diff --git a/src/daemon.ts b/src/daemon.ts
index f2c7d25..122cfe4 100644
--- a/src/daemon.ts
+++ b/src/daemon.ts
@@ -1222,6 +1222,7 @@ export class GlobalDaemon {
       toolName,
       toolUseId,
       toolInput,
+      conversationId: session.conversationId,
       displayName: toolDisplayName(toolName, toolInput),
     });
     session.pendingToolCalls.set(toolUseId, { span: toolSpan, toolName, toolInput });
diff --git a/src/genaiSpans.ts b/src/genaiSpans.ts
index 9d4813d..1678310 100644
--- a/src/genaiSpans.ts
+++ b/src/genaiSpans.ts
@@ -368,6 +368,8 @@ type ToolSpanArgs = {
   toolName: string;
   toolUseId: string;
   toolInput: Record<string, unknown>;
+  /** Stitching key — same as the enclosing turn's `gen_ai.conversation.id`. */
+  conversationId: string;
   displayName?: string;
 };
 
@@ -377,6 +379,7 @@ export function startToolSpan(tracer: Tracer, parentSpan: Span, args: ToolSpanAr
     [ATTR.TOOL_NAME]: args.toolName,
     [ATTR.TOOL_CALL_ID]: args.toolUseId,
     [ATTR.TOOL_CALL_ARGUMENTS]: jsonStr(args.toolInput),
+    [ATTR.CONVERSATION_ID]: args.conversationId,
   };
   if (args.displayName) attrs[ATTR.WEAVE_DISPLAY_NAME] = args.displayName;
 
diff --git a/tests/interleaved-assistant-spans.test.ts b/tests/interleaved-assistant-spans.test.ts
index fb9cf95..eb40369 100644
--- a/tests/interleaved-assistant-spans.test.ts
+++ b/tests/interleaved-assistant-spans.test.ts
@@ -74,6 +74,7 @@ test('chat span parents per-block assistant_text and execute_tool children in tr
     toolName: 'Edit',
     toolUseId: 'toolu_01',
     toolInput: { file_path: '/foo.ts' },
+    conversationId: 'conv-1',
   });
   t1.end();
   emitAssistantTextSpan(tracer, chat, {
@@ -84,6 +85,7 @@ test('chat span parents per-block assistant_text and execute_tool children in tr
     toolName: 'Edit',
     toolUseId: 'toolu_02',
     toolInput: { file_path: '/foo.test.ts' },
+    conversationId: 'conv-1',
   });
   t2.end();
   emitAssistantTextSpan(tracer, chat, {
diff --git a/tests/tool-span-conversation-id.test.ts b/tests/tool-span-conversation-id.test.ts
new file mode 100644
index 0000000..4bc4fa6
--- /dev/null
+++ b/tests/tool-span-conversation-id.test.ts
@@ -0,0 +1,59 @@
+// SPDX-FileCopyrightText: 2026 CoreWeave, Inc.
+// SPDX-License-Identifier: MIT
+// SPDX-PackageName: weave-claude-code
+
+// Regression test: a lost root span (hard crash) orphaned already-exported
+// tool spans that had no conversation id of their own.
+
+import { test } from 'node:test';
+import assert from 'node:assert/strict';
+import * as fs from 'node:fs';
+import * as os from 'node:os';
+import * as path from 'node:path';
+import {
+  BasicTracerProvider,
+  InMemorySpanExporter,
+  SimpleSpanProcessor,
+} from '@opentelemetry/sdk-trace-base';
+import { GlobalDaemon } from '../src/daemon.ts';
+import { ATTR, OP } from '../src/genaiSpans.ts';
+
+function setupTracer() {
+  const exporter = new InMemorySpanExporter();
+  const provider = new BasicTracerProvider({ spanProcessors: [new SimpleSpanProcessor(exporter)] });
+  return { tracer: provider.getTracer('test'), exporter, provider };
+}
+
+function makeDaemon(tracer: unknown) {
+  const logFile = path.join(os.tmpdir(), `wcp-toolconv-${process.pid}.log`);
+  const d = new GlobalDaemon('/tmp/unused-toolconv.sock', logFile, 'e/p', 'k', 'https://x', false, 'claude-code');
+  (d as unknown as { tracer: unknown }).tracer = tracer;
+  return d as unknown as { routeEvent(p: Record<string, unknown>): Promise<void> };
+}
+
+test('execute_tool spans carry gen_ai.conversation.id so they stitch even without their root', async () => {
+  const sid = 'sess-tool-conv';
+  const dir = fs.mkdtempSync(path.join(os.homedir(), '.weave-toolconv-itest-'));
+  const file = path.join(dir, `${sid}.jsonl`);
+  fs.writeFileSync(file, JSON.stringify({ type: 'user', timestamp: '2026-01-01T00:00:00.000Z', message: { role: 'user', content: [{ type: 'text', text: 'go' }] } }) + '\n');
+
+  const { tracer, exporter, provider } = setupTracer();
+  const d = makeDaemon(tracer);
+  try {
+    await d.routeEvent({ hook_event_name: 'SessionStart', session_id: sid, transcript_path: file, source: 'startup', cwd: '/x' });
+    await d.routeEvent({ hook_event_name: 'UserPromptSubmit', session_id: sid, prompt: 'go' });
+    // The assistant response the tool_use belongs to must be in the transcript before
+    // PreToolUse, so the daemon can parent the tool span under the right chat span.
+    fs.appendFileSync(file, JSON.stringify({ type: 'assistant', timestamp: '2026-01-01T00:00:02.000Z', message: { role: 'assistant', id: 'msgA', model: 'claude-opus-4-8', usage: { input_tokens: 1, output_tokens: 1 }, content: [{ type: 'tool_use', id: 'tool_1', name: 'Bash', input: { command: 'ls' } }], stop_reason: 'tool_use' } }) + '\n');
+    await d.routeEvent({ hook_event_name: 'PreToolUse', session_id: sid, tool_use_id: 'tool_1', tool_name: 'Bash', tool_input: { command: 'ls' } });
+    await d.routeEvent({ hook_event_name: 'PostToolUse', session_id: sid, tool_use_id: 'tool_1', tool_response: 'ok' });
+    await provider.forceFlush();
+
+    const tool = exporter.getFinishedSpans().find((s) => s.attributes[ATTR.OPERATION_NAME] === OP.EXECUTE_TOOL);
+    assert.ok(tool, 'tool span exported');
+    // conversationId === sessionId for a fresh (non-resumed) session.
+    assert.equal(tool.attributes[ATTR.CONVERSATION_ID], sid, 'tool span carries the conversation id');
+  } finally {
+    fs.rmSync(dir, { recursive: true, force: true });
+  }
+});