diff --git a/src/browser/WebSourceMapFetcher.ts b/src/browser/WebSourceMapFetcher.ts
index 47aa00aa6c..3e6dbe7e44 100644
--- a/src/browser/WebSourceMapFetcher.ts
+++ b/src/browser/WebSourceMapFetcher.ts
@@ -1,8 +1,5 @@
 import * as t from "@babel/types";
-import {
-  decodeValidatedSourceMapLeaves,
-  isBeforeSourceMapLeafStop,
-} from "../javascript/sourceMaps/DecodedSourceMap.js";
+import { decodeValidatedSourceMapLeaves } from "../javascript/sourceMaps/DecodedSourceMap.js";
 
 import { sanitizeBrowserUrl } from "../domain/browserObservation.js";
 import { AnalysisResourceConstraintError } from "../domain/analysisErrorCore.js";
@@ -653,7 +650,6 @@ const normalizeSourceMap = (
       for (const [line, row] of rows.entries()) {
         if ((line & 255) === 0) check();
         for (const segment of row) {
-          if (!isBeforeSourceMapLeafStop(leaf, line, segment[0])) continue;
           if (segment.length === 1) continue;
           const rawSource = leaf.map.sources[segment[1]];
           if (rawSource === undefined) continue;
@@ -741,7 +737,10 @@ const normalizeSourceMap = (
     return emptySourceMapItem(
       request,
       "invalid",
-      "Source-map JSON mappings could not be decoded safely.",
+      cause instanceof SourceMapFormatFailure &&
+        cause.reason === "section-overlap"
+        ? cause.message
+        : "Source-map JSON mappings could not be decoded safely.",
       fetchedUrl,
     );
   }
diff --git a/src/javascript/sourceMaps/DecodedSourceMap.ts b/src/javascript/sourceMaps/DecodedSourceMap.ts
index e9ed304e1a..cf431d6329 100644
--- a/src/javascript/sourceMaps/DecodedSourceMap.ts
+++ b/src/javascript/sourceMaps/DecodedSourceMap.ts
@@ -5,7 +5,7 @@ import {
 } from "./SourceMapFormat.js";
 
 /** Determine whether a leaf-local mapping falls before its enclosing boundary. */
-export const isBeforeSourceMapLeafStop = (
+const isBeforeSourceMapLeafStop = (
   leaf: SourceMapLeaf,
   line: number,
   column: number,
@@ -40,6 +40,11 @@ export const decodeValidatedSourceMapLeaves = (
         const generatedLine = leaf.offset.line + line;
         const generatedColumn =
           segment[0] + (line === 0 ? leaf.offset.column : 0);
+        if (!isBeforeSourceMapLeafStop(leaf, line, segment[0]))
+          throw new SourceMapFormatFailure(
+            "section-overlap",
+            "Embedded source-map mappings overlap the following section boundary.",
+          );
         if (
           !Number.isSafeInteger(generatedLine) ||
           !Number.isSafeInteger(generatedColumn) ||
diff --git a/src/javascript/sourceMaps/SourceMapCodecProcess.ts b/src/javascript/sourceMaps/SourceMapCodecProcess.ts
index cad362c57e..8cedb0a508 100644
--- a/src/javascript/sourceMaps/SourceMapCodecProcess.ts
+++ b/src/javascript/sourceMaps/SourceMapCodecProcess.ts
@@ -50,7 +50,11 @@ const execute = async (): Promise<string> => {
       throw cause;
     return JSON.stringify({
       state: "failure",
-      reason: cause instanceof SourceMapFormatFailure ? cause.reason : "format",
+      reason:
+        cause instanceof SourceMapFormatFailure &&
+        cause.reason !== "section-overlap"
+          ? cause.reason
+          : "format",
       message: cause.message,
     });
   }
diff --git a/src/javascript/sourceMaps/SourceMapFormat.ts b/src/javascript/sourceMaps/SourceMapFormat.ts
index 1db4aed60c..64579c63b1 100644
--- a/src/javascript/sourceMaps/SourceMapFormat.ts
+++ b/src/javascript/sourceMaps/SourceMapFormat.ts
@@ -55,7 +55,7 @@ export interface SourceMapStructureOptions {
 /** A decoder-profile failure with an actionable producer constraint. */
 export class SourceMapFormatFailure extends Error {
   constructor(
-    readonly reason: "format" | "unsupported" | "limit",
+    readonly reason: "format" | "unsupported" | "limit" | "section-overlap",
     message: string,
   ) {
     super(message);
diff --git a/src/javascript/sourceMaps/TraceSourceMap.test.ts b/src/javascript/sourceMaps/TraceSourceMap.test.ts
index d2ca915fd5..51377e7ec5 100644
