diff --git a/CHANGELOG.md b/CHANGELOG.md
index 3b2c3d7..a376701 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -7,6 +7,14 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased]
 
+### Added
+
+- **OpenTelemetry spans** around `protect()` and rule evaluation. Pass a tracer: `new WebDecoy({ tracer: trace.getTracer('webdecoy') })`. Injected rather than imported, so the package stays dependency-free and edge-safe — the `Tracer` type is a structural subset of OpenTelemetry's, so `trace.getTracer()` works with no adapter, and omitting it means no spans, no dependency and no behaviour change. Attributes cover the decision id (which joins a span to its dashboard row), the conclusion, the deciding rule, and whether the request cost a round trip to ingest. A tracer that throws cannot fail a request.
+
+### Changed
+
+- **One adapter core.** Express, Fastify, Next.js (middleware and Pages wrapper) and the fetch guard each carried their own copy of skip-path matching, the 429 and 403 payloads, and honeytoken arming — five copies of one set of decisions, and five places the next correction can fail to land. They now share `adapter-core.ts`; the framework-specific response mechanics are untouched, and every honeytoken-injection test passes unchanged. Fastify keeps its awaited arming, which has no window where early requests are served without the link.
+
 ## [0.13.0] - 2026-08-22
 
 ### Added
diff --git a/README.md b/README.md
index 7880bfd..3ec84a9 100644
--- a/README.md
+++ b/README.md
@@ -512,6 +512,31 @@ new WebDecoy({ logger: fromPino(pino()) });  // pino's argument order is reverse
 `fromPino()` exists because passing a pino instance directly type-checks and then
 silently drops every structured field.
 
+## Tracing
+
+Pass an OpenTelemetry tracer and `protect()` emits a span, with a child span for
+rule evaluation:
+
+```typescript
+import { trace } from '@opentelemetry/api';
+
+new WebDecoy({ tracer: trace.getTracer('webdecoy') });
+```
+
+The tracer is **injected, not imported** — this package has no dependencies and
+runs on Workers and Vercel Edge, where a stray transitive import is expensive.
+The `Tracer` type is a structural subset of OpenTelemetry's, so
+`trace.getTracer()` satisfies it with no adapter. Omit it and there are no spans,
+no dependency, and no behaviour change.
+
+Attributes are the questions an operator actually asks: `decision.id` (which
+joins the span to the dashboard row), `decision.conclusion`, `decision.rule`,
+`rules.evaluated`, and `webdecoy.remote` — whether the request cost a round trip
+to ingest or was settled locally.
+
+A tracer that throws cannot fail a request. Observability that can take the
+request path down is worse than none.
+
 ## Examples
 
 See [examples](./examples) for complete working setups — e.g. [express-basic](./examples/express-basic).
diff --git a/packages/webdecoy/src/index.ts b/packages/webdecoy/src/index.ts
index d782046..86ceea0 100644
--- a/packages/webdecoy/src/index.ts
+++ b/packages/webdecoy/src/index.ts
@@ -84,6 +84,7 @@ export type { BotVerdict, BotAgent, BotCategory } from './bots';
 // runtime with a fetch handler. `@webdecoy/hono` is a thin wrapper over it; Bun,
 // Deno, Astro and Nitro need no package at all.
 export { consoleLogger, silentLogger, fromPino } from './logger';
+export type { Tracer, Span } from './tracing';
 export type { Logger, LogFields } from './logger';
 
 // Browser signals, joined to the requests that follow them. See client-signals.ts
diff --git a/packages/webdecoy/src/sdk.ts b/packages/webdecoy/src/sdk.ts
index 03d2401..fa833e0 100644
--- a/packages/webdecoy/src/sdk.ts
+++ b/packages/webdecoy/src/sdk.ts
@@ -13,6 +13,8 @@ import { AgentVerifier } from './agent/verifier';
 import type { AgentRequestInput, AgentVerdict } from './agent/types';
 import { readEdgeVerdict } from './edge';
 import { resolveLogger } from './logger';
+import { startSpan, setAttribute, recordError, endSpan } from './tracing';
+import type { Tracer } from './tracing';
 import type { Logger } from './logger';
 import { Decision, newDecisionId } from './decision';
 import type { Conclusion } from './decision';
@@ -33,7 +35,7 @@ export class WebDecoy {
   private client: WebDecoyClient | null;
   private config: Omit<
     Required<WebDecoyConfig>,
-    'apiKey' | 'rules' | 'webBotAuth' | 'characteristics' | 'decisionCache' | 'logger'
+    'apiKey' | 'rules' | 'webBotAuth' | 'characteristics' | 'decisionCache' | 'logger' | 'tracer'
   > & {
     apiKey?: string;
   };
@@ -48,6 +50,8 @@ export class WebDecoy {
   private readonly characteristics: readonly import('./characteristics').Characteristic[];
   /** Where diagnostics go. Never console directly — see logger.ts. */
   readonly log: Logger;
+  /** Optional OpenTelemetry tracer. Absent means no spans and no cost. */
+  private readonly tracer?: Tracer;
   private readonly decisionCache: DecisionCache | null;
 
