docs(sdks): document SDK auto-versioning for self-hosted setups

.vale/styles/FernStyles/Acronyms.yml

@@ -108,3 +108,5 @@ exceptions:
   - SPDX
   - BCP
   - ISO
+  - AUTO
+  - SHA

fern/products/sdks/deep-dives/self-hosted.mdx

@@ -197,3 +197,176 @@ sequenceDiagram
     CLI->>CLI: Run container, generate SDK
     CLI->>CLI: Push to output (local/GitHub)
 ```
+
+## Determining SDK versions
+
+When self-hosting, your pipeline picks the version number for each SDK release. Cloud generation handles this automatically via [Fern Autorelease](/learn/sdks/overview/autorelease), but self-hosted setups need to compute the next version themselves.
+
+The CLI exposes two workflows for this. Both can be wired into the same generation pipeline:
+
+- **`--version AUTO`** is the recommended approach. Fern classifies the change and picks the next version for you. This is the path that continues to receive improvements.
+- **`fern ir` + `fern diff`** is the deterministic alternative. Use it when you need explicit control over how the version number is derived, or when you can't depend on Fern's hosted AI service.
+
+### `--version AUTO` (recommended)
+
+Pass `--version AUTO` to `fern generate` and Fern analyzes the diff between the previous and current SDK output, classifies the change as `MAJOR`, `MINOR`, or `PATCH`, and applies the next [semantic version](https://semver.org/) to the generated package. The same analysis also produces:
+
+- A changelog entry describing what changed
+- A PR description summarizing the release
+- A [conventional commit](https://www.conventionalcommits.org/) message
+
+```bash
+FERN_TOKEN=<token> fern generate --version AUTO
+```
+
+<Info>
+`--version AUTO` requires:
+- A `FERN_TOKEN` (the diff analysis runs as a hosted Fern service that authenticates your organization the same way [cloud generation](/learn/sdks/overview/how-it-works) does).
+- A self-hosted GitHub output configured in `generators.yml` (with `github.uri` and `github.token`, [as shown above](#setup)), because the auto-versioning pipeline pushes the version bump and changelog back to the SDK repo.
+</Info>
+
+This approach is designed for the [`fern-api/fern-generate`](https://github.com/fern-api/fern-generate) GitHub Action that powers Fern's automated release pipeline, but it works standalone anywhere that meets the requirements above.
+
+### Deterministic versioning with `fern ir` + `fern diff`
+
+If you'd rather derive the version number yourself, use the `fern ir` and `fern diff` commands to compute the bump from your API definition. This flow has no dependency on hosted AI: `fern diff` walks the intermediate representation (IR) of both specs and returns a deterministic result. It's the legacy path — `--version AUTO` will keep getting improvements first — but it remains fully supported for setups that need explicit control.
+
+The workflow has three steps:
+
+<Steps>
+<Step title="Generate the previous IR">
+
+Check out the spec as it was at the last SDK generation and write its IR to disk:
+
+```bash
+fern ir old-ir.json
+```
+
+Pass `--api <name>` if your [project defines multiple APIs](/learn/sdks/overview/project-structure).
+
+</Step>
+<Step title="Generate the current IR">
+
+Check out the spec at `HEAD` and write its IR to disk:
+
+```bash
+fern ir new-ir.json
+```
+
+</Step>
+<Step title="Diff the two IRs">
+
+Run `fern diff` with `--from-version` set to the SDK's current version. When `--from-version` is provided, the command prints a JSON object containing the computed bump and the next version:
+
+```bash
+fern diff \
+  --from old-ir.json \
+  --to new-ir.json \
+  --from-version 1.4.2
+```
+
+```json
+{
+  "bump": "minor",
+  "nextVersion": "1.5.0"
+}
+```
+
+`bump` is one of `major`, `minor`, `patch`, or `no_change`. Pass `nextVersion` to `fern generate` to release that exact version:
+
+<Tabs>
+<Tab title="CLI v1" language="cli-v1">
+```bash
+fern generate --version 1.5.0
+```
+</Tab>
+<Tab title="CLI v2" language="cli-v2">
+```bash
+fern generate --output-version 1.5.0
+```
+</Tab>
+</Tabs>
+
+</Step>
+</Steps>
+
+<Info>
+`fern diff` exits with a non-zero status when the computed bump is `major`. This makes it easy to gate breaking changes on a manual review step in CI.
+</Info>
+
+#### Wiring `fern ir` + `fern diff` into CI
+
+The non-obvious part is reproducing the IR for the *previous* spec. Each generated SDK records the config repo commit it was generated from in `.fern/metadata.json`:
+
+```json title=".fern/metadata.json"
+{
+  "cliVersion": "0.74.0",
+  "generatorName": "fernapi/fern-python-sdk",
+  "generatorVersion": "4.0.0",
+  "originGitCommit": "a1b2c3d4e5f6...",
+  "requestedVersion": "1.4.2",
+  "sdkVersion": "1.4.2"
+}
+```
+
+`originGitCommit` is the SHA in the **config repo** that produced the SDK at its current version. Check that commit out, run `fern ir`, then switch back to `HEAD` and run `fern ir` again to produce both inputs to `fern diff`.
+
+The following GitHub Actions snippet runs in the config repo on every push to `main` and computes the next version for a Python SDK whose `originGitCommit` and `sdkVersion` are read from the SDK repo:
+
+```yaml title=".github/workflows/release-sdk.yml"
+name: Release SDK
+
+on:
+  push:
+    branches: [main]
+
+jobs:
+  release:
+    runs-on: ubuntu-latest
+    env:
+      FERN_TOKEN: ${{ secrets.FERN_TOKEN }}
+      SDK_REPO: your-org/python-sdk
+    steps:
+      - name: Check out config repo
+        uses: actions/checkout@v4
+        with:
+          fetch-depth: 0
+
+      - name: Set up Fern CLI
+        uses: fern-api/setup-fern-cli@v1
+
+      - name: Read previous SDK metadata
+        id: meta
+        run: |
+          curl -fsSL \
+            -H "Authorization: token ${{ secrets.GITHUB_TOKEN }}" \
+            -H "Accept: application/vnd.github.raw" \
+            "https://api.github.com/repos/${SDK_REPO}/contents/.fern/metadata.json" \
+            > metadata.json
+          echo "old_sha=$(jq -r .originGitCommit metadata.json)" >> "$GITHUB_OUTPUT"
+          echo "from_version=$(jq -r .sdkVersion metadata.json)" >> "$GITHUB_OUTPUT"
+
+      - name: Generate previous IR
+        run: |
+          git checkout ${{ steps.meta.outputs.old_sha }}
+          fern ir old-ir.json --api plants
+
+      - name: Generate current IR
+        run: |
+          git checkout ${{ github.sha }}
+          fern ir new-ir.json --api plants
+
+      - name: Compute next version
+        id: diff
+        run: |
+          result=$(fern diff \
+            --from old-ir.json \
+            --to new-ir.json \
+            --from-version ${{ steps.meta.outputs.from_version }})
+          echo "next_version=$(echo "$result" | jq -r .nextVersion)" >> "$GITHUB_OUTPUT"
+
+      - name: Generate SDK
+        run: fern generate --group python-sdk --version ${{ steps.diff.outputs.next_version }}
+```
+
+`fetch-depth: 0` on the checkout step is required so the runner has the history needed to check out `originGitCommit`. Replace the metadata-fetch step with whatever mechanism gives your pipeline access to the SDK repo's `.fern/metadata.json` (a checkout of the SDK repo, a release artifact, an internal API, and so on).