fix(fallback)+docs(adr-0004): D16 — honor "usable chunks streamed" qualifier on SPAWN_FAILED

cold-audit catch from 2026-05-23

Cold-audit Finding 17 (P2 fallback correctness). ADR 0004 § Trigger
taxonomy Hard triggers bullet 3 says "Provider CLI exit code ≠ 0
**with no usable response chunks streamed**" — the qualifier was
not honored. Pre-D16 code in `collectAllChunks` re-raised SPAWN_FAILED
unconditionally, discarding any partial chunks. Concrete failure:
provider yields 1000 chars of completion then exits non-zero (e.g.,
post-stream cleanup error) → chunks dropped, fallback to next provider,
user pays double spawn cost and loses the original provider's output.

Coordinated change across two layers, single commit per the
ADR-with-code pattern (D11 / D15 precedent):

1. docs/adr/0004-fallback-engine.md — Amendment 1 (top of doc, matching
   D11/D15 placement convention):
   - Documents the "usable chunks streamed" semantics precisely
   - Behavior split: chunks.length > 0 + SPAWN_FAILED → synthesize stop
     + return (Case B); chunks.length === 0 + SPAWN_FAILED → re-throw
     (Case A, hard trigger fires as before)
   - finish_reason='length' rationale (4 reasons documented)
   - Streaming-path note: ADR 0004 first-chunk rule already handles the
     analogous case for D10's real-streaming branch; D16 applies to
     buffered path only
   - Cache behavior: write-through `getOrCompute` (preserves D4 singleflight
     during truncation event) then evict via `set(ttlMs=0)` (so future
     fresh callers re-spawn). `__truncated` non-enumerable marker
     travels with the chunks array for follower visibility

2. server.mjs `collectAllChunks` salvage path:
   - try/catch around the for-await loop
   - On SPAWN_FAILED with chunks.length > 0: synthesize stop chunk
     `{type:'stop', finish_reason:'length'}`, log warn event
     `spawn_failed_after_usable_chunks`, mark chunks array with
     non-enumerable `__truncated`, return (no re-throw)
   - On SPAWN_FAILED with chunks.length === 0 OR any other error:
     re-throw (preserves existing hard-trigger semantics)

3. server.mjs `executeHopFn` cache eviction:
   - After `cacheStore.getOrCompute(...)` returns, check `result.__truncated`
   - If truncated: `cacheStore.set(keyId, hopCacheKey, result, 0)` —
     ttlMs=0 causes `_isAlive` to treat the entry as expired on next
     read (verified in lib/cache/store.mjs)

4. test-features.mjs Suite 13 — 3 new tests:
   - Case A regression: SPAWN_FAILED at iter 0 + 2-hop chain → openai
     serves, X-OLP-Fallback-Hops: 1 (no behavior change)
   - Case B 2-hop: 2 deltas + SPAWN_FAILED → anthropic serves with
     synthesized stop, hops=0, finish_reason='length', content
     concatenates, openai NOT called
   - Case B single-hop: 1 delta + SPAWN_FAILED → HTTP 200 (not 502),
     finish_reason='length', partial content visible

Tests: 297 → 300 (+3). All pass on Node 20.

Pre-commit fold-ins (per evidence-first checkpoint #4 — fold-ins
themselves need second-pass review):

- **Error-chunk-in-chunks fold-in (sonnet flagged)**: pre-D12 code
  pushed error chunks BEFORE throwing. Post-D16's `chunks.length > 0`
  check would incorrectly include an error chunk and trigger Case B
  for a path that's actually Case A. Moved the `type === 'error'`
  check BEFORE the push, restoring the invariant that the chunks
  array contains only delta/stop chunks. Verified all 3 scenarios:
  (1) error at iter 0 → throws before push → length=0 → Case A
  (2) delta×2 + error at iter 3 → throws before push → length=2 → Case B
      with delta×2 + synthesized stop (no error chunk leaks)
  (3) delta + non-zero exit from outside loop → length=1 → Case B

- **ADR doc-code drift fold-in (D16 reviewer flagged)**: original
  Amendment 1 text said the salvaged result "bypasses
  cacheStore.getOrCompute and is returned directly, exactly as the
  cache-bypass path does." This was factually wrong — the code
  write-throughs via getOrCompute then evicts via ttlMs=0. The drift
  was ironic: D16 was about removing doc-code drift in ADR 0004
  bullet 3 itself, and the amendment was about to ship fresh drift.
  Corrected to accurately describe the write-then-evict pattern and
  the rationale (preserving D4 singleflight during truncation events).

