New Relic agent developer mode vs. local mock collector for integration tests without production credentials

Direct answer

Choose developer mode when you only need to verify that the agent’s instrumentation hooks are exercised and that no data is sent; choose a local mock collector when you must inspect the exact payload (headers, JSON/MsgPack body) to assert on metric or event structure.

Likely explanation

Developer mode initializes the agent, runs the normal metric collection and serialization pipeline, then discards the payload before transmission. This means the serialization code path is exercised, but older agents may short‑circuit or reject a placeholder license key, so the behavior is not uniform across versions. A mock collector receives the full payload, letting you verify structure, but you must prevent any fallback to the real collector.

Confirmed facts (from New Relic documentation and agent behavior)

• Developer mode (developer_mode: true or NEW_RELIC_DEVELOPER_MODE=1) initializes instrumentation and typically runs the full payload serialization path; data is dropped locally.
• Some agents released before ~2020 require a syntactically valid license key even in developer mode and will fail to start with a dummy key.
• A local mock collector (HTTP or gRPC stub) bound to 127.0.0.1 captures the exact payload; setting the agent’s host to 127.0.0.1 and providing any non‑empty license key prevents the agent from attempting the default collector.newrelic.com endpoint.
• If the mock is unreachable, the agent logs connection errors but does not fall back to the real collector when a host override is present.
• Startup log lines that confirm the configured mode or endpoint are language‑specific (see table below).

Steps for developer mode

1. Set developer_mode: true in the agent config or export NEW_RELIC_DEVELOPER_MODE=1.
2. Provide any non‑empty string as license_key (required for agents that validate the key).
3. Run your integration test suite.
4. Check the agent logs for the developer‑mode startup line (see table).
5. Optionally run netstat -tunp or tcpdump to verify no outbound traffic to collector.newrelic.com.

Steps for local mock collector

1. Start a stub HTTP (or gRPC) server listening on 127.0.0.1: that records incoming requests.
2. Configure the agent:
   • host: 127.0.0.1
   • port: <port>
   • license_key: dummy (any non‑empty value)
   • ssl: false (if the agent permits disabling TLS for the custom host)
3. Run the integration test.
4. Assert that the stub received the expected payload (headers, body).
5. Stop the stub and verify the agent logs connection errors but no attempts to reach collector.newrelic.com.

Startup log lines to assert on

Agent	Log line indicating developer mode or custom host
Java	New Relic Agent started in developer mode or Connecting to collector at 127.0.0.1:
Node.js	New Relic agent started in developer mode
Python	Developer mode enabled
Go	Starting agent in developer mode
.NET	New Relic Agent started in developer mode
Ruby	New Relic agent started in developer mode
PHP	New Relic agent started in developer mode

Missing diagnostic detail

To be certain that developer mode will not fail due to license‑key validation, please confirm the specific New Relic language agent and its version you are using. If it is a pre‑2020 release, you may need to provide a valid‑looking license key or switch to the mock collector approach.