diff --git a/.github/workflows/README.md b/.github/workflows/README.md
index 6154f60399..46013e7346 100644
--- a/.github/workflows/README.md
+++ b/.github/workflows/README.md
@@ -79,7 +79,7 @@ If those overrides are omitted, deployment targets are inferred from `release_ty
 ### `tidy3d-python-client-tests.yml`
 
 Primary CI workflow; it runs on PRs (`latest`, `develop`, `pre/*`), merge queue (`merge_group`), manual dispatch, and `workflow_call`. Highlights:
-- **Code quality**: `ruff format`, `ruff check`, `mypy`, `zizmor`, schema regeneration, commit/branch linting.
+- **Code quality**: `ruff format`, `ruff check`, `mypy`, `zizmor`, schema regeneration, commit/branch linting, and changelog policy enforcement (no direct `CHANGELOG.md` edits on regular PR branches).
 - **Local tests**: Self-hosted Slurm runners on Python 3.10 and 3.13 (coverage enforced, diff-coverage comments for 3.13).
 - **Remote tests**: GitHub-hosted matrix across Windows, Linux, and macOS for Python 3.10–3.13.
 - **Optional suites**: CLI tests, version consistency checks, submodule validation (non-RC release tags only), and `tidy3d-extras` integration tests can be toggled via inputs.
@@ -150,6 +150,26 @@ Manual or called workflow that updates `poetry.lock`, authenticates against AWS
 
 The workflow creates a PR with branch name `chore/update-poetry-lock-{source_branch}` targeting the specified source branch.
 
+### `tidy3d-python-client-build-changelog-pr.yml`
+
+Manual workflow that builds `CHANGELOG.md` from Towncrier fragments and opens a PR.
+
+**Key inputs:**
+- `source_branch` – branch to checkout and build changelog from (defaults to `develop`).
+- `target_branch` – branch to open the PR against (defaults to `develop`).
+- `release_version` – optional override for the release version. If omitted, it is derived from `pyproject.toml` by stripping `.devN`.
+- `release_date` – optional override in `YYYY-MM-DD`. If omitted, UTC `today` is used.
+- `previous_version` – optional override for the compare-link previous version. If omitted, the workflow uses the latest reachable stable `vX.Y.Z` tag, and falls back to the latest stable heading in `CHANGELOG.md` when no tag is available.
+- `run_workflow` – boolean guard to enable/disable execution.
+
+The workflow:
+1. Installs Poetry dependencies (`--extras dev`).
+2. Runs `towncrier build --yes`.
+3. Runs `scripts/changelog_refs.py` to update compare reference links.
+4. Opens a PR with the generated changelog updates.
+
+If no fragments are present in `changelog.d/`, the workflow exits without opening a PR.
+
 ## Documentation Workflows
 
 ### `tidy3d-docs-sync-readthedocs-repo.yml`
diff --git a/.github/workflows/tidy3d-python-client-build-changelog-pr.yml b/.github/workflows/tidy3d-python-client-build-changelog-pr.yml
new file mode 100644
index 0000000000..7256d14e5f
--- /dev/null
+++ b/.github/workflows/tidy3d-python-client-build-changelog-pr.yml
@@ -0,0 +1,164 @@
+name: public/tidy3d/python-client-build-changelog-pr
+
+on:
+  workflow_dispatch:
+    inputs:
+      run_workflow:
+        description: 'Set to true to build changelog and create a PR'
+        required: true
+        type: boolean
+        default: true
+      source_branch:
+        description: 'Source branch to checkout and build changelog from'
+        required: false
+        type: string
+        default: 'develop'
+      target_branch:
+        description: 'Target branch for the generated changelog PR'
+        required: false
+        type: string
+        default: 'develop'
+      release_version:
+        description: 'Optional release version override (for example: 2.11.0)'
+        required: false
+        type: string
+        default: ''
+      release_date:
+        description: 'Optional release date override in YYYY-MM-DD (defaults to UTC today)'
+        required: false
+        type: string
+        default: ''
+      previous_version:
+        description: 'Optional previous release version override (for example: 2.10.2)'
+        required: false
+        type: string
+        default: ''
+
+permissions:
+  contents: read
+
+jobs:
+  build-changelog-pr:
+    if: github.event.inputs.run_workflow == 'true'
+    runs-on: ubuntu-latest
+    permissions:
+      contents: write
+      pull-requests: write
+    steps:
+      - name: Checkout repository
+        uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6.0.2
+        with:
+          ref: ${{ github.event.inputs.source_branch || 'develop' }}
+          fetch-depth: 0
+          submodules: false
+          persist-credentials: false
+
+      - name: Set up Python
+        uses: actions/setup-python@a309ff8b426b58ec0e2a45f0f869d46889d02405 # v6.2.0
+        with:
+          python-version: '3.10'
+
+      - name: Install Poetry
+        uses: snok/install-poetry@76e04a911780d5b312d89783f7b1cd627778900a  # v1.4.1
+        with:
+          version: 2.1.1
+          virtualenvs-create: true
+          virtualenvs-in-project: true
+
+      - name: Install dependencies
+        run: |
+          set -e
+          poetry install --extras dev --no-interaction
+
+      - name: Build changelog
+        id: build-changelog
+        env:
+          INPUT_RELEASE_VERSION: ${{ github.event.inputs.release_version }}
+          INPUT_RELEASE_DATE: ${{ github.event.inputs.release_date }}
+          INPUT_PREVIOUS_VERSION: ${{ github.event.inputs.previous_version }}
+          SOURCE_BRANCH: ${{ github.event.inputs.source_branch || 'develop' }}
+        run: |
+          set -euo pipefail
+
+          if [[ ! -d changelog.d ]]; then
+            echo "changelog.d/ directory not found; skipping changelog build."
+            echo "changes_detected=false" >> "$GITHUB_OUTPUT"
+            exit 0
+          fi
+
+          fragment_count=$(find changelog.d -maxdepth 1 -type f -name '*.md' ! -name 'README.md' ! -name 'template.md' | wc -l)
+          if [[ "$fragment_count" -eq 0 ]]; then
+            echo "No changelog fragments found in changelog.d/."
+            echo "changes_detected=false" >> "$GITHUB_OUTPUT"
+            exit 0
+          fi
+
+          relver="${INPUT_RELEASE_VERSION}"
+          if [[ -z "$relver" ]]; then
+            relver=$(poetry version -s | sed -E 's/\.dev[0-9]+$//')
+          fi
+          relver=$(echo "$relver" | sed -E 's/^v//')
+          if [[ -z "$relver" ]]; then
+            echo "::error::Release version is empty after normalization."
+            exit 1
+          fi
+
+          reldate="${INPUT_RELEASE_DATE}"
+          if [[ -z "$reldate" ]]; then
+            reldate=$(date -u +%F)
+          fi
+
+          prevver="${INPUT_PREVIOUS_VERSION}"
+          prevver=$(echo "$prevver" | sed -E 's/^v//')
+          if [[ -z "$prevver" ]]; then
+            prevver=$(git tag --merged HEAD --list 'v*' --sort=-v:refname | sed 's/^v//' | grep -E '^[0-9]+\.[0-9]+\.[0-9]+$' | grep -v "^${relver}$" | head -n1 || true)
+          fi
+          if [[ -z "$prevver" ]]; then
+            prevver=$(grep -E '^## \[[0-9]+\.[0-9]+\.[0-9]+\] - ' CHANGELOG.md | sed -E 's/^## \[([0-9]+\.[0-9]+\.[0-9]+)\].*/\1/' | grep -v "^${relver}$" | head -n1 || true)
+          fi
+
+          safe_source_branch=$(echo "$SOURCE_BRANCH" | tr '/ ' '--')
+
+          echo "Building changelog for version ${relver} on ${reldate}"
+          poetry run towncrier build --yes --version "${relver}" --date "${reldate}"
+
+          if [[ -n "$prevver" ]]; then
+            poetry run python scripts/changelog_refs.py --version "${relver}" --previous-version "${prevver}"
+          else
+            poetry run python scripts/changelog_refs.py --version "${relver}"
+          fi
+
+          if git diff --quiet -- CHANGELOG.md changelog.d; then
+            echo "changes_detected=false" >> "$GITHUB_OUTPUT"
+          else
+            echo "changes_detected=true" >> "$GITHUB_OUTPUT"
+          fi
+          previous_version_output="${prevver}"
+          if [[ -z "$previous_version_output" ]]; then
+            previous_version_output="auto-detected"
+          fi
+          echo "release_version=${relver}" >> "$GITHUB_OUTPUT"
+          echo "release_date=${reldate}" >> "$GITHUB_OUTPUT"
+          echo "previous_version=${previous_version_output}" >> "$GITHUB_OUTPUT"
+          echo "safe_source_branch=${safe_source_branch}" >> "$GITHUB_OUTPUT"
+
+      - name: Create Pull Request
+        if: steps.build-changelog.outputs.changes_detected == 'true'
+        uses: peter-evans/create-pull-request@c0f553fe549906ede9cf27b5156039d195d2ece0 # v8.1.0
+        with:
+          token: ${{ secrets.GITHUB_TOKEN }}
+          commit-message: "chore(changelog): :robot: build release notes for ${{ steps.build-changelog.outputs.release_version }}"
+          title: "chore(changelog): :robot: build release notes for ${{ steps.build-changelog.outputs.release_version }}"
+          body: |
+            This pull request was automatically generated by a GitHub Action.
+
+            It builds `CHANGELOG.md` from Towncrier fragments and updates compare reference links.
+
+            Source branch: `${{ github.event.inputs.source_branch || 'develop' }}`
+            Target branch: `${{ github.event.inputs.target_branch || 'develop' }}`
+            Version: `${{ steps.build-changelog.outputs.release_version }}`
+            Previous version: `${{ steps.build-changelog.outputs.previous_version }}`
+            Date: `${{ steps.build-changelog.outputs.release_date }}`
+          branch: "chore/build-changelog-${{ steps.build-changelog.outputs.safe_source_branch }}-${{ github.run_id }}"
+          base: "${{ github.event.inputs.target_branch || 'develop' }}"
+          delete-branch: true
diff --git a/.github/workflows/tidy3d-python-client-tests.yml b/.github/workflows/tidy3d-python-client-tests.yml
index ef2c7ad16e..76619f5d8a 100644
--- a/.github/workflows/tidy3d-python-client-tests.yml
+++ b/.github/workflows/tidy3d-python-client-tests.yml
@@ -387,7 +387,10 @@ jobs:
 