Authority:
- ADR 0004 § Trigger taxonomy Hard triggers bullet 3 (the qualifier
  this amendment makes load-bearing)
  https://github.com/dtzp555-max/olp/blob/main/docs/adr/0004-fallback-engine.md
- ADR 0005 § Cache write conditions item 1 — "response completed
  successfully (no truncation, no error mid-stream)"
- OpenAI Chat Completions finish_reason enum (stop|length|tool_calls|
  content_filter|function_call|null)
  https://platform.openai.com/docs/api-reference/chat/object
- ADR 0004 § Fallback safety — first-chunk rule (already governs the
  analogous case in the real-streaming path)
- ALIGNMENT.md Rule 2(c) spirit — ADR amendment + code change land
  in same merge (D11 / D15 precedent)
- CC 开发铁律 v1.6 § 10.x — Cold Audit caught this; diff-review
  passes focused on first-chunk rule for streaming missed the
  buffered path's truncation-vs-fallback decision point

Reviewer (Iron Rule v1.6 § 10.x Mode A, fresh-context opus, independent
of drafter): APPROVE_WITH_MINOR. Folded the ADR doc-code drift minor
before commit. Walked all 3 error-chunk scenarios against actual code
to verify the pre-commit fold-in is correct. Analyzed the eviction
race window (sub-ms post-inflight pre-eviction window where a fresh
caller could hit the truncated cache before eviction lands) and
concluded it's structurally bounded — one-shot leak per truncation
event; subsequent callers re-spawn. Acceptable as v0.1.

Follow-up items (reviewer's non-blocking suggestions, NOT in this PR):
- 4th test asserting second identical request triggers fresh spawn
  (defense-in-depth around the eviction; store.mjs ttlMs=0 semantics
  are independently established)
- `cacheStore.delete()` API (cleaner than set-with-ttlMs=0 — leaves
  no dead entry in the namespace map; future PR)
- `cache_evicted_truncated` log event for dashboard observability
- SPAWN_TIMEOUT salvage parity — same architectural argument as
  SPAWN_FAILED (user paid for partial content); deferred as a
  separate cold-audit finding for a future D-stage

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

docs/adr/0004-fallback-engine.md

@@ -5,6 +5,26 @@
 - **Authors:** project maintainer (with AI drafting assistance)
 - **Related:** OLP v0.1 spec §4.3; ADR 0001 (project founding — fallback is OLP's core value proposition); ADR 0002 (plugin architecture); ADR 0003 (IR — fallback replays IR across chain hops)
 
