diff --git a/app/controllers/holdings_controller.rb b/app/controllers/holdings_controller.rb
index 96ce2cd49..cd6d6260c 100644
--- a/app/controllers/holdings_controller.rb
+++ b/app/controllers/holdings_controller.rb
@@ -1,5 +1,5 @@
 class HoldingsController < ApplicationController
-  before_action :set_holding, only: %i[show destroy]
+  before_action :set_holding, only: %i[show update destroy unlock_cost_basis]
 
   def index
     @account = Current.family.accounts.find(params[:account_id])
@@ -8,6 +8,31 @@ class HoldingsController < ApplicationController
   def show
   end
 
+  def update
+    total_cost_basis = holding_params[:cost_basis].to_d
+
+    if total_cost_basis >= 0 && @holding.qty.positive?
+      # Convert total cost basis to per-share cost (the cost_basis field stores per-share)
+      # Zero is valid for gifted/inherited shares
+      per_share_cost = total_cost_basis / @holding.qty
+      @holding.set_manual_cost_basis!(per_share_cost)
+      flash[:notice] = t(".success")
+    else
+      flash[:alert] = t(".error")
+    end
+
+    # Redirect to account page holdings tab to refresh list and close drawer
+    redirect_to account_path(@holding.account, tab: "holdings")
+  end
+
+  def unlock_cost_basis
+    @holding.unlock_cost_basis!
+    flash[:notice] = t(".success")
+
+    # Redirect to account page holdings tab to refresh list and close drawer
+    redirect_to account_path(@holding.account, tab: "holdings")
+  end
+
   def destroy
     if @holding.account.can_delete_holdings?
       @holding.destroy_holding_and_entries!
@@ -26,4 +51,8 @@ class HoldingsController < ApplicationController
     def set_holding
       @holding = Current.family.holdings.find(params[:id])
     end
+
+    def holding_params
+      params.require(:holding).permit(:cost_basis)
+    end
 end
diff --git a/app/javascript/controllers/cost_basis_form_controller.js b/app/javascript/controllers/cost_basis_form_controller.js
new file mode 100644
index 000000000..467cd5a9a
--- /dev/null
+++ b/app/javascript/controllers/cost_basis_form_controller.js
@@ -0,0 +1,30 @@
+import { Controller } from "@hotwired/stimulus"
+
+// Handles bidirectional conversion between total cost basis and per-share cost
+// in the manual cost basis entry form.
+export default class extends Controller {
+  static targets = ["total", "perShare", "perShareValue"]
+  static values = { qty: Number }
+
+  // Called when user types in the total cost basis field
+  // Updates the per-share display and input to show the calculated value
+  updatePerShare() {
+    const total = Number.parseFloat(this.totalTarget.value) || 0
+    const qty = this.qtyValue || 1
+    const perShare = qty > 0 ? (total / qty).toFixed(2) : "0.00"
+    this.perShareValueTarget.textContent = perShare
+    if (this.hasPerShareTarget) {
+      this.perShareTarget.value = perShare
+    }
+  }
+
+  // Called when user types in the per-share field
+  // Updates the total cost basis field with the calculated value
+  updateTotal() {
+    const perShare = Number.parseFloat(this.perShareTarget.value) || 0
+    const qty = this.qtyValue || 1
+    const total = (perShare * qty).toFixed(2)
+    this.totalTarget.value = total
+    this.perShareValueTarget.textContent = perShare.toFixed(2)
+  }
+}
diff --git a/app/javascript/controllers/drawer_cost_basis_controller.js b/app/javascript/controllers/drawer_cost_basis_controller.js
new file mode 100644
index 000000000..6091bc9cd
--- /dev/null
+++ b/app/javascript/controllers/drawer_cost_basis_controller.js
@@ -0,0 +1,33 @@
+import { Controller } from "@hotwired/stimulus"
+
+// Handles the inline cost basis editor in the holding drawer.
+// Shows/hides the form and handles bidirectional total <-> per-share conversion.
+export default class extends Controller {
+  static targets = ["form", "total", "perShare", "perShareValue"]
+  static values = { qty: Number }
+
+  toggle(event) {
+    event.preventDefault()
+    this.formTarget.classList.toggle("hidden")
+  }
+
+  // Called when user types in total cost basis field
+  updatePerShare() {
+    const total = Number.parseFloat(this.totalTarget.value) || 0
+    const qty = this.qtyValue || 1
+    const perShare = qty > 0 ? (total / qty).toFixed(2) : "0.00"
+    this.perShareValueTarget.textContent = perShare
+    if (this.hasPerShareTarget) {
+      this.perShareTarget.value = perShare
+    }
+  }
+
+  // Called when user types in per-share field
+  updateTotal() {
+    const perShare = Number.parseFloat(this.perShareTarget.value) || 0
+    const qty = this.qtyValue || 1
+    const total = (perShare * qty).toFixed(2)
+    this.totalTarget.value = total
+    this.perShareValueTarget.textContent = perShare.toFixed(2)
+  }
+}
diff --git a/app/models/account/provider_import_adapter.rb b/app/models/account/provider_import_adapter.rb
index d247e09e0..e74d1f5e6 100644
--- a/app/models/account/provider_import_adapter.rb
+++ b/app/models/account/provider_import_adapter.rb
@@ -313,17 +313,32 @@ class Account::ProviderImportAdapter
         end
       end
 
-      holding.assign_attributes(
+      # Reconcile cost_basis to respect priority hierarchy
+      reconciled = Holding::CostBasisReconciler.reconcile(
+        existing_holding: holding.persisted? ? holding : nil,
+        incoming_cost_basis: cost_basis,
+        incoming_source: "provider"
+      )
+
+      # Build base attributes
+      attributes = {
         security: security,
         date: date,
         currency: currency,
         qty: quantity,
         price: price,
         amount: amount,
-        cost_basis: cost_basis,
         account_provider_id: account_provider_id,
         external_id: external_id
-      )
+      }
+
+      # Only update cost_basis if reconciliation says to
+      if reconciled[:should_update]
+        attributes[:cost_basis] = reconciled[:cost_basis]
+        attributes[:cost_basis_source] = reconciled[:cost_basis_source]
+      end
+
+      holding.assign_attributes(attributes)
 
       begin
         Holding.transaction(requires_new: true) do
@@ -353,11 +368,22 @@ class Account::ProviderImportAdapter
             updates = {
               qty: quantity,
               price: price,
-              amount: amount,
-              cost_basis: cost_basis
+              amount: amount
             }
 