   constructor(config: WebDecoyConfig) {
@@ -74,6 +78,7 @@ export class WebDecoy {
     };
 
     this.log = resolveLogger(config.logger, this.config.debug);
+    this.tracer = config.tracer;
 
     // Initialize API client only when apiKey is provided
     if (hasApiKey) {
@@ -253,7 +258,15 @@ export class WebDecoy {
   /** Evaluate rules against a prepared context and report any violations. */
   private runRules(context: RuleContext): RuleEngineResult | null {
     if (!this.ruleEngine) return null;
+
+    const span = startSpan(this.tracer, 'webdecoy.rules');
     const result = this.ruleEngine.evaluate(context);
+    setAttribute(span, 'webdecoy.rules.action', result.action);
+    setAttribute(span, 'webdecoy.rules.evaluated', result.results.length);
+    setAttribute(span, 'webdecoy.rules.violations', result.violations.length);
+    if (result.rule) setAttribute(span, 'webdecoy.rules.deciding', result.rule);
+    endSpan(span);
+
     if (result.violations.length > 0 && this.violationReporter) {
       this.violationReporter.report(result.violations);
     }
@@ -335,7 +348,41 @@ export class WebDecoy {
     // present on every outcome — and a per-return copy is a line someone would
     // eventually forget on the branch that mattered.
     const edge = readEdgeVerdict(metadata.headers);
-    return (await this.decide(metadata, options)).withEdge(edge);
+
+    const span = startSpan(this.tracer, 'webdecoy.protect');
+    try {
+      const decision = (await this.decide(metadata, options)).withEdge(edge);
+
+      // Attributes chosen so a trace answers the questions an operator actually
+      // asks: what did we decide, which rule decided it, and did this request
+      // cost a round trip to ingest. The decision id joins the span to the
+      // dashboard row.
+      setAttribute(span, 'webdecoy.decision.id', decision.id);
+      setAttribute(span, 'webdecoy.decision.conclusion', decision.conclusion);
+      setAttribute(span, 'webdecoy.decision.allowed', decision.allowed);
+      setAttribute(span, 'webdecoy.rules.evaluated', decision.results.length);
+      if (decision.ruleResult?.rule) {
+        setAttribute(span, 'webdecoy.decision.rule', decision.ruleResult.rule);
+      }
+      // A detection id that is not the decision id means the verdict came back
+      // from ingest rather than being settled locally.
+      setAttribute(
+        span,
+        'webdecoy.remote',
+        decision.detection.detection_id !== decision.id,
+      );
+      if (decision.error) {
+        setAttribute(span, 'webdecoy.error', decision.error);
+      }
+      return decision;
+    } catch (error) {
+      // decide() fails open rather than throwing, so this is a bug rather than
+      // a bad day — worth marking on the span rather than swallowing.
+      recordError(span, error);
+      throw error;
+    } finally {
+      endSpan(span);
+    }
   }
 