       - name: enforce-jira-key
         id: enforce-jira-key
-        if: github.event_name == 'pull_request' && github.event.pull_request.user.login != 'dependabot[bot]'
+        if: >
+          github.event_name == 'pull_request' &&
+          github.event.pull_request.user.login != 'dependabot[bot]' &&
+          !startsWith(github.event.pull_request.head.ref, 'chore/build-changelog-')
         env:
           STEPS_EXTRACT_BRANCH_NAME_OUTPUTS_BRANCH_NAME: ${{ steps.extract-branch-name.outputs.branch_name }}
         run: |
@@ -425,6 +428,44 @@ jobs:
               fi
           fi
 
+  enforce-changelog-policy:
+    needs: determine-test-scope
+    runs-on: ubuntu-latest
+    if: needs.determine-test-scope.outputs.pr_review_tests == 'true'
+    name: enforce-changelog-policy
+    steps:
+      - name: Check out source code
+        uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6.0.2
+        with:
+          fetch-depth: 0
+          persist-credentials: false
+
+      - name: disallow-manual-changelog-edits
+        env:
+          PR_BASE_SHA: ${{ github.event.pull_request.base.sha }}
+          PR_HEAD_SHA: ${{ github.event.pull_request.head.sha }}
+          PR_BRANCH: ${{ github.event.pull_request.head.ref }}
+        run: |
+          set -euo pipefail
+
+          if ! git cat-file -e "${PR_BASE_SHA}:changelog.d/README.md" 2>/dev/null; then
+            echo "Towncrier changelog workflow not detected in base commit; skipping check."
+            exit 0
+          fi
+
+          if [[ "$PR_BRANCH" == chore/build-changelog-* ]]; then
+            echo "Auto-generated changelog branch detected; allowing CHANGELOG.md edits."
+            exit 0
+          fi
+
+          if git diff --name-only "$PR_BASE_SHA" "$PR_HEAD_SHA" | grep -Fxq "CHANGELOG.md"; then
+            echo "❌ Manual edits to CHANGELOG.md are not allowed."
+            echo "Add changelog fragments under changelog.d/ instead."
+            exit 1
+          fi
+
+          echo "✅ No direct CHANGELOG.md edits detected."
+
   lint-commit-messages:
     needs: determine-test-scope
     runs-on: ubuntu-latest
@@ -1365,6 +1406,7 @@ jobs:
       - verify-schema-change
       - lint-commit-messages
       - lint-branch-name
+      - enforce-changelog-policy
       - zizmor
       - develop-cli-tests
       - verify-version-consistency
@@ -1420,6 +1462,12 @@ jobs:
           echo "❌ Branch name linting failed."
           exit 1
 
+      - name: check-changelog-policy
+        if: ${{ needs.determine-test-scope.outputs.pr_review_tests == 'true' && needs.enforce-changelog-policy.result != 'success' && needs.enforce-changelog-policy.result != 'skipped' }}
+        run: |
+          echo "❌ Changelog policy check failed."
+          exit 1
+
       - name: check-zizmor-static-analysis
         if: ${{ needs.determine-test-scope.outputs.code_quality_tests == 'true' && needs.zizmor.result != 'success' && needs.zizmor.result != 'skipped' }}
         run: |
diff --git a/AGENTS.md b/AGENTS.md
index 232c920a37..5a39282c3d 100644
--- a/AGENTS.md
+++ b/AGENTS.md
@@ -48,6 +48,7 @@
 - Follow Conventional Commits per `.commitlintrc.json`.
 - Branch names must use an allowed prefix (`chore`, `hotfix`, `daily-chore`) or include a Jira key to satisfy CI.
 - PRs should link issues, summarize behavior changes, list the `poetry run …` checks you executed, and call out docs/schema updates.
-- Add a changelog entry under `## [Unreleased]` in `CHANGELOG.md` for user-facing changes (new features, bug fixes, breaking changes).
+- For user-facing changes (new features, bug fixes, breaking changes), add a changelog fragment under `changelog.d/` using the pattern `<PR_NUMBER>.<type>.md` (for example `1234.added.md`) instead of editing `CHANGELOG.md` directly; CI rejects direct `CHANGELOG.md` edits on regular PR branches.
+- Release managers can use the GitHub Actions workflow `public/tidy3d/python-client-build-changelog-pr` to generate `CHANGELOG.md` from fragments and open a PR (defaults source/target to `develop`).
 
