adkgo v2.1.0 uses incompatible Go builder version

[Antupis]

🔴 Required Information

Describe the Bug:
adkgo v2.1.0 deploy agentengine generates an InlineSource Dockerfile using golang:1.25, but google.golang.org/adk/v2 v2.1.0 declares Go 1.26.5 in its go.mod.

The generated Dockerfile is defined here:

adk-go/cmd/adkgo/internal/deploy/agentengine/agentengine.go

Line 187 in f4c7670

FROM golang:1.25 as builder

As a result, a Go ADK v2.1.0 application that builds successfully with Go 1.26.5 fails during the managed Agent Engine source-image build.

Steps to Reproduce:

1. Create a Go agent using google.golang.org/adk/v2 v2.1.0.
2. Ensure its go.mod contains:

   go 1.26.5
   
   require google.golang.org/adk/v2 v2.1.0

3. Confirm the project builds locally:
   go mod tidy
   go test ./...
   go vet ./...
   go build ./...
4. Deploy it using the pinned v2.1.0 CLI:
   go run google.golang.org/adk/v2/cmd/adkgo@v2.1.0 deploy agentengine
   --entry_point_path=./main.go
   --name=adk-go-v2-inline-test
   --project_name="$GOOGLE_CLOUD_PROJECT"
   --region=europe-west1
   --source_dir=.
5. The source archive uploads successfully, but the managed build fails:
   Sending CreateReasoningEngine request...
   Waiting for operation to complete...

Error: operation failed: rpc error: code = InvalidArgument desc = Build failed.
The issue might be caused by incorrect code, requirements.txt file or other dependencies.
6. Inspect the Dockerfile generated by adkgo. It contains:
FROM golang:1.25 as builder
WORKDIR /app
COPY . .
RUN CGO_ENABLED=0 GOOS=linux GOARCH=amd64 go build ...
Expected Behavior:
adkgo v2.1.0 deploy agentengine should generate a builder image compatible with the Go version required by ADK v2.1.0, allowing a locally valid ADK Go v2.1.0 application to deploy successfully.
For example:
FROM golang:1.26.5 AS builder
Alternatively, the builder Go version could be derived from the application or ADK module requirements rather than being hardcoded.
Observed Behavior:
The generated Dockerfile hardcodes:
FROM golang:1.25 as builder
The managed Agent Engine image build subsequently fails with:
Error: operation failed: rpc error: code = InvalidArgument desc = Build failed.
The issue might be caused by incorrect code, requirements.txt file or other dependencies.
The same source passes go test, go vet, and go build with Go 1.26.5.
Environment Details:

• ADK Library Version: google.golang.org/adk/v2 v2.1.0
• OS: Linux, arm64
• Go Version: go1.26.5 linux/arm64
• Deployment region: europe-west1
• Deployment command: go run google.golang.org/adk/v2/cmd/adkgo@v2.1.0 deploy agentengine

[bilus]

I can confirm; ran into the same issue today. Downgrading to 2.0.0 makes the problem go away.

The blocker is primarily caused by GOTOOLCHAIN=local. Ideally, that would be changed to auto so this problem doesn't reappear once the ADK library upgrades to Go 1.27 (but I don't know what other considerations went into the decision to use local).

[indurireddy-TF]

I was able to reproduce this issue locally. The build fails with go: go.mod requires go >= 1.26.5 (running go 1.25.12; GOTOOLCHAIN=local), confirming the Go version mismatch between go.mod and the generated Agent Engine Dockerfile (FROM golang:1.25 as builder).

[indurireddy-TF]

Possible solutions:

1.Update the hardcoded builder image from golang:1.25 to golang:1.26.5 to match the current Go version required by the module.

2.Instead of hardcoding the builder image, read the Go version from the deployed application's go.mod and generate FROM golang: AS builder, so it stays in sync with future Go version updates.