--- a/src/javascript/sourceMaps/TraceSourceMap.test.ts
+++ b/src/javascript/sourceMaps/TraceSourceMap.test.ts
@@ -310,7 +310,7 @@ describe("source-map layout admission", () => {
           { offset: { line: 0, column: 5 }, map: leaf },
         ],
       }),
-    ).toThrow(/overlap/u);
+    ).toThrow(expect.objectContaining({ reason: "section-overlap" }));
   });
   it("accepts absent optional names and null contents without changing reported identity", () => {
     expect(
diff --git a/src/javascript/sourceMaps/TraceSourceMap.ts b/src/javascript/sourceMaps/TraceSourceMap.ts
index d85f79b36a..56c188e95d 100644
--- a/src/javascript/sourceMaps/TraceSourceMap.ts
+++ b/src/javascript/sourceMaps/TraceSourceMap.ts
@@ -22,10 +22,7 @@ import {
   SourceMapFormatFailure,
   type SourceMapSourceDeclaration,
 } from "./SourceMapFormat.js";
-import {
-  decodeValidatedSourceMapLeaves,
-  isBeforeSourceMapLeafStop,
-} from "./DecodedSourceMap.js";
+import { decodeValidatedSourceMapLeaves } from "./DecodedSourceMap.js";
 
 /** Decode with the pinned upstream codec; preserve every equal-position candidate. */
 export const traceSourceMap = (
@@ -35,14 +32,6 @@ export const traceSourceMap = (
 ): WebSourceMapReport => {
   const inspected = inspectSourceMap(text);
   const decodedLeaves = decodeValidatedSourceMapLeaves(inspected.leaves, url);
-  for (const { leaf, rows } of decodedLeaves)
-    for (const [line, row] of rows.entries())
-      for (const segment of row)
-        if (!isBeforeSourceMapLeafStop(leaf, line, segment[0]))
-          throw new SourceMapFormatFailure(
-            "format",
-            "Embedded source-map mappings overlap the following section boundary.",
-          );
   let map: TraceMap;
   if (inspected.format === "regular") {
     const decoded = decodedLeaves[0];
diff --git a/tests/boundary/browser/webSourceMapNestedCoordinates.test.ts b/tests/boundary/browser/webSourceMapNestedCoordinates.test.ts
index 3ed94f72df..7408a48354 100644
--- a/tests/boundary/browser/webSourceMapNestedCoordinates.test.ts
+++ b/tests/boundary/browser/webSourceMapNestedCoordinates.test.ts
@@ -127,6 +127,7 @@ const cases = [
       [1, 10, "b.ts"],
     ],
     dedicatedControl: false,
+    expectedStatus: "invalid",
   },
   {
     name: "nested gap clips previous parent leaf",
@@ -177,6 +178,7 @@ const cases = [
       [1, 8, "b.ts"],
     ],
     dedicatedControl: false,
+    expectedStatus: "invalid",
   },
   {
     name: "empty indexed sibling clips previous leaf",
@@ -210,6 +212,7 @@ const cases = [
     },
     expected: [[1, 0, "a.ts"]],
     dedicatedControl: false,
+    expectedStatus: "invalid",
   },
   {
     name: "depth-three same-line composition",
@@ -307,12 +310,16 @@ const cases = [
       [4, 1, "b.ts"],
     ],
     dedicatedControl: false,
+    expectedStatus: "invalid",
   },
 ] as const;
 
 it.each(cases)(
   "preserves indexed coordinate contract: $name",
-  async ({ map, expected, dedicatedControl }) => {
+  async (testCase) => {
+    const { map, expected, dedicatedControl } = testCase;
+    const expectedStatus =
+      "expectedStatus" in testCase ? testCase.expectedStatus : "included";
     const origin = "https://example.test";
     const url = origin + "/app.js.map";
     const text = JSON.stringify(map);
@@ -340,6 +347,15 @@ it.each(cases)(
       { fetch: () => Promise.resolve(new Response(text, { status: 200 })) },
     );
     const item = result.items[0];
+    if (expectedStatus === "invalid") {
+      expect(item).toMatchObject({
+        status: "invalid",
+        artifact: null,
+        mappings: [],
+        limitation: expect.stringContaining("overlap"),
+      });
+      return;
+    }
     if (item?.status !== "included") throw new Error(JSON.stringify(result));
     expect(
       item.mappings.map((entry) => [
@@ -358,3 +374,76 @@ it.each(cases)(
     ]);
   },
 );
+
+it("rejects a source mapping that crosses an indexed section boundary", async () => {
+  const origin = "https://example.test";
+  const url = `${origin}/app.js.map`;
+  const makeMap = (firstMappings: string) => ({
