fix(deploy): only warn about --region on v0 projects when it was explicitly passed

[jariy17]

What

Follow-up to #2601 (merged). That PR added a warning for --region being silently ignored on v0 deploys, but the warning fired on every v0 deploy (and every agentcore dev on a v0 project), even when --region was never passed at all.

Why

#2601 checked ctx.value(RegionKey) !== undefined to decide whether to warn. That's never undefined by the time a handler runs: withRegion() (src/middleware/withRegion.tsx) always overwrites RegionKey with a fully-resolved value (flag → env → config → DEFAULT_REGION) before any handler sees it. So the check was always true, and the warning became unconditional noise on every v0 deploy — caught in review on #2601 before merge, filed here as the fix.

How

• Added RegionExplicitKey (src/handlers/keys.tsx) — a separate context key holding whether --region was actually passed, set by withRegion() from the raw flag value before it resolves and overwrites RegionKey. RegionKey itself and every other reader of it (deploy/index.ts's v1 native path, dev/index.ts, resolveRegion()) are untouched.
• The warning now checks RegionExplicitKey instead.
• Moved the warning (and the assertNoPendingMigration check it was duplicated around) into delegateV0Deploy itself, called once from both entry points, instead of being duplicated at each call site.
• The warning now prints even under --json — stderr carries informational lines regardless of --json (matching the existing deployedMessage precedent), so a scripted caller passing --region on a v0 project still finds out.
• Reworded the message: v0 also falls back to AWS_REGION/the AWS profile, not only aws-targets.json.
• Added the missing no-flag regression test (asserts the warning does not fire with no --region), and tightened the existing dev test from toContain(...) to an exact match so it actually catches a spurious warning like this one.

Testing

• bun test (4574 pass, 0 fail)
• bun run typecheck
• bun run lint:check
• bun run format:check
• bun run build
• e2e — not applicable, same rationale as fix(deploy): stop forwarding --region to the v0 deploy shim #2601

Checklist

• Read CONTRIBUTING
• Added tests proving the fix
• Docs — not applicable, no documented behavior change
• No new warnings
• Dependent changes merged (fix(deploy): stop forwarding --region to the v0 deploy shim #2601)

[agentcore-devx-automation]

Claude Security Review: no high-confidence findings. (run)

[agentcore-devx-automation]

AgentCore Harness Review

Verdict: Looks good

Nice fix. The prior warnIfRegionIgnored check (ctx.value(RegionKey) !== undefined) was effectively always true after withRegion ran, which meant the warning fired on every v0 deploy regardless of whether --region was actually passed. Splitting the "did the user ask?" bit into a dedicated RegionExplicitKey captured before withRegion resolves the default is the right shape, and threading it through the middleware keeps the detection away from flag parsing internals.

A few things I checked that look correct:

• withRegion is installed at the root (src/handlers/index.tsx), so every routed command sees RegionExplicitKey set to a real boolean before the deploy handler runs. The agentcore dev → createDeployProjectHandler(...).handle(ctx, ...) passthrough also inherits the same ctx, so dev invoking a v0 deploy will still warn correctly when --region was passed to dev.
• Computing explicit from ctx.value(RegionKey) before the ctx is replaced with the resolved region is the only correct ordering, and that's what the diff does.
• Emitting the warning even with --json is right — stderr is informational and isn't part of the JSON output contract; the test matrix now covers both --yes and --json paths.
• The new "does not warn about --region when it was never passed" test closes the regression the previous implementation would have missed.

No telemetry gap worth flagging (warning-message bug fix), and the tests use real temp dirs rather than fs mocks.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.58%. Comparing base (3fb8f16) to head (99af320).

Additional details and impacted files

@@             Coverage Diff              @@
##           refactor    #2603      +/-   ##
============================================
- Coverage     96.58%   96.58%   -0.01%     
============================================
  Files           700      700              
  Lines         57639    57638       -1     
============================================
- Hits          55673    55672       -1     
  Misses         1966     1966              

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[nborges-aws]

LGTM