 _Reminder: update this AGENTS.md whenever workflow, tooling, or review expectations change so agents stay in sync with the repo._
diff --git a/CHANGELOG.md b/CHANGELOG.md
index c2cb6cf7fd..fae209ac84 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -5,67 +5,8 @@ All notable changes to this project will be documented in this file.
 The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
 and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
-## [Unreleased]
-
-### Added
-- Added `GeometryArray` class for efficiently representing multiple copies of a base geometry at specified offsets with transformation matrices. Includes a convenience method `geometry.array(offsets=..., transforms=...)` on all geometry objects.
-- Added `ModeSortSpec.keep_modes` which can be set to `"all"` to keep all modes in the mode solver (the default), `"filtered"` to keep only modes passing the filter defined by the `ModeSortSpec`, or an integer `N` to keep only the top `N` modes after filtering and sorting.
-- Added `fill_fraction_box` as a new filtering and sorting key which computes the field-energy fill fraction within a specified bounding box (`ModeSortSpec.bounding_box`).
-- Added `Grid.fine_mesh_info` property to identify and report locations where grid cell sizes are fine for understanding meshing hotspots.
-- Added visualization of finest grid regions in `Simulation.plot_grid()` with shaded regions highlighting areas of fine meshing.
-- Added autograd support for `Sphere`.
-- Added validation warning in `HeatChargeSimulation` for very small `Cylinder` radii to help users avoid meshing and numerical issues.
-- Added `GaussianOverlapMonitor` and `AstigmaticGaussianOverlapMonitor` for decomposing electromagnetic fields onto Gaussian beam profiles.
-- Added `GaussianPort` and `AstigmaticGaussianPort` for S-matrix calculations using Gaussian beam sources and overlap monitors.
-- Added `symmetric_pseudo` option for `s_param_def` in `TerminalComponentModeler` which applies a scaling factor that ensures the S-matrix is symmetric in reciprocal systems.
-- Added deprecation warning for ``TemperatureMonitor`` and ``SteadyPotentialMonitor`` when ``unstructured`` parameter is not explicitly set. The default value of ``unstructured`` will change from ``False`` to ``True`` after the 2.11 release.
-- Added flag `remove_fragments` to the base `UnstructuredGrid` to remove fragments in unstructured grids. This can ease meshing by eliminating internal boundaries in overlapping structures.
-- Added deprecation warning for `conformal` in TCAD heat/charge monitors when explicitly set; this option is ignored (treated as `False`) when meshing with `remove_fragments=True`.
-- Added in-memory caching for downloaded batch results, configurable via ``config.batch_data_cache``.
-- Added `DesignSpace` support for sweeping `WorkflowType` objects, including mode/EME simulations and component modelers.
-- Added `current_amplitude_definition` parameter to `UniformCurrentSource` for size-independent total current injection. Set to `"total"` to interpret the source amplitude as total current rather than current density.
-- Added baseband source time classes (`BasebandStep`, `BasebandGaussianPulse`, `BasebandRectangularPulse`, `BasebandCustomSourceTime`) for transient RF simulations with real-valued time signals.
-- Added config versioning with automatic backward migrations by default; forward‑compat is best‑effort unless strict mode is enabled.
-
-### Breaking Changes
-- `web.Batch(simulations=...)` now requires string task names when simulations are passed as a dictionary. Numeric keys (for example `0`, `1`) are no longer converted automatically; convert them to strings first (for example `"0"`, `"1"`).
-- Added optional automatic extrusion of structures at the simulation boundaries into/through PML/Absorber layers via `extrude_structures` field in class `AbsorberSpec`.
-- Added `structure_priority_mode` for `TerminalComponentModeler` and default to `"conductor"` to ensure metal structures override dielectrics regardless of structure order, preventing order-dependent results in RF simulations.
-- 1D lumped elements (with zero lateral extent) are no longer allowed. Use a small finite lateral extent (e.g., `1e-6`) instead.
-- `ModeSortSpec.sort_key` is now required with a default of `"n_eff"` (previously optional with `None` default). `ModeSortSpec.sort_order` is now optional with a default of `None`, which automatically selects the natural order based on `sort_key` and `sort_reference`: ascending when a reference is provided (closest first), otherwise descending for `n_eff` and polarization fractions (higher values first), ascending for `k_eff` and `mode_area` (lower values first).
-- Changed the interpretation of `waist_distance` and `waist_distances` for backward-propagating Gaussian beams (`GaussianBeam`, `AstigmaticGaussianBeam`, `GaussianBeamProfile`, `AstigmaticGaussianBeamProfile`). Previously, the waist position was interpreted relative to the directed propagation axis, meaning switching `direction` from `+` to `-` would also flip the waist position in the global reference frame. Now, the waist position is defined consistently for both directions: a positive `waist_distance` always places the beam waist behind the source/monitor plane (toward the negative normal axis), regardless of propagation direction. This ensures reciprocity between Gaussian sources and overlap monitors used in port-based S-matrix calculations. Users with existing simulations using backward-propagating Gaussian beams with non-zero waist distances may need to adjust their values.
-
-### Changed
-- Changed the default value of `fill_value` in `UnstructuredGridDataset.interp()` from `0` to `"extrapolate"`. This means points outside the mesh will now use nearest-neighbor extrapolation instead of being filled with zeros.
-- `ModeSortSpec.sort_key` is now required with a default of `"n_eff"` (previously optional with `None` default). `ModeSortSpec.sort_order` is now optional with a default of `None`, which automatically selects the natural order based on `sort_key` and `sort_reference`: ascending when a reference is provided (closest first), otherwise descending for `n_eff` and polarization fractions (higher values first), ascending for `k_eff` and `mode_area` (lower values first).
-- Added `symmetric_pseudo` option for `s_param_def` in `TerminalComponentModeler` which applies a scaling factor that ensures the S-matrix is symmetric in reciprocal systems.
-- Added deprecation warning for `TemperatureMonitor` and `SteadyPotentialMonitor` when `unstructured` parameter is not explicitly set. The default value of `unstructured` will change from `False` to `True` in the next release.
-- Added deprecation warning for ``TemperatureMonitor`` and ``SteadyPotentialMonitor`` when ``unstructured`` parameter is not explicitly set. The default value of ``unstructured`` will change from ``False`` to ``True`` after the 2.11 release.
-- Added validation to `GaussianDoping` to ensure `ref_con < concentration`, validate `source` face identifier, and warn the user when the box size is not sufficient for the specified transition width.
-- Local caching is now enabled by default (set `td.config.local_cache.enabled=False` to opt out).
-- Reduced computation time of `adaptive_vjp_spacing` for `GeometryGroup` by allowing permittivity based spacing value to be cached.
-- Added warning in `LayerRefinementSpec` when `dl_min_from_gaps` (derived from automatic gap refinement) is very small relative to the lateral grid size for identifying cases where excessive grid refinement may occur due to very small detected gaps.
-- Added `custom_vjp` and new custom run functions that provide hooks into adjoint for custom gradient calculations.
-- Changed default `num_points` in `EMEModeSpec.interp_spec` from 3 to 5 for improved accuracy of frequency interpolation.
-- Unstructured data plots are now "crinkled", showing the full mesh elements that cover given monitor boundaries. Previously, the mesh elements were "clipped" to the monitor boundaries.
-
-### Fixed
-- Fixed intermittent "API key not found" errors in parallel job launches by making configuration directory detection race-safe.
-- Fixed frequency accumulation of gradients for custom dispersive media.
-- Fixed `snap_box_to_grid` producing zero-size boxes when using `Expand` behavior with very small intervals centered on a grid point.
-- Fixed sliver polygon artifacts in 2D material subdivision by filtering polygons based on grid cell size, preventing numerical issues with large-coordinate geometries.
-- Fixed `CustomMedium` gradient calculation when field coordinates exactly align with boundaries.
-- Fixed adjoint simulation `grid_spec` to align exactly with forward simulation for correct `FieldData` adjoint source power.
-- Fixed `TerminalComponentModeler` serialization failure with `CustomGridBoundaries` by storing JSON-serializable grid metadata in `GridSpec.attrs`.
-- Fixed redundant logging when `Batch.download()` skips existing files, and added `replace_existing` to `Batch.run()` so overwrite behavior can be controlled directly.
-- Fixed redundant server lookups when loading simulation results.
-- Updated docstrings for `DerivativeInfo` to more accurately reflect dataclass fields.
-- Fixed local cache race conditions causing `FileNotFoundError`.
-- Improved `ModeSolver.solve()` to suppress the accuracy warning when local subpixel averaging is enabled via `tidy3d-extras`, and expanded docstrings for `ModeSolver.solve()`, `ModeSimulation.run_local()`, `Simulation.epsilon()`, and `Simulation.epsilon_on_grid()` to document the `config.simulation.use_local_subpixel` option.
-- Fixed `ModeSolver.validate_pre_upload` not validating `MicrowaveModeSpec` with `AutoImpedanceSpec`, causing cryptic server-side errors for invalid conductor geometries.
-- Fixed local cache not storing results for `ModalComponentModeler` (and `TerminalComponentModeler`) runs, causing cache misses on repeated `web.run()` calls.
-- Fixed spurious `ModeSimulation` error logs during deserialization of `SimulationDataMap` by adding discriminators to `SimulationType` and `SimulationDataType` unions.
-- Fixed unintended model output in Jupyter notebooks during `import tidy3d` by preventing `Tidy3dBaseModel.__str__` from triggering notebook display side effects.
+<!-- Maintainer note: do not edit this file directly in regular PRs. Add Towncrier fragments in changelog.d/ and follow changelog.d/README.md. -->
+<!-- towncrier release notes start -->
 
 ## [2.10.2] - 2026-01-21
 
@@ -2011,7 +1952,6 @@ which fields are to be projected is now determined automatically based on the me
 - Job and Batch classes for better simulation handling (eventually to fully replace webapi functions).
 - A large number of small improvements and bug fixes.
 
