Output quality is important when working with AI coding agents, but true efficiency comes from getting work done quickly, efficiently, and with the right context.
That’s why token count of individual interactions alone isn’t a meaningful measure of efficiency. The goal shouldn’t be to use fewer tokens, but to tap into the right amount of context to move a task forward. A concise tool response can sometimes require additional calls or work if it leaves out information the agent needs, ultimately making the task slower and more expensive.
That’s why we want to optimize for the outcome rather than the tool call. This post examines four changes in GitHub Copilot that put that principle into practice:
- Preserve useful context while reducing repetitive output.
- Remove formatting that adds no value to the task.
- Shorten instructions without changing useful behavior.
- Deliver completed background work without an extra retrieval step.
Possible changes were evaluated offline using agentic coding benchmarks. The most promising changes were then validated through controlled online experiments before shipping. The examples in this post come from GitHub Copilot CLI. Multiple other Copilot products, such as the GitHub Copilot app and Copilot code review, use the same underlying harness and also become more efficient through these improvements.
The local metric trap
It’s common to shorten the output from each tool call as a way to reduce agent costs. RTK (Rust Token Killer) is a utility that shortens shell output before an agent reads it. We evaluated its effect on GitHub Copilot using our agentic coding benchmarks.
In our harness and benchmark configuration, RTK shortened some responses, but when the omitted text mattered, the model sometimes reopened the original output or reran the command to recover what it needed.
Those recovery steps added turns and carried more context forward. The individual tool response was shorter, but on average, the task used more tokens and took longer. We saved tokens locally and spent more globally.
This result applies to the integration and workloads we tested, not to every RTK configuration or to output compression in general. This meant that tokens per tool call is the wrong objective. An efficiency change has to be evaluated across the complete task, from the user’s request through the final result.
More useful was to look at what can we remove without making the model repeat work.
Compress noise, preserve useful information
The goal was to shorten repetitive output while preserving the context an agent needs to complete its task without retracing steps.
Analysis of benchmark runs showed that install, build, test, and lint output often contains repetitive noise, while source-like output and arbitrary command results are more likely to contain the information an agent needs. That analysis informed a selective output compressor, informed in part by RTK and similar approaches.
The prototype was evaluated on agentic coding benchmarks and a range of open source repositories, exercising their build, test, and lint systems.
Early versions were too aggressive. They made the model repeat work or read the full saved output, increasing end-to-end cost and reducing task success. For example, we initially compressed git diff but removed that filter after benchmark tasks showed agents reopening the original output to recover missing information.
Those early failures led to a three-part policy:
- Preserve source-like and arbitrary output. Commands such as
cat,git diff,git show, and arbitrary scripts are returned unchanged. - Reorganize search results without dropping content. Matches and file lists from tools such as
grepcan be grouped more efficiently while retaining every result. - Compress repetitive noise selectively. Install, build, test, and progress output is compressed only when the savings are substantial.
The shipped version emerged through repeated evaluation and refinement. It is conservative not because the goal was to build a conservative compressor, but because that is what the evaluations supported.
When output is compressed, the agent can still retrieve the complete original through a direct recovery path.
That recovery path is both a safety mechanism and an evaluation signal. We tracked whether the agent opened the saved original, reran commands, repeated exploration, narrowed its searches, or took additional turns. Frequent recovery would indicate that the compressor had removed something valuable.
On offline tasks where output compression triggered, no statistically significant task-success regression was detected, and agents extremely rarely opened the saved originals. In the online experiment, average cost decreased slightly with no material regression detected in the tracked quality metrics.
Remove formatting before removing information
One clean token optimization came from the view tool, which agents use to read file contents into context.
Previously, view prefixed every line with a number before showing the contents to the model. Earlier file-editing tools used those numbers to target changes, but current tools instead match surrounding code and do not use line numbers. The line-number prefixes remained even though the normal workflow no longer used them.
Each prefix was small. Repeated across every line and every file read, however, that unused formatting accumulated throughout a session. So, we removed it.
Line numbers remain useful in diffs and short snippets. They were wasteful here because they were attached to every file read without serving the current editing workflow.
Removing them caused model-inference cost to fall by roughly 5% in offline agentic coding benchmarks. Success rates stayed within the expected run-to-run variance, and edit failures did not increase.
