feat: add --fields flag to filter JSON output

[qtzx06]

Problem

The CLI's JSON output includes every field from the API response. A single market object has 80+ fields. When agents or scripts consume this output, they're processing (and paying for) data they don't need.

This matters because LLM-based agents pay per token. Feeding an agent 80 fields per market when it only needs question and volumeNum wastes context window and increases cost for every call.

Solution

Adds a global --fields flag that filters JSON output to only the requested keys:

# before: returns 80+ fields per market
polymarket markets list -o json

# after: returns only what you need
polymarket markets list -o json --fields question,volumeNum,slug

The filtering happens at the print_json layer, so it works with every command that produces JSON output — no per-command changes needed.

How it works

• --fields accepts a comma-separated list of JSON field names
• Filters top-level keys from objects
• For arrays, filters each element individually
• When --fields is not set, output is unchanged (fully backwards compatible)
• Has no effect on table output

Implementation

Two files changed, 74 lines added:

• src/main.rs — adds --fields global arg, stores the field list at startup via OnceLock
• src/output/mod.rs — print_json reads the stored fields and filters before printing; includes filter_fields helper with 4 unit tests

Test plan

• cargo test — 135 tests pass (82 unit + 49 integration + 4 new)
• cargo clippy -- -D warnings — clean
• cargo fmt --check — clean
• Verified --fields appears in --help output

---

Note

Medium Risk
Medium risk because it changes CLI argument parsing and request-building for markets list/events list, which can alter query results and JSON output shape for downstream scripts.

Overview
Adds a global --fields flag that filters JSON output only to a specified subset of keys, implemented centrally in output::print_json via a per-invocation field list set from main.

Extends markets list and events list with additional filtering flags (volume/liquidity min/max and start/end date ranges; plus market tag id filtering) and wires them into the respective *Request::builder() calls.

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Cursor Bugbot has reviewed your changes and found 1 potential issue.

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[cursor]

Silent failure in set_json_fields hides broken --fields

Low Severity

set_json_fields silently swallows a RwLock write failure with if let Ok(...), meaning the --fields flag is quietly ignored and the previous filter value persists. Meanwhile, print_json calls .unwrap() on the read lock, which would panic on the same poisoned-lock condition. The writer silently no-ops while the reader crashes — these two call sites need consistent error handling. If the write silently fails, users get unfiltered output with no indication that --fields was ignored.

Additional Locations (1)

• src/output/mod.rs#L77-L78