+## Amendments
+
+### Amendment 1 — 2026-05-24: Buffered-path SPAWN_FAILED with usable chunks must NOT trigger fallback (D16)
+
+- **Finding:** Cold-audit Finding 17 (P2 fallback correctness) — Hard triggers bullet 3 in the Decision section below reads: "Provider CLI exit code ≠ 0 **with no usable response chunks streamed**." The qualifier "with no usable response chunks streamed" is load-bearing. Pre-D16 code in `server.mjs` `collectAllChunks` had no catch block: if the provider generator threw `ProviderError(SPAWN_FAILED)` after yielding N>0 IR delta/stop chunks, the throw propagated out of the `for await` loop, discarding the accumulated chunks. The fallback engine then treated SPAWN_FAILED as a hard trigger unconditionally, advanced the chain, and served the next provider's response. The original provider's actual partial output — content the user had already paid for — was silently dropped.
+
+- **Clarification — "usable response chunks streamed":** A chunks array has usable chunks if `chunks.length > 0` where the counted chunks are IR delta or stop chunks (i.e., `type === 'delta'` or `type === 'stop'`). Chunks of `type === 'error'` are excluded from this count — they are failure signals, not usable content. In the buffered path (`collectAllChunks` in `server.mjs`), `type === 'error'` chunks cause an immediate throw before being pushed to the array (see existing code), so in practice `chunks.length > 0` after the loop implies the array contains only delta/stop chunks and the count correctly reflects usable content. The N=1 boundary is intentional: even a single non-empty delta chunk represents content the provider has committed and the user has paid for in quota; dropping it to serve an alternate provider's response is strict waste and potentially misleading (the second provider may produce different content for the same prompt).
+
+- **Behavior change (D16):** In the buffered path (`collectAllChunks` in `server.mjs`), when the provider generator throws `ProviderError` with `code === 'SPAWN_FAILED'` after yielding one or more chunks:
+  - If `chunks.length > 0`: synthesize a final stop chunk `{ type: 'stop', finish_reason: 'length' }`, push it to the chunks array, log a `warn` event `spawn_failed_after_usable_chunks`, and **return the chunks** (do not re-throw). The fallback engine sees a successful return value and does not advance the chain. The client receives the partial response with `finish_reason: 'length'`. `X-OLP-Fallback-Hops: 0` is emitted.
+  - If `chunks.length === 0`: the SPAWN_FAILED error propagates as before. The fallback engine fires the hard trigger and advances the chain (Case A — correct behavior unchanged).
+  - Any other error code (not SPAWN_FAILED): always re-thrown, behavior unchanged.
+  - **Cache behavior:** truncated responses (those produced by the SPAWN_FAILED salvage path) are NOT persistently cached. ADR 0005 § "Cache write conditions" item 1 requires "response completed successfully (no truncation, no error mid-stream)"; a salvaged partial response does not meet this condition. In `executeHopFn`, the salvaged result is written through `cacheStore.getOrCompute` normally so that concurrent inflight callers (D4 singleflight) share the spawn and all receive the partial response — then the entry is immediately evicted via `cacheStore.set(keyId, hopCacheKey, result, ttlMs=0)`, which causes `_isAlive` to treat the entry as expired on the next read. The write-then-evict pattern preserves D4 singleflight during the truncation event (the right behavior — concurrent callers should share the partial spawn rather than each triggering their own) while ensuring future fresh callers re-spawn the provider rather than serving cached truncated content. The salvage path uses a non-enumerable `__truncated` marker on the returned chunks array to signal the eviction; the marker does not appear in `JSON.stringify` or in any client-visible surface.
+
+- **finish_reason choice:** The synthesized stop chunk uses `finish_reason: 'length'` because: (a) it is in the OpenAI spec enum (`stop | length | tool_calls | content_filter | function_call | null`); (b) it semantically maps to "output was truncated due to resource constraints," which is the closest available enum value to "truncated because the provider process exited non-zero during cleanup after streaming output"; (c) `null` is inferior — some clients retry on `null` finish_reason expecting additional chunks, which would cause spurious re-requests; (d) `stop` would be misleading — it implies the model naturally terminated at a sentence boundary, which is false. Using `length` correctly signals to the client that the output was cut short and they may wish to re-request or continue.
+
+- **Streaming path note:** The D10 real-streaming branch (single-hop, `server.mjs` lines 401–510) already handles the analogous case correctly via ADR 0004's first-chunk rule: once `firstChunkEmitted === true`, any subsequent error truncates the response with `res.end()` (no re-throw, no fallback). This amendment applies specifically to the **buffered path** (`collectAllChunks` + multi-hop fallback chains). The streaming path is not changed by D16.
+
+- **Procedural mechanism:** CC 开发铁律 v1.6 § 10.x (Cold Audit). Diff-review reviewers on earlier passes focused on the first-chunk rule for the real-streaming path; the buffered path has its own truncation-vs-fallback decision point, which the cold-audit pass on 2026-05-23 identified as Finding 17.
+
 ## Context
 
 A multi-provider proxy is only valuable if it fails over gracefully. Per spec §1, OLP's core value proposition is "your IDEs and family clients keep working as long as *any* of your subscriptions has quota left." If Anthropic returns 529 (overloaded), or OpenAI returns 429 with `insufficient_quota`, or `claude -p` exits non-zero, the client should not see an error; the client should see the next provider in the chain transparently take over.

