tdd

name	tdd
description	Write code through a disciplined, BDD-framed Test-Driven Development loop: build a behavior test list, then drive each behavior through red-green-refactor with an enforced observed-failure gate. Use when the user wants to implement, build, or write code test-first, "do TDD", follow "red-green-refactor", drive code from tests, choose the next test by the Transformation Priority Premise (TPP) or ZOMBIES ordering, or grow a feature behavior-by-behavior with tests leading. This skill writes and changes code; it does not produce a test plan document (use test-planning), review or audit existing code (use code-review), restructure existing code outside a TDD loop (use refactor), specify what a feature should do (use plan-a-feature), or find the root cause of a bug (use investigate).
argument-hint	[what to build, a behavior to drive, or a path to a spec/plan]
allowed-tools	Read, Write, Edit, Glob, Grep, Agent, Bash(git *), Bash(find *), Bash(npm *), Bash(npx *), Bash(pnpm *), Bash(yarn *), Bash(pytest *), Bash(python3 *), Bash(go *), Bash(cargo *), Bash(make *), Bash(bundle *), Bash(rake *)

Project Context

• git installed: !which git 2>/dev/null || echo "not installed"
• current branch: !git branch --show-current 2>/dev/null || echo unknown
• CLAUDE.md: !find . -maxdepth 1 -name "CLAUDE.md" -type f
• project-discovery.md: !find . -maxdepth 3 -name "project-discovery.md" -type f

Constraints (read before anything else)

This skill writes production and test code in your working tree. It is an execution skill, not a document generator. These constraints shape every step and override any instinct to move faster.

• The observed-failure gate is load-bearing. No production-code change until a test has been run and observed to fail for the intended reason in this loop. A test that passes on first run is a stop-and-diagnose signal, not progress. This single rule is what separates real TDD from TDD-flavored code. The verbatim Three Laws and Canon TDD steps it derives from are in references/tdd-loop.md; pull that reference when a step needs the canon or the implementation gears.
• Two hats. Never refactor while any test is red. See references/tdd-loop.md for the canonical statement.
• One behavior at a time. Exactly one test list item becomes one runnable test per loop. Newly discovered scenarios are written to the list and deferred, never implemented in the current loop.
• BDD framing. Tests describe observable behavior, named in the project's existing test-naming convention, asserting outcomes through the public interface — never private state. The behavior-naming and Given/When/Then protocol is in references/bdd-framing.md; pull it when Step 2 needs it.
• You will be tempted to fake this. The specific ways an agent fakes TDD, and the discipline that catches each, are in references/failure-modes.md; pull it when a loop feels off (a test passes on first run, no red is shown, the implementation has outrun the test, refactor is being skipped).
• YAGNI governs the refactor step and the test list. Apply the rule in ../../references/yagni-rule.md: remove duplication, but do not add abstractions, configuration, or indirection without evidence. Speculative structure added "for flexibility" during refactor is a YAGNI candidate. Speculative scenarios on the test list are deferred with a reopen trigger, never silently added.

Test-Driven Development

Step 1: Resolve Project Config and Confirm Scope

Resolve commands. Read CLAUDE.md's ## Project Discovery section for the test command (under ### Commands and Tests, not ### Frameworks and Tooling), the lint command, the build command, language, and framework. If absent, fall back to project-discovery.md. If still absent, run ${CLAUDE_SKILL_DIR}/scripts/detect-tdd-context.sh and parse its output for git state and manifest-inferred commands. Store the resolved test, lint, and build commands for use in every later step.

Resolve standards and decisions. Resolve the coding-standards directory and ADR directory the same way: read CLAUDE.md's ## Project Discovery section; fall back to project-discovery.md; fall back to Glob defaults (docs/, docs/adr/, docs/coding-standards/, docs/decisions/). Also check CLAUDE.md and AGENTS.md for inline standards. Read the standards and ADRs whose titles, paths, or one-line summaries indicate they govern the area being built. Cap at five documents; if more than five look relevant, list them and read only the five with the strongest apparent relevance — defer the rest until refactor surfaces a need. These govern the green and refactor steps. If none exist, state that plainly and plan to infer conventions from the surrounding code instead.

