GitHub Copilot 工程复盘：优化 AI 编码成本与质量平衡

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.

Figure 1: Four independent A/B experiments using the same AI-credit metric. The segments are shown together for comparison; their effects are not necessarily strictly additive.

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.

Figure 2: A shorter tool response can make the completed task more expensive when missing details force the agent to reread output, rerun commands, and carry more context forward.

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 grep can 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.

Figure 3: The shipped compressor preserves source-like output, reorganizes search results without loss, and compresses only predictable repetitive noise while retaining the full original.

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.

Figure 4: Removing line-number prefixes preserves the source exactly while eliminating formatting that was repeated across every file read.

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.