(watch) `cdk watch` hangs indefinitely (no error) on a stack with a `CfnParameter` that has no default

[ShaMan123]

Summary

cdk watch hangs indefinitely — with no error, progress, or fallback — when a watched stack declares a CfnParameter that has no default. The hotswap template evaluator can't resolve the parameter, and instead of erroring or falling back, watch silently sits at deploying... forever.

Environment

• aws-cdk (CLI): 2.1128.0 (build 7daa104)
• aws-cdk-lib: 2.258.1
• node: v24.12.0
• OS: macOS 26.5.1 (Darwin)

Reproduction

A stack that declares a parameter without a default, plus a hotswappable resource:

new cdk.CfnParameter(this, 'Foo', { type: 'String' }); // no default
new lambda.Function(this, 'Fn', { /* inline code */ });

cdk watch (after the stack already exists, so watch attempts a hotswap) prints MyStack: deploying... and then hangs forever — no progress, no error, no hotswap, no fallback. CPU idle, no network.

• cdk deploy --hotswap MyStack of the same stack does not hang.
• Adding default: '' to the parameter, or removing the parameter, fixes the hang.
• Passing --parameters does not help (hotswap's read-only template evaluation appears to resolve from template defaults, not CLI --parameters).

Analysis

This looks like the hotswap template evaluator failing to resolve a parameter with no default (cf. aws/aws-cdk#22323), but in watch it manifests as a silent infinite hang rather than an error or fallback.

Expected

Error out (or fall back to a full CloudFormation deployment), not hang silently.

Workaround

Give every CfnParameter a default so the hotswap evaluator can resolve it; the real value is still supplied at deploy via --parameters (which overrides the default), and cdk deploy falls back correctly when there's a non-hotswappable change:

new cdk.CfnParameter(this, 'Foo', { type: 'String', default: '' }); // unblocks watch

Better still (AWS recommends against CfnParameter with CDK): avoid the parameter entirely — resolve the value at synth (context/env) or, for secrets, via a CloudFormation dynamic reference such as cdk.SecretValue.secretsManager(...) / {{resolve:secretsmanager:...}}, which keeps the value out of the template and leaves no CfnParameter for the evaluator to choke on. (Note: Cognito UserPoolIdentityProvider.ProviderDetails accepts secretsmanager dynamic references but not ssm-secure ones.)

Related

• aws/aws-cdk#22323 — hotswap deploy fails on CfnEvaluationException (same template-evaluator root cause; here it surfaces as a silent hang in watch rather than an error).