-            # Adopt the row to this provider if it’s currently unowned
+            # Reconcile cost_basis to respect priority hierarchy
+            collision_reconciled = Holding::CostBasisReconciler.reconcile(
+              existing_holding: existing,
+              incoming_cost_basis: cost_basis,
+              incoming_source: "provider"
+            )
+
+            if collision_reconciled[:should_update]
+              updates[:cost_basis] = collision_reconciled[:cost_basis]
+              updates[:cost_basis_source] = collision_reconciled[:cost_basis_source]
+            end
+
+            # Adopt the row to this provider if it's currently unowned
             if account_provider_id.present? && existing.account_provider_id.nil?
               updates[:account_provider_id] = account_provider_id
             end
diff --git a/app/models/holding.rb b/app/models/holding.rb
index c2297db50..fd7fe4284 100644
--- a/app/models/holding.rb
+++ b/app/models/holding.rb
@@ -3,6 +3,16 @@ class Holding < ApplicationRecord
 
   monetize :amount
 
+  # Cost basis source priority (higher = takes precedence)
+  COST_BASIS_SOURCE_PRIORITY = {
+    nil => 0,
+    "provider" => 1,
+    "calculated" => 2,
+    "manual" => 3
+  }.freeze
+
+  COST_BASIS_SOURCES = %w[manual calculated provider].freeze
+
   belongs_to :account
   belongs_to :security
   belongs_to :account_provider, optional: true
@@ -10,9 +20,12 @@ class Holding < ApplicationRecord
   validates :qty, :currency, :date, :price, :amount, presence: true
   validates :qty, :price, :amount, numericality: { greater_than_or_equal_to: 0 }
   validates :external_id, uniqueness: { scope: :account_id }, allow_blank: true
+  validates :cost_basis_source, inclusion: { in: COST_BASIS_SOURCES }, allow_nil: true
 
   scope :chronological, -> { order(:date) }
   scope :for, ->(security) { where(security_id: security).order(:date) }
+  scope :with_locked_cost_basis, -> { where(cost_basis_locked: true) }
+  scope :with_unlocked_cost_basis, -> { where(cost_basis_locked: false) }
 
   delegate :ticker, to: :security
 
@@ -76,6 +89,53 @@ class Holding < ApplicationRecord
     account.sync_later
   end
 
+  # Returns the priority level for the current source (higher = better)
+  def cost_basis_source_priority
+    COST_BASIS_SOURCE_PRIORITY[cost_basis_source] || 0
+  end
+
+  # Check if this holding's cost_basis can be overwritten by the given source
+  def cost_basis_replaceable_by?(new_source)
+    return false if cost_basis_locked?
+
+    new_priority = COST_BASIS_SOURCE_PRIORITY[new_source] || 0
+
+    # Special case: when user unlocks a manual cost_basis, they're opting into
+    # recalculation. Allow only "calculated" source to replace it (from trades).
+    # This is the whole point of the unlock action.
+    if cost_basis_source == "manual"
+      return new_source == "calculated"
+    end
+
+    new_priority > cost_basis_source_priority
+  end
+
+  # Set cost_basis from user input (locks the value)
+  def set_manual_cost_basis!(value)
+    update!(
+      cost_basis: value,
+      cost_basis_source: "manual",
+      cost_basis_locked: true
+    )
+  end
+
+  # Unlock cost_basis to allow provider/calculated updates
+  def unlock_cost_basis!
+    update!(cost_basis_locked: false)
+  end
+
+  # Check if cost_basis is known (has a source and positive value)
+  def cost_basis_known?
+    cost_basis.present? && cost_basis.positive? && cost_basis_source.present?
+  end
+
+  # Human-readable source label for UI display
+  def cost_basis_source_label
+    return nil unless cost_basis_source.present?
+
+    I18n.t("holdings.cost_basis_sources.#{cost_basis_source}")
+  end
+
   private
     def calculate_trend
       return nil unless amount_money
diff --git a/app/models/holding/cost_basis_reconciler.rb b/app/models/holding/cost_basis_reconciler.rb
new file mode 100644
index 000000000..88bd9b635
--- /dev/null
+++ b/app/models/holding/cost_basis_reconciler.rb
@@ -0,0 +1,58 @@
+# Determines the appropriate cost_basis value and source when updating a holding.
+#
+# Used by both Materializer (for trade-derived calculations) and
+# ProviderImportAdapter (for provider-supplied values) to ensure consistent
+# reconciliation logic across all data sources.
+#
+# Priority hierarchy: manual > calculated > provider > unknown
+#
+class Holding::CostBasisReconciler
+  # Determines the appropriate cost_basis value and source for a holding update
+  #
+  # @param existing_holding [Holding, nil] The existing holding record (nil for new)
+  # @param incoming_cost_basis [BigDecimal, nil] The incoming cost_basis value
+  # @param incoming_source [String] The source of incoming data ('calculated', 'provider')
+  # @return [Hash] { cost_basis: value, cost_basis_source: source, should_update: boolean }
+  def self.reconcile(existing_holding:, incoming_cost_basis:, incoming_source:)
+    # Treat zero cost_basis from provider as unknown
+    if incoming_source == "provider" && (incoming_cost_basis.nil? || incoming_cost_basis.zero?)
+      incoming_cost_basis = nil
+    end
+
+    # New holding - use whatever we have
+    if existing_holding.nil?
+      return {
+        cost_basis: incoming_cost_basis,
+        cost_basis_source: incoming_cost_basis.present? ? incoming_source : nil,
+        should_update: true
+      }
+    end
+
+    # Locked - never overwrite
+    if existing_holding.cost_basis_locked?
+      return {
+        cost_basis: existing_holding.cost_basis,
+        cost_basis_source: existing_holding.cost_basis_source,
+        should_update: false
+      }
+    end
+
+    # Check priority - can the incoming source replace the existing?
+    if existing_holding.cost_basis_replaceable_by?(incoming_source)
+      if incoming_cost_basis.present?
+        return {
+          cost_basis: incoming_cost_basis,
+          cost_basis_source: incoming_source,
+          should_update: true
+        }
+      end
+    end
+
+    # Keep existing (equal or lower priority, or incoming is nil)
+    {
+      cost_basis: existing_holding.cost_basis,
+      cost_basis_source: existing_holding.cost_basis_source,
+      should_update: false
+    }
+  end
+end
diff --git a/app/models/holding/materializer.rb b/app/models/holding/materializer.rb
index da2950fa3..3fb704ca7 100644
--- a/app/models/holding/materializer.rb
+++ b/app/models/holding/materializer.rb
@@ -31,36 +31,73 @@ class Holding::Materializer
 
       current_time = Time.now
 
-      # Separate holdings into those with and without computed cost_basis
-      holdings_with_cost_basis, holdings_without_cost_basis = @holdings.partition { |h| h.cost_basis.present? }
+      # Load existing holdings to check locked status and source priority
+      existing_holdings_map = load_existing_holdings_map
 