Report scope, then proceed (no gate). This skill runs autonomously after the initial request: it does not stop for confirmation. State to the user, in a few lines: the behavior or feature to be built, whether this is net-new behavior or a fix to existing broken behavior (a reported bug, a failing case, a fix being driven back in after /investigate, or code that already exhibits the error — recognize the fix case from those signals, not only from the word "bug"), the resolved test/lint/build commands, the standards and ADRs found (or that none were), the current branch, and that the skill will now write code in a red-green-refactor loop. If current branch from Project Context is the repository's default branch (main or master), recommend working on a branch, but do not wait for an answer. This is a report the user reads while the work runs, not a gate. Continue immediately to Step 2 without waiting for a response.

The one exception. If the initial request or the provided context explicitly states the human wants to review, verify, or approve the plan or test list before implementation, then this becomes a gate: build the test list in Step 2, present it together with this scope report, and wait for approval before starting the Step 3 loop. Absent an explicit request like that, the skill runs to completion without further human input.

The one input that can still block is a missing test command: if it could not be resolved from CLAUDE.md, project-discovery.md, the discovery script, or manifest inference, ask the user for it, because TDD is impossible without a way to run tests. Exhaust inference before asking; this is a hard dependency, not a discretionary checkpoint.

Step 2: Build the BDD Test List

Turn the requested feature or behavior into a test list (Kent Beck's "test list" pattern). Each item is one observable behavior, phrased as a behavior sentence, not as an implementation note. "Returns the unrounded fee for a sub-dollar charge" is a list item; "use a BigDecimal" is not. Follow references/bdd-framing.md for how to phrase and name behaviors, and which test-naming convention to adopt (the project's existing convention and any discovered coding standard win over a literal "should" default).

Fixing existing broken behavior is a regression test, not a bug-asserting test. When the work fixes broken behavior, the list item names the desired correct behavior, not the current broken one: "returns the rounded total for a refund" (red now because the bug is present, green once the fix lands), never "raises ArgumentError on a refund" — a test that asserts the error the bug produces passes while the bug is present and breaks when you fix it, locking the bug in. The regression test asserts what the code should do. The boundary: asserting that the code raises is the correct test when raising is the specified desired behavior (raise on invalid input); it is wrong only when the raised error is the bug being fixed.

Order the list outside-in by user value: the next item is the most important thing the system does not yet do. When one behavior expands into several candidate tests (the empty case, the single case, the many case, the boundaries), order those tests simplest-first — Zero → One → Many — so each test forces the smallest generalization of the code. The ranking behind that order is in references/test-selection.md; pull it when the order is not obvious. For an item that is user-observable behavior at a system boundary, write the outer acceptance test for it first (it will be red until its inner behaviors exist) and record it as the outer loop for that item. For internal or utility behavior with no meaningful system boundary, the outer acceptance test is optional; the inner loop alone is correct.

Apply YAGNI to the list itself. A scenario earns a place only with evidence it is needed now (a user-described need, a named dependency, an existing code path that breaks, a regulation, a real incident). Scenarios that fail the evidence test go to a deferred list with the trigger that would reopen them. Do not pad the list for symmetry or completeness.

Report the test list to the user. Unless the verify-plan exception from Step 1 applies, continue to Step 3 immediately without waiting for approval. When that exception applies, present the test list together with the Step 1 scope report and wait for approval before entering the loop.

Step 3: The Red-Green-Refactor Loop

Pick exactly one item from the list. Choose one that teaches you something and that you are confident you can implement in one cycle (Beck's "one step test"). When several items qualify, prefer the one whose passing requires the simplest transformation of the code: a test needing only a constant return comes before one forcing a conditional, and a conditional before a loop — the Transformation Priority Premise, made concrete by the ZOMBIES ordering, both in references/test-selection.md; pull that reference when the choice is not obvious. If every remaining item forces a big leap (a loop or recursion with no smaller test in between), a simpler test is missing: add it to the list and pick it. Then run these three phases in order. Do not collapse them.

Read once, don't reread. Within a single loop iteration, do not reread a file you have already read in this iteration unless you have edited it. When grep returns a line number, use Read with offset and limit to read 20-40 lines around the target — not the entire file. Rereading whole source files between Red and Green of the same behavior is overhead, not discipline.

Red

Write exactly one test for the chosen behavior. Name it for the behavior in the project's convention. Assert an observable outcome through the public interface (Given = arrange the state before; When = the one action under test; Then = assert the observable result). Write no more of the test than is sufficient to fail; a compilation failure is a failure.