server.mjs

@@ -368,18 +368,55 @@ async function handleChatCompletions(req, res) {
     // Collect all chunks from this provider, throwing on error chunks.
     // Error semantics: ProviderError thrown here propagates to executeWithFallback
     // which decides whether to advance the chain.
+    //
+    // D16 (ADR 0004 Amendment 1): if the generator throws ProviderError SPAWN_FAILED
+    // after yielding one or more chunks, those chunks are usable content the provider
+    // committed — dropping them and falling back to the next hop is strict waste.
+    // In that case: synthesize a stop chunk with finish_reason='length', return the
+    // partial chunks, and do NOT re-throw (fallback engine sees a success, hops=0).
+    // If chunks.length===0 on SPAWN_FAILED, re-throw as before (hard trigger fires).
+    // Any other error code always re-throws unchanged.
+    //
+    // D16 cache note: truncated responses are NOT cached (ADR 0005 § cache write
+    // conditions item 1 requires no truncation). A non-enumerable __truncated marker
+    // is set on the returned array so executeHopFn can evict the cache entry below.
     async function collectAllChunks() {
       const chunks = [];
+      try {
         for await (const irChunk of hopProviderPlugin.spawn(irReq, authContext)) {
-        chunks.push(irChunk);
+          // D16: check error chunks BEFORE pushing — preserves the invariant that
+          // chunks array contains only delta/stop chunks. Without this, the catch
+          // block's `chunks.length > 0` would mistake a single error chunk for
+          // "usable content streamed" (Case B) and synthesize a stop + return,
+          // sending an empty body to the client when the correct behavior is to
+          // re-throw and let the fallback engine advance the chain.
           if (irChunk.type === 'error') {
             throw new ProviderError(
               irChunk.error ?? 'Provider emitted error chunk',
               'SPAWN_FAILED',
             );
           }
+          chunks.push(irChunk);
           if (irChunk.type === 'stop') break;
         }
+      } catch (spawnErr) {
+        if (spawnErr instanceof ProviderError && spawnErr.code === 'SPAWN_FAILED' && chunks.length > 0) {
+          // Case B (ADR 0004 Amendment 1): provider emitted usable chunks then exited
+          // non-zero. Synthesize a truncated stop and surface the partial response.
+          chunks.push({ type: 'stop', finish_reason: 'length' });
+          logEvent('warn', 'spawn_failed_after_usable_chunks', {
+            chunks_count: chunks.length - 1, // exclude the synthesized stop
+            provider: hopProvider,
+            model: hopModel,
+          });
+          // Mark as truncated so the caller can evict this entry from cache.
+          Object.defineProperty(chunks, '__truncated', { value: true, enumerable: false });
+          return chunks;
+        }
+        // Case A (SPAWN_FAILED with no chunks) or any other error: re-throw.
+        // Fallback engine fires hard trigger and advances chain as before.
+        throw spawnErr;
+      }
       return chunks;
     }
 
@@ -398,7 +435,21 @@ async function handleChatCompletions(req, res) {
     // D4 singleflight + D1 per-key isolation per ADR 0005.
     // Each hop has its own (provider, model) key — cross-provider contamination
     // is structurally impossible (ADR 0005 § Per-model isolation).
-    return cacheStore.getOrCompute(keyId, hopCacheKey, collectAllChunks);
+    //
+    // D16: after getOrCompute returns, check if collectAllChunks used the salvage
+    // path (chunks.__truncated===true). If so, evict the just-written cache entry
+    // to satisfy ADR 0005 § "Cache write conditions" item 1 (no truncation).
+    // D4 singleflight is preserved: getOrCompute still deduplicates concurrent
+    // requests during the spawn; the eviction only affects persistent caching.
+    const result = await cacheStore.getOrCompute(keyId, hopCacheKey, collectAllChunks);
+    if (result.__truncated) {
+      // Evict the truncated entry so future requests get a fresh spawn.
+      // ADR 0005 § "Cache write conditions" item 1: truncated responses must not
+      // persist in cache. Overwrite with ttlMs=0 so any subsequent get/peek
+      // finds the entry already-expired and deletes it.
+      await cacheStore.set(keyId, hopCacheKey, result, 0);
+    }
+    return result;
   }
 
   // ── Execute with fallback (ADR 0004) ────────────────────────────────────

test-features.mjs

@@ -4153,6 +4153,282 @@ describe('Fallback engine — HTTP integration (D9)', () => {
     }
   });
 
