diff --git a/src/browser/assets/LocalWebScriptArtifacts.ts b/src/browser/assets/LocalWebScriptArtifacts.ts
index bd2bc1bc88..68873e0290 100644
--- a/src/browser/assets/LocalWebScriptArtifacts.ts
+++ b/src/browser/assets/LocalWebScriptArtifacts.ts
@@ -6,7 +6,10 @@ import { readStableArtifact } from "../../artifacts/readStableArtifact.js";
 import { normalizeArtifactPath } from "../../artifacts/ArtifactPaths.js";
 import type { WebScriptArtifactPort } from "../../application/WebScriptArtifactPort.js";
 import type { ExecutionOptions } from "../../application/AnalysisProvider.js";
-import { AnalysisCapabilityUnavailableError } from "../../domain/analysisErrorCore.js";
+import {
+  AnalysisCapabilityUnavailableError,
+  AnalysisInputError,
+} from "../../domain/analysisErrorCore.js";
 import { isPathWithinRoot } from "../../domain/localPath.js";
 import { err, ok } from "../../domain/result.js";
 import type { SelectedWebScriptInput } from "../../domain/webScriptArtifacts.js";
@@ -15,7 +18,6 @@ import {
   decodeWebArtifact,
   readWebArtifactJson,
   webArtifactReadError,
-  WebArtifactFormatFailure,
   type WebArtifactReadContext,
 } from "./WebArtifactReads.js";
 
@@ -64,9 +66,24 @@ export class LocalWebScriptArtifacts implements WebScriptArtifactPort {
       const manifest = webScriptExportManifestSchema.parse(manifestBytes.value);
       field = ["script_index"];
       const selected = manifest.scripts[input.script_index];
+      // A valid manifest without this index is a caller selection fault.
       if (selected === undefined)
-        throw new WebArtifactFormatFailure(
-          `No script at index ${String(input.script_index)}; manifest contains ${String(manifest.scripts.length)} scripts.`,
+        return err(
+          new AnalysisInputError(this.operation, undefined, [
+            manifest.scripts.length === 0
+              ? {
+                  path: ["script_index"],
+                  reason: "invalid_value",
+                  message: "The selected manifest contains no scripts.",
+                }
+              : {
+                  path: ["script_index"],
+                  reason: "out_of_range",
+                  minimum: 0,
+                  maximum: manifest.scripts.length - 1,
+                  message: `No script at index ${String(input.script_index)}; the manifest contains ${String(manifest.scripts.length)} scripts.`,
+                },
+          ]),
         );
       if (selected.content.state !== "exported")
         return err(
diff --git a/tests/boundary/filesystem/webModuleArtifacts.test.ts b/tests/boundary/filesystem/webModuleArtifacts.test.ts
index 70b3d1b6db..72bf5184c4 100644
--- a/tests/boundary/filesystem/webModuleArtifacts.test.ts
+++ b/tests/boundary/filesystem/webModuleArtifacts.test.ts
@@ -294,3 +294,58 @@ describe("caller-selected web module files", () => {
     );
   });
 });
+
+describe("selected captured script index", () => {
+  it("reports an index past the manifest's scripts as an out-of-range selection", async () => {
+    const f = await fixture();
+    const response = await new LocalWebModuleArtifacts().load({
+      manifest_path: f.manifestPath,
+      script_index: 5,
+    });
+    if (response.ok) throw new Error("expected an out-of-range index");
+    expect(projectAnalysisError(response.error)).toMatchObject({
+      code: "invalid_request",
+      details: {
+        issues: [
+          {
+            path: ["script_index"],
+            reason: "out_of_range",
+            minimum: 0,
+            maximum: 0,
+          },
+        ],
+      },
+    });
+  });
+
+  it("distinguishes an empty valid manifest from a malformed manifest", async () => {
+    const f = await fixture();
+    const parsed: unknown = JSON.parse(await readFile(f.manifestPath, "utf8"));
+    const manifest = webScriptExportResultSchema
+      .omit({ manifest: true })
+      .parse(parsed);
+    manifest.scripts = [];
+    await writeFile(f.manifestPath, JSON.stringify(manifest));
+
+    const artifacts = new LocalWebModuleArtifacts();
+    const input = { manifest_path: f.manifestPath, script_index: 0 };
+    const empty = await artifacts.load(input);
+    if (empty.ok) throw new Error("expected an empty-manifest selection fault");
+    expect(projectAnalysisError(empty.error)).toMatchObject({
+      code: "invalid_request",
+      details: {
+        issues: [{ path: ["script_index"], reason: "invalid_value" }],
+      },
+    });
+
+    await writeFile(f.manifestPath, "{");
+    const malformed = await artifacts.load(input);
+    if (malformed.ok) throw new Error("expected a malformed manifest fault");
+    expect(projectAnalysisError(malformed.error)).toMatchObject({
+      code: "invalid_request",
+      details: {
+        issues: [{ path: ["manifest_path"], reason: "invalid_format" }],
+      },
+    });
+  });
+});