-[Unreleased]: https://github.com/flexcompute/tidy3d/compare/v2.10.2...develop
 [2.10.2]: https://github.com/flexcompute/tidy3d/compare/v2.10.1...v2.10.2
 [2.10.1]: https://github.com/flexcompute/tidy3d/compare/v2.10.0...v2.10.1
 [2.10.0]: https://github.com/flexcompute/tidy3d/compare/v2.9.3...v2.10.0
diff --git a/changelog.d/3015.changed.md b/changelog.d/3015.changed.md
new file mode 100644
index 0000000000..6cc805b208
--- /dev/null
+++ b/changelog.d/3015.changed.md
@@ -0,0 +1 @@
+Added `custom_vjp` and new custom run functions that provide hooks into adjoint for custom gradient calculations.
diff --git a/changelog.d/3026.breaking.md b/changelog.d/3026.breaking.md
new file mode 100644
index 0000000000..76db3c6202
--- /dev/null
+++ b/changelog.d/3026.breaking.md
@@ -0,0 +1 @@
+Added optional automatic extrusion of structures at the simulation boundaries into/through PML/Absorber layers via `extrude_structures` field in class `AbsorberSpec`.
diff --git a/changelog.d/3055.1.added.md b/changelog.d/3055.1.added.md
new file mode 100644
index 0000000000..890fc467da
--- /dev/null
+++ b/changelog.d/3055.1.added.md
@@ -0,0 +1 @@
+Added `ModeSortSpec.keep_modes` which can be set to `"all"` to keep all modes in the mode solver (the default), `"filtered"` to keep only modes passing the filter defined by the `ModeSortSpec`, or an integer `N` to keep only the top `N` modes after filtering and sorting.
diff --git a/changelog.d/3055.2.added.md b/changelog.d/3055.2.added.md
new file mode 100644
index 0000000000..74a753f17e
--- /dev/null
+++ b/changelog.d/3055.2.added.md
@@ -0,0 +1 @@
+Added `fill_fraction_box` as a new filtering and sorting key which computes the field-energy fill fraction within a specified bounding box (`ModeSortSpec.bounding_box`).
diff --git a/changelog.d/3082.added.md b/changelog.d/3082.added.md
new file mode 100644
index 0000000000..f82a1bf9e3
--- /dev/null
+++ b/changelog.d/3082.added.md
@@ -0,0 +1 @@
+Added autograd support for `Sphere`.
diff --git a/changelog.d/3168.changed.md b/changelog.d/3168.changed.md
new file mode 100644
index 0000000000..965802d05c
--- /dev/null
+++ b/changelog.d/3168.changed.md
@@ -0,0 +1 @@
+Added validation to `GaussianDoping` to ensure `ref_con < concentration`, validate `source` face identifier, and warn the user when the box size is not sufficient for the specified transition width.
diff --git a/changelog.d/3179.fixed.md b/changelog.d/3179.fixed.md
new file mode 100644
index 0000000000..f8b435ea2e
--- /dev/null
+++ b/changelog.d/3179.fixed.md
@@ -0,0 +1 @@
+Fixed frequency accumulation of gradients for custom dispersive media.
diff --git a/changelog.d/3185.breaking.md b/changelog.d/3185.breaking.md
new file mode 100644
index 0000000000..7f2c5286e5
--- /dev/null
+++ b/changelog.d/3185.breaking.md
@@ -0,0 +1 @@
+1D lumped elements (with zero lateral extent) are no longer allowed. Use a small finite lateral extent (e.g., `1e-6`) instead.
diff --git a/changelog.d/3185.fixed.md b/changelog.d/3185.fixed.md
new file mode 100644
index 0000000000..8d8a901182
--- /dev/null
+++ b/changelog.d/3185.fixed.md
@@ -0,0 +1 @@
+Fixed `snap_box_to_grid` producing zero-size boxes when using `Expand` behavior with very small intervals centered on a grid point.
diff --git a/changelog.d/3190.fixed.md b/changelog.d/3190.fixed.md
new file mode 100644
index 0000000000..3c641e4b19
--- /dev/null
+++ b/changelog.d/3190.fixed.md
@@ -0,0 +1 @@
+Fixed sliver polygon artifacts in 2D material subdivision by filtering polygons based on grid cell size, preventing numerical issues with large-coordinate geometries.
diff --git a/changelog.d/3192.1.added.md b/changelog.d/3192.1.added.md
new file mode 100644
index 0000000000..7c83bab81f
--- /dev/null
+++ b/changelog.d/3192.1.added.md
@@ -0,0 +1 @@
+Added `Grid.fine_mesh_info` property to identify and report locations where grid cell sizes are fine for understanding meshing hotspots.
diff --git a/changelog.d/3192.2.added.md b/changelog.d/3192.2.added.md
new file mode 100644
index 0000000000..3040474576
--- /dev/null
+++ b/changelog.d/3192.2.added.md
@@ -0,0 +1 @@
+Added visualization of finest grid regions in `Simulation.plot_grid()` with shaded regions highlighting areas of fine meshing.
diff --git a/changelog.d/3200.breaking.md b/changelog.d/3200.breaking.md
new file mode 100644
index 0000000000..412f47c012
--- /dev/null
+++ b/changelog.d/3200.breaking.md
@@ -0,0 +1 @@
+Added `structure_priority_mode` for `TerminalComponentModeler` and default to `"conductor"` to ensure metal structures override dielectrics regardless of structure order, preventing order-dependent results in RF simulations.
diff --git a/changelog.d/3203.fixed.md b/changelog.d/3203.fixed.md
new file mode 100644
index 0000000000..07f2dfc0c9
--- /dev/null
+++ b/changelog.d/3203.fixed.md
@@ -0,0 +1 @@
+Fixed intermittent "API key not found" errors in parallel job launches by making configuration directory detection race-safe.
diff --git a/changelog.d/3206.changed.md b/changelog.d/3206.changed.md
new file mode 100644
index 0000000000..82b7c9d949
--- /dev/null
+++ b/changelog.d/3206.changed.md
@@ -0,0 +1 @@
+Added warning in `LayerRefinementSpec` when `dl_min_from_gaps` (derived from automatic gap refinement) is very small relative to the lateral grid size for identifying cases where excessive grid refinement may occur due to very small detected gaps.
diff --git a/changelog.d/3211.2.changed.md b/changelog.d/3211.2.changed.md
new file mode 100644
index 0000000000..8abedd6feb
--- /dev/null
+++ b/changelog.d/3211.2.changed.md
@@ -0,0 +1 @@
+Added deprecation warning for `TemperatureMonitor` and `SteadyPotentialMonitor` when `unstructured` parameter is not explicitly set. The default value of `unstructured` will change from `False` to `True` after the 2.11 release.
diff --git a/changelog.d/3215.added.md b/changelog.d/3215.added.md
new file mode 100644
index 0000000000..b0fe5dc254
--- /dev/null
+++ b/changelog.d/3215.added.md
@@ -0,0 +1 @@
+Added `GeometryArray` class for efficiently representing multiple copies of a base geometry at specified offsets with transformation matrices. Includes a convenience method `geometry.array(offsets=..., transforms=...)` on all geometry objects.
diff --git a/changelog.d/3216.changed.md b/changelog.d/3216.changed.md
new file mode 100644
index 0000000000..4603ff1c77
--- /dev/null
+++ b/changelog.d/3216.changed.md
@@ -0,0 +1 @@
+Reduced computation time of `adaptive_vjp_spacing` for `GeometryGroup` by allowing permittivity based spacing value to be cached.
diff --git a/changelog.d/3217.added.md b/changelog.d/3217.added.md
new file mode 100644
index 0000000000..f2d36c6c59
--- /dev/null
+++ b/changelog.d/3217.added.md
@@ -0,0 +1 @@
+Added validation warning in `HeatChargeSimulation` for very small `Cylinder` radii to help users avoid meshing and numerical issues.
diff --git a/changelog.d/3219.1.added.md b/changelog.d/3219.1.added.md
new file mode 100644
index 0000000000..c59091ee02
--- /dev/null
+++ b/changelog.d/3219.1.added.md
@@ -0,0 +1 @@
+Added `GaussianOverlapMonitor` and `AstigmaticGaussianOverlapMonitor` for decomposing electromagnetic fields onto Gaussian beam profiles.
diff --git a/changelog.d/3219.1.breaking.md b/changelog.d/3219.1.breaking.md
new file mode 100644
index 0000000000..04f48cca80
--- /dev/null
+++ b/changelog.d/3219.1.breaking.md
@@ -0,0 +1 @@
+`ModeSortSpec.sort_key` is now required with a default of `"n_eff"` (previously optional with `None` default). `ModeSortSpec.sort_order` is now optional with a default of `None`, which automatically selects the natural order based on `sort_key` and `sort_reference`: ascending when a reference is provided (closest first), otherwise descending for `n_eff` and polarization fractions (higher values first), ascending for `k_eff` and `mode_area` (lower values first).
diff --git a/changelog.d/3219.2.added.md b/changelog.d/3219.2.added.md
new file mode 100644
index 0000000000..07f04df648
--- /dev/null
+++ b/changelog.d/3219.2.added.md
@@ -0,0 +1 @@
+Added `GaussianPort` and `AstigmaticGaussianPort` for S-matrix calculations using Gaussian beam sources and overlap monitors.
diff --git a/changelog.d/3219.2.breaking.md b/changelog.d/3219.2.breaking.md
new file mode 100644
index 0000000000..99172120e8
--- /dev/null
+++ b/changelog.d/3219.2.breaking.md
@@ -0,0 +1 @@
+Changed the interpretation of `waist_distance` and `waist_distances` for backward-propagating Gaussian beams (`GaussianBeam`, `AstigmaticGaussianBeam`, `GaussianBeamProfile`, `AstigmaticGaussianBeamProfile`). Previously, the waist position was interpreted relative to the directed propagation axis, meaning switching `direction` from `+` to `-` would also flip the waist position in the global reference frame. Now, the waist position is defined consistently for both directions: a positive `waist_distance` always places the beam waist behind the source/monitor plane (toward the negative normal axis), regardless of propagation direction. This ensures reciprocity between Gaussian sources and overlap monitors used in port-based S-matrix calculations. Users with existing simulations using backward-propagating Gaussian beams with non-zero waist distances may need to adjust their values.
diff --git a/changelog.d/3219.3.added.md b/changelog.d/3219.3.added.md
new file mode 100644
index 0000000000..6e37f358c4
--- /dev/null
+++ b/changelog.d/3219.3.added.md
@@ -0,0 +1 @@
+Added `symmetric_pseudo` option for `s_param_def` in `TerminalComponentModeler` which applies a scaling factor that ensures the S-matrix is symmetric in reciprocal systems.
diff --git a/changelog.d/3224.added.md b/changelog.d/3224.added.md
new file mode 100644
index 0000000000..5827645c5a
--- /dev/null
+++ b/changelog.d/3224.added.md
@@ -0,0 +1 @@
+Added `DesignSpace` support for sweeping `WorkflowType` objects, including mode/EME simulations and component modelers.
diff --git a/changelog.d/3225.added.md b/changelog.d/3225.added.md
new file mode 100644
index 0000000000..188d31850c
--- /dev/null
+++ b/changelog.d/3225.added.md
@@ -0,0 +1 @@
+Added config versioning with automatic backward migrations by default; forward-compat is best-effort unless strict mode is enabled.