+  // ── D16 tests: SPAWN_FAILED partial-response salvage (ADR 0004 Amendment 1) ─
+
+  it('D16/Case-A regression guard: SPAWN_FAILED with no chunks → hard fallback fires → openai serves (2-hop)', async () => {
+    // Case A: provider exits non-zero BEFORE emitting any content.
+    // Fallback engine must still advance the chain — this is the pre-D16 behavior
+    // that must remain unchanged. Emitting zero chunks then exit-1 must still trigger
+    // the hard fallback.
+    __clearCache();
+
+    const { writeFileSync, unlinkSync } = await import('node:fs');
+    const { tmpdir } = await import('node:os');
+    const { join: pathJoin } = await import('node:path');
+    const tmpAuthFile = pathJoin(tmpdir(), `olp-test-d16-caseA-auth-${Date.now()}.json`);
+    writeFileSync(tmpAuthFile, JSON.stringify({ accessToken: 'fake-d16-caseA-codex-token' }), 'utf8');
+    const savedCodexAuthPath = process.env.OPENAI_CODEX_AUTH_PATH;
+    process.env.OPENAI_CODEX_AUTH_PATH = tmpAuthFile;
+
+    __setFallbackConfig({
+      chains: {
+        'claude-sonnet-4-6': [
+          { provider: 'anthropic', model: 'claude-sonnet-4-6' },
+          { provider: 'openai', model: 'gpt-5.5' },
+        ],
+      },
+      soft_triggers: {},
+    });
+
+    // Anthropic mock: emits NO stdout, exits with code 1 → SPAWN_FAILED with chunks=[]
+    // This is Case A — hard trigger must fire, chain must advance.
+    __setSpawnImpl(function (_bin, _args, _opts) {
+      const proc = new EventEmitter();
+      proc.stdout = new EventEmitter();
+      proc.stderr = new EventEmitter();
+      proc.stdin = {
+        write: () => {},
+        end: () => {
+          setImmediate(() => {
+            // No stdout data emitted before close
+            proc.stdout.emit('end');
+            proc.stderr.emit('end');
+            proc.emit('close', 1, null); // exit 1 → SPAWN_FAILED, no usable chunks
+          });
+        },
+      };
+      proc.killed = false;
+      proc.kill = () => {};
+      return proc;
+    });
+
+    // OpenAI (codex) mock: succeeds with a real response
+    codexSetSpawnImpl(function (_bin, _args, _opts) {
+      const proc = new EventEmitter();
+      proc.stdout = new EventEmitter();
+      proc.stderr = new EventEmitter();
+      proc.stdin = {
+        write: () => {},
+        end: () => {
+          setImmediate(() => {
+            proc.stdout.emit('data', Buffer.from('{"content":"D16-caseA-fallback-openai-response"}\n'));
+            proc.stdout.emit('data', Buffer.from('{"type":"stop"}\n'));
+            proc.stdout.emit('end');
+            proc.stderr.emit('end');
+            proc.emit('close', 0, null);
+          });
+        },
+      };
+      proc.killed = false;
+      proc.kill = () => {};
+      return proc;
+    });
+
+    try {
+      const r = await fetch({
+        port,
+        method: 'POST',
+        path: '/v1/chat/completions',
+        body: {
+          model: 'claude-sonnet-4-6',
+          messages: [{ role: 'user', content: 'Hi' }],
+          max_tokens: 10,
+        },
+      });
+      assert.equal(r.status, 200, `Expected 200 from openai fallback, got ${r.status}: ${r.body.slice(0, 300)}`);
+      // Hard trigger fired → chain advanced → openai served → hops=1
+      assert.equal(r.headers['x-olp-fallback-hops'], '1', `Expected fallback-hops=1 (Case A hard trigger), got: ${r.headers['x-olp-fallback-hops']}`);
+      assert.equal(r.headers['x-olp-provider-used'], 'openai', `Expected openai served, got: ${r.headers['x-olp-provider-used']}`);
+    } finally {
+      __resetFallbackConfig();
+      __resetSpawnImpl();
+      codexResetSpawnImpl();
+      if (savedCodexAuthPath !== undefined) {
+        process.env.OPENAI_CODEX_AUTH_PATH = savedCodexAuthPath;
+      } else {
+        delete process.env.OPENAI_CODEX_AUTH_PATH;
+      }
+      try { unlinkSync(tmpAuthFile); } catch { /* ignore */ }
