diff --git a/docs/README.md b/docs/README.md
index 465ba8ab..591c2001 100644
--- a/docs/README.md
+++ b/docs/README.md
@@ -86,6 +86,7 @@ Pre-flight checks for critical operations:
 
 LLM-specific task guides for AI assistants:
 
+- [test-writing.md](skills/test-writing.md) - Decision tree for choosing test types and placement
 - [testing-local-dev.md](skills/testing-local-dev.md) - Validate Skaffold dev environment (required before Skaffold changes)
 
 ## Related Documentation
diff --git a/docs/playbooks/backend-test-patterns.md b/docs/playbooks/backend-test-patterns.md
index fd4e25ba..451dfc78 100644
--- a/docs/playbooks/backend-test-patterns.md
+++ b/docs/playbooks/backend-test-patterns.md
@@ -426,8 +426,9 @@ Migrations haven't run. The test framework runs them automatically, but check:
 
 ## See Also
 
-- `docs/playbooks/frontend-test-patterns.md` - Frontend testing guide
-- `docs/playbooks/adding-migration.md` - Database migration workflow
+- [Test Writing Skill](../skills/test-writing.md) - LLM decision tree for test placement
+- [Frontend Test Patterns](./frontend-test-patterns.md) - Frontend testing guide
+- [Adding Migration](./adding-migration.md) - Database migration workflow
 - `service/tests/common/mod.rs` - Test DB infrastructure (`test_transaction`, `isolated_db`)
 - `service/tests/common/app_builder.rs` - Test Axum app builder
 - `service/tests/common/graphql.rs` - GraphQL response helpers
diff --git a/docs/playbooks/fixing-flaky-tests.md b/docs/playbooks/fixing-flaky-tests.md
index 83200c08..38254c42 100644
--- a/docs/playbooks/fixing-flaky-tests.md
+++ b/docs/playbooks/fixing-flaky-tests.md
@@ -147,6 +147,7 @@ test('sometimes flaky', async ({ page }) => {
 
 ## See Also
 
+- [Test Writing Skill](../skills/test-writing.md) - LLM decision tree for test placement
 - [Playwright Best Practices](https://playwright.dev/docs/best-practices)
-- `docs/playbooks/frontend-test-patterns.md` - General frontend testing
+- [Frontend Test Patterns](./frontend-test-patterns.md) - General frontend testing
 - `web/playwright.config.ts` - Test configuration
diff --git a/docs/playbooks/frontend-test-patterns.md b/docs/playbooks/frontend-test-patterns.md
index d150a4cf..1674b8aa 100644
--- a/docs/playbooks/frontend-test-patterns.md
+++ b/docs/playbooks/frontend-test-patterns.md
@@ -142,6 +142,8 @@ Coverage reports uploaded as artifacts.
 | "Target closed" | Page navigated during test | Wait for navigation to complete |
 
 ## See also
+- [Test Writing Skill](../skills/test-writing.md) - LLM decision tree for test placement
+- [Backend Test Patterns](./backend-test-patterns.md) - Rust/database testing guide
 - `web/test-utils/` - Shared test utilities
 - `web/vitest.setup.mjs` - Test configuration
 - `web/playwright.config.ts` - E2E configuration
diff --git a/docs/playbooks/migration-testing.md b/docs/playbooks/migration-testing.md
index 385fe87e..ddf04eeb 100644
--- a/docs/playbooks/migration-testing.md
+++ b/docs/playbooks/migration-testing.md
@@ -69,7 +69,9 @@ A migration file was deleted after being applied.
 
 ## See Also
 
+- [Test Writing Skill](../skills/test-writing.md) - LLM decision tree for test placement
+- [Backend Test Patterns](./backend-test-patterns.md) - Database testing patterns
+- [Adding Migration](./adding-migration.md) - How to add new migrations
 - `service/tests/common/mod.rs` - Test infrastructure documentation
 - `service/tests/migration_tests.rs` - Migration test implementations
 - `service/tests/schema_snapshot.rs` - Schema drift detection
-- `docs/playbooks/adding-migration.md` - How to add new migrations
diff --git a/docs/playbooks/new-graphql-endpoint.md b/docs/playbooks/new-graphql-endpoint.md
index 771bb1d2..e3be1c26 100644
--- a/docs/playbooks/new-graphql-endpoint.md
+++ b/docs/playbooks/new-graphql-endpoint.md
@@ -45,11 +45,17 @@
 
 5. **Add tests** in `service/tests/`:
    ```rust
-   #[tokio::test]
+   use tc_test_macros::shared_runtime_test;
+   use common::test_db::test_transaction;
+
+   #[shared_runtime_test]
    async fn test_my_new_query() {
-       // Setup and assertions
+       let mut tx = test_transaction().await;
+       // Setup and assertions using &mut *tx
    }
    ```
+   Use `#[shared_runtime_test]` with `test_transaction()` for database tests.
+   See [Backend Test Patterns](./backend-test-patterns.md) for detailed guidance.
 
