diff --git a/workers/src/index.ts b/workers/src/index.ts
index bf7476b..c247301 100644
--- a/workers/src/index.ts
+++ b/workers/src/index.ts
@@ -978,7 +978,15 @@ export default {
       // response. The helper handles clone failures safely.
       if (telemetryClone) {
         const durationMs = Date.now() - startTime;
-        const cacheTier = tracer.indexSource;
+        // NOTE: Do NOT read tracer.indexSource here. The MCP handler returns
+        // a streaming Response — `await handler(...)` resolves with the
+        // Response object before the tool handler closure has finished
+        // running, so the tracer has not yet recorded the `index` span at
+        // this point. Reading here yields "none" for every tool. The tracer
+        // is only fully populated once the response body has been consumed
+        // (which forces the streaming tool handler to complete). The read
+        // therefore happens inside the waitUntil callback below, after
+        // `await responseClone.text()` resolves.
         // Clone the response synchronously before returning so the body is
         // still available to read inside the deferred waitUntil callback.
         const responseClone = response.clone();
@@ -997,6 +1005,13 @@ export default {
               } catch {
                 // Fall through with empty string; bytes_out / tokens_out will be 0.
               }
+              // Read tracer.indexSource AFTER the response body has been
+              // consumed. By this point the streaming tool handler has
+              // completed and any "index" / "index-build" spans have been
+              // recorded. Reading earlier (e.g. immediately after `await
+              // handler()` returned) was the streaming-race bug that caused
+              // every tool call to record cache_tier="none" in production.
+              const cacheTier = tracer.indexSource;
               const shape = await measurePayloadShape(requestText, responseText);
               recordTelemetry(request, requestText, env, durationMs, cacheTier, shape);
             } catch {
diff --git a/workers/test/telemetry-integration.test.mjs b/workers/test/telemetry-integration.test.mjs
index b79e50c..284fcbc 100644
--- a/workers/test/telemetry-integration.test.mjs
+++ b/workers/test/telemetry-integration.test.mjs
@@ -49,6 +49,7 @@ const tsconfig = {
   include: [
     join(WORKERS_ROOT, "src", "tokenize.ts"),
     join(WORKERS_ROOT, "src", "telemetry.ts"),
+    join(WORKERS_ROOT, "src", "tracing.ts"),
     join(WORKERS_ROOT, "src", "zip-baseline-fetcher.ts"),
   ],
 };
@@ -75,8 +76,9 @@ const compile = spawnSync("npx", ["--yes", "tsc", "-p", tsconfigPath], {
 // actually need weren't emitted.
 const tokenizeJs = join(tmp, "build", "tokenize.js");
 const telemetryJs = join(tmp, "build", "telemetry.js");
+const tracingJs = join(tmp, "build", "tracing.js");
 const zipFetcherJs = join(tmp, "build", "zip-baseline-fetcher.js");
-if (!existsSync(tokenizeJs) || !existsSync(telemetryJs) || !existsSync(zipFetcherJs)) {
+if (!existsSync(tokenizeJs) || !existsSync(telemetryJs) || !existsSync(tracingJs) || !existsSync(zipFetcherJs)) {
   console.error("TypeScript compile failed (target files not emitted):");
   console.error(compile.stdout);
   console.error(compile.stderr);
@@ -110,6 +112,7 @@ for (const f of rds(buildDir).filter(n => n.endsWith(".js"))) {
 
 const { measurePayloadShape } = await import(tokenizeJs);
 const { recordTelemetry } = await import(telemetryJs);
+const { RequestTracer } = await import(tracingJs);
 
 // ─── Mock env with writeDataPoint capture ──────────────────────────────────
 
@@ -536,5 +539,83 @@ await test("missing env.ODDKIT_TELEMETRY is a graceful no-op", async () => {
   recordTelemetry(mockRequest(), requestBody, env, 5, "memory", shape);
 });
 
+// ─── Test 7: Streaming-race regression — cacheTier must be read AFTER body ──
+
+await test("cache_tier reads must happen after the streaming response body completes", async () => {
+  // The MCP handler from agents/mcp returns a streaming Response. `await
+  // handler(...)` resolves with the Response object before the tool handler
+  // closure has finished populating the tracer. Reading `tracer.indexSource`
+  // immediately after the await yields "none" for every tool because the
+  // "index" / "index-build" span has not been recorded yet. The fix in
+  // workers/src/index.ts moves the read inside the waitUntil callback,
+  // after the response body has been consumed (which forces the streaming
+  // tool handler to complete).
+  //
+  // This test simulates the timing pattern. A tracer is created, then a
+  // deferred task adds the "index" span asynchronously (mimicking the tool
+  // handler running while the response body streams). We assert:
+  //   (a) the OLD pattern (read immediately) returns "none" — this is the
+  //       observable bug the fix exists to prevent
+  //   (b) the FIXED pattern (read after a microtask flush) returns the
+  //       actual span source — this proves the fix recovers the value
+
+  const tracer = new RequestTracer();
+
+  // Schedule the "index" span for the next tick — this models a streaming
+  // tool handler that has not yet recorded its index access at the moment
+  // the outer handler's `await` resolves.
+  const handlerDone = new Promise((resolve) => {
+    setImmediate(() => {
+      tracer.addSpan("index", 12, "cache");
+      resolve();
+    });
+  });
+
+  // (a) OLD pattern: read tracer.indexSource synchronously, before the
+  // deferred span has been added. This reproduces the production bug.
+  const oldPatternRead = tracer.indexSource;
+  assert.equal(
+    oldPatternRead,
+    "none",
+    "OLD pattern (read immediately after await) returns 'none' — this is the streaming-race bug",
+  );
+
+  // Wait for the deferred span to land (modeling `await responseClone.text()`
+  // forcing the streaming tool handler to finish).
+  await handlerDone;
+
+  // (b) FIXED pattern: read tracer.indexSource AFTER the deferred work has
+  // completed. The tracer now reflects the actual cache tier.
+  const fixedPatternRead = tracer.indexSource;
+  assert.equal(
+    fixedPatternRead,
+    "cache",
+    "FIXED pattern (read after body consumption) returns the actual span source",
+  );
+
+  // Round-trip: feed the fixed value through recordTelemetry and verify it
+  // lands in blob9.
+  const env = mockEnv();
+  const requestBody = JSON.stringify({ jsonrpc: "2.0", id: 1, method: "tools/call", params: { name: "oddkit_search", arguments: { input: "test" } } });
+  const responseBody = JSON.stringify({ jsonrpc: "2.0", id: 1, result: { content: [{ type: "text", text: "ok" }] } });
+  const shape = await measurePayloadShape(requestBody, responseBody);
+  recordTelemetry(mockRequest(), requestBody, env, 42, fixedPatternRead, shape);
+  assert.equal(env.ODDKIT_TELEMETRY.writes.length, 1, "exactly one data point written");
+  assert.equal(
+    env.ODDKIT_TELEMETRY.writes[0].blobs[8],
+    "cache",
+    "blob9 (cache_tier) carries the post-body-consumption tracer value",
+  );
+
+  // Sanity: if we had used the broken old-pattern read, blob9 would be "none"
+  const env2 = mockEnv();
+  recordTelemetry(mockRequest(), requestBody, env2, 42, oldPatternRead, shape);
+  assert.equal(
+    env2.ODDKIT_TELEMETRY.writes[0].blobs[8],
+    "none",
+    "blob9 with the OLD-pattern read would be 'none' — what production has been recording",
+  );
+});
+
 console.log(`\n${pass} passed, ${fail} failed`);
 process.exit(fail > 0 ? 1 : 0);