diff --git a/docker-compose.yml b/docker-compose.yml
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+# Local dev topology: backend (FastAPI) + frontend (Vite dev server) + Jaeger
+# (OTLP collector + UI). One command to a working stack with traces.
+#
+# `docker compose up` brings the three services up on their published ports.
+# Backend at http://localhost:8000, frontend at http://localhost:5173, Jaeger
+# UI at http://localhost:16686. The frontend image (built by ticket #21) ships
+# the Vite dev server bound to 0.0.0.0:5173 so HMR works through the published
+# port.
+
+services:
+  app:
+    build: .
+    ports:
+      - "8000:8000"
+    env_file:
+      - path: .env
+        required: false
+    depends_on:
+      - jaeger
+    environment:
+      # OTLP gRPC endpoint inside the compose network — the SDK auto-detects
+      # OTEL_EXPORTER_OTLP_ENDPOINT and emits to Jaeger's OTLP receiver.
+      - OTEL_EXPORTER_OTLP_ENDPOINT=http://jaeger:4317
+      - OTEL_EXPORTER_OTLP_PROTOCOL=grpc
+      - OTEL_SERVICE_NAME=harness-python-react
+
+  frontend:
+    build: ./frontend
+    ports:
+      - "5173:5173"
+    depends_on:
+      - app
+    # Bind-mount the source so Vite HMR sees host edits. node_modules stays
+    # inside the container so platform-native deps aren't shadowed.
+    volumes:
+      - ./frontend:/app
+      - /app/node_modules
+
+  jaeger:
+    image: jaegertracing/all-in-one:latest
+    ports:
+      - "16686:16686"  # Jaeger UI
+      - "4317:4317"    # OTLP gRPC
+      - "4318:4318"    # OTLP HTTP