Before you run it, check the assertion direction for a fix to broken behavior. The assertion must target the desired correct result, so the red you are about to observe is "correct behavior not yet produced" — not "the error the bug raises was raised successfully." A test that asserts the buggy behavior either passes immediately or goes red for the wrong reason; both look like a satisfied gate and both lock the bug in. (Asserting a raise is still correct when raising is the specified desired behavior; the trap is asserting the error that is the bug.)

Run the resolved test command directly with Bash. Paste the failing assertion plus enough surrounding output (5-10 lines) to confirm the failure reason — the assertion text or the missing symbol you expect, not an unrelated error. If the test passed on its first run, paste only the runner's summary line and stop to diagnose: the observed-failure gate has tripped.

If the test passes on its first run, the observed-failure gate has tripped. Stop. Diagnose one of three causes: the test is not exercising the behavior; the behavior already exists; or — for a fix to broken behavior — the test is asserting the current broken behavior (the error the bug raises), which passes precisely because the bug is still present. If the behavior already exists, cross the item off and pick the next one. If the test is asserting the bug, do not cross the item off — rewrite it to assert the desired correct behavior, so it goes red until the fix lands. Do not write production code off an unobserved red.

Green

Write the minimum production code that makes this one test pass. Use the smallest gear that works: Obvious Implementation when you are certain, Fake It (return a constant, generalize later) when you are not, Triangulate (force the abstraction with a second example) only when you are really unsure. Gears are described in references/tdd-loop.md.

While going green, respect the coding standards and ADRs that govern correctness and architectural placement: where this code is allowed to live, which boundary or client it must go through, which contract it must honor. Violating an ADR boundary is not a sin you clean up later — it is the wrong code. Do not apply stylistic or structural polish here (naming sweeps, extraction, formatting passes). That is the refactor hat, and wearing it now violates "no more code than is sufficient to pass the test."

Run the full test suite with Bash. Paste the runner's summary line (pass and fail counts). Paste full output only if a previously passing test broke or something unexpected appears. The gate to leave green is: the new test passes and every previously passing test still passes. If a prior test broke, you are not green — fix it before refactoring.

Refactor (non-skippable)

Only with every test green. Neglecting this step is the most common way to ruin TDD, so it is not optional: either you change something, or you state explicitly "no duplication, structure, or standards issue this cycle" and move on.

Eliminate the duplication you just created. Bring the code into full conformance with the resolved coding standards and ADRs — this is the home for the stylistic and structural standards you deliberately skipped in green.

Apply YAGNI per ../../references/yagni-rule.md: remove duplication, do not add speculative abstraction. Defer speculative structure with the trigger that would reopen it; never add silently, never drop silently.

Change no behavior. Re-run the full suite after the refactor. Paste the runner's summary line — paste full output only if something unexpected appears. The suite must stay green. If a refactor reddened a test, revert it — a refactor that changes behavior is a defect, not a refactor.

Close the cycle

Cross the completed item off the list. Append any scenarios you discovered while implementing (deferred, with their reopen trigger if speculative), but do not implement them now. If the open list has grown past roughly ten items, do not stop for input: flag it prominently as a scope warning, keep going, and record in the final summary that the work exceeded the recommended size and should be split next time. A runaway list is a scope signal, not a reason to pause for a human.

Return to the top of Step 3 with the next item. Continue until the list is empty.

Step 4: Close the Outer Loop

For any item that had an outer acceptance test (Step 2), run that test now. It should pass only because its inner behaviors are all implemented with real code (not mocks). If it is still red, the gap is a missing inner behavior: add the missing scenario to the test list and return to Step 3. The acceptance test going green is the signal the user-facing behavior is actually delivered.

Step 5: Final Verification and Summary

Run the full test suite, then the lint command, then the build command, using the resolved commands from Step 1. Paste the summary line from each. Paste full output only when one of them fails. If lint or build fails, that is in scope — fix it (a lint or build break is not a "pre-existing error" to wave off) and re-run.

Summarize for the user:

• Behaviors implemented, and the state of the test list (done, and any deferred items with their reopen triggers).
• Which coding standards and ADRs were applied, and where they shaped the code.
• Any YAGNI deferrals from refactor, each with its reopen trigger.
• A scope warning if the test list exceeded roughly ten open items, with a recommendation to split future work.
• The final test, lint, and build status, with output shown, not asserted.