Public

Update Engine for Self-Targeting Counter EffectsPublic

The user aimed to update a game engine to correctly handle adding and removing counters on cards when no specific targets are provided, effectively allowing self-targeting counter effects. They investigated existing engine behavior, implemented the necessary changes, added tests to verify the new behavior, and fixed all related cards accordingly. The process concluded with successful compilation, passing tests, and updated documentation.

Agent

Claude Code

Model

Claude Opus 4.6

Shared

Feb 14, 7:03 PM

Messages245

Session command

Read docs/work-queue.md and do the next unchecked batch. For each batch:

Read the engine files (mtg-engine/src/abilities.rs, mtg-engine/src/game.rs) to understand current Effect/StaticEffect variants
If the batch has a NOTE about uncertainty, investigate first and update the queue with findings before committing the fix
If engine work is needed: add the variant, constructor, and match arm
Add a test for any new engine effect (follow the pattern in game.rs mod tests)
Fix all cards listed in the batch (grep to find exact lines, fix across all 4 set files)
cargo check -p mtg-cards && cargo test --lib
Update docs: mark cards in docs/{fdn,tla,tdm,ecl}-remediation.md, update ROADMAP.md
Check off the batch in docs/work-queue.md with today's date
If you discover new cards that fit the current or a future batch while working, add them to the queue

If a batch turns out to be wrong or more complex than described, update the queue with what you learned and move on to the next one.

I'll start by reading the work queue to understand what's next.

Readwork-queue.md

Batch 2 is next - "Easy swaps — existing typed effects" with sub-batches 2a through 2f. Let me read the engine files and investigate the cards.

Readabilities.rs

Readgame.rs

Now let me investigate each sub-batch by finding the exact card lines. Let me search in parallel.

GreppedCustom\("Destroy all creatures\."\)

GreppedCustom\("Enters tapped\."\)

GreppedCustom\("Target creature can't block this turn\."\)

GreppedCustom\("Put.*-1/-1 counter

GreppedCustom\("Remove a -1/-1 counter

GreppedCustom\("draw a card, then discard a card"\)

GreppedCustom\("When this creature enters, draw a card\."\)

Some patterns weren't found — line numbers shifted or text changed. Let me search more broadly for the remaining items.

GreppedDay of Judgment|DayOfJudgment

GreppedDiregraf Ghoul|DiregrafGhoul

GreppedFrenzied Goblin|FrenziedGoblin

GreppedSkoa Veteran|SkoaVeteran

Greppeddraw a card.*discard a card|loot

GreppedCustom.*draw a card

Let me get more context on the specific cards that need fixing.

Readfdn.rs15 lines

Readfdn.rs25 lines

Readecl.rs20 lines

Greppedcan't block this turn

Readecl.rs30 lines

Let me check how AddCounters and EntersTapped are handled in game.rs, and look at the remaining card contexts.

GreppedAddCounters

GreppedEntersTapped

GreppedRemoveCounters

Readecl.rs50 lines

Readecl.rs15 lines

Now let me check how AddCounters resolves when TargetSpec::None — the key question for batch 2d.

Greppedfn resolve_effect|fn resolve_ability

Greppedfn resolve

Readgame.rs120 lines

Greppedfn execute_effects

Readgame.rs120 lines

Let me check how the stack item stores source information.

Greppedstruct StackItem|source_id|pub id:

The StackItemKind::Ability has a source_id. I need to pass this through to execute_effects so AddCounters and RemoveCounters can fall back to the source when there are no targets. Let me look at how the stack item passes data during resolution.

Readgame.rs35 lines

Let me check all callers of execute_effects to plan the source_id plumbing.

Greppedexecute_effects\(

Let me understand how triggered abilities get their source_id onto the stack.

Greppedcheck_triggers|process_triggers|push_triggered

Greppedtrigger

Greppedtrigger|Trigger

Let me check existing EntersTapped usage in cards and how it's handled by the engine.

GreppedEntersTapped

Good — EntersTapped { filter: "self".into() } is the established pattern. Now let me look at the Sting-Slinger cost issue and understand how Blight cost works in game.rs.

GreppedBlight

Greppedpay_costs|check_costs|fn pay

Readgame.rs80 lines

Let me check the remaining cards I found and check for more matches.

Readecl.rs20 lines

Readfdn.rs20 lines

Let me check for any other StaticEffect::Custom("Enters tapped") patterns across all sets.

GreppedCustom\("Enters tapped

Only the Diregraf Ghoul has the simple "Enters tapped" Custom — the TDM ones are conditional (batch 6). Good.

Now I have the full picture. Let me also check what the RemoveCounters match arm looks like more carefully and read the rest of execute_effects.

Readgame.rs120 lines

Good. I now have a complete picture. Let me implement everything. I'll start with the engine change to execute_effects, then fix all the cards.

Let me first modify the engine to support "add/remove counters on self" when no targets are provided.

Readgame.rs15 lines

Now I'll make the changes. The approach:

Add source: Option<ObjectId> to execute_effects
For AddCounters/RemoveCounters: fall back to source when targets is empty
Pass source from resolve callers
Fix all cards

Now update the execute_effects signature and add the source fallback for AddCounters/RemoveCounters.

Now update the AddCounters match arm to fall back to source:

Now update RemoveCounters the same way:

Now update the test callers of execute_effects:

Readgame.rs30 lines

Good. Now let me add a test for the self-counter behavior, then fix all the cards. Let me first check compilation and tests.

Engine compiles. Now let me add a test for self-counters, then fix all the cards. Let me find the test section in game.rs.

Readgame.rs40 lines

Readgame.rs30 lines

Now let me add a test for self-counters at the end of the test module.

All 159 engine tests pass. Now let me fix all the cards. I'll do them in parallel using sed for efficiency.

Now the 2d self-counter fixes for ECL:

Now the additional RemoveCounters cards found:

Now batch 2e (loot — Icewind Elemental in FDN) and 2f (ETB draw — Mistmeadow Council in ECL):

Now let me also fix the bonus card I found — Refute (FDN) which has counter spell + loot:

Also need to fix Refute's target — it says "Counter target spell" but has TargetSpec::None:

Readfdn.rs12 lines

Now let me verify all changes compiled correctly:

Now run all tests:

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