report hybrid leg failure when survivor is empty (cppalliance#231)

* report hybrid leg failure when survivor is empty

* hybrid empty-survivor degradation and PR cppalliance#231 review follow-ups

* moved fixed items on top

---------

Co-authored-by: zho <jornathanm910923@gmail.com>

Author: jonathanMLDev

CHANGELOG.md

@@ -26,6 +26,10 @@ Tagged releases are published to npm from GitHub Actions when a **GitHub Release
 
 - **Breaking (library):** Trimmed internal-only re-exports from the package root and `/alliance` entry to shrink the public surface and blast radius: `trimOptional`, `createUnconfiguredAllianceContext` (core), and the concrete URL generators `generatorMailing`, `generatorSlackCpplang` (alliance). These were internal helpers, not part of the documented API; register built-ins via `registerBuiltinUrlGenerators` and build contexts via `createServer` / `createIsolatedContext`. A snapshot test now guards the runtime export surface so internal symbols cannot leak back in. See [MIGRATION.md](docs/MIGRATION.md#internal-only-re-exports-removed-203). (#203)
 
+### Fixed
+
+- **Hybrid query degradation:** When exactly one search leg fails and the surviving leg returns zero hits, `query` / `query_documents` / `guided_query` now set `experimental.hybrid_leg_failed` and `experimental.degraded: true` with `degradation_reason` `dense_leg_failed` or `sparse_leg_failed`, so empty results from a leg failure are distinguishable from a legitimately empty namespace. (#228)
+
 ## [0.4.0] - 2026-06-24
 
 ### Added

docs/TOOLS.md

@@ -211,13 +211,15 @@ Each row: `document_id`, `paper_number` (deprecated alias), `title`, `author`, `
 
 ### Rerank and hybrid degradation
 
-When reranking is requested but the rerank API fails, the server still returns **`status: 'success'`** with rows where `reranked: false`, plus **experimental** envelope fields:
+Hybrid `query` / `query_documents` responses use **`status: 'success'`** even when confidence is reduced. The server sets **experimental** envelope fields when reranking fails or when exactly one hybrid leg fails and the surviving leg returns zero hits:
 
 | Field | When set | Meaning |
 | ----- | -------- | ------- |
-| `experimental.degraded` | `true` | Rerank was attempted and failed (or another degradation path fired) |
-| `experimental.degradation_reason` | string | Human-readable detail for MCP/LLM clients (e.g. `rerank_failed: timeout after 5000ms`) |
-| `experimental.hybrid_leg_failed` | `'dense'` \| `'sparse'` | Exactly one hybrid search leg failed while the other returned hits |
+| `experimental.degraded` | `true` | Rerank was attempted and failed, **or** one hybrid leg failed with an empty survivor |
+| `experimental.degradation_reason` | string | Human-readable detail (e.g. `rerank_failed: timeout after 5000ms`, `dense_leg_failed`, `sparse_leg_failed`) |
+| `experimental.hybrid_leg_failed` | `'dense'` \| `'sparse'` | Exactly one hybrid search leg failed (survivor may have hits or be empty) |
+
+When `hybrid_leg_failed` is set and `degraded` is `false`, the survivor leg returned hits (partial hybrid). When both are set with zero `results`, a leg failed — not a confidently empty namespace.
 
 Treat **`experimental.degraded: true`** as lower confidence even when `status` is `success`. Combine with per-row `reranked`, `preset`, and `use_reranking`. Structured stderr logs may include additional detail.
 