-      # Upsert holdings that have computed cost_basis (from trades)
-      # These will overwrite any existing provider cost_basis with the trade-derived value
-      if holdings_with_cost_basis.any?
+      # Separate holdings into categories based on cost_basis reconciliation
+      holdings_to_upsert_with_cost = []
+      holdings_to_upsert_without_cost = []
+
+      @holdings.each do |holding|
+        key = holding_key(holding)
+        existing = existing_holdings_map[key]
+
+        reconciled = Holding::CostBasisReconciler.reconcile(
+          existing_holding: existing,
+          incoming_cost_basis: holding.cost_basis,
+          incoming_source: "calculated"
+        )
+
+        base_attrs = holding.attributes
+          .slice("date", "currency", "qty", "price", "amount", "security_id")
+          .merge("account_id" => account.id, "updated_at" => current_time)
+
+        if existing&.cost_basis_locked?
+          # For locked holdings, preserve ALL cost_basis fields
+          holdings_to_upsert_without_cost << base_attrs
+        elsif reconciled[:should_update] && reconciled[:cost_basis].present?
+          # Update with new cost_basis and source
+          holdings_to_upsert_with_cost << base_attrs.merge(
+            "cost_basis" => reconciled[:cost_basis],
+            "cost_basis_source" => reconciled[:cost_basis_source]
+          )
+        else
+          # No cost_basis to set, or existing is better - don't touch cost_basis fields
+          holdings_to_upsert_without_cost << base_attrs
+        end
+      end
+
+      # Upsert with cost_basis updates
+      if holdings_to_upsert_with_cost.any?
         account.holdings.upsert_all(
-          holdings_with_cost_basis.map { |h|
-            h.attributes
-              .slice("date", "currency", "qty", "price", "amount", "security_id", "cost_basis")
-              .merge("account_id" => account.id, "updated_at" => current_time)
-          },
+          holdings_to_upsert_with_cost,
           unique_by: %i[account_id security_id date currency]
         )
       end
 
-      # Upsert holdings WITHOUT cost_basis column - preserves existing provider cost_basis
-      # This handles securities that have no trades (e.g., SimpleFIN-only holdings)
-      if holdings_without_cost_basis.any?
+      # Upsert without cost_basis (preserves existing)
+      if holdings_to_upsert_without_cost.any?
         account.holdings.upsert_all(
-          holdings_without_cost_basis.map { |h|
-            h.attributes
-              .slice("date", "currency", "qty", "price", "amount", "security_id")
-              .merge("account_id" => account.id, "updated_at" => current_time)
-          },
+          holdings_to_upsert_without_cost,
           unique_by: %i[account_id security_id date currency]
         )
       end
     end
 
+    def load_existing_holdings_map
+      # Load holdings that might affect reconciliation:
+      # - Locked holdings (must preserve their cost_basis)
+      # - Holdings with a source (need to check priority)
+      account.holdings
+        .where(cost_basis_locked: true)
+        .or(account.holdings.where.not(cost_basis_source: nil))
+        .index_by { |h| holding_key(h) }
+    end
+
+    def holding_key(holding)
+      [ holding.account_id || account.id, holding.security_id, holding.date, holding.currency ]
+    end
+
     def purge_stale_holdings
       portfolio_security_ids = account.entries.trades.map { |entry| entry.entryable.security_id }.uniq
 
diff --git a/app/views/holdings/_cost_basis_cell.html.erb b/app/views/holdings/_cost_basis_cell.html.erb
new file mode 100644
index 000000000..24401b8bb
--- /dev/null
+++ b/app/views/holdings/_cost_basis_cell.html.erb
@@ -0,0 +1,109 @@
+<%# locals: (holding:, editable: true) %>
+
+<%
+  # Pre-calculate values for the form
+  # Note: cost_basis field stores per-share cost, so calculate total for display
+  current_per_share = holding.cost_basis.present? && holding.cost_basis.positive? ? holding.cost_basis : nil
+  current_total = current_per_share && holding.qty.positive? ? (current_per_share * holding.qty).round(2) : nil
+  currency = Money::Currency.new(holding.currency)
+%>
+
+<%= turbo_frame_tag dom_id(holding, :cost_basis) do %>
+  <% if holding.cost_basis_locked? && !editable %>
+    <%# Locked and not editable (from holdings list) - just show value, right-aligned %>
+    <div class="flex items-center justify-end gap-1">
+      <%= tag.span format_money(holding.avg_cost) %>
+      <%= icon "lock", size: "xs", class: "text-secondary" %>
+    </div>
+  <% else %>
+    <%# Unlocked OR editable context (drawer) - show clickable menu %>
+    <%= render DS::Menu.new(variant: :button, placement: "bottom-end") do |menu| %>
+      <% menu.with_button(class: "hover:text-primary cursor-pointer group") do %>
+        <% if holding.avg_cost %>
+          <div class="flex items-center gap-1">
+            <%= tag.span format_money(holding.avg_cost) %>
+            <% if holding.cost_basis_locked? %>
+              <%= icon "lock", size: "xs", class: "text-secondary" %>
+            <% end %>
+            <%= icon "pencil", size: "xs", class: "text-secondary opacity-0 group-hover:opacity-100 transition-opacity" %>
+          </div>
+        <% else %>
+          <div class="flex items-center gap-1 px-2 py-0.5 rounded text-secondary hover:text-primary hover:bg-gray-100 theme-dark:hover:bg-gray-700 transition-colors">
+            <%= icon "pencil", size: "xs" %>
+            <span class="text-xs">Set</span>
+          </div>
+        <% end %>
+      <% end %>
+      <% menu.with_custom_content do %>
+        <div class="p-4 min-w-[280px]"
+             data-controller="cost-basis-form"
+             data-cost-basis-form-qty-value="<%= holding.qty %>">
+          <h4 class="font-medium text-sm mb-3">
+            <%= t(".set_cost_basis_header", ticker: holding.ticker, qty: number_with_precision(holding.qty, precision: 2)) %>
+          </h4>
+          <%
+            form_data = { turbo: false }
+            if holding.avg_cost
+              form_data[:turbo_confirm] = {
+                title: t(".overwrite_confirm_title"),
+                body: t(".overwrite_confirm_body", current: format_money(holding.avg_cost))
+              }
+            end
+          %>
+          <%= styled_form_with model: holding,
+                url: holding_path(holding),
+                method: :patch,
+                class: "space-y-3",
+                data: form_data do |f| %>
+            <!-- Primary: Total cost basis (custom input, no spinners) -->
+            <div class="form-field">
+              <div class="form-field__body">
+                <label class="form-field__label"><%= t(".total_cost_basis_label") %></label>
+                <div class="flex items-center gap-1">
+                  <span class="text-secondary text-sm font-medium"><%= currency.symbol %></span>
+                  <input type="text" inputmode="decimal"
+                         name="holding[cost_basis]"
+                         class="form-field__input grow"
+                         placeholder="0.00"
+                         autocomplete="off"
+                         value="<%= number_with_precision(current_total, precision: 2) if current_total %>"
+                         data-action="input->cost-basis-form#updatePerShare"
+                         data-cost-basis-form-target="total">
+                  <span class="text-secondary text-sm"><%= currency.iso_code %></span>
+                </div>
+              </div>
+            </div>
+            <p class="text-xs text-secondary -mt-2" data-cost-basis-form-target="perShareDisplay">
+              = <%= currency.symbol %><span data-cost-basis-form-target="perShareValue"><%= number_with_precision(current_per_share, precision: 2) || "0.00" %></span> <%= t(".per_share") %>
+            </p>
+
+            <!-- Alternative: Per-share input -->
+            <div class="pt-2 border-t border-tertiary">
+              <label class="text-xs text-secondary block mb-1"><%= t(".or_per_share_label") %></label>
+              <div class="flex items-center gap-1">
+                <span class="text-secondary text-sm font-medium"><%= currency.symbol %></span>
+                <input type="text" inputmode="decimal"
+                       class="form-field__input grow"
+                       placeholder="0.00"
+                       autocomplete="off"
+                       value="<%= number_with_precision(current_per_share, precision: 2) if current_per_share %>"
+                       data-action="input->cost-basis-form#updateTotal"
+                       data-cost-basis-form-target="perShare">
+                <span class="text-secondary text-sm"><%= currency.iso_code %></span>
+              </div>
+            </div>
+
+            <div class="flex justify-end gap-2 pt-2">
+              <button type="button"
+                      class="inline-flex items-center gap-1 px-2 py-1 rounded-md text-sm font-medium text-primary bg-gray-200 hover:bg-gray-300 theme-dark:bg-gray-700 theme-dark:hover:bg-gray-600"
+                      data-action="click->DS--menu#close">
+                <%= t(".cancel") %>
+              </button>
+              <%= f.submit t(".save"), class: "inline-flex items-center gap-1 px-2 py-1 rounded-md text-sm font-medium text-inverse bg-inverse hover:bg-inverse-hover" %>
+            </div>
+          <% end %>
+        </div>
+      <% end %>
+    <% end %>
+  <% end %>
+<% end %>
diff --git a/app/views/holdings/_holding.html.erb b/app/views/holdings/_holding.html.erb
index 45ff4bf81..85d9474ac 100644
--- a/app/views/holdings/_holding.html.erb
+++ b/app/views/holdings/_holding.html.erb
@@ -31,7 +31,7 @@
     </div>
 