   private async decide(
diff --git a/packages/webdecoy/src/tracing.test.ts b/packages/webdecoy/src/tracing.test.ts
new file mode 100644
index 0000000..3709024
--- /dev/null
+++ b/packages/webdecoy/src/tracing.test.ts
@@ -0,0 +1,148 @@
+import { WebDecoy } from './sdk';
+import { tripwire, rateLimit } from './rules';
+import type { Span, Tracer } from './tracing';
+import type { RequestMetadata } from './types';
+
+const req = (over: Partial<RequestMetadata> = {}): RequestMetadata => ({
+  method: 'GET',
+  path: '/',
+  ip: '203.0.113.9',
+  headers: {},
+  timestamp: Date.now(),
+  ...over,
+});
+
+interface Recorded {
+  name: string;
+  attributes: Record<string, unknown>;
+  ended: boolean;
+  errors: unknown[];
+}
+
+function recordingTracer() {
+  const spans: Recorded[] = [];
+  const tracer: Tracer = {
+    startSpan(name: string): Span {
+      const rec: Recorded = { name, attributes: {}, ended: false, errors: [] };
+      spans.push(rec);
+      return {
+        setAttribute: (k, v) => {
+          rec.attributes[k] = v;
+        },
+        recordException: (e) => rec.errors.push(e),
+        setStatus: () => undefined,
+        end: () => {
+          rec.ended = true;
+        },
+      };
+    },
+  };
+  return { tracer, spans };
+}
+
+describe('tracing', () => {
+  it('emits a span for protect() and one for rule evaluation', async () => {
+    const { tracer, spans } = recordingTracer();
+    const wd = new WebDecoy({ tracer, rules: [tripwire()] });
+
+    await wd.protect(req({ path: '/.env' }));
+
+    expect(spans.map((s) => s.name).sort()).toEqual(['webdecoy.protect', 'webdecoy.rules']);
+    expect(spans.every((s) => s.ended)).toBe(true);
+  });
+
+  it('records what an operator actually asks a trace', async () => {
+    const { tracer, spans } = recordingTracer();
+    const wd = new WebDecoy({ tracer, rules: [tripwire()] });
+
+    const decision = await wd.protect(req({ path: '/.env' }));
+    const protectSpan = spans.find((s) => s.name === 'webdecoy.protect')!;
+
+    // The id is what joins this span to the dashboard row.
+    expect(protectSpan.attributes['webdecoy.decision.id']).toBe(decision.id);
+    expect(protectSpan.attributes['webdecoy.decision.conclusion']).toBe('DENY');
+    expect(protectSpan.attributes['webdecoy.decision.allowed']).toBe(false);
+    expect(protectSpan.attributes['webdecoy.decision.rule']).toBe('tripwire');
+    // Settled locally: no round trip to ingest.
+    expect(protectSpan.attributes['webdecoy.remote']).toBe(false);
+  });
+
+  it('names the deciding rule on the rules span', async () => {
+    const { tracer, spans } = recordingTracer();
+    const wd = new WebDecoy({
+      tracer,
+      rules: [rateLimit({ max: 1, window: 60, action: 'DENY' })],
+    });
+
+    await wd.protect(req());
+    await wd.protect(req());
+
+    const ruleSpans = spans.filter((s) => s.name === 'webdecoy.rules');
+    expect(ruleSpans[0].attributes['webdecoy.rules.action']).toBe('ALLOW');
+    expect(ruleSpans[1].attributes['webdecoy.rules.action']).toBe('DENY');
+    expect(ruleSpans[1].attributes['webdecoy.rules.deciding']).toBe('rate-limit:1/60s');
+  });
+
+  it('ends the span even when the decision is an error', async () => {
+    // A leaked span is worse than a missing one: it holds memory and never
+    // reaches the exporter, so the trace is silently incomplete.
+    const { tracer, spans } = recordingTracer();
+    const wd = new WebDecoy({ tracer, rules: [] });
+
+    await wd.protect(req({ ip: '' })); // malformed — decide() fails open
+
+    const protectSpan = spans.find((s) => s.name === 'webdecoy.protect')!;
+    expect(protectSpan.ended).toBe(true);
+    expect(protectSpan.attributes['webdecoy.decision.conclusion']).toBe('ERROR');
+    expect(protectSpan.attributes['webdecoy.error']).toBeTruthy();
+  });
+});
+
+describe('a tracer must never be able to break a request', () => {
+  it('survives a tracer that throws on startSpan', async () => {
+    const wd = new WebDecoy({
+      tracer: {
+        startSpan() {
+          throw new Error('exporter misconfigured');
+        },
+      },
+      rules: [tripwire()],
+    });
+