src/constants.ts

@@ -77,3 +77,11 @@ export const ALLIANCE_SERVER_INSTRUCTIONS =
  * @deprecated Use {@link ALLIANCE_SERVER_INSTRUCTIONS} or {@link CORE_SERVER_INSTRUCTIONS}.
  */
 export const SERVER_INSTRUCTIONS = ALLIANCE_SERVER_INSTRUCTIONS;
+
+export const DENSE_LEG_FAILED_REASON = 'dense_leg_failed' as const;
+export const SPARSE_LEG_FAILED_REASON = 'sparse_leg_failed' as const;
+
+export const HYBRID_LEG_FAILED_REASON = {
+  dense: DENSE_LEG_FAILED_REASON,
+  sparse: SPARSE_LEG_FAILED_REASON,
+} as const satisfies Record<'dense' | 'sparse', string>;

src/core/pinecone-client.test.ts

@@ -3,6 +3,7 @@ import { PineconeClient } from './pinecone-client.js';
 import { resolveConfig } from './config.js';
 import type { SearchableIndex, PineconeHit } from '../types.js';
 import * as rerankModule from './pinecone/rerank.js';
+import { DENSE_LEG_FAILED_REASON, SPARSE_LEG_FAILED_REASON } from '../constants.js';
 
 /** Stubs for private methods (assigned at runtime; avoid intersecting private `PineconeClient` members). */
 type PineconeClientMethodStubs = {
@@ -31,6 +32,29 @@ function stubDualLegSearchFailure(testClient: PineconeClientMethodStubs, searchE
   };
 }
 
+function stubSingleLegHybridFailure(
+  testClient: PineconeClientMethodStubs,
+  options: {
+    failedLeg: 'dense' | 'sparse';
+    survivorHits: PineconeHit[];
+  }
+): void {
+  const denseRef = {} as SearchableIndex;
+  const sparseRef = {} as SearchableIndex;
+  const failedRef = options.failedLeg === 'dense' ? denseRef : sparseRef;
+
+  testClient.ensureIndexes = async () => ({
+    denseIndex: denseRef,
+    sparseIndex: sparseRef,
+  });
+  testClient.searchIndex = async (index) => {
+    if (index === failedRef) {
+      throw new Error(`${options.failedLeg} failure`);
+    }
+    return options.survivorHits;
+  };
+}
+
 describe('PineconeClient', () => {
   let client: PineconeClient;
 
@@ -120,26 +144,16 @@ describe('PineconeClient', () => {
 
     it('should continue hybrid search when one index fails', async () => {
       const testClient = stubPineconeClient(client);
-      const denseRef = {} as SearchableIndex;
-      const sparseRef = {} as SearchableIndex;
-
-      testClient.ensureIndexes = async () => ({
-        denseIndex: denseRef,
-        sparseIndex: sparseRef,
-      });
-
-      testClient.searchIndex = async (index) => {
-        if (index === denseRef) {
-          throw new Error('dense failure');
-        }
-        return [
+      stubSingleLegHybridFailure(testClient, {
+        failedLeg: 'dense',
+        survivorHits: [
           {
             _id: 'doc-1',
             _score: 0.9,
             fields: { chunk_text: 'hybrid content', author: 'tester' },
           },
-        ];
-      };
+        ],
+      });
 
       const out = await client.query({
         query: 'hybrid search',
@@ -155,6 +169,82 @@ describe('PineconeClient', () => {
       expect(out.degraded).toBe(false);
     });
 