+    }
+  });
+
+  it('D16/Case-B fix: SPAWN_FAILED after N chunks → fallback does NOT fire → anthropic partial response served (2-hop)', async () => {
+    // Case B (the D16 fix): provider emits content then exits non-zero.
+    // The partial chunks are usable — fallback must NOT advance to openai.
+    // Instead, the partial response is returned with finish_reason='length'.
+    // X-OLP-Fallback-Hops must be 0 (anthropic served, openai not called).
+    __clearCache();
+
+    const { writeFileSync, unlinkSync } = await import('node:fs');
+    const { tmpdir } = await import('node:os');
+    const { join: pathJoin } = await import('node:path');
+    const tmpAuthFile = pathJoin(tmpdir(), `olp-test-d16-caseB-auth-${Date.now()}.json`);
+    writeFileSync(tmpAuthFile, JSON.stringify({ accessToken: 'fake-d16-caseB-codex-token' }), 'utf8');
+    const savedCodexAuthPath = process.env.OPENAI_CODEX_AUTH_PATH;
+    process.env.OPENAI_CODEX_AUTH_PATH = tmpAuthFile;
+
+    let openaiMockCalled = false;
+
+    __setFallbackConfig({
+      chains: {
+        'claude-sonnet-4-6': [
+          { provider: 'anthropic', model: 'claude-sonnet-4-6' },
+          { provider: 'openai', model: 'gpt-5.5' },
+        ],
+      },
+      soft_triggers: {},
+    });
+
+    // Anthropic mock: emits 2 raw text chunks then exits non-zero (cleanup error).
+    // The anthropic plugin processes raw stdout as text → IR delta chunks.
+    // exit code 1 after yielding content → Case B: SPAWN_FAILED with chunks.length>0.
+    __setSpawnImpl(function (_bin, _args, _opts) {
+      const proc = new EventEmitter();
+      proc.stdout = new EventEmitter();
+      proc.stderr = new EventEmitter();
+      proc.stdin = {
+        write: () => {},
+        end: () => {
+          setImmediate(() => {
+            // Emit 2 content chunks as raw text (anthropic --output-format text)
+            proc.stdout.emit('data', Buffer.from('Hello '));
+            proc.stdout.emit('data', Buffer.from('world'));
+            proc.stdout.emit('end');
+            proc.stderr.emit('end');
+            // Non-zero exit AFTER emitting content → SPAWN_FAILED, chunks.length=2
+            proc.emit('close', 1, null);
+          });
+        },
+      };
+      proc.killed = false;
+      proc.kill = () => {};
+      return proc;
+    });
+
+    // OpenAI mock: should NOT be called (D16 must prevent fallback here)
+    codexSetSpawnImpl(function (_bin, _args, _opts) {
+      openaiMockCalled = true;
+      const proc = new EventEmitter();
+      proc.stdout = new EventEmitter();
+      proc.stderr = new EventEmitter();
+      proc.stdin = {
+        write: () => {},
+        end: () => {
+          setImmediate(() => {
+            proc.stdout.emit('data', Buffer.from('{"content":"unexpected-openai-response"}\n'));
+            proc.stdout.emit('data', Buffer.from('{"type":"stop"}\n'));
+            proc.stdout.emit('end');
+            proc.stderr.emit('end');
+            proc.emit('close', 0, null);
+          });
+        },
+      };
+      proc.killed = false;
+      proc.kill = () => {};
+      return proc;
+    });
+
+    try {
+      const r = await fetch({
+        port,
+        method: 'POST',
+        path: '/v1/chat/completions',
+        body: {
+          model: 'claude-sonnet-4-6',
+          messages: [{ role: 'user', content: 'Hi' }],
+          max_tokens: 10,
+        },
+      });
+      // D16: anthropic partial response surfaced instead of falling back
+      assert.equal(r.status, 200, `Expected 200 with partial anthropic response, got ${r.status}: ${r.body.slice(0, 300)}`);
+      // No fallback should have fired — anthropic's partial chunks were usable
+      assert.equal(r.headers['x-olp-fallback-hops'], '0', `Expected fallback-hops=0 (no fallback, Case B), got: ${r.headers['x-olp-fallback-hops']}`);