diff --git a/changelog.d/3227.fixed.md b/changelog.d/3227.fixed.md
new file mode 100644
index 0000000000..3a1c6b61a2
--- /dev/null
+++ b/changelog.d/3227.fixed.md
@@ -0,0 +1 @@
+Fixed local cache race conditions causing `FileNotFoundError`.
diff --git a/changelog.d/3228.fixed.md b/changelog.d/3228.fixed.md
new file mode 100644
index 0000000000..9c8a598973
--- /dev/null
+++ b/changelog.d/3228.fixed.md
@@ -0,0 +1 @@
+Fixed redundant logging when `Batch.download()` skips existing files, and added `replace_existing` to `Batch.run()` so overwrite behavior can be controlled directly.
diff --git a/changelog.d/3229.fixed.md b/changelog.d/3229.fixed.md
new file mode 100644
index 0000000000..ac2bc38da6
--- /dev/null
+++ b/changelog.d/3229.fixed.md
@@ -0,0 +1 @@
+Fixed redundant server lookups when loading simulation results.
diff --git a/changelog.d/3233.1.added.md b/changelog.d/3233.1.added.md
new file mode 100644
index 0000000000..5ec83b1af5
--- /dev/null
+++ b/changelog.d/3233.1.added.md
@@ -0,0 +1 @@
+Added flag `remove_fragments` to the base `UnstructuredGrid` to remove fragments in unstructured grids. This can ease meshing by eliminating internal boundaries in overlapping structures.
diff --git a/changelog.d/3233.2.added.md b/changelog.d/3233.2.added.md
new file mode 100644
index 0000000000..4b34016794
--- /dev/null
+++ b/changelog.d/3233.2.added.md
@@ -0,0 +1 @@
+Added deprecation warning for `conformal` in TCAD heat/charge monitors when explicitly set; this option is ignored (treated as `False`) when meshing with `remove_fragments=True`.
diff --git a/changelog.d/3234.added.md b/changelog.d/3234.added.md
new file mode 100644
index 0000000000..77f30d26f2
--- /dev/null
+++ b/changelog.d/3234.added.md
@@ -0,0 +1 @@
+Added in-memory caching for downloaded batch results, configurable via ``config.batch_data_cache``.
diff --git a/changelog.d/3236.added.md b/changelog.d/3236.added.md
new file mode 100644
index 0000000000..20c350cfaa
--- /dev/null
+++ b/changelog.d/3236.added.md
@@ -0,0 +1 @@
+Added `current_amplitude_definition` parameter to `UniformCurrentSource` for size-independent total current injection. Set to `"total"` to interpret the source amplitude as total current rather than current density.
diff --git a/changelog.d/3237.1.fixed.md b/changelog.d/3237.1.fixed.md
new file mode 100644
index 0000000000..5a700a62c8
--- /dev/null
+++ b/changelog.d/3237.1.fixed.md
@@ -0,0 +1 @@
+Fixed `CustomMedium` gradient calculation when field coordinates exactly align with boundaries.
diff --git a/changelog.d/3237.2.fixed.md b/changelog.d/3237.2.fixed.md
new file mode 100644
index 0000000000..2b1dd30f1b
--- /dev/null
+++ b/changelog.d/3237.2.fixed.md
@@ -0,0 +1 @@
+Fixed adjoint simulation `grid_spec` to align exactly with forward simulation for correct `FieldData` adjoint source power.
diff --git a/changelog.d/3239.changed.md b/changelog.d/3239.changed.md
new file mode 100644
index 0000000000..612fa7dc42
--- /dev/null
+++ b/changelog.d/3239.changed.md
@@ -0,0 +1 @@
+Local caching is now enabled by default (set `td.config.local_cache.enabled=False` to opt out).
diff --git a/changelog.d/3241.fixed.md b/changelog.d/3241.fixed.md
new file mode 100644
index 0000000000..86f8e7ceeb
--- /dev/null
+++ b/changelog.d/3241.fixed.md
@@ -0,0 +1 @@
+Updated docstrings for `DerivativeInfo` to more accurately reflect dataclass fields.
diff --git a/changelog.d/3244.fixed.md b/changelog.d/3244.fixed.md
new file mode 100644
index 0000000000..d32b5211da
--- /dev/null
+++ b/changelog.d/3244.fixed.md
@@ -0,0 +1 @@
+Fixed `TerminalComponentModeler` serialization failure with `CustomGridBoundaries` by storing JSON-serializable grid metadata in `GridSpec.attrs`.
diff --git a/changelog.d/3250.added.md b/changelog.d/3250.added.md
new file mode 100644
index 0000000000..5ba16c90a6
--- /dev/null
+++ b/changelog.d/3250.added.md
@@ -0,0 +1 @@
+Added baseband source time classes (`BasebandStep`, `BasebandGaussianPulse`, `BasebandRectangularPulse`, `BasebandCustomSourceTime`) for transient RF simulations with real-valued time signals.
diff --git a/changelog.d/3252.fixed.md b/changelog.d/3252.fixed.md
new file mode 100644
index 0000000000..6db5bd13f5
--- /dev/null
+++ b/changelog.d/3252.fixed.md
@@ -0,0 +1 @@
+Improved `ModeSolver.solve()` to suppress the accuracy warning when local subpixel averaging is enabled via `tidy3d-extras`, and expanded docstrings for `ModeSolver.solve()`, `ModeSimulation.run_local()`, `Simulation.epsilon()`, and `Simulation.epsilon_on_grid()` to document the `config.simulation.use_local_subpixel` option.
diff --git a/changelog.d/3253.changed.md b/changelog.d/3253.changed.md
new file mode 100644
index 0000000000..620eeb8e6e
--- /dev/null
+++ b/changelog.d/3253.changed.md
@@ -0,0 +1 @@
+Changed default `num_points` in `EMEModeSpec.interp_spec` from 3 to 5 for improved accuracy of frequency interpolation.
diff --git a/changelog.d/3259.changed.md b/changelog.d/3259.changed.md
new file mode 100644
index 0000000000..b429ef7e94
--- /dev/null
+++ b/changelog.d/3259.changed.md
@@ -0,0 +1 @@
+Unstructured data plots are now "crinkled", showing the full mesh elements that cover given monitor boundaries. Previously, the mesh elements were "clipped" to the monitor boundaries.
diff --git a/changelog.d/3260.fixed.md b/changelog.d/3260.fixed.md
new file mode 100644
index 0000000000..9ff9f7b7b9
--- /dev/null
+++ b/changelog.d/3260.fixed.md
@@ -0,0 +1 @@
+Fixed `ModeSolver.validate_pre_upload` not validating `MicrowaveModeSpec` with `AutoImpedanceSpec`, causing cryptic server-side errors for invalid conductor geometries.
diff --git a/changelog.d/3268.fixed.md b/changelog.d/3268.fixed.md
new file mode 100644
index 0000000000..6dfecc3cf7
--- /dev/null
+++ b/changelog.d/3268.fixed.md
@@ -0,0 +1 @@
+Fixed local cache not storing results for `ModalComponentModeler` (and `TerminalComponentModeler`) runs, causing cache misses on repeated `web.run()` calls.
diff --git a/changelog.d/3275.fixed.md b/changelog.d/3275.fixed.md
new file mode 100644
index 0000000000..76702d55e2
--- /dev/null
+++ b/changelog.d/3275.fixed.md
@@ -0,0 +1 @@
+Fixed spurious `ModeSimulation` error logs during deserialization of `SimulationDataMap` by adding discriminators to `SimulationType` and `SimulationDataType` unions.
diff --git a/changelog.d/3294.fixed.md b/changelog.d/3294.fixed.md
new file mode 100644
index 0000000000..f48ba9decf
--- /dev/null
+++ b/changelog.d/3294.fixed.md
@@ -0,0 +1 @@
+Fixed unintended model output in Jupyter notebooks during `import tidy3d` by preventing `Tidy3dBaseModel.__str__` from triggering notebook display side effects.
diff --git a/changelog.d/3296.breaking.md b/changelog.d/3296.breaking.md
new file mode 100644
index 0000000000..42557fc6f7
--- /dev/null
+++ b/changelog.d/3296.breaking.md
@@ -0,0 +1 @@
+`web.Batch(simulations=...)` now requires string task names when simulations are passed as a dictionary. Numeric keys (for example `0`, `1`) are no longer converted automatically; convert them to strings first (for example `"0"`, `"1"`).
diff --git a/changelog.d/3301.changed.md b/changelog.d/3301.changed.md
new file mode 100644
index 0000000000..278c53f3c1
--- /dev/null
+++ b/changelog.d/3301.changed.md
@@ -0,0 +1 @@
+Changed the default value of `fill_value` in `UnstructuredGridDataset.interp()` from `0` to `"extrapolate"`. This means points outside the mesh will now use nearest-neighbor extrapolation instead of being filled with zeros.
diff --git a/changelog.d/README.md b/changelog.d/README.md
new file mode 100644
index 0000000000..360fa2fbdd
--- /dev/null
+++ b/changelog.d/README.md
@@ -0,0 +1,47 @@
+# Changelog Fragments
+
+Use Towncrier fragments for user-visible changes instead of editing `CHANGELOG.md` directly.
+`CHANGELOG.md` is release-generated; regular PRs should add fragment files in this directory.
+CI rejects direct `CHANGELOG.md` edits in regular PRs (auto-generated `chore/build-changelog-*` branches are exempt).
+
+## File naming
+
+Create one file per change using:
+
+- Single entry for a PR/type: `<PR_NUMBER>.<type>.md`
+- Multiple entries for the same PR/type: `<PR_NUMBER>.<N>.<type>.md` where `N` starts at `1`
+
+Examples:
+
+- `1234.added.md`
+- `1235.1.added.md`
+- `1235.2.added.md`
+- `1236.changed.md`
+- `1237.fixed.md`
+- `1238.removed.md`
+- `1239.breaking.md`
+- `1240.planned_deprecation.md`
+
+## Allowed fragment types
+
+- `added`
+- `changed`
+- `fixed`
+- `removed`
+- `breaking`
+- `planned_deprecation`
+
+## Content format
+
+- Write plain text only (no leading `-` bullet).
+- Keep it to one short sentence per fragment file.
+- Focus on the user-visible impact.
+
+Examples:
+
+- `added`: `Added GeometryArray for efficiently representing repeated geometry instances.`
+- `changed`: `Improved local cache performance for repeated result loads.`
+- `fixed`: `Fixed race conditions when reading the local configuration directory in parallel jobs.`
+- `removed`: `Removed the deprecated legacy material alias from the public API.`
+- `breaking`: `ModeSortSpec.sort_key is now required; update any code relying on None defaults.`
+- `planned_deprecation`: `CurrentIntegralAxisAligned is deprecated and will be removed in a future release; use AxisAlignedCurrentIntegral.`
diff --git a/changelog.d/template.md b/changelog.d/template.md
new file mode 100644
index 0000000000..34c3c38418
--- /dev/null
+++ b/changelog.d/template.md
@@ -0,0 +1,16 @@
+{% for section, _ in sections.items() %}
+{% set section_name = definitions[section]["name"] if section else "" %}
+{% if section_name %}
+### {{ section_name }}
+
+{% endif %}
+{% for category, val in definitions.items() if category in sections[section] %}
+
+### {{ definitions[category]["name"] }}
+
+{% for text, values in sections[section][category].items() %}
+- {{ text }}
+{% endfor %}
+{% endfor %}
+{% endfor %}
+{{ "\n" -}}
diff --git a/docs/development/release/flow.rst b/docs/development/release/flow.rst
index 55df801e72..cc8815a466 100644
--- a/docs/development/release/flow.rst
+++ b/docs/development/release/flow.rst
@@ -60,7 +60,7 @@ a clean branch on your machine.
 
