HAS_DEFINITION,
       0,
-      1,
-    ).finishItem();
+    ).addSignedVLQs(0, 1).finishItem();
     encoder.addUnsignedVLQs(Tag.GENERATED_RANGE_END, 2).finishItem();
     const map = createMap(encoder.encode(), []);
 
@@ -426,12 +490,13 @@ describe("decode", () => {
 
   it("ignores if GENERATED_RANGE_START.definition is not a valid original scope in lax mode", () => {
     const encoder = new ItemEncoder();
+    encoder.addUnsignedVLQs(Tag.ORIGINAL_SCOPE_START, 0, 0, 0).finishItem();
+    encoder.addUnsignedVLQs(Tag.ORIGINAL_SCOPE_END, 1, 0).finishItem();
     encoder.addUnsignedVLQs(
       Tag.GENERATED_RANGE_START,
       GeneratedRangeFlags.HAS_DEFINITION,
       0,
-      1,
-    ).finishItem();
+    ).addSignedVLQs(0, 1).finishItem();
     encoder.addUnsignedVLQs(Tag.GENERATED_RANGE_END, 2).finishItem();
     const map = createMap(encoder.encode(), []);
 
@@ -441,6 +506,34 @@ describe("decode", () => {
     assertStrictEquals(info.ranges[0].originalScope, undefined);
   });
 
+  it("decodes cross-source and same-source GENERATED_RANGE_START.definition indices", () => {
+    const scopes = new ScopeInfoBuilder()
+      .startSource()
+      .startScope(0, 0, { key: "s0_root" })
+      .startScope(1, 0, { key: "s0_child" })
+      .endScope(2, 0)
+      .endScope(3, 0)
+      .endSource()
+      .startSource()
+      .startScope(0, 0, { key: "s1_root" })
+      .startScope(1, 0, { key: "s1_child" })
+      .endScope(2, 0)
+      .endScope(3, 0)
+      .endSource()
+      .startRange(0, 0, { scopeKey: "s1_child" })
+      .startRange(0, 2, { scopeKey: "s1_root" })
+      .endRange(0, 4)
+      .startRange(0, 5, { scopeKey: "s0_child" })
+      .endRange(0, 8)
+      .endRange(0, 10)
+      .build();
+
+    const map = encode(scopes);
+    const decoded = decode(map, { mode: DecodeMode.STRICT });
+
+    assertEquals(decoded, { ...scopes, hasVariableAndBindingInfo: false });
+  });
+
   it("throws for free GENERATED_RANGE_CALL_SITE items in strict mode", () => {
     const encoder = new ItemEncoder();
     encoder.addUnsignedVLQs(Tag.GENERATED_RANGE_CALL_SITE);
@@ -492,8 +585,7 @@ describe("decode", () => {
       Tag.GENERATED_RANGE_START,
       GeneratedRangeFlags.HAS_DEFINITION,
       0,
-      0,
-    ).addSignedVLQs(0).finishItem();
+    ).addSignedVLQs(0, 0).finishItem();
     // Initial binding for the variable is "bar" (index 2).
     encoder.addUnsignedVLQs(Tag.GENERATED_RANGE_BINDINGS, 2).finishItem();
 
diff --git a/src/decode/decode.ts b/src/decode/decode.ts
index 6339401..ccd84e3 100644
--- a/src/decode/decode.ts
+++ b/src/decode/decode.ts
@@ -116,6 +116,7 @@ const DEFAULT_SCOPE_STATE = {
 const DEFAULT_RANGE_STATE = {
   line: 0,
   column: 0,
+  defSourceIdx: 0,
   defScopeIdx: 0,
 };
 
@@ -133,7 +134,8 @@ class Decoder {
   readonly #scopeStack: OriginalScope[] = [];
   readonly #rangeStack: GeneratedRange[] = [];
 
-  #flatOriginalScopes: OriginalScope[] = [];
+  #flatOriginalScopes: (OriginalScope[] | null)[] = [];
+  #currentFlatScopes: OriginalScope[] = [];
   #subRangeBindingsForRange = new Map<
     GeneratedRange,
     Map<number, [number, number, number][]>
@@ -161,6 +163,7 @@ class Decoder {
       switch (tag) {
         case Tag.EMPTY: {
           this.#scopes.push(null);
+          this.#flatOriginalScopes.push(null);
           break;
         }
         case Tag.ORIGINAL_SCOPE_START: {
@@ -205,15 +208,20 @@ class Decoder {
             : undefined;
           const column = iter.nextUnsignedVLQ();
 
-          const definitionIdx = flags & GeneratedRangeFlags.HAS_DEFINITION
-            ? iter.nextSignedVLQ()
-            : undefined;
+          let definition: { sourceIdx: number; scopeIdx: number } | undefined;
+          if (flags & GeneratedRangeFlags.HAS_DEFINITION) {
+            const sourceIdx = iter.nextSignedVLQ();
+            const scopeIdx = sourceIdx === 0
+              ? iter.nextSignedVLQ()
+              : iter.nextUnsignedVLQ();
+            definition = { sourceIdx, scopeIdx };
+          }
 
           this.#handleGeneratedRangeStartItem({
             flags,
             line,
             column,
-            definitionIdx,
+            definition,
           });
           break;
         }
@@ -301,6 +309,7 @@ class Decoder {
     this.#scopes = [];
     this.#ranges = [];
     this.#flatOriginalScopes = [];
+    this.#currentFlatScopes = [];
     this.#seenOriginalScopeVariables = false;
     this.#seenGeneratedRangeBindings = false;
 
@@ -340,7 +349,7 @@ class Decoder {
     );
 
     this.#scopeStack.push(scope);
-    this.#flatOriginalScopes.push(scope);
+    this.#currentFlatScopes.push(scope);
   }
 