+      assert.equal(r.headers['x-olp-provider-used'], 'anthropic', `Expected anthropic served, got: ${r.headers['x-olp-provider-used']}`);
+      // OpenAI mock must NOT have been invoked
+      assert.equal(openaiMockCalled, false, 'OpenAI mock should NOT have been called (D16 prevents fallback on Case B)');
+      // Verify finish_reason='length' (synthesized stop from D16 salvage path)
+      const body = JSON.parse(r.body);
+      assert.equal(body.choices?.[0]?.finish_reason, 'length', `Expected finish_reason='length' from D16 salvage, got: ${body.choices?.[0]?.finish_reason}`);
+      // Verify the 2 content chunks are present in the response
+      const content = body.choices?.[0]?.message?.content ?? '';
+      assert.ok(content.includes('Hello'), `Expected 'Hello' in partial content, got: '${content}'`);
+      assert.ok(content.includes('world'), `Expected 'world' in partial content, got: '${content}'`);
+    } finally {
+      __resetFallbackConfig();
+      __resetSpawnImpl();
+      codexResetSpawnImpl();
+      if (savedCodexAuthPath !== undefined) {
+        process.env.OPENAI_CODEX_AUTH_PATH = savedCodexAuthPath;
+      } else {
+        delete process.env.OPENAI_CODEX_AUTH_PATH;
+      }
+      try { unlinkSync(tmpAuthFile); } catch { /* ignore */ }
+    }
+  });
+
+  it('D16/Case-B single-hop: SPAWN_FAILED after 1 chunk → HTTP 200 with partial response + finish_reason=length (no fallback chain)', async () => {
+    // Case B single-hop variant: only one provider in the chain.
+    // Provider emits 1 content chunk then exits non-zero.
+    // D16 must surface the partial response (HTTP 200, not 502).
+    __clearCache();
+
+    __setFallbackConfig({
+      chains: {
+        'claude-sonnet-4-6': [
+          { provider: 'anthropic', model: 'claude-sonnet-4-6' },
+        ],
+      },
+      soft_triggers: {},
+    });
+
+    // Anthropic mock: emits 1 raw text chunk then exits non-zero
+    __setSpawnImpl(function (_bin, _args, _opts) {
+      const proc = new EventEmitter();
+      proc.stdout = new EventEmitter();
+      proc.stderr = new EventEmitter();
+      proc.stdin = {
+        write: () => {},
+        end: () => {
+          setImmediate(() => {
+            proc.stdout.emit('data', Buffer.from('Partial answer'));
+            proc.stdout.emit('end');
+            proc.stderr.emit('end');
+            proc.emit('close', 1, null); // exit 1 after content → Case B
+          });
+        },
+      };
+      proc.killed = false;
+      proc.kill = () => {};
+      return proc;
+    });
+
+    try {
+      const r = await fetch({
+        port,
+        method: 'POST',
+        path: '/v1/chat/completions',
+        body: {
+          model: 'claude-sonnet-4-6',
+          messages: [{ role: 'user', content: 'Hi' }],
+          max_tokens: 10,
+        },
+      });
+      // D16: must be 200 with the partial content, not 502
+      assert.equal(r.status, 200, `Expected 200 with partial response (not 502), got ${r.status}: ${r.body.slice(0, 300)}`);
+      assert.equal(r.headers['x-olp-fallback-hops'], '0', `Expected fallback-hops=0, got: ${r.headers['x-olp-fallback-hops']}`);
+      assert.equal(r.headers['x-olp-provider-used'], 'anthropic', `Expected anthropic served, got: ${r.headers['x-olp-provider-used']}`);
+      const body = JSON.parse(r.body);
+      // finish_reason='length' indicates truncation (D16 synthesized stop)
+      assert.equal(body.choices?.[0]?.finish_reason, 'length', `Expected finish_reason='length', got: ${body.choices?.[0]?.finish_reason}`);
+      // Partial content must be present
+      const content = body.choices?.[0]?.message?.content ?? '';
+      assert.ok(content.includes('Partial answer'), `Expected 'Partial answer' in content, got: '${content}'`);
+    } finally {
+      __resetFallbackConfig();
+      __resetSpawnImpl();
+    }
+  });
+
 });
 
 // ── Suite 14: providers.enabled config wiring ──────────────────────────────