 Develop your code in this new branch, committing your changes when it seems like a natural time to “save your progress”.
 
-If you are working on a new feature, make sure you add a line in the `CHANGELOG.md <https://github.com/flexcompute/Tidy3D-client-revamp/blob/develop/CHANGELOG.md>`_ file (if it exists in that repository) to summarize your changes.
+If your PR has a user-visible change, add a changelog fragment in ``changelog.d/`` instead of editing ``CHANGELOG.md`` directly. Use ``<PR_NUMBER>.<type>.md`` for a single entry, or ``<PR_NUMBER>.<N>.<type>.md`` when a PR has multiple entries of the same type (with ``N`` starting at ``1``). ``type`` is one of ``added``, ``changed``, ``fixed``, ``removed``, ``breaking``, or ``planned_deprecation``. Write one short plain sentence without a leading bullet (Towncrier adds bullets during release builds).
 
 
 3. Create a pull request on GitHub
@@ -118,7 +118,7 @@ rebasing has changed its history.
 Every PR must have the following before it can be merged:
 
 - At least one review.
-- A description in the CHANGELOG of what has been done.
+- For user-visible changes, at least one changelog fragment in ``changelog.d/``.
 
 Every new major feature must also pass all of the following before it can be merged:
 
diff --git a/docs/development/release/version.rst b/docs/development/release/version.rst
index ae01557d85..cf0074f840 100644
--- a/docs/development/release/version.rst
+++ b/docs/development/release/version.rst
@@ -143,6 +143,37 @@ Examples
    workflow_control: start-deploy
    # Skips tag creation and tests
 
+Release Changelog Build
+^^^^^^^^^^^^^^^^^^^^^^^
+
+Before running the release workflow, generate release notes from ``changelog.d`` fragments:
+
+.. code-block:: bash
+
+   RELVER=$(poetry version -s | sed -E 's/\.dev[0-9]+$//')
+   RELDATE=$(date -u +%F)
+   poetry run towncrier build --yes --version "${RELVER}" --date "${RELDATE}"
+   poetry run python scripts/changelog_refs.py --version "${RELVER}"
+
+This sequence:
+
+- Derives the release version from ``pyproject.toml`` (for example ``2.11.0.dev0`` -> ``2.11.0``).
+- Builds ``CHANGELOG.md`` and consumes fragment files.
+- Adds or updates the reference-style compare link for ``[RELVER]`` using the latest reachable stable ``vX.Y.Z`` tag as the previous version.
+
+Commit the updated ``CHANGELOG.md`` and removed fragment files in the same release commit.
+
+Automated Changelog PR Workflow
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+You can automate changelog generation and PR creation with GitHub Actions workflow ``public/tidy3d/python-client-build-changelog-pr``.
+
+- Default source branch: ``develop``
+- Default target branch: ``develop``
+- Optional overrides: ``source_branch``, ``target_branch``, ``release_version``, ``release_date``, ``previous_version``
+
+This workflow runs Towncrier, updates compare reference links, and opens a PR with the resulting changes.
+
 Best Practices
 ^^^^^^^^^^^^^^
 
diff --git a/poetry.lock b/poetry.lock
index ae4830e7b8..97be8451cb 100644
--- a/poetry.lock
+++ b/poetry.lock
@@ -7643,6 +7643,27 @@ files = [
     {file = "tornado-6.5.4.tar.gz", hash = "sha256:a22fa9047405d03260b483980635f0b041989d8bcc9a313f8fe18b411d84b1d7"},
 ]
 
