From 6b37e6e2282c968a1805972d3f987716b56fab48 Mon Sep 17 00:00:00 2001
From: Nathan Hangen <nhangen@gmail.com>
Date: Sun, 10 May 2026 21:33:55 -0400
Subject: [PATCH] fix(mcp): skip bytes histogram when sizer throws

Previously a sizer exception (e.g. upstream result shape change) recorded
bytes=0, silently poisoning the 0-bucket with synthetic data. Skip the
histogram observe entirely so distribution stats stay honest.
---
 gitnexus/src/mcp/metrics.ts            |  7 +++----
 gitnexus/test/unit/mcp/metrics.test.ts | 22 ++++++++++++++++++++++
 2 files changed, 25 insertions(+), 4 deletions(-)

diff --git a/gitnexus/src/mcp/metrics.ts b/gitnexus/src/mcp/metrics.ts
index ee428341f..0dc417fb4 100644
--- a/gitnexus/src/mcp/metrics.ts
+++ b/gitnexus/src/mcp/metrics.ts
@@ -158,13 +158,12 @@ export async function observe<T>(
     requestsCounter?.add(1, labels);
     durationHistogram?.record(durationSeconds, labels);
     if (!errored) {
-      let bytes = 0;
       try {
-        bytes = sizer(result);
+        const bytes = sizer(result);
+        resultBytesHistogram?.record(bytes, { tool });
       } catch {
-        bytes = 0;
+        // Skip the histogram rather than poison the 0-bucket on a shape change.
       }
-      resultBytesHistogram?.record(bytes, { tool });
     }
     return result;
   } catch (err) {
diff --git a/gitnexus/test/unit/mcp/metrics.test.ts b/gitnexus/test/unit/mcp/metrics.test.ts
index 3bfdbbed2..9c04ea0bf 100644
--- a/gitnexus/test/unit/mcp/metrics.test.ts
+++ b/gitnexus/test/unit/mcp/metrics.test.ts
@@ -226,6 +226,28 @@ describe('privacy: target_info / scope_info suppression', () => {
   });
 });
 
+describe('observe() — sizer fault tolerance', () => {
+  it('skips the bytes histogram when sizer throws (does not record bytes=0)', async () => {
+    process.env['GITNEXUS_OTEL_METRICS'] = 'on';
+    await withRandomPort(async () => {
+      const init = await initMetrics();
+      await observe(
+        'list_repos',
+        async () => ({ content: [{ type: 'text', text: 'ok' }] }),
+        () => {
+          throw new Error('shape changed');
+        },
+        () => false,
+      );
+      const text = await fetch(`http://127.0.0.1:${init.port}/metrics`).then((r) => r.text());
+      expect(text).toContain(
+        'gitnexus_mcp_tool_requests_total{tool="list_repos",error="false"} 1',
+      );
+      expect(text).not.toMatch(/gitnexus_mcp_tool_result_bytes_count\{tool="list_repos"\}/);
+    });
+  });
+});
+
 describe('bucket boundary contract', () => {
   it('publishes the documented duration buckets verbatim', () => {
     expect(DURATION_BUCKETS_SECONDS).toEqual([0.001, 0.005, 0.025, 0.1, 0.5, 2.5, 10, 30]);