tldr: The Software Testing Life Cycle (STLC) is six phases: requirements analysis, test planning, test case design, environment setup, test execution, and test closure. Each has clear inputs, outputs, and exit criteria. Teams that skip phases pay for it in production.
Why STLC exists separately from SDLC
The Software Development Life Cycle covers the whole build process. STLC zooms in on testing.
Treating testing as an SDLC phase, "testing" between coding and release, hides how much real work it is. STLC breaks that lump into the actual activities a QA team performs in parallel with development. The two cycles run together; STLC is not sequential after SDLC.
The 6 phases
1. Requirements analysis
QA reads the requirements alongside engineering and product. The output is a list of testable requirements: what features need verification, what acceptance criteria look like, what risks need extra coverage.
A requirement like "the system should be fast" is not testable. "P95 page load under 2 seconds on a 4G network" is.
The deliverable: a Requirements Traceability Matrix mapping each requirement to one or more planned tests. See requirements-based testing for the deeper guide.
2. Test planning
Define scope, strategy, environment needs, schedule, and exit criteria. The plan answers: what will we test, how, with what tools, by when, and how do we know we are done?
A common mistake is making the test plan a 40-page formal document. Engineers do not read 40-page documents. A useful test plan is one or two pages with the parts the team actually needs.
The deliverable: a test plan with effort estimates, risk assessment, and exit criteria. See test planning.
3. Test case design
Convert testable requirements into specific test cases. Each test case has preconditions, steps, expected results, and traceability back to a requirement.
This is where techniques like equivalence partitioning, boundary value analysis, and decision tables earn their keep. Without them, test case design becomes guessing.
The deliverable: a test case suite organized by feature, with priority and risk tags. See test design.
4. Test environment setup
Provision the infrastructure tests will run in: data, services, configurations, network, monitoring. The environment must mirror production closely enough that test results predict production behavior.
This is the phase teams underinvest in most. A flaky environment produces flaky tests, and flaky tests get ignored. See test bed for what a good environment includes.
The deliverable: a working environment with seed data, test accounts, and access controls.
5. Test execution
Run the tests. Record results. File defects for failures. Re-run after fixes. Track defect aging and pass rate over time.
In modern teams this phase is mostly automated. AI testing platforms like Bug0 execute end-to-end suites continuously instead of in batches. The phase shifts from "execute on a schedule" to "execute on every change."
The deliverable: a test execution report with pass/fail counts, defect summary, and risk assessment.
6. Test closure
Review what worked, what did not, and what the team learned. Update test cases, retire obsolete ones, archive artifacts, write the lessons-learned doc.
Most teams skip this. The result is the same problems repeating in the next release.
The deliverable: a closure report with metrics, lessons, and updated artifacts.
What good exit criteria look like
Each phase needs an exit criterion. Vague criteria like "QA is complete" produce arguments. Specific criteria produce decisions.
Examples that work:
- Phase 3 exits when 100% of in-scope requirements have at least one passing test.
- Phase 5 exits when zero P0 defects remain open, fewer than 3 P1 defects remain open, and 95% of planned test cases have executed at least once.
- Phase 6 exits when the closure report is reviewed and the team has agreed on at least one process improvement for next release.
Phases without exit criteria do not exit. They drift.
STLC in agile and DevOps
The classic STLC was written for waterfall. In agile and DevOps, the phases compress and overlap.
In a one-week sprint, requirements analysis, test design, and execution all happen inside the sprint. Closure is a 15-minute retrospective.
In continuous delivery, the cycle runs continuously. Tests are designed alongside code, executed on every commit, and closure happens at quarterly review. The phases are still there. They are just not labeled.
How AI testing compresses STLC
Traditional STLC assumes humans write each test case. That makes design and execution slow.
With AI testing, you describe the goal in plain English. The agent generates the test, executes it, and reports failures. Phases 3, 4, and 5 collapse from days to minutes.
This does not eliminate STLC. Requirements analysis, planning, and closure still need humans. But the bottleneck phases get cheap, which lets you test more flows more often. Bug0 builds this compression into a done-for-you QA service.
FAQs
What is the difference between SDLC and STLC?
SDLC covers the whole development process. STLC covers the testing activities that happen alongside it. The two run in parallel, not in sequence.
Are all six STLC phases mandatory?
In a regulated environment, yes. In a fast-moving startup, the spirit of each phase matters more than the formality. Skipping closure entirely, however, is a common mistake regardless of team maturity.
How long does each phase take?
Highly variable. As a rough rule, planning is 10% of total testing effort, design is 30%, execution is 50%, the rest split across the others.
Where does test automation fit?
Automation is a strategy applied across phases 3 and 5: design and execution. Planning automation strategy belongs in phase 2.
Can Bug0 cover the entire STLC?
No tool covers requirements analysis or closure: those need human judgment. Bug0 compresses phases 3 to 5 dramatically by generating, executing, and triaging tests on every change.