+[[package]]
+name = "towncrier"
+version = "25.8.0"
+description = "Building newsfiles for your project."
+optional = true
+python-versions = ">=3.9"
+groups = ["main"]
+markers = "extra == \"dev\""
+files = [
+    {file = "towncrier-25.8.0-py3-none-any.whl", hash = "sha256:b953d133d98f9aeae9084b56a3563fd2519dfc6ec33f61c9cd2c61ff243fb513"},
+    {file = "towncrier-25.8.0.tar.gz", hash = "sha256:eef16d29f831ad57abb3ae32a0565739866219f1ebfbdd297d32894eb9940eb1"},
+]
+
+[package.dependencies]
+click = "*"
+jinja2 = "*"
+tomli = {version = "*", markers = "python_version < \"3.11\""}
+
+[package.extras]
+dev = ["furo (>=2024.05.06)", "nox", "packaging", "sphinx (>=5)", "twisted"]
+
 [[package]]
 name = "tox"
 version = "4.34.1"
@@ -8129,7 +8150,7 @@ files = [
 
 [extras]
 design = ["bayesian-optimization", "pygad", "pyswarms"]
-dev = ["bayesian-optimization", "cma", "coverage", "diff-cover", "dill", "gdstk", "grcwa", "ipython", "ipython", "jinja2", "jupyter", "libcst", "memory_profiler", "mypy", "myst-parser", "nbconvert", "nbdime", "nbsphinx", "networkx", "openpyxl", "optax", "pre-commit", "psutil", "pydata-sphinx-theme", "pygad", "pylint", "pyswarms", "pytest", "pytest-cov", "pytest-env", "pytest-testmon", "pytest-timeout", "pytest-xdist", "rtree", "ruff", "sax", "scikit-rf", "signac", "sphinx", "sphinx-book-theme", "sphinx-copybutton", "sphinx-design", "sphinx-favicon", "sphinx-notfound-page", "sphinx-sitemap", "sphinx-tabs", "sphinxemoji", "tmm", "torch", "torch", "tox", "trimesh", "vtk", "zizmor"]
+dev = ["bayesian-optimization", "cma", "coverage", "diff-cover", "dill", "gdstk", "grcwa", "ipython", "ipython", "jinja2", "jupyter", "libcst", "memory_profiler", "mypy", "myst-parser", "nbconvert", "nbdime", "nbsphinx", "networkx", "openpyxl", "optax", "pre-commit", "psutil", "pydata-sphinx-theme", "pygad", "pylint", "pyswarms", "pytest", "pytest-cov", "pytest-env", "pytest-testmon", "pytest-timeout", "pytest-xdist", "rtree", "ruff", "sax", "scikit-rf", "signac", "sphinx", "sphinx-book-theme", "sphinx-copybutton", "sphinx-design", "sphinx-favicon", "sphinx-notfound-page", "sphinx-sitemap", "sphinx-tabs", "sphinxemoji", "tmm", "torch", "torch", "towncrier", "tox", "trimesh", "vtk", "zizmor"]
 docs = ["cma", "gdstk", "grcwa", "ipython", "jinja2", "jupyter", "myst-parser", "nbconvert", "nbdime", "nbsphinx", "openpyxl", "optax", "pydata-sphinx-theme", "pylint", "sax", "signac", "sphinx", "sphinx-book-theme", "sphinx-copybutton", "sphinx-design", "sphinx-favicon", "sphinx-notfound-page", "sphinx-sitemap", "sphinx-tabs", "sphinxemoji", "tmm"]
 extras = ["tidy3d-extras"]
 gdstk = ["gdstk"]
@@ -8144,4 +8165,4 @@ vtk = ["vtk"]
 [metadata]
 lock-version = "2.1"
 python-versions = ">=3.10,<3.14"
-content-hash = "a09616de367b2f46ad52109dc990ad23613c4cb0008bc1ed510e372594d8733a"
+content-hash = "b373985f28814d5d05ddab1800864563dbc586b85b8b7d8b98292573b64d1cee"
diff --git a/pyproject.toml b/pyproject.toml
index f8b1fd312b..d2ed1f00b7 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -69,6 +69,7 @@ pytest-cov = "^6.0.0"
 pytest-env = "^1.1.5"
 pytest-testmon = { version = "^2.1.3", optional = true }
 tox = { version = "^4.33.0", optional = true }
+towncrier = { version = "^25.8.0", optional = true }
 diff-cover = { version = "^10.1.0", optional = true }
 zizmor = { version = "^1.20.0", optional = true }
 mypy = { version = "1.13.0", optional = true }
@@ -173,6 +174,7 @@ dev = [
   'sphinxemoji',
   'tmm',
   'tox',
+  'towncrier',
   'trimesh',
   'scikit-rf',
   'vtk',
@@ -253,6 +255,43 @@ name = "codeartifact"
 url = "https://flexcompute-625554095313.d.codeartifact.us-east-1.amazonaws.com/pypi/pypi-releases/simple/"
 priority = "supplemental"
 
