TestState does not report variables assigned by the state under test

[gjoseph]

TestState does not report variables assigned by the state under test

Is there an existing issue for this?

• I have searched the existing issues

Summary

The Step Functions TestState API returns inspectionData.variables containing only the
variables supplied in the request's variables parameter. Variables the state under test
assigns via Assign are missing. AWS returns the post-state set — request variables merged
with everything the state assigned.

Per the AWS API reference for InspectionData.variables:

JSON string that contains the set of workflow variables after execution of the state. The
set will include variables assigned in the state and variables set up as test state input.

The returned string preserves the request payload's whitespace verbatim
({"fromInput": "kept"}), which suggests the request parameter is echoed back rather than the
post-state environment being serialized.

Affects both JSONata and JSONPath states.

Environment

• LocalStack 2026.8.0:ad728e1ae (localstack/localstack-pro:latest), pro edition, licensed.
  Also reproduced on 2026.7.0.dev21:0e6b6e128.
• Docker Desktop, 7.0.12-linuxkit, aarch64
• awscli 2.36.31

Minimal reproduction

export AWS_ENDPOINT_URL=http://localhost.localstack.cloud:4566 \
       AWS_ACCESS_KEY_ID=test AWS_SECRET_ACCESS_KEY=test AWS_REGION=ap-southeast-2

aws stepfunctions test-state \
  --definition '{"QueryLanguage":"JSONata","Type":"Pass","Assign":{"assignedHere":"{% 42 %}"},"End":true}' \
  --input '{}' --variables '{"fromInput": "kept"}' --inspection-level DEBUG

LocalStack — the assignment is absent:

"inspectionData": { "input": "{}", "variables": "{\"fromInput\": \"kept\"}" }

Real AWS, same call:

"inspectionData": { "input": "{}", "variables": "{\"assignedHere\":42,\"fromInput\":\"kept\"}" }

Identical divergence with JSONPath, so it is not specific to the JSONata evaluator:

--definition '{"Type":"Pass","Assign":{"assignedHere":42},"End":true}'

The same request with the value moved into Output returns it correctly
({"fromInput": "kept", "assignedHere": 42}), so the evaluation is fine — only the reporting of
assignments is not.

Scope: the assignment itself works

Only TestState's reporting is affected — a real execution propagates the assigned variable
correctly, so this is not a gap in Assign support:

aws stepfunctions create-state-machine --name assign-probe \
  --role-arn arn:aws:iam::000000000000:role/r \
  --definition '{"QueryLanguage":"JSONata","StartAt":"S1","States":{
    "S1":{"Type":"Pass","Assign":{"x":"{% 42 %}"},"Next":"S2"},
    "S2":{"Type":"Pass","Output":"{% {\"seenByNextState\": $x} %}","End":true}}}'
# execution output: {"seenByNextState":42}

Relation to existing issues

#13215 reported request variables not being interpolated at all, fixed by #13827 ("Set
variables from API call parameter to test state execution environment"). That fixed the read
path — {% $fromInput %} now resolves. This report is the write-back path, which #13827 did not
cover. #13215 does note "variables is null in inspectionData" in passing, but its repro and
fix are both about reading request variables.

Impact

It fails silently rather than erroring, which makes it worse than an unsupported feature. A test
that exercises an Assign expression and asserts on inspectionData.variables reads its own
input back and passes — hiding both a broken expression and this gap.

We hit it with a JSONata expression stamping {$states.context.State.Name: $now()} into an
accumulating variable. The probe passed against real Step Functions and "passed" against
LocalStack while reporting one key where two were expected; it only surfaced because the
assertion happened to check the key count. Every JSONata feature involved — $now(),
$states.context.State.Name, computed object keys, $merge — works correctly on LocalStack.

Expected

inspectionData.variables should contain the workflow variables after the state executes:
request-supplied variables merged with those the state assigns.

Workaround

Evaluate the expression through Output rather than Assign and read the response's output
field; that path is correct on LocalStack.

full disclosure: this report was written by/with Claude but I believe it's correct