android-probe-logging

Android Probe Logging — Ephemeral Log.d Instrumentation

When to use

• Confirming a branch, callback, or lambda actually ran
• Tracing the order of async / coroutine operations
• Inspecting the actual values flowing through a function (params, return, intermediate state)
• Disambiguating "the fix didn't work" from "the fix never ran"
• Verifying a when / if arm landed where you expected

When NOT to use

• The question is about UI state — use verify-android-layout instead
• The question is about main-thread blocking or leaks — use android-strictmode-probe
• The question is "is this code expensive / on which thread?" — use android-trace-sections
• The question is "which of 1000 commits broke it?" — use android-regression-diff-scan

Pre-flight: detect what your project supports

Before instrumenting, confirm two things:

# 1. adb is connected and the device is authorized
adb devices                                    # expect "<id>  device"

# 2. The package builds and installs in debug
ls app/src/main/java app/src/main/kotlin 2>/dev/null    # which language?

Language note. The examples below use Kotlin (private const val PROBE = ...). For a Java codebase, the equivalent is:

private static final String PROBE = "AGENT_PROBE_a4f9c2e1";
// ...
Log.d(PROBE, "fetchUser id=" + id + " cached=" + cache.contains(id));

The cleanup grep for AGENT_PROBE_ catches both forms — sentinel hygiene is language-agnostic.

Mixed Kotlin/Java module. Place the constant in whichever language the file you're probing uses; mixing is fine since String is String on the JVM.

No Android Log import in the file? Add import android.util.Log (Kotlin) or it's resolvable as Log.d(...) once android.util.Log is on the classpath (always true for an app module). Lint may flag the import on cleanup — make sure rg 'AGENT_PROBE_' is empty before relying on lint, since stale probes can keep an unused import alive.

The pattern: probe → run → observe → remove

The shape is non-negotiable. The most common failure mode is leaving probes in committed code. The unique sentinel tag is what makes cleanup tractable.

1. Pick a sentinel tag for this investigation

Generate a short unique id and bake it into the tag:

private const val PROBE = "AGENT_PROBE_a4f9c2e1"

One id per investigation. Don't reuse across sessions — leftover probes from yesterday will pollute today's logs.

2. Insert probes at decision boundaries

Probe at points where execution order or state is non-obvious:

• Branch entries (if, when, else)
• Before and after suspect calls
• Inside lambdas, callbacks, coroutine launch / collect blocks
• Early returns

Log inputs and outputs, not "got here":

// Bad — tells you nothing you didn't already guess
Log.d(PROBE, "in fetchUser")

// Good — captures the actual data
Log.d(PROBE, "fetchUser id=$id cached=${cache.contains(id)}")
// ... call ...
Log.d(PROBE, "fetchUser id=$id -> ${result.javaClass.simpleName}")

For exceptions, log the type and message but not full stack — adb logcat will already have the stack from AndroidRuntime if it propagates:

runCatching { suspectCall() }
    .onFailure { Log.d(PROBE, "suspectCall threw ${it::class.simpleName}: ${it.message}") }
    .onSuccess { Log.d(PROBE, "suspectCall ok=$it") }

3. Redeploy and drive the app

android run                             # or your usual deploy command
adb logcat -c                           # clear stale logs
# ... drive the app to the suspect state via taps/swipes/input ...
adb logcat -d -s AGENT_PROBE_a4f9c2e1 > /tmp/probe-a4f9c2e1.log

-c clears, -d dumps and exits, -s <tag> filters to your sentinel only. The output file should contain only your probe lines.

4. Delegate parsing for anything non-trivial

Inline-read the log if it's under ~30 lines. Otherwise spawn a Sonnet sub-agent with a self-contained prompt:

Read /tmp/probe-a4f9c2e1.log. Confirm the sequence: (a) fetchUser called with id=42, (b) cache miss, (c) network branch entered, (d) result logged with class User. Answer YES/NO + one sentence on what's missing or out of order. Under 40 words.

Pass model: "sonnet" — narrow text parsing doesn't need Opus.

5. Cleanup gate (BLOCKING — do not skip)

Before declaring the task done, run from the repo root:

rg 'AGENT_PROBE_a4f9c2e1'

(Substitute your investigation's id.) The expected output is empty. If it's not, remove every match — including the private const val PROBE = ... declaration line — and run again. Only then is the task done.

If you used multiple sentinel ids across one investigation, grep for the shared prefix to catch them all:

rg 'AGENT_PROBE_'

This is the single most-skipped step. Sentinel tags exist precisely to make this gate trivial; use them.

Probe placement examples

Coroutine flow — confirm collection actually fires:

viewModelScope.launch {
    Log.d(PROBE, "collect start state=${_state.value}")
    repository.observeUser()
        .onEach { Log.d(PROBE, "emit ${it.id}") }
        .collect {
            Log.d(PROBE, "collect received ${it.id}")
            _state.value = it
        }
}

Conditional logic — confirm which arm executes:

when (status) {
    Status.Loading -> { Log.d(PROBE, "Loading arm"); /* ... */ }
    Status.Error -> { Log.d(PROBE, "Error arm err=${error?.message}"); /* ... */ }
    Status.Success -> { Log.d(PROBE, "Success arm n=${data.size}"); /* ... */ }
}

Compose recomposition — confirm a composable re-runs:

@Composable
fun UserRow(user: User) {
    Log.d(PROBE, "UserRow recompose id=${user.id} v=${user.version}")
    // ...
}

Common mistakes

Mistake	Fix
Skipping the cleanup gate	rg 'AGENT_PROBE_' must return zero before commit; this is non-negotiable
Generic tag like "DEBUG" or "TAG"	Use a unique AGENT_PROBE_<8-char-id> per investigation so cleanup is greppable
Probe says "got here" with no data	Log the actual inputs/outputs; "got here" tells you nothing you didn't already guess
Reading a 500-line probe log inline	Delegate to a Sonnet sub-agent with explicit criteria and a return-format cap
Forgetting adb logcat -c before driving the app	Stale logs from earlier runs will mix in and confuse the sub-agent's verdict
Reusing yesterday's sentinel id	Old probes from a prior investigation pollute today's results — fresh id each time
Letting the sub-agent default to Opus	Always pass model: "sonnet" — narrow text parsing
Committing the private const val PROBE line	The sentinel constant counts as a probe; the cleanup grep catches it, remove it too