+    it('reports hybrid_leg_failed and degraded when dense fails and sparse returns empty', async () => {
+      const testClient = stubPineconeClient(client);
+      stubSingleLegHybridFailure(testClient, { failedLeg: 'dense', survivorHits: [] });
+
+      const out = await client.query({
+        query: 'hybrid search',
+        namespace: 'test',
+        topK: 5,
+        useReranking: false,
+      });
+
+      expect(out.results).toHaveLength(0);
+      expect(out.hybrid_leg_failed).toBe('dense');
+      expect(out.degraded).toBe(true);
+      expect(out.degradation_reason).toBe(DENSE_LEG_FAILED_REASON);
+    });
+
+    it('reports hybrid_leg_failed and degraded when sparse fails and dense returns empty', async () => {
+      const testClient = stubPineconeClient(client);
+      stubSingleLegHybridFailure(testClient, { failedLeg: 'sparse', survivorHits: [] });
+
+      const out = await client.query({
+        query: 'hybrid search',
+        namespace: 'test',
+        topK: 5,
+        useReranking: false,
+      });
+
+      expect(out.results).toHaveLength(0);
+      expect(out.hybrid_leg_failed).toBe('sparse');
+      expect(out.degraded).toBe(true);
+      expect(out.degradation_reason).toBe(SPARSE_LEG_FAILED_REASON);
+    });
+
+    it('returns no degradation when both legs succeed with empty hits', async () => {
+      const testClient = stubPineconeClient(client);
+      testClient.ensureIndexes = async () => ({
+        denseIndex: {} as SearchableIndex,
+        sparseIndex: {} as SearchableIndex,
+      });
+      testClient.searchIndex = async () => [];
+
+      const out = await client.query({
+        query: 'hybrid search',
+        namespace: 'test',
+        topK: 5,
+        useReranking: false,
+      });
+
+      expect(out.results).toHaveLength(0);
+      expect(out.hybrid_leg_failed).toBeNull();
+      expect(out.degraded).toBe(false);
+      expect(out.degradation_reason).toBeUndefined();
+    });
+
+    it('prioritizes leg-failure degradation_reason over rerank_skipped_no_model when both apply', async () => {
+      const noModelClient = new PineconeClient({
+        apiKey: 'test-api-key',
+        indexName: 'test-index',
+      });
+      const testClient = stubPineconeClient(noModelClient);
+      stubSingleLegHybridFailure(testClient, { failedLeg: 'dense', survivorHits: [] });
+
+      const out = await noModelClient.query({
+        query: 'hybrid search',
+        namespace: 'test',
+        topK: 5,
+        useReranking: true,
+      });
+
+      expect(out.hybrid_leg_failed).toBe('dense');
+      expect(out.degraded).toBe(true);
+      expect(out.degradation_reason).toBe(DENSE_LEG_FAILED_REASON);
+      expect(out.rerank_skipped_reason).toBe('no_model');
+    });
+
     it('should throw when both dense and sparse searches fail', async () => {
       const testClient = stubPineconeClient(client);
       stubDualLegSearchFailure(testClient, new Error('index failure'));

src/core/pinecone-client.ts

@@ -14,7 +14,13 @@ import type {
   HybridQueryResult,
   HybridLegFailed,
 } from '../types.js';
-import { DEFAULT_TOP_K, MAX_TOP_K, COUNT_TOP_K, COUNT_FIELDS } from '../constants.js';
+import {
+  DEFAULT_TOP_K,
+  MAX_TOP_K,
+  COUNT_TOP_K,
+  COUNT_FIELDS,
+  HYBRID_LEG_FAILED_REASON,
+} from '../constants.js';
 import { DEFAULT_REQUEST_TIMEOUT_MS } from './config.js';
 import { PineconeIndexSession, type NamespacesWithMetadataResult } from './pinecone/indexes.js';
 import {
@@ -164,17 +170,9 @@ export class PineconeClient {
     }
 
     let hybridLegFailed: HybridLegFailed = null;
-    if (
-      denseResult.status === 'rejected' &&
-      sparseResult.status === 'fulfilled' &&
-      sparseHits.length > 0
-    ) {
+    if (denseResult.status === 'rejected' && sparseResult.status === 'fulfilled') {
       hybridLegFailed = 'dense';
-    } else if (
-      sparseResult.status === 'rejected' &&
-      denseResult.status === 'fulfilled' &&
-      denseHits.length > 0
-    ) {
+    } else if (sparseResult.status === 'rejected' && denseResult.status === 'fulfilled') {
       hybridLegFailed = 'sparse';
     }
 
