golang/go

cmd/go: go mod tidy can automatically maintain toolchain directive when godebug default requires a higher toolchain

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#79153 opened on May 1, 2026

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FeatureRequestFixPendingGoCommandNeedsInvestigationToolProposalhelp wanted

Description

This is a minor friction report and a suggested feature request to address it, similar to #74739, except it's about the godebug directive and its default key.

Consider a module with a go 1.25.0 directive. If a godebug default directive is added with something higher than go1.25.0, then that becomes the minimum toolchain when working inside the module. It'd be good if the go command noticed that and automatically maintained the toolchain directive. As is, it leaves the module in a state where if an older go toolchain is used, it fails to parse the go.mod file:

$ GOTOOLCHAIN=local go version
go version go1.25.0 darwin/arm64
$ cd $(mktemp -d)
$ go mod init test
$ go get go@1.25.0
$ go mod edit -godebug default=go1.26.0
$ go mod tidy
$ go list -m
go: error loading go.mod:
go.mod:5: default=go1.26.0 too new (toolchain is go1.25.0)

If go mod tidy were to add a toolchain go1.26.0, then go list -m could succeed via the usual https://go.dev/doc/toolchain mechanism:

$ go list -m
go: downloading go1.26.0 (darwin/arm64)
test

The workaround for now is to manually maintain the toolchain directive (as was done in CL 772660 for example).

CC @golang/command-line.

Contributor guide

Tech stack
go
Domain
toolingdeveloper experience
Issue type
feature
DifficultyEstimated implementation difficulty for a new contributor, from 1 for very small changes to 5 for expert-level work.
3
Estimated timeA rough time range for an experienced contributor to investigate, implement, test, and prepare a pull request.
1-3 hours
Activity statusHow available the issue appears right now: fresh, active, stale, blocked, or waiting on maintainer input.
active
ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.
clear
Prerequisites
Understanding of Go module systemFamiliarity with go.mod directivesKnowledge of Go toolchain versioning
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
35
Research direction
Examine the existing handling of toolchain directives in go mod tidy (see #74739 and CL 772660). Understand how the godebug default key sets the minimum toolchain. Then modify the tidy command to automatically add or update the toolchain directive when necessary. Test with a module where godebug default is higher than go directive.