feat(usage): track prompt tokens and cost saved by request rewriters

ext/ext.go

@@ -55,6 +55,13 @@ type Result struct {
 	// chain and never sent upstream; it must never contain secrets or
 	// request credentials.
 	Detail any
+	// TokensSaved is the rewriter's estimate of prompt tokens its body
+	// change removed from the request. When positive and the rewritten body
+	// is applied, core adds it to the request's usage record together with
+	// the input cost those tokens would have incurred, and the dashboard
+	// aggregates both as rewrite savings. Leave zero when the rewrite does
+	// not shrink the prompt.
+	TokensSaved int
 }
 
 // RequestRewriter rewrites raw JSON request bodies at ingress, after

internal/admin/dashboard/static/js/modules/usage.js

@@ -23,7 +23,9 @@
                     cache_write_input_tokens: 0,
                     total_input_cost: null,
                     total_output_cost: null,
-                    total_cost: null
+                    total_cost: null,
+                    rewrite_tokens_saved: 0,
+                    rewrite_cost_saved: null
                 };
             },
 
@@ -519,6 +521,29 @@
                 return this.formatCost(summary.total_input_cost) + ' input + ' + this.formatCost(summary.total_output_cost) + ' output';
             },
 
+            // --- Rewrite savings (request rewriters, e.g. token compression) ---
+            // Savings ride on provider usage rows, so the uncached summary holds
+            // the full totals. Cards only appear once a rewriter reported savings.
+            usagePageRewriteTokensSaved() {
+                const saved = Number((this.usageSummary && this.usageSummary.rewrite_tokens_saved) || 0);
+                return Number.isFinite(saved) && saved > 0 ? saved : 0;
+            },
+
+            usagePageRewriteSavingsVisible() {
+                return this.usagePageRewriteTokensSaved() > 0;
+            },
+
+            usagePageRewriteCostSaved() {
+                const summary = this.usageSummary || {};
+                return summary.rewrite_cost_saved === undefined ? null : summary.rewrite_cost_saved;
+            },
+
+            usagePageRewriteSavedTitle() {
+                const tokens = this.usagePageRewriteTokensSaved();
+                if (tokens <= 0) return '';
+                return this.formatNumber(tokens) + ' prompt tokens removed by request rewriters before reaching providers';
+            },
+
             async fetchModelUsage() {
                 let controller = null;
                 try {

internal/admin/dashboard/static/js/modules/usage.test.cjs

@@ -388,6 +388,39 @@ test('usage page stat cards follow the log cache scope and derive hits from the
     assert.equal(module.usagePageCostTitle(), '');
 });
 
+test('usage page rewrite savings cards show only when rewriters saved tokens', () => {
+    const module = createUsageModule();
+    module.formatNumber = (n) => String(n);
+
+    // No savings reported: cards hidden, zero-safe values.
+    module.usageSummary = { total_requests: 10 };
+    assert.equal(module.usagePageRewriteSavingsVisible(), false);
+    assert.equal(module.usagePageRewriteTokensSaved(), 0);
+    assert.equal(module.usagePageRewriteCostSaved(), null);
+    assert.equal(module.usagePageRewriteSavedTitle(), '');
+
+    // Savings with pricing: both cards visible with tokens and cost.
+    module.usageSummary = { rewrite_tokens_saved: 4200, rewrite_cost_saved: 0.0125 };
+    assert.equal(module.usagePageRewriteSavingsVisible(), true);
+    assert.equal(module.usagePageRewriteTokensSaved(), 4200);
+    assert.equal(module.usagePageRewriteCostSaved(), 0.0125);
+    assert.equal(
+        module.usagePageRewriteSavedTitle(),
+        '4200 prompt tokens removed by request rewriters before reaching providers'
+    );
+
+    // Savings without pricing: tokens card carries the value, cost stays null.
+    module.usageSummary = { rewrite_tokens_saved: 100, rewrite_cost_saved: null };
+    assert.equal(module.usagePageRewriteSavingsVisible(), true);
+    assert.equal(module.usagePageRewriteCostSaved(), null);
+
+    // Defensive: negative or garbage totals never surface.
+    module.usageSummary = { rewrite_tokens_saved: -5 };
+    assert.equal(module.usagePageRewriteSavingsVisible(), false);
+    module.usageSummary = { rewrite_tokens_saved: 'NaN?' };
+    assert.equal(module.usagePageRewriteSavingsVisible(), false);
+});
+
 test('fetchUsagePageSummary loads uncached and all cache modes with the page filters', async () => {
     const { app, fetchCalls } = createUsageLogApp({ usageFilterLabel: 'env:prod' });
 