@@ -205,6 +203,14 @@ export class PineconeClient {
       }
     }
 
+    const survivorEmpty =
+      (hybridLegFailed === 'dense' && sparseHits.length === 0) ||
+      (hybridLegFailed === 'sparse' && denseHits.length === 0);
+    if (hybridLegFailed && survivorEmpty) {
+      degraded = true;
+      degradation_reason = HYBRID_LEG_FAILED_REASON[hybridLegFailed];
+    }
+
     logInfo(
       `Retrieved ${documents.length} documents from hybrid search (dense: ${denseHits.length}, sparse: ${sparseHits.length})`
     );

src/core/rerank-trace.test.ts

@@ -1,4 +1,5 @@
 import { describe, expect, it } from 'vitest';
+import { SPARSE_LEG_FAILED_REASON } from '../constants.js';
 import { guidedRerankStatus } from './rerank-trace.js';
 import { makeHybridQueryResult } from './server/tools/test-helpers.js';
 
@@ -25,6 +26,19 @@ describe('guidedRerankStatus', () => {
     ).toBe('failed');
   });
 
+  it('returns success when degraded from empty-survivor hybrid leg failure', () => {
+    expect(
+      guidedRerankStatus(
+        true,
+        makeHybridQueryResult({
+          degraded: true,
+          degradation_reason: SPARSE_LEG_FAILED_REASON,
+          hybrid_leg_failed: 'sparse',
+        })
+      )
+    ).toBe('success');
+  });
+
   it('returns success when rerank was requested and not skipped or failed', () => {
     expect(guidedRerankStatus(true, makeHybridQueryResult())).toBe('success');
   });

src/core/server/tools/guided-query-tool.context.test.ts

@@ -1,4 +1,5 @@
 import { describe, expect, it, vi } from 'vitest';
+import { SPARSE_LEG_FAILED_REASON } from '../../../constants.js';
 import { registerBuiltinUrlGenerators } from '../../../alliance/url-builtins.js';
 import { guidedQueryResponseSchema } from '../response-schemas.js';
 import { registerQueryTool } from './query-tool.js';
@@ -94,7 +95,7 @@ describe('guided_query tool handler (ServerContext instance path)', () => {
     const query = vi.fn().mockResolvedValue(
       makeHybridQueryResult({
         degraded: true,
-        degradation_reason: 'sparse_leg_empty',
+        degradation_reason: SPARSE_LEG_FAILED_REASON,
         hybrid_leg_failed: 'sparse',
       })
     );
@@ -115,7 +116,7 @@ describe('guided_query tool handler (ServerContext instance path)', () => {
     const resultExperimental = result['experimental'] as Record<string, unknown>;
     expect(resultExperimental['degraded']).toBe(true);
     expect(resultExperimental['hybrid_leg_failed']).toBe('sparse');
-    expect(resultExperimental['degradation_reason']).toBe('sparse_leg_empty');
+    expect(resultExperimental['degradation_reason']).toBe(SPARSE_LEG_FAILED_REASON);
   });
 
   it('enriches urls via ctx builtins when enrich_urls is true', async () => {

src/types.ts

@@ -36,7 +36,7 @@ export interface SearchResult {
   reranked: boolean;
 }
 
-/** Which hybrid leg failed when the other produced hits (partial hybrid success). */
+/** Which hybrid leg failed when exactly one of dense/sparse search rejected. */
 export type HybridLegFailed = 'dense' | 'sparse' | null;
 
 /**
@@ -47,11 +47,14 @@ export type RerankSkippedReason = 'no_model';
 
 export interface HybridQueryResult {
   results: SearchResult[];
-  /** True when reranking was attempted and failed (rows may have `reranked: false`). */
+  /**
+   * True when reranking was attempted and failed (rows may have `reranked: false`),
+   * or when exactly one hybrid leg failed and the surviving leg returned zero hits.
+   */
   degraded: boolean;
   /** Present when {@link degraded} is true; suitable for LLM-facing tool output. */
   degradation_reason?: string;
-  /** Set when exactly one of dense/sparse search failed but the other succeeded. */
+  /** Set when exactly one of dense/sparse search failed (regardless of survivor hit count). */
   hybrid_leg_failed: HybridLegFailed;
   /**
    * Set when `useReranking` was true but no rerank model is configured on the client