2.10Communication Failures

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By the end of this section you can answer

The team has a channel and still fails. Which cause is it?

Communication can fail even when a channel exists and every agent is trained successfully.

In this section you will

• Distinguish missing information from irrelevant information
• Distinguish a lost message from an ambiguous one
• Recognize a protocol mismatch between agents
• Match each symptom to the intervention that can actually repair it

Missing Information

The sender holds something useful and does not transmit it.

Agent 1 can see the order ticket and never mentions the dish. The information exists in the system, the channel exists, and the two are not connected.

How to spot it: compare each agent’s private information against what it sends. Anything the receiver could act on and never hears is a candidate.

Where it comes from: usually a communication policy that was never pressured to find the useful message, either because $\lambda$ was high enough to discourage sending anything, or because the bootstrapping problem from Learning Communication Protocols never resolved.

Irrelevant Information

Messages are sent and change no decision.

Agent 1 reports the pot temperature, which agent 2 can already see. Or reports the chopping board, which agent 2 cannot use. Bandwidth is spent, nothing changes.

How to spot it: ablate. Replace a message with $\varnothing$ and see whether performance moves. If it does not, that message was not doing anything.

Why it matters even when it seems harmless: it costs $\lambda c_t$ every step, it makes the protocol unreadable by burying the useful part, and under a narrower channel it displaces something that mattered.

Message Loss

The protocol assumes reliable delivery and messages disappear.

Communication Constraints covered the mechanism. The sharp version is that a lost message becomes $\varnothing$, which is a legitimate message meaning “say nothing”, so a protocol in which silence carries meaning misreads loss as content.

How to spot it: sweep $p_{\text{loss}}$ and look at the shape of the decline. A protocol robust to loss degrades gently. One that depends on silence falls off a cliff.

Message Ambiguity

The message arrives and does not narrow the receiver’s options enough.

Agent 1 sends 1, meaning “help needed”. Agent 2 does not know whether to fetch an ingredient, take over the stove, or start plating. It has been told that something is wrong and not what.

How to spot it: look for messages after which the receiver’s action distribution stays wide. If receiving a message leaves the receiver as uncertain as before, it did not resolve anything.

Where it comes from: most often a capacity limit. With two messages available, several situations must share one symbol, and the receiver has to disambiguate from context. That is a design choice with a known cost, not necessarily a bug.

Protocol Mismatch

Two individually successful agents interpret the same message differently.

Key takeaway

This is the important one, and it is unlike the other four.

In the first four, something is wrong: information withheld, bandwidth wasted, a channel dropping, a distinction unmade. Fix the sender, the channel, or the capacity and the problem goes away.

In protocol mismatch nothing is wrong with either agent. Each has a complete, working, well-tested protocol. What is missing is agreement, and agreement is not a property either agent can hold by itself.

Agent A learned 2 means bring an ingredient. Agent D learned 2 means serve the dish. Both scored highly with their own partners. Paired, they fail, and no amount of retraining either one alone identifies the fault.

How to spot it: cross-play. Pair policies that were not trained together and compare against their within-team scores. A large drop is the signature. the Adapt chapter makes this a measurement discipline.

Why it has no single-agent analogue: there is no such thing as a convention held by one agent. The failure lives in the relationship, so it is invisible to any evaluation that tests agents only with the partners they trained with.

Failure Diagnosis

Failure	What is wrong	First thing to try
Missing information	useful facts never sent	check private information against messages sent
Irrelevant information	messages change no decision	ablate messages and watch performance
Message loss	protocol assumes delivery	sweep $p_{\text{loss}}$
Ambiguity	message does not narrow options	check the receiver’s action spread after a message
Protocol mismatch	nothing, in either agent	cross-play against unfamiliar partners

Knowledge check

A team scores 94% together. Each agent, paired with an equally strong agent from a different training run, scores 31%. Which failure is this?

Select one answer.

AProtocol mismatch. Both protocols work internally and assign symbols differently, so nothing is wrong with either agent.BAmbiguity. The messages are not specific enough for the new partner.CMessage loss on the new channel.DIrrelevant information. The messages were never carrying anything useful.

Check answer

Communication Failures Summary

• Missing information: useful private facts are never sent.
• Irrelevant information: messages arrive and change no decision. Ablate to detect.
• Message loss: the protocol assumed delivery. Worst where silence carries meaning.
• Ambiguity: the message does not narrow the receiver’s options, usually a consequence of limited capacity.
• Protocol mismatch: two working protocols disagree. Nothing is wrong with either agent, the failure lives in the relationship, and only cross-play reveals it.