The user wants me to retrieve the contribution_rate for the case. I need to:
1. List documents to find the case inputs
2. Read the document to find the payroll month
3. Look up the reference (first-half or second-half) based on the payroll month
4. Return the contribution_rate as a decimal string

Let me start by listing documents.

There's one payroll document. Let me read it.

The payroll month is 2026-05. May is in the first half of the year (months 1-6 presumably). Let me look up the first-half reference.

The first-half reference covers months 2026-01 through 2026-06 (the "months" field shows the range endpoints). Payroll month 2026-05 falls within this range. The contribution_rate is 0.04.

The answer format for retrieval: `decision: "retrieve"`, `field`, `value`, and `clauses`. Field is `contribution_rate`, value is "0.04" (decimal string, not percentage). Cite R1 (selecting rule) and R3 (source rule for contribution_rate).

Value should be a decimal string: "0.04".

The task is a retrieval task: retrieve `contribution_rate` for the supplied payroll month. I need to:

1. List documents to find the case inputs (payroll month).
2. Read the document to get the payroll month.
3. Look up the reference row for that month via lookup_reference.
4. Return decision "retrieve", field "contribution_rate", value as decimal string, clauses citing R1 and R3.

First, list documents.

The payroll month is 2026-05, which is covered by reference rows (only 2026 is covered). Now I need to look up the reference. The month 2026-05 is in the first half (January–June presumably) or second half. Let me check both halves? The lookup_reference takes "first-half" or "second-half". May 2026 is in the first half of the year (Jan–Jun). Let me look up first-half.

The first-half reference covers months "2026-01" and "2026-06" — interesting, it lists only those two months. 2026-05 is not in that list. Let me check the second half to see if 2026-05 is covered there.

Hmm. The first-half schedule covers months ["2026-01", "2026-06"] and the second-half covers ["2026-07", "2026-12"]. Wait — that's odd. The first-half lists 2026-01 and 2026-06, and the second-half lists 2026-07 and 2026-12. It seems the "months" arrays represent the range endpoints: first half covers 2026-01 through 2026-06, and second half covers 2026-07 through 2026-12. So 2026-05 falls within the first half range (2026-01 to 2026-06).

So the applicable row for 2026-05 is the first-half row: rate 0.04, cap 4000.00, deduction 400.00, free 1200.00, upper 2800.00, middle_rate 0.14, upper_rate 0.24.

The retrieval field is contribution_rate. Value: "0.04" (decimal string, not a percentage). Clauses: R1 (selecting the row by payroll month) and R3 (contribution rate source). Not R8 per instructions ("Cite the selecting rule R1 and the source rule, not the answer-format rule R8").

The value is a decimal string: "0.04". Should I include trailing zeros? The reference gives "0.04". That's fine as-is.

No calculation needed for retrieval. Return JSON:

{"decision": "retrieve", "field": "contribution_rate", "value": "0.04", "clauses": ["R1", "R3"]}

Maybe an explanation string is optional; I could add a brief explanation. The rules say an optional explanation string is permitted. I'll include a short one noting the month maps to the first-half row.

The task: Retrieve contribution_rate from the supplied rulebook. Return the R8 JSON answer. First, I need to get the case inputs via list_documents and read_document, then lookup_reference for the payroll month.

Let me start by listing documents.

The payroll month is 2026-05. That's in the second half of the year (months 7-12 are second half? Actually "first-half" and "second-half" — likely first half is Jan-Jun and second half is Jul-Dec). 2026-05 is May, so first half. But let me check both to be safe — actually, May is in the first half (Jan–Jun). Let me look up the first-half reference.

Actually, the reference is described as "first-half or second-half payroll reference rates and effective months." So I should look up first-half for May 2026. Let me call lookup_reference with first-half. I could also call second-half in parallel to verify boundaries, but May is clearly first half. To be safe, I'll call both in one block since they're independent.

The payroll month is 2026-05. The first-half schedule covers months "2026-01" through "2026-06" (the months array shows the range endpoints: 2026-01 to 2026-06). So 2026-05 falls in the first-half row. The second-half covers 2026-07 to 2026-12.

So contribution_rate for 2026-05 is "0.04" from the first-half row.

Return R8 JSON: decision: "retrieve", field: "contribution_rate", value: "0.04", clauses: ["R1", "R3"]. Values are decimal strings, not percentages. Cite R1 (selecting rule) and R3 (source rule), not R8.