cli: /usage (alias /cost) REPL command

[akrentsel]

Summary

Adds a /usage command (alias /cost) to the chat REPL so you can see, at any point in a conversation, what it has cost so far. This is the demo surface for the per-message cost tracking in #16.

Example output:

usage (this conversation):
  model calls : 3
  input       : 14,500 tokens (1,200 cached, 800 cache-write)
  output      : 2,100 tokens (450 reasoning)
  cost        : $0.0182
  model time  : 8.3s

If some calls used a model not in the LiteLLM price table, their tokens still count but they're excluded from the dollar total, with an honest note:

  note        : 1 of 3 call(s) had no price in the table; cost is a partial total

How it works

• New /usage / /cost branch in the REPL input loop (tui.rs), alongside the existing /history, /quit, /exit.
• print_usage reads the conversation's messages events via get_events and folds the UsageRecord on each — no new persistence, it's pure read-side.
• Aggregation/formatting split into pure functions (summarize_usage, render_usage, with_commas) so they're unit-testable without a live model.
• priced_calls is tracked separately from calls so the partial-total case is reported rather than silently undercounted.

Stacking

Stacked on #16 (feature/message-cost-tracking) — this reads the UsageRecord that #16 introduces, so its base is that branch, not main. Will retarget to main once #16 merges. Also adds a one-line re-export of UsageRecord from executor for the CLI.

Test plan

• cargo test --workspace — +4 unit tests in tui::tests:
  • summarize_usage_aggregates_priced_calls_and_skips_usageless_events (user-input events without usage are ignored; totals sum correctly)
  • summarize_usage_counts_unpriced_calls_separately (None cost → counted but not priced; partial-total note rendered)
  • render_usage_reports_empty_conversation
  • with_commas_groups_thousands
• cargo fmt --all -- --check
• cargo clippy --workspace --all-targets -- -D warnings

Not in scope

• Per-turn breakdown / live inline display after each turn (this is a cumulative on-demand summary).
• A REPL command help banner (none exists today; /usage follows the same undocumented-but-discoverable convention as /history).

🤖 Generated with Claude Code

[ankrgyl]

why not just reuse the const struct from Lingua? The only difference appears to be the priced_calls counter which seems like it could be derived, or you could make a composite that embeds the lingua struct?

Pushing on this a bit because usage definitions are changing rapidly and getting more complicated (Eg anthropic recently introduced tiers of cached tokens)

[akrentsel]

yup, that makes sense, can just embed it here + add costs. done.

[Alexsun1one]

clean read-side feature, the priced_calls / partial-total note is a nice touch for honesty. one small UX thing inline on the cost formatting.

[Alexsun1one]

${:.4} looks fine for the example values but rounds small costs to misleading shapes:

• a cached call ending in $0.00004 prints cost : $0.0000, which reads as free
• a $0.00005001 call prints $0.0001, off by ~2x in display terms

few options if it's worth caring about:

• dynamic precision: 4 digits when >= $0.001, 6 digits below
• swap the unit when total < $0.01 (e.g. 42.5 ¢ or 5,432 micro-USD)
• print exact {:e} for very small totals

not blocking, just flagging because the partial-total note already shows that getting cost honest matters here.

[akrentsel]

thanks, addresse

[akrentsel]

updated to address both comments