     <div class="col-span-2 text-right">
-      <%= tag.p holding.avg_cost ? format_money(holding.avg_cost) : t(".unknown"), class: holding.avg_cost ? nil : "text-secondary" %>
+      <%= render "holdings/cost_basis_cell", holding: holding, editable: false %>
       <%= tag.p t(".per_share"), class: "font-normal text-secondary" %>
     </div>
 
@@ -45,13 +45,13 @@
     </div>
 
     <div class="col-span-2 text-right">
-      <%# Show Total Return (unrealized G/L) when cost basis exists %>
-      <% if holding.trades.any? && holding.trend %>
+      <%# Show Total Return (unrealized G/L) when cost basis exists (from trades or manual) %>
+      <% if holding.trend %>
         <%= tag.p format_money(holding.trend.value), style: "color: #{holding.trend.color};" %>
         <%= tag.p "(#{number_to_percentage(holding.trend.percent, precision: 1)})", style: "color: #{holding.trend.color};" %>
       <% else %>
         <%= tag.p "--", class: "text-secondary" %>
-        <%= tag.p "No cost basis", class: "text-xs text-secondary" %>
+        <%= tag.p t(".no_cost_basis"), class: "text-xs text-secondary" %>
       <% end %>
     </div>
   </div>
diff --git a/app/views/holdings/show.html.erb b/app/views/holdings/show.html.erb
index 32918b49e..868b77f1d 100644
--- a/app/views/holdings/show.html.erb
+++ b/app/views/holdings/show.html.erb
@@ -35,16 +35,107 @@
             <dd class="text-primary"><%= @holding.weight ? number_to_percentage(@holding.weight, precision: 2) : t(".unknown") %></dd>
           </div>
 
-          <div class="flex items-center justify-between text-sm">
-            <dt class="text-secondary"><%= t(".avg_cost_label") %></dt>
-            <dd class="text-primary"><%= @holding.avg_cost ? format_money(@holding.avg_cost) : t(".unknown") %></dd>
+          <%# Average Cost with inline editor %>
+          <%
+            currency = Money::Currency.new(@holding.currency)
+            current_per_share = @holding.cost_basis.present? && @holding.cost_basis.positive? ? @holding.cost_basis : nil
+            current_total = current_per_share && @holding.qty.positive? ? (current_per_share * @holding.qty).round(2) : nil
+          %>
+          <div data-controller="drawer-cost-basis" data-drawer-cost-basis-qty-value="<%= @holding.qty %>">
+            <div class="flex items-center justify-between text-sm">
+              <dt class="text-secondary"><%= t(".avg_cost_label") %></dt>
+              <dd class="text-primary flex items-center gap-1">
+                <%= @holding.avg_cost ? format_money(@holding.avg_cost) : t(".unknown") %>
+                <% if @holding.cost_basis_locked? %>
+                  <%= icon "lock", size: "xs", class: "text-secondary" %>
+                <% end %>
+                <% if @holding.cost_basis_source.present? %>
+                  <span class="text-xs text-secondary">(<%= @holding.cost_basis_source_label %>)</span>
+                <% end %>
+                <button type="button" class="ml-1" data-action="click->drawer-cost-basis#toggle">
+                  <%= icon "pencil", size: "xs", class: "text-secondary hover:text-primary" %>
+                </button>
+              </dd>
+            </div>
+
+            <%# Inline cost basis editor (hidden by default) %>
+            <div class="hidden mt-3 space-y-3" data-drawer-cost-basis-target="form">
+              <%
+                drawer_form_data = { turbo: false }
+                if @holding.avg_cost
+                  drawer_form_data[:turbo_confirm] = {
+                    title: t("holdings.cost_basis_cell.overwrite_confirm_title"),
+                    body: t("holdings.cost_basis_cell.overwrite_confirm_body", current: format_money(@holding.avg_cost))
+                  }
+                end
+              %>
+              <%= styled_form_with model: @holding,
+                    url: holding_path(@holding),
+                    method: :patch,
+                    class: "space-y-3",
+                    data: drawer_form_data do |f| %>
+                <p class="text-xs text-secondary mb-2">
+                  <%= t("holdings.cost_basis_cell.set_cost_basis_header", ticker: @holding.ticker, qty: number_with_precision(@holding.qty, precision: 4)) %>
+                </p>
+                <!-- Total cost basis input -->
+                <div class="form-field">
+                  <div class="form-field__body">
+                    <label class="form-field__label"><%= t("holdings.cost_basis_cell.total_cost_basis_label") %></label>
+                    <div class="flex items-center gap-1">
+                      <span class="text-secondary text-sm font-medium"><%= currency.symbol %></span>
+                      <input type="text" inputmode="decimal"
+                             name="holding[cost_basis]"
+                             class="form-field__input grow"
+                             placeholder="0.00"
+                             autocomplete="off"
+                             value="<%= number_with_precision(current_total, precision: 2) if current_total %>"
+                             data-action="input->drawer-cost-basis#updatePerShare"
+                             data-drawer-cost-basis-target="total">
+                      <span class="text-secondary text-sm"><%= currency.iso_code %></span>
+                    </div>
+                  </div>
+                </div>
+                <p class="text-xs text-secondary -mt-2">
+                  = <%= currency.symbol %><span data-drawer-cost-basis-target="perShareValue"><%= number_with_precision(current_per_share, precision: 2) || "0.00" %></span> <%= t("holdings.cost_basis_cell.per_share") %>
+                </p>
+
+                <!-- Per-share input -->
+                <div class="pt-2 border-t border-tertiary">
+                  <label class="text-xs text-secondary block mb-1"><%= t("holdings.cost_basis_cell.or_per_share_label") %></label>
+                  <div class="flex items-center gap-1">
+                    <span class="text-secondary text-sm font-medium"><%= currency.symbol %></span>
+                    <input type="text" inputmode="decimal"
+                           class="form-field__input grow"
+                           placeholder="0.00"
+                           autocomplete="off"
+                           value="<%= number_with_precision(current_per_share, precision: 2) if current_per_share %>"
+                           data-action="input->drawer-cost-basis#updateTotal"
+                           data-drawer-cost-basis-target="perShare">
+                    <span class="text-secondary text-sm"><%= currency.iso_code %></span>
+                  </div>
+                </div>
+
+                <div class="flex justify-end gap-2 pt-2">
+                  <button type="button"
+                          class="inline-flex items-center gap-1 px-3 py-2 rounded-lg text-sm font-medium text-primary bg-gray-200 hover:bg-gray-300 theme-dark:bg-gray-700 theme-dark:hover:bg-gray-600"
+                          data-action="click->drawer-cost-basis#toggle">
+                    <%= t("holdings.cost_basis_cell.cancel") %>
+                  </button>
+                  <%= f.submit t("holdings.cost_basis_cell.save"), class: "inline-flex items-center gap-1 px-3 py-2 rounded-lg text-sm font-medium text-inverse bg-inverse hover:bg-inverse-hover" %>
+                </div>
+              <% end %>
+            </div>
           </div>
 
