diff --git a/knowledge-fs/packages/api/src/llm-semantic-chunker.test.ts b/knowledge-fs/packages/api/src/llm-semantic-chunker.test.ts
index e204d9fab9d..b592a13eeca 100644
--- a/knowledge-fs/packages/api/src/llm-semantic-chunker.test.ts
+++ b/knowledge-fs/packages/api/src/llm-semantic-chunker.test.ts
@@ -13,6 +13,7 @@ import { createInMemoryDocumentSemanticWindowCheckpointRepository } from "./docu
 
 import {
   DEFAULT_MAX_SEMANTIC_WINDOWS,
+  LlmSemanticChunkingOutputError,
   type LlmSemanticCompletionCatalogEntry,
   type LlmSemanticWindowManifestEntry,
   type SemanticChunkingLlmProvider,
@@ -1513,6 +1514,36 @@ describe("LLM semantic chunker", () => {
     ).rejects.toThrow(error);
   });
 
+  it("classifies an unknown response unit ID as an invalid model-runtime response", async () => {
+    const chunker = createLlmSemanticChunker({
+      maxChunkChars: 20,
+      reasoningProviderFactory: () =>
+        new ScriptedProvider([
+          ({ units }) => ({ chunks: [chunkRange(units[0]?.id, "u-missing")] }),
+        ]),
+    });
+
+    const failure = chunker.chunk({
+      knowledgeSpaceId: KNOWLEDGE_SPACE_ID,
+      parseArtifact: artifact([
+        {
+          id: "paragraph",
+          metadata: {},
+          sectionPath: ["Validation"],
+          text: "Alpha. Beta.",
+          type: "paragraph",
+        },
+      ]),
+      retrievalProfile: profile(),
+    });
+
+    await expect(failure).rejects.toBeInstanceOf(LlmSemanticChunkingOutputError);
+    await expect(failure).rejects.toMatchObject({
+      code: "MODEL_RUNTIME_RESPONSE_INVALID",
+      retryable: false,
+    });
+  });
+
   it("drops ungrounded image OCR entities and their relations without discarding the chunk", async () => {
     const provider = new ScriptedProvider([
       ({ units }) => ({
diff --git a/knowledge-fs/packages/api/src/llm-semantic-chunker.ts b/knowledge-fs/packages/api/src/llm-semantic-chunker.ts
index 5eba1cefc71..2a47353fa4b 100644
--- a/knowledge-fs/packages/api/src/llm-semantic-chunker.ts
+++ b/knowledge-fs/packages/api/src/llm-semantic-chunker.ts
@@ -2836,7 +2836,9 @@ function validateAndMaterializeWindowOutput({
     const start = unitIndex.get(candidate.startUnitId);
     const end = unitIndex.get(candidate.endUnitId);
     if (start === undefined || end === undefined) {
-      throw new Error("LLM semantic chunking response referenced an unknown unit ID");
+      throw new LlmSemanticChunkingOutputError(
+        "LLM semantic chunking response referenced an unknown unit ID",
+      );
     }
     if (start !== expectedStart || end < start) {
       throw new Error(