We then tested the change with Copilot CLI users. The online experiment reduced average daily model-inference cost per user by about 3%, with no material regression detected in the quality or satisfaction metrics we tracked.
For developers, that means more of the context window is available for the work itself rather than formatting the agent does not use.
This was the ideal change: no new instructions for the model, no source of information to recover, and no additional decision to make. The file contents reached the model unchanged.
Compress prompts without compressing intent
Prompts carry instructions that shape how an agent works, and they are sent to the model on every turn. Shortening them only improves efficiency if the agent keeps the behaviors developers depend on.
In GitHub Copilot, the task tool launches specialized agents for parallel work. Its guidance had accumulated across tool descriptions, schemas, agent definitions, system instructions, and companion tools.
A meta-prompting loop, in which Copilot iteratively wrote its own prompt, reduced that prompt by roughly half. Copilot produced and refined smaller candidates, and targeted behavioral tests checked the requirements we wanted to preserve.
The first online experiment found a regression that the initial offline evaluations had missed. The meta-prompting loop had rewritten cautious parallelism guidance into a hard scheduling policy, causing independent custom agents to run sequentially.
We stopped the experiment. Before changing the prompt again, we wrote a regression evaluation for the behavior users had exposed. The eventual fix replaced an explicit allowlist and denylist with one sentence:
Independent agents can run in parallel; consider side effects.
That sentence was shorter and less restrictive; it deferred the choice of whether to run sub-agents in parallel to the model instead of the previous explicit guidance. With it, our new behavior test passed without causing any existing behavioral tests to fail.
Prompt behavior needs tests. If a behavior is not tested, a shorter prompt can remove it without anyone noticing.
The shipped prompt removes about 1,300 task-tool prompt tokens per turn, corresponding to approximately 1.8% fewer total prompt tokens per session and 2.9% lower normalized cost per active hour, with no quality regression detected in the measured evaluations.
Agents often run independent work in the background, such as a long-running shell command alongside a sub-agent investigation. Notifications let the agent continue until that work is ready without spending a tool call waiting.
If the agent does not explicitly wait for either task, the harness wakes the model and notifies it when the shell command or sub-agent finishes.
Previously, that notification did not include the completed result, so the agent had to spend another turn retrieving output Copilot had already received. When several tasks finished close together, that detour could repeat. Copilot now batches eligible completion notifications and delivers completed results directly in the existing tool-result format. The agent can continue with the information it needs, without spending an extra turn asking for it again. Explicit reads for work that is still running behave as before.
Before this change, each completed task required one model call to request its result and another to process it. For the shell command and sub-agent shown above, that meant four model calls before work could continue.
Now, the harness batches both completions and supplies their results together, so a single model call can process both. Removing those retrieval detours also avoids carrying the full session context through unnecessary calls.
By delivering completed results directly, without compressing, summarizing, or withholding anything, the harness reduced average token-related usage, as measured in AI Credits, by about 2.3%.
Measure changes in context
A change that saves tokens in one Copilot workflow can increase costs in another.
For example, a tighter set of file-tool instructions was inspired by positive results in Copilot code review. In a Copilot CLI online experiment, it increased cost, so we did not ship it.
By contrast, removing line-number prefixes and selectively compressing output each reduced average prompt tokens per review by roughly 5% in independent evaluations across a large set of Copilot code review tasks using the production model. We detected no material change in the tracked review-quality metrics.
These findings are separate from the earlier migration of Copilot code review to the shared file tools, which, together with review-instruction tuning, reduced code review cost by about 20%.
Each change needs to be measured in the workflow where it runs.
Five lessons for building efficient AI coding agents
- Optimize the completed task, not the tool call. Shorter output is not cheaper if the agent spends more turns recovering what was removed.
- Optimize orchestration, not just model output. Eliminate model turns that perform work the harness can complete deterministically.
- Compress by what the output represents. Preserve exact content, prefer lossless transformations, and measure how often agents use the recovery path.
- Prompt rewrites sometimes have unintended consequences. Validate that intended behavior is preserved.
- Evidence is local to the workload. Re-evaluate changes in offline benchmarks, online experiments, and every product surface where they ship.
None of these changes made the model smarter. They removed work the model never needed to do.
The changes described in this post are shipping across GitHub Copilot experiences that use the same underlying harness.
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