internal/admin/dashboard/templates/page-usage.html

@@ -54,6 +54,14 @@ <h2>Usage Analytics</h2>
             <div class="card-label">Estimated Cost</div>
             <div class="card-value" x-text="formatCost(usageSummary.total_cost)" :title="usagePageCostTitle()">-</div>
         </div>
+        <div class="card" x-show="usagePageRewriteSavingsVisible()">
+            <div class="card-label">Rewrite Saved</div>
+            <div class="card-value" x-text="formatCost(usagePageRewriteCostSaved())" :title="usagePageRewriteSavedTitle()">-</div>
+        </div>
+        <div class="card" x-show="usagePageRewriteSavingsVisible()">
+            <div class="card-label">Tokens Saved</div>
+            <div class="card-value" x-text="formatNumber(usagePageRewriteTokensSaved())" :title="usagePageRewriteSavedTitle()">-</div>
+        </div>
         <div class="card" x-show="cacheAnalyticsEnabled()">
             <div class="card-label">Cache Saved</div>
             <div class="card-value" x-text="formatCost(cacheOverview.summary.total_saved_cost)">-</div>

internal/core/context.go

@@ -49,6 +49,11 @@ const (
 	// requestOriginKey stores the logical request origin for internal execution
 	// flows that still reuse the translated request pipeline.
 	requestOriginKey contextKey = "request-origin"
+
+	// rewriteTokensSavedKey stores the total prompt tokens that applied
+	// request rewriters estimate they removed from the request body. Usage
+	// recording folds it into the request's usage entry as rewrite savings.
+	rewriteTokensSavedKey contextKey = "rewrite-tokens-saved"
 )
 
 // RequestOrigin identifies whether a request came from an external caller or an
@@ -226,6 +231,27 @@ func GetFailoverUsed(ctx context.Context) bool {
 	return false
 }
 
+// WithRewriteTokensSaved returns a new context carrying the total prompt
+// tokens that applied request rewriters estimate they removed. Non-positive
+// totals leave the context unchanged.
+func WithRewriteTokensSaved(ctx context.Context, tokensSaved int) context.Context {
+	if tokensSaved <= 0 {
+		return ctx
+	}
+	return context.WithValue(ctx, rewriteTokensSavedKey, tokensSaved)
+}
+
+// RewriteTokensSavedFromContext retrieves the request's rewrite savings
+// estimate, or zero when no rewriter reported savings.
+func RewriteTokensSavedFromContext(ctx context.Context) int {
+	if v := ctx.Value(rewriteTokensSavedKey); v != nil {
+		if saved, ok := v.(int); ok && saved > 0 {
+			return saved
+		}
+	}
+	return 0
+}
+
 // WithRequestOrigin returns a new context with the logical request origin attached.
 func WithRequestOrigin(ctx context.Context, origin RequestOrigin) context.Context {
 	return context.WithValue(ctx, requestOriginKey, origin)

internal/gateway/usage.go

@@ -35,6 +35,7 @@ func (o *InferenceOrchestrator) logUsage(
 		entry.ProviderName = strings.TrimSpace(providerName)
 		entry.UserPath = core.UserPathFromContext(ctx)
 		entry.Labels = core.RequestLabelsFromContext(ctx)
+		usage.ApplyRewriteSavings(entry, core.RewriteTokensSavedFromContext(ctx), pricing)
 		o.usageLogger.Write(entry)
 	}
 }

internal/server/passthrough_support.go

@@ -298,6 +298,7 @@ func (s *passthroughService) proxyPassthroughResponse(c *echo.Context, providerT
 			if observer := usage.NewStreamUsageObserver(s.usageLogger, model, providerType, requestID, usagePath, s.pricingResolver, core.UserPathFromContext(c.Request().Context())); observer != nil {
 				observer.SetProviderName(providerName)
 				observer.SetLabels(core.RequestLabelsFromContext(c.Request().Context()))
+				observer.SetRewriteTokensSaved(core.RewriteTokensSavedFromContext(c.Request().Context()))
 				observers = append(observers, observer)
 			}
 		}

internal/server/request_rewrite.go

@@ -45,6 +45,7 @@ func RequestRewriteMiddleware(rewriters []ext.RequestRewriter, auditLogger audit
 			}
 
 			changed := false
+			tokensSaved := 0
 			for _, rw := range rewriters {
 				res, rwErr := rw.Rewrite(c.Request().Context(), in)
 				if rwErr != nil {
@@ -58,12 +59,19 @@ func RequestRewriteMiddleware(rewriters []ext.RequestRewriter, auditLogger audit
 					recordRequestRevision(c, auditLogger, rw.Name(), len(in.Body), res)
 					in.Body = res.Body
 					changed = true
+					if res.TokensSaved > 0 {
+						tokensSaved += res.TokensSaved
+					}
 				}
 			}
 
 			if changed {
 				pinOriginalAuditRequestBody(c, auditLogger)
 				applyRewrittenBody(c, in.Body)
+				if tokensSaved > 0 {
+					req := c.Request()
+					c.SetRequest(req.WithContext(core.WithRewriteTokensSaved(req.Context(), tokensSaved)))
+				}
 			}
 			return next(c)
 		}