+    version: 3,
+    sections: [
+      {
+        offset: { line: 0, column: 0 },
+        map: {
+          version: 3,
+          sources: ["a.js"],
+          sourcesContent: ["x();"],
+          names: [],
+          mappings: firstMappings,
+        },
+      },
+      {
+        offset: { line: 0, column: 1 },
+        map: {
+          version: 3,
+          sources: ["b.js"],
+          sourcesContent: ["y();"],
+          names: [],
+          mappings: "AAAA",
+        },
+      },
+    ],
+  });
+  const input = analyzeWebBundleInputSchema.parse({
+    cdp_endpoint: "http://127.0.0.1:9222",
+    allowed_origins: [origin],
+    target_id: "page-1",
+    fetch_source_maps: true,
+  });
+  const request = {
+    scriptKey: "scr_" + "1".repeat(64),
+    declaredUrl: url,
+    fetchUrl: url,
+  };
+  const fetchMap = (map: ReturnType<typeof makeMap>) =>
+    fetchWebSourceMaps([request], input, undefined, {
+      fetch: () =>
+        Promise.resolve(new Response(JSON.stringify(map), { status: 200 })),
+    });
+  const overlapping = makeMap("AAAA,EAAA");
+  const overlapText = JSON.stringify(overlapping);
+  expect(() =>
+    traceSourceMap(overlapText, url, { line: 1, column: 2 }),
+  ).toThrow(/overlap/u);
+  const overlappingResult = await fetchMap(overlapping);
+  expect(overlappingResult).toMatchObject({
+    status: "unavailable",
+    requested: 1,
+    processed: 1,
+  });
+  expect(overlappingResult.items[0]).toMatchObject({
+    status: "invalid",
+    artifact: null,
+    mappings: [],
+    limitation: expect.stringContaining("overlap"),
+  });
+
+  const controlResult = await fetchMap(makeMap("AAAA"));
+  expect(controlResult.items[0]).toMatchObject({
+    status: "included",
+    limitation: null,
+    mappings: [
+      { generated_line: 1, generated_column: 0, source: `${origin}/a.js` },
+      { generated_line: 1, generated_column: 1, source: `${origin}/b.js` },
+    ],
+  });
+});
diff --git a/tests/boundary/javascript/sourceMapDecoder.test.ts b/tests/boundary/javascript/sourceMapDecoder.test.ts
index 974fa33a34..d388c31ed5 100644
--- a/tests/boundary/javascript/sourceMapDecoder.test.ts
+++ b/tests/boundary/javascript/sourceMapDecoder.test.ts
@@ -1,6 +1,7 @@
 import { fileURLToPath } from "node:url";
 import { access } from "node:fs/promises";
 import { expect, it, vi } from "vitest";
+import { AnalysisInputError } from "../../../src/domain/analysisErrorCore.js";
 import { SourceMapDecoder } from "../../../src/javascript/sourceMaps/SourceMapDecoder.js";
 import {
   SOURCE_MAP_CODEC_MEMORY_LIMIT_BYTES,
@@ -139,6 +140,47 @@ it.each([
   if (response.ok) throw new Error("expected failed map");
   expect(response.error._tag).toBe(tag);
 });
+
+it("keeps indexed section overlap typed as a malformed source map", async () => {
+  const text = JSON.stringify({
+    version: 3,
+    sections: [
+      {
+        offset: { line: 0, column: 0 },
+        map: {
+          version: 3,
+          sources: ["a.js"],
+          sourcesContent: ["x();"],
+          names: [],
+          mappings: "AAAA,EAAA",
+        },
+      },
+      {
+        offset: { line: 0, column: 1 },
+        map: {
+          version: 3,
+          sources: ["b.js"],
+          sourcesContent: ["y();"],
+          names: [],
+          mappings: "AAAA",
+        },
+      },
+    ],
+  });
+  const response = await realDecoder().trace({ ...input, text });
+  if (response.ok) throw new Error("expected overlapping map rejection");
+  expect(response.error).toBeInstanceOf(AnalysisInputError);
+  if (!(response.error instanceof AnalysisInputError))
+    throw new Error("expected malformed source-map input error");
+  expect(response.error.issues).toEqual(
+    expect.arrayContaining([
+      expect.objectContaining({
+        reason: "invalid_format",
+        message: expect.stringContaining("overlap"),
+      }),
+    ]),
+  );
+});
 it("reports cleanup failure with the actual owning operation", async () => {
   const response = await realDecoder({
     createRuntime: async () => {