           <div class="flex items-center justify-between text-sm">
             <dt class="text-secondary"><%= t(".total_return_label") %></dt>
-            <dd style="color: <%= @holding.trend&.color %>;">
-              <%= @holding.trend ? render("shared/trend_change", trend: @holding.trend) : t(".unknown") %>
-            </dd>
+            <% if @holding.trend %>
+              <dd style="color: <%= @holding.trend.color %>;">
+                <%= render("shared/trend_change", trend: @holding.trend) %>
+              </dd>
+            <% else %>
+              <dd class="text-secondary"><%= t(".unknown") %></dd>
+            <% end %>
           </div>
         </dl>
       </div>
@@ -85,21 +176,39 @@
       </div>
     <% end %>
 
-    <% if @holding.account.can_delete_holdings? %>
+    <% if @holding.cost_basis_locked? || @holding.account.can_delete_holdings? %>
       <% dialog.with_section(title: t(".settings"), open: true) do %>
         <div class="pb-4">
-          <div class="flex items-center justify-between gap-2 p-3">
-            <div class="text-sm space-y-1">
-              <h4 class="text-primary"><%= t(".delete_title") %></h4>
-              <p class="text-secondary"><%= t(".delete_subtitle") %></p>
-            </div>
+          <% if @holding.cost_basis_locked? %>
+            <div class="flex items-center justify-between gap-2 p-3 border-b border-tertiary">
+              <div class="text-sm space-y-1">
+                <h4 class="text-primary"><%= t(".cost_basis_locked_label") %></h4>
+                <p class="text-secondary"><%= t(".cost_basis_locked_description") %></p>
+              </div>
 
-            <%= button_to t(".delete"),
-                holding_path(@holding),
-                method: :delete,
-                class: "rounded-lg px-3 py-2 text-red-500 text-sm font-medium border border-secondary",
-                data: { turbo_confirm: true } %>
-          </div>
+              <%= button_to t(".unlock_cost_basis"),
+                  unlock_cost_basis_holding_path(@holding),
+                  method: :post,
+                  class: "inline-flex items-center gap-1 px-3 py-2 rounded-lg text-sm font-medium text-primary bg-gray-200 hover:bg-gray-300 theme-dark:bg-gray-700 theme-dark:hover:bg-gray-600",
+                  form: { data: { turbo: false } },
+                  data: { turbo_confirm: { title: t(".unlock_confirm_title"), body: t(".unlock_confirm_body") } } %>
+            </div>
+          <% end %>
+
+          <% if @holding.account.can_delete_holdings? %>
+            <div class="flex items-center justify-between gap-2 p-3">
+              <div class="text-sm space-y-1">
+                <h4 class="text-primary"><%= t(".delete_title") %></h4>
+                <p class="text-secondary"><%= t(".delete_subtitle") %></p>
+              </div>
+
+              <%= button_to t(".delete"),
+                  holding_path(@holding),
+                  method: :delete,
+                  class: "rounded-lg px-3 py-2 text-red-500 text-sm font-medium border border-secondary",
+                  data: { turbo_confirm: true } %>
+            </div>
+          <% end %>
         </div>
       <% end %>
     <% end %>
diff --git a/app/views/reports/_investment_performance.html.erb b/app/views/reports/_investment_performance.html.erb
index f88fa9fd2..c727c819a 100644
--- a/app/views/reports/_investment_performance.html.erb
+++ b/app/views/reports/_investment_performance.html.erb
@@ -119,7 +119,7 @@
 
         <div class="grid grid-cols-1 md:grid-cols-2 lg:grid-cols-3 gap-3">
           <% investment_metrics[:accounts].each do |account| %>
-            <div class="bg-container-inset rounded-lg p-4 flex items-center justify-between">
+            <%= link_to account_path(account), class: "bg-container-inset rounded-lg p-4 flex items-center justify-between hover:bg-container-hover transition-colors" do %>
               <div class="flex items-center gap-3">
                 <%= render "accounts/logo", account: account, size: "sm" %>
                 <div>
@@ -128,7 +128,7 @@
                 </div>
               </div>
               <p class="font-medium text-primary"><%= format_money(account.balance_money) %></p>
-            </div>
+            <% end %>
           <% end %>
         </div>
       </div>
diff --git a/config/locales/views/holdings/en.yml b/config/locales/views/holdings/en.yml
index 10b648965..0e6d588d4 100644
--- a/config/locales/views/holdings/en.yml
+++ b/config/locales/views/holdings/en.yml
@@ -5,10 +5,30 @@ en:
       brokerage_cash: Brokerage cash
     destroy:
       success: Holding deleted
+    update:
+      success: Cost basis saved.
+      error: Invalid cost basis value.
+    unlock_cost_basis:
+      success: Cost basis unlocked. It may be updated on next sync.
+    cost_basis_sources:
+      manual: User set
+      calculated: From trades
+      provider: From provider
+    cost_basis_cell:
+      unknown: "--"
+      set_cost_basis_header: "Set cost basis for %{ticker} (%{qty} shares)"
+      total_cost_basis_label: Total cost basis
+      or_per_share_label: "Or enter per share:"
+      per_share: per share
+      cancel: Cancel
+      save: Save
+      overwrite_confirm_title: Overwrite cost basis?
+      overwrite_confirm_body: "This will replace the current cost basis of %{current}."
     holding:
       per_share: per share
       shares: "%{qty} shares"
       unknown: "--"
+      no_cost_basis: No cost basis
     index:
       average_cost: Average cost
       holdings: Holdings
@@ -36,3 +56,8 @@ en:
       trade_history_entry: "%{qty} shares of %{security} at %{price}"
       total_return_label: Total Return
       unknown: Unknown
+      cost_basis_locked_label: Cost basis is locked
+      cost_basis_locked_description: Your manually set cost basis won't be changed by syncs.
+      unlock_cost_basis: Unlock
+      unlock_confirm_title: Unlock cost basis?
+      unlock_confirm_body: This will allow the cost basis to be updated by provider syncs or trade calculations.
diff --git a/config/routes.rb b/config/routes.rb
index f6f6fdd66..ced0f676a 100644
--- a/config/routes.rb
+++ b/config/routes.rb
@@ -168,7 +168,11 @@ Rails.application.routes.draw do
     resources :mappings, only: :update, module: :import
   end
 