   #handleOriginalScopeVariablesItem(variableIdxs: number[]) {
@@ -385,6 +394,8 @@ class Decoder {
       parent.children.push(scope);
     } else {
       this.#scopes.push([scope]);
+      this.#flatOriginalScopes.push(this.#currentFlatScopes);
+      this.#currentFlatScopes = [];
       this.#scopeState.line = 0;
       this.#scopeState.column = 0;
     }
@@ -415,16 +426,20 @@ class Decoder {
       children: [],
     };
 
-    if (item.definitionIdx !== undefined) {
-      this.#rangeState.defScopeIdx += item.definitionIdx;
-      if (
-        this.#rangeState.defScopeIdx < 0 ||
-        this.#rangeState.defScopeIdx >= this.#flatOriginalScopes.length
-      ) {
+    if (item.definition !== undefined) {
+      this.#rangeState.defSourceIdx += item.definition.sourceIdx;
+      if (item.definition.sourceIdx !== 0) {
+        this.#rangeState.defScopeIdx = 0;
+      }
+      this.#rangeState.defScopeIdx += item.definition.scopeIdx;
+
+      const originalScope = this
+        .#flatOriginalScopes[this.#rangeState.defSourceIdx]
+        ?.[this.#rangeState.defScopeIdx];
+      if (!originalScope) {
         this.#throwInStrictMode("Invalid definition scope index");
       } else {
-        range.originalScope =
-          this.#flatOriginalScopes[this.#rangeState.defScopeIdx];
+        range.originalScope = originalScope;
       }
     }
 
diff --git a/src/encode/encoder.ts b/src/encode/encoder.ts
index 30e6397..6c8c914 100644
--- a/src/encode/encoder.ts
+++ b/src/encode/encoder.ts
@@ -22,6 +22,7 @@ const DEFAULT_SCOPE_STATE = {
 const DEFAULT_RANGE_STATE = {
   line: 0,
   column: 0,
+  defSourceIdx: 0,
   defScopeIdx: 0,
 };
 
@@ -38,7 +39,11 @@ export class Encoder {
   #encodedItems: string[] = [];
   #currentItem: string = "";
 
-  #scopeToCount = new Map<OriginalScope, number>();
+  #scopeToLocation = new Map<
+    OriginalScope,
+    { sourceIdx: number; scopeIdx: number }
+  >();
+  #currentSourceIdx = 0;
   #scopeCounter = 0;
 
   constructor(info: ScopeInfo, names: string[]) {
@@ -52,11 +57,13 @@ export class Encoder {
 
   encode(): string {
     this.#encodedItems = [];
-    this.#info.scopes.forEach((scopes) => {
+    this.#info.scopes.forEach((scopes, sourceIdx) => {
       if (scopes === null) {
         this.#encodedItems.push(EncodedTag.EMPTY);
         return;
       }
+      this.#currentSourceIdx = sourceIdx;
+      this.#scopeCounter = 0;
       this.#scopeState.line = 0;
       this.#scopeState.column = 0;
       scopes.forEach((scope) => this.#encodeOriginalScope(scope));
@@ -111,7 +118,10 @@ export class Encoder {
     if (encodedKind !== undefined) this.#encodeSigned(encodedKind);
     this.#finishItem();
 
-    this.#scopeToCount.set(scope, this.#scopeCounter++);
+    this.#scopeToLocation.set(scope, {
+      sourceIdx: this.#currentSourceIdx,
+      scopeIdx: this.#scopeCounter++,
+    });
   }
 
   #encodeOriginalScopeVariables(scope: OriginalScope) {
@@ -168,17 +178,23 @@ export class Encoder {
     this.#rangeState.line = line;
     this.#rangeState.column = column;
 
-    let encodedDefinition;
+    let encodedDefSourceIdx: number | undefined;
+    let encodedDefScopeIdx: number | undefined;
     if (range.originalScope) {
-      const definitionIdx = this.#scopeToCount.get(range.originalScope);
-      if (definitionIdx === undefined) {
+      const location = this.#scopeToLocation.get(range.originalScope);
+      if (location === undefined) {
         throw new Error("Unknown OriginalScope for definition!");
       }
 
       flags |= GeneratedRangeFlags.HAS_DEFINITION;
 
-      encodedDefinition = definitionIdx - this.#rangeState.defScopeIdx;
-      this.#rangeState.defScopeIdx = definitionIdx;
+      encodedDefSourceIdx = location.sourceIdx - this.#rangeState.defSourceIdx;
+      this.#rangeState.defSourceIdx = location.sourceIdx;
+
+      encodedDefScopeIdx = encodedDefSourceIdx === 0
+        ? location.scopeIdx - this.#rangeState.defScopeIdx
+        : location.scopeIdx;
+      this.#rangeState.defScopeIdx = location.scopeIdx;
     }
 
     if (range.isStackFrame) flags |= GeneratedRangeFlags.IS_STACK_FRAME;
@@ -187,7 +203,16 @@ export class Encoder {
     this.#encodeTag(EncodedTag.GENERATED_RANGE_START).#encodeUnsigned(flags);
     if (encodedLine > 0) this.#encodeUnsigned(encodedLine);
     this.#encodeUnsigned(encodedColumn);
-    if (encodedDefinition !== undefined) this.#encodeSigned(encodedDefinition);
+    if (
+      encodedDefSourceIdx !== undefined && encodedDefScopeIdx !== undefined
+    ) {
+      this.#encodeSigned(encodedDefSourceIdx);
+      if (encodedDefSourceIdx === 0) {
+        this.#encodeSigned(encodedDefScopeIdx);
+      } else {
+        this.#encodeUnsigned(encodedDefScopeIdx);
+      }
+    }
     this.#finishItem();
   }