PR tools re-fetch data without conditional requests (ETag / If-None-Match) #3025

Summary

The REST client is built with a transport chain (http.DefaultTransport → user-agent → bearer-auth) that has no response-cache or conditional-request layer, so pull-request tools (pull_request_read with get / get_files / get_commits / get_reviews / get_status, and list_pull_requests) issue unconditional GET requests. Repeated identical reads within or across sessions re-download full responses even when nothing has changed; the ETag GitHub returns is never captured or replayed as If-None-Match.

Where

• Client/transport construction (single site): internal/ghmcp/server.go — createGitHubClients() wires UserAgentTransport{ Transport: http.DefaultTransport } (and BearerAuthTransport) with no caching transport.
• Transport package has only user-agent / bearer / graphql-features round-trippers: pkg/http/transport/.
• PR tool handlers call client.PullRequests.Get/ListFiles/ListCommits/ListReviews/List unconditionally: pkg/github/pullrequests.go.

Impact on users

GitHub's REST API supports conditional requests: replaying the previous ETag as If-None-Match returns 304 Not Modified when nothing changed, and 304 responses don't count against the token's primary rate limit. Agentic sessions frequently re-read the same pull request (e.g. get → get_files → get_reviews, retries, multi-step reasoning). Without conditional requests, each re-read is a full response that spends rate-limit budget and bandwidth, making the token's limit easier to hit and slowing tool calls.

Example prompts that trigger repeated reads

• "Summarize PR Issue: Fix incorrect VS Code installation URL (both buttons point to Insiders) #123, then list its changed files, then show review comments." (multiple reads of the same PR)
• "Watch PR Issue: Fix incorrect VS Code installation URL (both buttons point to Insiders) #123 and tell me when checks pass." (repeated get_status / get)
• "Compare the files and commits in PR Issue: Fix incorrect VS Code installation URL (both buttons point to Insiders) #123 and PR Fix VS Code installation link #124." (list + per-PR reads)

Suggested change

Insert an ETag-revalidating http.RoundTripper at the single client-construction site in createGitHubClients(), below the user-agent transport: cache {etag, body} keyed by (auth scope, method, URL), send If-None-Match on GETs, and return the cached body on 304. An always-revalidate ETag transport keeps tool results fresh (no stale window) while turning unchanged reads into cheap 304s. The GraphQL client is unaffected (POST). This keeps results identical while reducing rate-limit pressure and bandwidth.