-  resources :holdings, only: %i[index new show destroy]
+  resources :holdings, only: %i[index new show update destroy] do
+    member do
+      post :unlock_cost_basis
+    end
+  end
   resources :trades, only: %i[show new create update destroy]
   resources :valuations, only: %i[show new create update destroy] do
     post :confirm_create, on: :collection
diff --git a/db/migrate/20260112011546_add_cost_basis_source_tracking_to_holdings.rb b/db/migrate/20260112011546_add_cost_basis_source_tracking_to_holdings.rb
new file mode 100644
index 000000000..f805dfbe4
--- /dev/null
+++ b/db/migrate/20260112011546_add_cost_basis_source_tracking_to_holdings.rb
@@ -0,0 +1,6 @@
+class AddCostBasisSourceTrackingToHoldings < ActiveRecord::Migration[7.2]
+  def change
+    add_column :holdings, :cost_basis_source, :string
+    add_column :holdings, :cost_basis_locked, :boolean, default: false, null: false
+  end
+end
diff --git a/db/migrate/20260112065106_backfill_cost_basis_source_for_holdings.rb b/db/migrate/20260112065106_backfill_cost_basis_source_for_holdings.rb
new file mode 100644
index 000000000..b81f92156
--- /dev/null
+++ b/db/migrate/20260112065106_backfill_cost_basis_source_for_holdings.rb
@@ -0,0 +1,42 @@
+class BackfillCostBasisSourceForHoldings < ActiveRecord::Migration[7.2]
+  disable_ddl_transaction!
+
+  def up
+    # Backfill cost_basis_source for existing holdings that have cost_basis but no source
+    # This is safe - it only adds metadata, doesn't change actual cost_basis values
+    # Locks existing data by default to protect it - users can unlock if they want syncs to update
+
+    say_with_time "Backfilling cost_basis_source for holdings" do
+      updated = 0
+
+      # Process in batches to avoid locking issues
+      Holding.where.not(cost_basis: nil)
+             .where(cost_basis_source: nil)
+             .where("cost_basis > 0")
+             .find_each do |holding|
+        # Heuristic: If holding's account has buy trades for this security, likely calculated
+        # Otherwise, likely from provider (SimpleFIN/Plaid/Lunchflow)
+        has_trades = holding.account.trades
+                           .where(security_id: holding.security_id)
+                           .where("qty > 0")
+                           .exists?
+
+        source = has_trades ? "calculated" : "provider"
+
+        # Lock existing data to protect it - users can unlock via UI if they want syncs to update
+        holding.update_columns(cost_basis_source: source, cost_basis_locked: true)
+        updated += 1
+      end
+
+      updated
+    end
+  end
+
+  def down
+    # Reversible: clear the source and unlock for holdings that were backfilled
+    # We can't know for sure which ones were backfilled vs manually set,
+    # but clearing all non-manual sources is safe since they'd be re-detected
+    Holding.where(cost_basis_source: %w[calculated provider])
+           .update_all(cost_basis_source: nil, cost_basis_locked: false)
+  end
+end
diff --git a/db/schema.rb b/db/schema.rb
index 6465a2c16..067d2c9f6 100644
--- a/db/schema.rb
+++ b/db/schema.rb
@@ -10,7 +10,7 @@
 #
 # It's strongly recommended that you check this file into your version control system.
 
-ActiveRecord::Schema[7.2].define(version: 2026_01_10_122603) do
+ActiveRecord::Schema[7.2].define(version: 2026_01_12_065106) do
   # These are extensions that must be enabled in order to support this database
   enable_extension "pgcrypto"
   enable_extension "plpgsql"
@@ -49,6 +49,8 @@ ActiveRecord::Schema[7.2].define(version: 2026_01_10_122603) do
     t.string "institution_name"
     t.string "institution_domain"
     t.text "notes"
+    t.jsonb "holdings_snapshot_data"
+    t.datetime "holdings_snapshot_at"
     t.index ["accountable_id", "accountable_type"], name: "index_accounts_on_accountable_id_and_accountable_type"
     t.index ["accountable_type"], name: "index_accounts_on_accountable_type"
     t.index ["currency"], name: "index_accounts_on_currency"
@@ -340,12 +342,14 @@ ActiveRecord::Schema[7.2].define(version: 2026_01_10_122603) do
     t.jsonb "locked_attributes", default: {}
     t.string "external_id"
     t.string "source"
+    t.boolean "exclude_from_cashflow", default: false, null: false
     t.index "lower((name)::text)", name: "index_entries_on_lower_name"
     t.index ["account_id", "date"], name: "index_entries_on_account_id_and_date"
     t.index ["account_id", "source", "external_id"], name: "index_entries_on_account_source_and_external_id", unique: true, where: "((external_id IS NOT NULL) AND (source IS NOT NULL))"
     t.index ["account_id"], name: "index_entries_on_account_id"
     t.index ["date"], name: "index_entries_on_date"
     t.index ["entryable_type"], name: "index_entries_on_entryable_type"
+    t.index ["exclude_from_cashflow"], name: "index_entries_on_exclude_from_cashflow"
     t.index ["import_id"], name: "index_entries_on_import_id"
   end
 
@@ -485,6 +489,8 @@ ActiveRecord::Schema[7.2].define(version: 2026_01_10_122603) do
     t.string "external_id"
     t.decimal "cost_basis", precision: 19, scale: 4
     t.uuid "account_provider_id"
+    t.string "cost_basis_source"
+    t.boolean "cost_basis_locked", default: false, null: false
     t.index ["account_id", "external_id"], name: "idx_holdings_on_account_id_external_id_unique", unique: true, where: "(external_id IS NOT NULL)"
     t.index ["account_id", "security_id", "date", "currency"], name: "idx_on_account_id_security_id_date_currency_5323e39f8b", unique: true
     t.index ["account_id"], name: "index_holdings_on_account_id"
@@ -1125,7 +1131,14 @@ ActiveRecord::Schema[7.2].define(version: 2026_01_10_122603) do
     t.string "currency"
     t.jsonb "locked_attributes", default: {}
     t.uuid "category_id"
+    t.decimal "realized_gain", precision: 19, scale: 4
+    t.decimal "cost_basis_amount", precision: 19, scale: 4
+    t.string "cost_basis_currency"
+    t.integer "holding_period_days"
+    t.string "realized_gain_confidence"
+    t.string "realized_gain_currency"
     t.index ["category_id"], name: "index_trades_on_category_id"
+    t.index ["realized_gain"], name: "index_trades_on_realized_gain_not_null", where: "(realized_gain IS NOT NULL)"
     t.index ["security_id"], name: "index_trades_on_security_id"
   end
 