 6. **Regenerate TypeScript types**:
    ```bash
@@ -81,4 +87,6 @@
 ## See also
 - `service/src/` - existing resolvers
 - [GraphQL Codegen Workflow](./graphql-codegen.md) - type generation process
+- [Backend Test Patterns](./backend-test-patterns.md) - database and resolver testing
+- [Test Writing Skill](../skills/test-writing.md) - LLM decision tree for test placement
 - async-graphql docs for patterns
diff --git a/docs/playbooks/test-data-factories.md b/docs/playbooks/test-data-factories.md
index cab7dbbc..067e2615 100644
--- a/docs/playbooks/test-data-factories.md
+++ b/docs/playbooks/test-data-factories.md
@@ -141,3 +141,9 @@ impl Default for EntityFactory {
 3. **Builder pattern**: Methods return `Self` for fluent chaining
 4. **Executor agnostic**: Works with transactions, connections, and pools
 5. **Return Result**: Factories return `Result` types, letting callers decide how to handle errors (use `.expect()` for simple cases, `?` for propagation)
+
+## See Also
+
+- [Test Writing Skill](../skills/test-writing.md) - LLM decision tree for test placement
+- [Backend Test Patterns](./backend-test-patterns.md) - Database testing patterns
+- `service/tests/common/factories/` - Existing factory implementations
diff --git a/docs/skills/test-writing.md b/docs/skills/test-writing.md
new file mode 100644
index 00000000..4a922c1b
--- /dev/null
+++ b/docs/skills/test-writing.md
@@ -0,0 +1,105 @@
+# Test Writing
+
+Use this skill when writing tests for new code or bug fixes. This guide helps you choose the right test type and location.
+
+## Decision Tree
+
+```
+What are you testing?
+│
+├─► Backend (Rust code in service/)
+│   │
+│   ├─► Database interaction?
+│   │   │
+│   │   ├─► Query logic, CRUD, business logic
+│   │   │   → Use `test_transaction()` with `#[shared_runtime_test]`
+│   │   │
+│   │   └─► Migration, transaction isolation, DB features
+│   │       → Use `isolated_db()` with `#[shared_runtime_test]`
+│   │
+│   ├─► GraphQL resolver?
+│   │   → Test via schema.execute() or full HTTP test
+│   │   → See backend-test-patterns.md#graphql-resolver-tests
+│   │
+│   ├─► Pure function, no DB?
+│   │   → Use `#[test]` or `#[tokio::test]`
+│   │
+│   └─► Invariants that should hold for all inputs?
+│       → Use proptest for property-based testing
+│
+└─► Frontend (TypeScript code in web/)
+    │
+    ├─► Component rendering/interaction?
+    │   → Vitest test in same directory: `Component.test.tsx`
+    │   → Use `@test-utils` for render/screen/userEvent
+    │
+    ├─► User flow across pages?
+    │   → Playwright E2E in `web/tests/`
+    │   → Use `./fixtures` for shared helpers
+    │
+    └─► Time-dependent behavior?
+        → Use fake timers (vi.useFakeTimers or page.clock)
+```
+
+## Quick Reference
+
+| Scenario | Test Type | Location | Macro/Tool |
+|----------|-----------|----------|------------|
+| DB query/CRUD | Integration | `service/tests/*_tests.rs` | `#[shared_runtime_test]` + `test_transaction()` |
+| Migration testing | Isolated DB | `service/tests/*_tests.rs` | `#[shared_runtime_test]` + `isolated_db()` |
+| GraphQL resolver | Integration | `service/tests/graphql_tests.rs` | `#[shared_runtime_test]` or `#[tokio::test]` |
+| Pure Rust function | Unit | `service/tests/*_tests.rs` | `#[test]` |
+| Async without DB | Unit | `service/tests/*_tests.rs` | `#[tokio::test]` |
+| Property/invariant | Property | Same file, `mod proptests` | `proptest!` |
+| React component | Unit | `web/src/**/*.test.tsx` | Vitest |
+| User flow E2E | E2E | `web/tests/*.spec.ts` | Playwright |
+
+## Backend: Which Database Pattern?
+
+**95% of tests:** Use `test_transaction()`
+- Fast (~1-5ms setup)
+- Auto-rollback on drop
+- Ideal for query logic, CRUD, business rules
+
+**5% of tests:** Use `isolated_db()`
+- Slower (~15-30ms setup)
+- Full database isolation
+- Use for: migrations, concurrent transactions, LISTEN/NOTIFY, advisory locks
+
+**Never use `get_test_db()` for tests that write data** - changes persist and cause flaky tests.
+
+## Frontend: Component vs E2E
+
+**Component tests (Vitest):**
+- Single component behavior
+- Unit interactions (clicks, inputs)
+- Fast, no browser needed
+- Co-located with component
+
+**E2E tests (Playwright):**
+- Multi-page user journeys
+- Integration with real API
+- Visual verification
+- In `web/tests/` directory
+
+## Naming Conventions
+
+```
+# Backend
+service/tests/
+  {feature}_tests.rs    # e.g., voting_tests.rs, auth_tests.rs
+
+# Frontend
+web/src/components/
+  MyComponent.tsx
+  MyComponent.test.tsx  # Co-located
+
+web/tests/
+  user-signup.spec.ts   # E2E kebab-case
+```
+
+## See Also
+
+- [Backend Test Patterns](../playbooks/backend-test-patterns.md) - Detailed Rust testing guide
+- [Frontend Test Patterns](../playbooks/frontend-test-patterns.md) - Vitest and Playwright patterns
+- [Test Data Factories](../playbooks/test-data-factories.md) - Creating backend test data