+[tool.towncrier]
+directory = "changelog.d"
+filename = "CHANGELOG.md"
+start_string = "<!-- towncrier release notes start -->\n"
+template = "changelog.d/template.md"
+title_format = "## [{version}] - {project_date}"
+
+[[tool.towncrier.type]]
+directory = "added"
+name = "Added"
+showcontent = true
+
+[[tool.towncrier.type]]
+directory = "changed"
+name = "Changed"
+showcontent = true
+
+[[tool.towncrier.type]]
+directory = "fixed"
+name = "Fixed"
+showcontent = true
+
+[[tool.towncrier.type]]
+directory = "removed"
+name = "Removed"
+showcontent = true
+
+[[tool.towncrier.type]]
+directory = "breaking"
+name = "Breaking Changes"
+showcontent = true
+
+[[tool.towncrier.type]]
+directory = "planned_deprecation"
+name = "Planned Deprecation"
+showcontent = true
+
 [build-system]
 requires = ["poetry-core>=1.0.0"]
 build-backend = "poetry.core.masonry.api"
diff --git a/scripts/changelog_refs.py b/scripts/changelog_refs.py
new file mode 100644
index 0000000000..a80e37b726
--- /dev/null
+++ b/scripts/changelog_refs.py
@@ -0,0 +1,199 @@
+from __future__ import annotations
+
+import argparse
+import re
+import subprocess
+import sys
+from collections import OrderedDict
+from pathlib import Path
+
+try:
+    import tomllib
+except ModuleNotFoundError:  # pragma: no cover - Python < 3.11 fallback
+    tomllib = None  # type: ignore[assignment]
+
+import toml
+
+REFERENCE_RE = re.compile(r"^\[([^\]]+)\]:\s+(\S+)\s*$")
+RELEASE_HEADING_RE = re.compile(r"^##\s+\[([^\]]+)\]")
+DEV_SUFFIX_RE = re.compile(r"\.dev\d+$")
+RELEASE_VERSION_RE = re.compile(r"^\d+\.\d+\.\d+$")
+
+
+def _run_git_command(*args: str) -> str:
+    """Run a git command and return stdout."""
+    completed = subprocess.run(
+        ["git", *args],
+        check=True,
+        capture_output=True,
+        text=True,
+    )
+    return completed.stdout.strip()
+
+
+def _read_pyproject_version(pyproject_path: Path) -> str:
+    """Read the Poetry version from pyproject.toml."""
+    if tomllib is not None:
+        data = tomllib.loads(pyproject_path.read_text(encoding="utf-8"))
+    else:
+        data = toml.loads(pyproject_path.read_text(encoding="utf-8"))
+    return data["tool"]["poetry"]["version"]
+
+
+def _derive_release_version(pyproject_path: Path) -> str:
+    """Derive a release version by stripping any .devN suffix."""
+    version = _read_pyproject_version(pyproject_path)
+    return DEV_SUFFIX_RE.sub("", version)
+
+
+def _find_previous_version(new_version: str) -> str:
+    """Find the latest reachable stable vX.Y.Z tag excluding the new version."""
+    tags = _run_git_command(
+        "tag", "--merged", "HEAD", "--list", "v*", "--sort=-v:refname"
+    ).splitlines()
+    for tag in tags:
+        tag = tag.strip()
+        if not tag:
+            continue
+        candidate_version = tag.removeprefix("v")
+        if not RELEASE_VERSION_RE.fullmatch(candidate_version):
+            continue
+        if candidate_version == new_version:
+            continue
+        return candidate_version
+    raise RuntimeError("Could not determine previous stable vX.Y.Z tag reachable from HEAD.")
+
+
+def _find_previous_version_from_changelog(changelog_path: Path, new_version: str) -> str:
+    """Find the latest stable release heading in CHANGELOG.md excluding the new version."""
+    for line in changelog_path.read_text(encoding="utf-8").splitlines():
+        match = RELEASE_HEADING_RE.match(line)
+        if not match:
+            continue
+        candidate_version = match.group(1).strip()
+        if not RELEASE_VERSION_RE.fullmatch(candidate_version):
+            continue
+        if candidate_version == new_version:
+            continue
+        return candidate_version
+    raise RuntimeError("Could not determine previous stable release from CHANGELOG.md headings.")
+
+
+def _update_reference_links(
+    changelog_path: Path,
+    new_version: str,
+    previous_version: str,
+    repo_url: str,
+) -> None:
+    """Add or update the release compare URL reference at the bottom of CHANGELOG.md."""
+    lines = changelog_path.read_text(encoding="utf-8").splitlines()
+    first_ref_index = next(
+        (index for index, line in enumerate(lines) if REFERENCE_RE.match(line)), None
+    )
+
+    if first_ref_index is None:
+        body_lines = lines
+        existing_refs: list[str] = []
+    else:
+        body_lines = lines[:first_ref_index]
+        existing_refs = lines[first_ref_index:]
+
+    ordered_refs: OrderedDict[str, str] = OrderedDict()
+    for ref_line in existing_refs:
+        match = REFERENCE_RE.match(ref_line)
+        if not match:
+            continue
+        reference_name, url = match.groups()
+        if reference_name in {"Unreleased", new_version}:
+            continue
+        ordered_refs[reference_name] = url
+
+    compare_url = f"{repo_url.rstrip('/')}/compare/v{previous_version}...v{new_version}"
+    ordered_refs = OrderedDict([(new_version, compare_url), *ordered_refs.items()])
+
+    rendered_refs = [f"[{name}]: {url}" for name, url in ordered_refs.items()]
+    body = "\n".join(body_lines).rstrip("\n")
+    output = f"{body}\n\n" + "\n".join(rendered_refs) + "\n"
+    changelog_path.write_text(output, encoding="utf-8")
+
+
+def main() -> int:
+    """CLI entry point."""
+    parser = argparse.ArgumentParser(
+        description="Update CHANGELOG.md reference-style release compare links.",
+    )
+    parser.add_argument(
+        "--version",
+        help="New release version (for example: 2.11.0). Defaults to pyproject version without .devN.",
+    )
+    parser.add_argument(
+        "--previous-version",
+        help=(
+            "Previous release version (with or without leading v). "
+            "Defaults to latest reachable stable vX.Y.Z tag; if unavailable, "
+            "falls back to latest stable release heading in CHANGELOG.md."
+        ),
+    )
+    parser.add_argument(
+        "--changelog",
+        default="CHANGELOG.md",
+        help="Path to CHANGELOG.md.",
+    )
+    parser.add_argument(
+        "--pyproject",
+        default="pyproject.toml",
+        help="Path to pyproject.toml.",
+    )
+    parser.add_argument(
+        "--repo-url",
+        default="https://github.com/flexcompute/tidy3d",
+        help="Repository URL used to build compare links.",
+    )
+    args = parser.parse_args()
+
+    changelog_path = Path(args.changelog)
+    pyproject_path = Path(args.pyproject)
+
+    if not changelog_path.exists():
+        raise FileNotFoundError(f"Changelog path does not exist: {changelog_path}")
+    if not pyproject_path.exists():
+        raise FileNotFoundError(f"pyproject path does not exist: {pyproject_path}")
+
+    new_version = args.version or _derive_release_version(pyproject_path)
+    new_version = DEV_SUFFIX_RE.sub("", new_version).strip().removeprefix("v")
+    if not new_version:
+        raise ValueError("New release version is empty.")
+
+    previous_version = None
+    if args.previous_version:
+        previous_version = args.previous_version.strip().removeprefix("v")
+        if not previous_version:
+            raise ValueError("Previous release version is empty after normalization.")
+    if previous_version is None:
+        try:
+            previous_version = _find_previous_version(new_version)
+        except (RuntimeError, subprocess.CalledProcessError):
+            previous_version = _find_previous_version_from_changelog(changelog_path, new_version)
+            print(
+                (
+                    "No stable git tag found; "
+                    f"falling back to latest stable release in {changelog_path}: "
+                    f"{previous_version}"
+                ),
+                file=sys.stderr,
+            )
+    if previous_version == new_version:
+        raise ValueError("Previous version must differ from new version.")
+
+    _update_reference_links(
+        changelog_path=changelog_path,
+        new_version=new_version,
+        previous_version=previous_version,
+        repo_url=args.repo_url,
+    )
+    print(f"Updated changelog references for {new_version} (previous {previous_version}).")
+    return 0
+
+
+if __name__ == "__main__":
+    sys.exit(main())
diff --git a/tests/test_package/test_changelog_refs.py b/tests/test_package/test_changelog_refs.py
new file mode 100644
index 0000000000..b874fd2ecb
--- /dev/null
+++ b/tests/test_package/test_changelog_refs.py
@@ -0,0 +1,96 @@
+"""Regression tests for the changelog reference updater script."""
+
+from __future__ import annotations
+
+import subprocess
+import sys
+from pathlib import Path
+
+
+def test_changelog_refs_normalizes_leading_v_in_version(tmp_path: Path) -> None:
+    """Ensure --version values like v2.10.2 do not generate vv compare links."""
+
+    repo_root = Path(__file__).resolve().parents[2]
+    script_path = repo_root / "scripts" / "changelog_refs.py"
+    pyproject_path = repo_root / "pyproject.toml"
+
+    changelog_path = tmp_path / "CHANGELOG.md"
+    changelog_path.write_text(
+        (
+            "# Changelog\n\n"
+            "<!-- towncrier release notes start -->\n\n"
+            "## [2.10.2] - 2026-01-21\n\n"
+            "### Added\n"
+            "- Existing entry.\n\n"
+            "[2.10.2]: "
+            "https://github.com/flexcompute/tidy3d/compare/v2.10.1...v2.10.2\n"
+        ),
+        encoding="utf-8",
+    )
+
+    result = subprocess.run(
+        [
+            sys.executable,
+            str(script_path),
+            "--changelog",
+            str(changelog_path),
+            "--pyproject",
+            str(pyproject_path),
+            "--version",
+            "v2.10.2",
+            "--previous-version",
+            "2.10.1",
+        ],
+        capture_output=True,
+        text=True,
+        check=True,
+    )
+
+    content = changelog_path.read_text(encoding="utf-8")
+    assert "[2.10.2]:" in content
+    assert "[v2.10.2]:" not in content
+    assert "...vv2.10.2" not in content
+    assert "compare/v2.10.1...v2.10.2" in content
+    assert "Updated changelog references for 2.10.2" in result.stdout
+
+
+def test_changelog_refs_rejects_empty_previous_version_after_normalization(tmp_path: Path) -> None:
+    """Reject previous-version values that normalize to an empty string."""
+
+    repo_root = Path(__file__).resolve().parents[2]
+    script_path = repo_root / "scripts" / "changelog_refs.py"
+    pyproject_path = repo_root / "pyproject.toml"
+
+    changelog_path = tmp_path / "CHANGELOG.md"
+    changelog_path.write_text(
+        (
+            "# Changelog\n\n"
+            "<!-- towncrier release notes start -->\n\n"
+            "## [2.10.2] - 2026-01-21\n\n"
+            "### Added\n"
+            "- Existing entry.\n\n"
+            "[2.10.2]: "
+            "https://github.com/flexcompute/tidy3d/compare/v2.10.1...v2.10.2\n"
+        ),
+        encoding="utf-8",
+    )
+
+    result = subprocess.run(
+        [
+            sys.executable,
+            str(script_path),
+            "--changelog",
+            str(changelog_path),
+            "--pyproject",
+            str(pyproject_path),
+            "--version",
+            "2.10.2",
+            "--previous-version",
+            " v ",
+        ],
+        capture_output=True,
+        text=True,
+    )
+
+    assert result.returncode != 0
+    assert "Previous release version is empty after normalization." in result.stderr