@@ -1138,9 +1151,11 @@ ActiveRecord::Schema[7.2].define(version: 2026_01_10_122603) do
     t.string "kind", default: "standard", null: false
     t.string "external_id"
     t.jsonb "extra", default: {}, null: false
+    t.string "investment_activity_label"
     t.index ["category_id"], name: "index_transactions_on_category_id"
     t.index ["external_id"], name: "index_transactions_on_external_id"
     t.index ["extra"], name: "index_transactions_on_extra", using: :gin
+    t.index ["investment_activity_label"], name: "index_transactions_on_investment_activity_label"
     t.index ["kind"], name: "index_transactions_on_kind"
     t.index ["merchant_id"], name: "index_transactions_on_merchant_id"
   end
diff --git a/test/controllers/holdings_controller_test.rb b/test/controllers/holdings_controller_test.rb
index 2e03b9f46..a73680d54 100644
--- a/test/controllers/holdings_controller_test.rb
+++ b/test/controllers/holdings_controller_test.rb
@@ -27,4 +27,38 @@ class HoldingsControllerTest < ActionDispatch::IntegrationTest
     assert_redirected_to account_path(@holding.account)
     assert_empty @holding.account.entries.where(entryable: @holding.account.trades.where(security: @holding.security))
   end
+
+  test "updates cost basis with total amount divided by qty" do
+    # Given: holding with 10 shares
+    @holding.update!(qty: 10, cost_basis: nil, cost_basis_source: nil, cost_basis_locked: false)
+
+    # When: user submits total cost basis of $100 (should become $10 per share)
+    patch holding_path(@holding), params: { holding: { cost_basis: "100.00" } }
+
+    # Redirects to account page holdings tab to refresh list
+    assert_redirected_to account_path(@holding.account, tab: "holdings")
+    @holding.reload
+
+    # Then: cost_basis should be per-share ($10), not total
+    assert_equal 10.0, @holding.cost_basis.to_f
+    assert_equal "manual", @holding.cost_basis_source
+    assert @holding.cost_basis_locked?
+  end
+
+  test "unlock_cost_basis removes lock" do
+    # Given: locked holding
+    @holding.update!(cost_basis: 50.0, cost_basis_source: "manual", cost_basis_locked: true)
+
+    # When: user unlocks
+    post unlock_cost_basis_holding_path(@holding)
+
+    # Redirects to account page holdings tab to refresh list
+    assert_redirected_to account_path(@holding.account, tab: "holdings")
+    @holding.reload
+
+    # Then: lock is removed but cost_basis and source remain
+    assert_not @holding.cost_basis_locked?
+    assert_equal 50.0, @holding.cost_basis.to_f
+    assert_equal "manual", @holding.cost_basis_source
+  end
 end
diff --git a/test/models/holding/cost_basis_reconciler_test.rb b/test/models/holding/cost_basis_reconciler_test.rb
new file mode 100644
index 000000000..c8413b0c1
--- /dev/null
+++ b/test/models/holding/cost_basis_reconciler_test.rb
@@ -0,0 +1,171 @@
+require "test_helper"
+
+class Holding::CostBasisReconcilerTest < ActiveSupport::TestCase
+  setup do
+    @family = families(:empty)
+    @account = @family.accounts.create!(
+      name: "Test Investment",
+      balance: 20000,
+      currency: "USD",
+      accountable: Investment.new
+    )
+    @security = securities(:aapl)
+  end
+
+  test "new holding uses incoming cost_basis" do
+    result = Holding::CostBasisReconciler.reconcile(
+      existing_holding: nil,
+      incoming_cost_basis: BigDecimal("150"),
+      incoming_source: "provider"
+    )
+
+    assert result[:should_update]
+    assert_equal BigDecimal("150"), result[:cost_basis]
+    assert_equal "provider", result[:cost_basis_source]
+  end
+
+  test "new holding with nil cost_basis gets nil source" do
+    result = Holding::CostBasisReconciler.reconcile(
+      existing_holding: nil,
+      incoming_cost_basis: nil,
+      incoming_source: "provider"
+    )
+
+    assert result[:should_update]
+    assert_nil result[:cost_basis]
+    assert_nil result[:cost_basis_source]
+  end
+
+  test "locked holding is never overwritten" do
+    holding = @account.holdings.create!(
+      security: @security,
+      date: Date.current,
+      qty: 10,
+      price: 200,
+      amount: 2000,
+      currency: "USD",
+      cost_basis: BigDecimal("175"),
+      cost_basis_source: "manual",
+      cost_basis_locked: true
+    )
+
+    result = Holding::CostBasisReconciler.reconcile(
+      existing_holding: holding,
+      incoming_cost_basis: BigDecimal("200"),
+      incoming_source: "calculated"
+    )
+
+    assert_not result[:should_update]
+    assert_equal BigDecimal("175"), result[:cost_basis]
+    assert_equal "manual", result[:cost_basis_source]
+  end
+
+  test "calculated overwrites provider" do
+    holding = @account.holdings.create!(
+      security: @security,
+      date: Date.current,
+      qty: 10,
+      price: 200,
+      amount: 2000,
+      currency: "USD",
+      cost_basis: BigDecimal("150"),
+      cost_basis_source: "provider",
+      cost_basis_locked: false
+    )
+
+    result = Holding::CostBasisReconciler.reconcile(
+      existing_holding: holding,
+      incoming_cost_basis: BigDecimal("175"),
+      incoming_source: "calculated"
+    )
+
+    assert result[:should_update]
+    assert_equal BigDecimal("175"), result[:cost_basis]
+    assert_equal "calculated", result[:cost_basis_source]
+  end
+
+  test "provider does not overwrite calculated" do
+    holding = @account.holdings.create!(
+      security: @security,
+      date: Date.current,
+      qty: 10,
+      price: 200,
+      amount: 2000,
+      currency: "USD",
+      cost_basis: BigDecimal("175"),
+      cost_basis_source: "calculated",
+      cost_basis_locked: false
+    )
+
+    result = Holding::CostBasisReconciler.reconcile(
+      existing_holding: holding,
+      incoming_cost_basis: BigDecimal("150"),
+      incoming_source: "provider"
+    )
+
+    assert_not result[:should_update]
+    assert_equal BigDecimal("175"), result[:cost_basis]
+    assert_equal "calculated", result[:cost_basis_source]
+  end
+
+  test "provider does not overwrite manual" do
+    holding = @account.holdings.create!(
+      security: @security,
+      date: Date.current,
+      qty: 10,
+      price: 200,
+      amount: 2000,
+      currency: "USD",
+      cost_basis: BigDecimal("175"),
+      cost_basis_source: "manual",
+      cost_basis_locked: false
+    )
+
+    result = Holding::CostBasisReconciler.reconcile(
+      existing_holding: holding,
+      incoming_cost_basis: BigDecimal("150"),
+      incoming_source: "provider"
+    )
+
+    assert_not result[:should_update]
+    assert_equal BigDecimal("175"), result[:cost_basis]
+    assert_equal "manual", result[:cost_basis_source]
+  end
+
+  test "zero provider cost_basis treated as unknown" do
+    result = Holding::CostBasisReconciler.reconcile(
+      existing_holding: nil,
+      incoming_cost_basis: BigDecimal("0"),
+      incoming_source: "provider"
+    )
+
+    assert result[:should_update]
+    assert_nil result[:cost_basis]
+    assert_nil result[:cost_basis_source]
+  end
+
+  test "nil incoming cost_basis does not overwrite existing" do
+    holding = @account.holdings.create!(
+      security: @security,
+      date: Date.current,
+      qty: 10,
+      price: 200,
+      amount: 2000,
+      currency: "USD",
+      cost_basis: BigDecimal("175"),
+      cost_basis_source: "provider",
+      cost_basis_locked: false
+    )
+
+    result = Holding::CostBasisReconciler.reconcile(
+      existing_holding: holding,
+      incoming_cost_basis: nil,
+      incoming_source: "calculated"
+    )
+
+    # Even though calculated > provider, nil incoming shouldn't overwrite existing value
+    assert_not result[:should_update]
+    assert_equal BigDecimal("175"), result[:cost_basis]
+    assert_equal "provider", result[:cost_basis_source]
+  end
+end
diff --git a/test/models/holding_test.rb b/test/models/holding_test.rb
index a3f4e2039..7e49fe6ba 100644
--- a/test/models/holding_test.rb
+++ b/test/models/holding_test.rb
@@ -112,6 +112,132 @@ class HoldingTest < ActiveSupport::TestCase
     assert_equal Money.new(30), @amzn.trend.value
   end
 
