diff --git a/docs/opsx.md b/docs/opsx.md
index bebe0a51d..f1b49ed3f 100644
--- a/docs/opsx.md
+++ b/docs/opsx.md
@@ -163,6 +163,7 @@ rules:
 | `/opsx:continue` | Create the next artifact (expanded workflow) |
 | `/opsx:ff` | Fast-forward planning artifacts (expanded workflow) |
 | `/opsx:apply` | Implement tasks, updating artifacts as needed |
+| `/opsx:tdd` | Write failing tests from `tests.md` before implementing (TDD gate) |
 | `/opsx:verify` | Validate implementation against artifacts (expanded workflow) |
 | `/opsx:sync` | Sync delta specs to main (default workflow, optional) |
 | `/opsx:archive` | Archive when done |
diff --git a/openspec/specs/opsx-tdd-skill/spec.md b/openspec/specs/opsx-tdd-skill/spec.md
new file mode 100644
index 000000000..86b8bf319
--- /dev/null
+++ b/openspec/specs/opsx-tdd-skill/spec.md
@@ -0,0 +1,128 @@
+# OPSX TDD Skill Spec
+
+## Purpose
+
+Define the expected behavior for the `/opsx:tdd` skill, which converts a change's `tests.md` artifact into failing test skeletons before implementation begins. This enforces a test-first gate between `opsx:propose` and `opsx:apply`.
+
+## Requirements
+
+### Requirement: OPSX TDD Skill
+
+The system SHALL provide an `/opsx:tdd` skill that writes failing test skeletons from the `tests.md` artifact, confirms RED status, and marks the tests artifact done before `opsx:apply` begins.
+
+#### Scenario: tests.md not found
+
+- **WHEN** agent executes `/opsx:tdd` with a change name
+- **AND** `openspec/changes/<name>/tests.md` does not exist
+- **THEN** display warning: "`tests.md` not found for change <name>. Was this change proposed with opsx:propose?"
+- **AND** stop execution
+
+#### Scenario: Automated section absent
+
+- **WHEN** agent reads `tests.md`
+- **AND** no `## Automated` section exists
+- **THEN** display warning: "tests.md has no Automated section — nothing to write. Add scenarios to tests.md first."
+- **AND** stop execution
+
+#### Scenario: Zero scenarios in Automated section
+
+- **WHEN** agent reads `tests.md`
+- **AND** `## Automated` section exists but contains zero scenarios
+- **THEN** display warning: "tests.md Automated section has no scenarios — nothing to write."
+- **AND** stop execution
+
+#### Scenario: Successful test skeleton generation
+
+- **WHEN** agent executes `/opsx:tdd` for a change with Automated scenarios in `tests.md`
+- **THEN** agent writes a failing test skeleton for each scenario
+- **AND** each skeleton uses forced RED: `expect(true).toBe(false)`
+- **AND** agent does NOT write real assertions (those are written during `opsx:apply`)
+- **AND** agent runs `bun test --run <file>` for each written file
+- **AND** agent confirms each new test is RED (failing)
+- **AND** agent reports count of tests written, RED count, and unexpected passes
+
+#### Scenario: Test passes immediately (unexpected)
+
+- **WHEN** agent runs tests after writing skeletons
+- **AND** a new test passes without implementation
+- **THEN** display warning: "WARNING: test '<name>' passes without implementation — scenario may already be covered or the test is wrong. Review before proceeding."
+
+#### Scenario: Mark tests artifact done
+
+- **WHEN** all test skeletons are written and confirmed RED
+- **THEN** agent marks tests artifact done via `openspec done tests --change "<name>"`
+- **AND** if command fails (artifact not in schema), skip silently
+
+#### Scenario: Existing describe block in target file
+
+- **WHEN** target test file already contains a `describe` block with the exact matching name
+- **THEN** agent appends new `it()` blocks inside the existing describe block
+- **AND** does NOT create a duplicate describe block
+
+### Requirement: tests.md Artifact in opsx:propose
+
+The `opsx:propose` skill SHALL generate a `tests.md` artifact after all `applyRequires` artifacts are complete.
+
+#### Scenario: tests.md generated from BDD scenarios
+
+- **WHEN** `opsx:propose` completes all required artifacts
+- **AND** `proposal.md` or `design.md` contains BDD scenarios or acceptance criteria
+- **THEN** agent generates `openspec/changes/<name>/tests.md`
+- **AND** `tests.md` contains an `## Automated` section with scenario groups
+- **AND** each scenario group declares `File:`, `Describe:`, and `Scenarios:` fields
+- **AND** `tests.md` contains a `## Manual` section for non-automatable behaviors
+- **AND** each scenario maps to exactly one `it()` block
+- **AND** agent shows progress: "Created tests.md"
+- **AND** agent does NOT ask the user for input during generation
+
+#### Scenario: No BDD scenarios found
+
+- **WHEN** `opsx:propose` completes all required artifacts
+- **AND** neither `proposal.md` nor `design.md` contains BDD scenarios or acceptance criteria
+- **THEN** agent generates `tests.md` with a `## Manual` section only
+- **AND** includes HTML comment: `<!-- No automatable scenarios found — add scenarios before running opsx:tdd -->`
+
+#### Scenario: File path derivation — page component
+
+- **WHEN** the change affects a page component (e.g. `app/(innerPages)/foo/page.tsx`)
+- **THEN** `File:` in `tests.md` is `app/(innerPages)/foo/page.test.tsx`
+
+#### Scenario: File path derivation — non-page source
+
+- **WHEN** the change affects a non-page source file (query hook, utility, shared lib)
+- **THEN** `File:` in `tests.md` mirrors the source path with `.test` inserted before the extension
+- **EXAMPLE** `lib/queries/players.ts` → `lib/queries/players.test.ts`
+
+### Requirement: TDD Guard in opsx:apply
+
+The `opsx:apply` skill SHALL check for `tests.md` before beginning implementation.
+
+#### Scenario: tests.md absent — proceed silently
+
+- **WHEN** `opsx:apply` is invoked
+- **AND** `openspec/changes/<name>/tests.md` does not exist
+- **THEN** proceed with implementation silently (backward compatible — change was proposed without TDD mode)
+
+#### Scenario: tests artifact done — proceed
+
+- **WHEN** `opsx:apply` is invoked
+- **AND** `tests.md` exists
+- **AND** tests artifact status is `"done"`
+- **THEN** proceed with implementation (tests are written; implement to make them GREEN)
+
+#### Scenario: tests artifact not done — warn and prompt
+
+- **WHEN** `opsx:apply` is invoked
+- **AND** `tests.md` exists
+- **AND** tests artifact status is any value other than `"done"` (e.g. `"pending"`, `"ready"`, `"in_progress"`)
+- **THEN** display: "⚠️ tests.md found but tests not yet written for change <name>. Run `/opsx:tdd` first to write failing tests before implementing."
+- **AND** prompt user with options: `["Yes, proceed without tests", "No, I'll run opsx:tdd first"]`
+- **AND** if user chooses "Yes, proceed without tests" → proceed
+- **AND** if user chooses "No" → stop and remind: "Run `/opsx:tdd <name>` to write tests first."
+
+#### Scenario: tests artifact not in schema — proceed silently
+
+- **WHEN** `opsx:apply` is invoked
+- **AND** `tests.md` exists
+- **AND** the `tests` artifact ID is not present in the change's schema
+- **THEN** treat as "not applicable" and proceed silently