+  # Cost basis source tracking tests
+
+  test "cost_basis_replaceable_by? returns false when locked" do
+    @amzn.update!(cost_basis: 200, cost_basis_source: "manual", cost_basis_locked: true)
+
+    assert_not @amzn.cost_basis_replaceable_by?("calculated")
+    assert_not @amzn.cost_basis_replaceable_by?("provider")
+    assert_not @amzn.cost_basis_replaceable_by?("manual")
+  end
+
+  test "cost_basis_replaceable_by? respects priority hierarchy" do
+    # Provider data can be replaced by calculated or manual
+    @amzn.update!(cost_basis: 200, cost_basis_source: "provider", cost_basis_locked: false)
+    assert @amzn.cost_basis_replaceable_by?("calculated")
+    assert @amzn.cost_basis_replaceable_by?("manual")
+    assert_not @amzn.cost_basis_replaceable_by?("provider")
+
+    # Calculated data can be replaced by manual only
+    @amzn.update!(cost_basis: 200, cost_basis_source: "calculated", cost_basis_locked: false)
+    assert @amzn.cost_basis_replaceable_by?("manual")
+    assert_not @amzn.cost_basis_replaceable_by?("calculated")
+    assert_not @amzn.cost_basis_replaceable_by?("provider")
+
+    # Manual data when LOCKED cannot be replaced by anything
+    @amzn.update!(cost_basis: 200, cost_basis_source: "manual", cost_basis_locked: true)
+    assert_not @amzn.cost_basis_replaceable_by?("manual")
+    assert_not @amzn.cost_basis_replaceable_by?("calculated")
+    assert_not @amzn.cost_basis_replaceable_by?("provider")
+
+    # Manual data when UNLOCKED can be replaced by calculated (enables recalculation)
+    @amzn.update!(cost_basis: 200, cost_basis_source: "manual", cost_basis_locked: false)
+    assert_not @amzn.cost_basis_replaceable_by?("manual")
+    assert @amzn.cost_basis_replaceable_by?("calculated")
+    assert_not @amzn.cost_basis_replaceable_by?("provider")
+  end
+
+  test "set_manual_cost_basis! sets value and locks" do
+    @amzn.set_manual_cost_basis!(BigDecimal("175.50"))
+
+    assert_equal BigDecimal("175.50"), @amzn.cost_basis
+    assert_equal "manual", @amzn.cost_basis_source
+    assert @amzn.cost_basis_locked?
+  end
+
+  test "unlock_cost_basis! allows future updates" do
+    @amzn.set_manual_cost_basis!(BigDecimal("175.50"))
+    @amzn.unlock_cost_basis!
+
+    assert_not @amzn.cost_basis_locked?
+    # Source remains manual but since unlocked, calculated could now overwrite
+    assert @amzn.cost_basis_replaceable_by?("calculated")
+  end
+
+  test "cost_basis_source_label returns correct translation" do
+    @amzn.update!(cost_basis_source: "manual")
+    assert_equal I18n.t("holdings.cost_basis_sources.manual"), @amzn.cost_basis_source_label
+
+    @amzn.update!(cost_basis_source: "calculated")
+    assert_equal I18n.t("holdings.cost_basis_sources.calculated"), @amzn.cost_basis_source_label
+
+    @amzn.update!(cost_basis_source: "provider")
+    assert_equal I18n.t("holdings.cost_basis_sources.provider"), @amzn.cost_basis_source_label
+
+    @amzn.update!(cost_basis_source: nil)
+    assert_nil @amzn.cost_basis_source_label
+  end
+
+  test "cost_basis_known? returns true only when source and positive value exist" do
+    @amzn.update!(cost_basis: nil, cost_basis_source: nil)
+    assert_not @amzn.cost_basis_known?
+
+    @amzn.update!(cost_basis: 200, cost_basis_source: nil)
+    assert_not @amzn.cost_basis_known?
+
+    @amzn.update!(cost_basis: nil, cost_basis_source: "provider")
+    assert_not @amzn.cost_basis_known?
+
+    @amzn.update!(cost_basis: 0, cost_basis_source: "provider")
+    assert_not @amzn.cost_basis_known?
+
+    @amzn.update!(cost_basis: 200, cost_basis_source: "provider")
+    assert @amzn.cost_basis_known?
+  end
+
+  # Precision and edge case tests
+
+  test "cost_basis precision is maintained with fractional shares" do
+    @amzn.update!(qty: BigDecimal("0.123456"))
+    @amzn.set_manual_cost_basis!(BigDecimal("100.123456"))
+    @amzn.reload
+
+    assert_in_delta 100.123456, @amzn.cost_basis.to_f, 0.0001
+  end
+
+  test "set_manual_cost_basis! with zero qty does not raise but saves the value" do
+    @amzn.update!(qty: 0)
+    @amzn.set_manual_cost_basis!(BigDecimal("100"))
+
+    # Value is stored but effectively meaningless with zero qty
+    assert_equal BigDecimal("100"), @amzn.cost_basis
+    assert @amzn.cost_basis_locked?
+  end
+
+  test "cost_basis_locked prevents all sources from overwriting" do
+    @amzn.set_manual_cost_basis!(BigDecimal("100"))
+    assert @amzn.cost_basis_locked?
+
+    # Verify all sources are blocked when locked
+    assert_not @amzn.cost_basis_replaceable_by?("provider")
+    assert_not @amzn.cost_basis_replaceable_by?("calculated")
+    assert_not @amzn.cost_basis_replaceable_by?("manual")
+
+    # Value should remain unchanged
+    assert_equal BigDecimal("100"), @amzn.cost_basis
+  end
+
+  test "unlocked manual allows only calculated to replace" do
+    @amzn.set_manual_cost_basis!(BigDecimal("100"))
+    @amzn.unlock_cost_basis!
+
+    assert_not @amzn.cost_basis_locked?
+    assert @amzn.cost_basis_replaceable_by?("calculated")
+    assert_not @amzn.cost_basis_replaceable_by?("provider")
+    assert_not @amzn.cost_basis_replaceable_by?("manual")
+  end
+
   private
 
     def load_holdings