+    // Observability that can take down the request path is worse than none.
+    const decision = await wd.protect(req({ path: '/.env' }));
+    expect(decision.conclusion).toBe('DENY');
+  });
+
+  it('survives a tracer that throws on every method', async () => {
+    const hostile: Tracer = {
+      startSpan: () =>
+        ({
+          setAttribute() {
+            throw new Error('nope');
+          },
+          end() {
+            throw new Error('nope');
+          },
+        }) as unknown as Span,
+    };
+    const wd = new WebDecoy({ tracer: hostile, rules: [tripwire()] });
+
+    const decision = await wd.protect(req({ path: '/.env' }));
+    expect(decision.conclusion).toBe('DENY');
+  });
+
+  it('survives a tracer that returns nothing', async () => {
+    const wd = new WebDecoy({
+      tracer: { startSpan: () => undefined as unknown as Span },
+      rules: [tripwire()],
+    });
+    expect((await wd.protect(req({ path: '/.env' }))).conclusion).toBe('DENY');
+  });
+
+  it('costs nothing when no tracer is configured', async () => {
+    // The majority case. No spans, no dependency, no behaviour change.
+    const wd = new WebDecoy({ rules: [tripwire()] });
+    expect((await wd.protect(req({ path: '/.env' }))).conclusion).toBe('DENY');
+  });
+});
diff --git a/packages/webdecoy/src/tracing.ts b/packages/webdecoy/src/tracing.ts
new file mode 100644
index 0000000..8c73832
--- /dev/null
+++ b/packages/webdecoy/src/tracing.ts
@@ -0,0 +1,99 @@
+/**
+ * OpenTelemetry spans, without depending on OpenTelemetry.
+ *
+ * WHY INJECTED RATHER THAN IMPORTED
+ *
+ * An optional peer dependency was the obvious route and the wrong one. This
+ * package is dependency-free and passes an edge-compatibility gate, and most of
+ * the runtimes it targets — Workers, Vercel Edge — are exactly where a stray
+ * transitive import hurts. A conditional `import('@opentelemetry/api')` also
+ * bundles badly: the bundler either resolves it, adding weight for the majority
+ * who do not use it, or fails on a module that is legitimately absent.
+ *
+ * So the tracer is passed in. The interface below is a structural subset of
+ * OpenTelemetry's, which means `trace.getTracer('webdecoy')` satisfies it
+ * directly with no adapter:
+ *
+ * ```ts
+ * import { trace } from '@opentelemetry/api';
+ * new WebDecoy({ tracer: trace.getTracer('webdecoy') });
+ * ```
+ *
+ * An app that passes nothing gets no spans, no dependency, and no behaviour
+ * change — which is the majority, and they should not pay for this.
+ */
+
+/** A span, structurally compatible with OpenTelemetry's. */
+export interface Span {
+  setAttribute(key: string, value: string | number | boolean): unknown;
+  recordException?(error: unknown): unknown;
+  setStatus?(status: { code: number; message?: string }): unknown;
+  end(): unknown;
+}
+
+/** A tracer, structurally compatible with OpenTelemetry's. */
+export interface Tracer {
+  startSpan(name: string): Span;
+}
+
+/** OpenTelemetry's SpanStatusCode.ERROR, inlined so the enum need not be imported. */
+const STATUS_ERROR = 2;
+
+/**
+ * A span that does nothing, so call sites need no null checks.
+ *
+ * Every `if (span)` is a branch that can be forgotten on the path that
+ * mattered, and a span left unended leaks. One object costs less than the
+ * discipline.
+ */
+const NOOP_SPAN: Span = {
+  setAttribute: () => undefined,
+  end: () => undefined,
+};
+
+/**
+ * Start a span, or hand back a no-op.
+ *
+ * Never throws. A tracer is observability, and observability that can take down
+ * the request path is worse than none — a misconfigured exporter must not
+ * become a 500 on a customer's site.
+ */
+export function startSpan(tracer: Tracer | undefined, name: string): Span {
+  if (!tracer) return NOOP_SPAN;
+  try {
+    return tracer.startSpan(name) ?? NOOP_SPAN;
+  } catch {
+    return NOOP_SPAN;
+  }
+}
+
+/** Set an attribute, swallowing anything the tracer throws. */
+export function setAttribute(span: Span, key: string, value: string | number | boolean): void {
+  try {
+    span.setAttribute(key, value);
+  } catch {
+    // See startSpan: instrumentation must not be able to fail a request.
+  }
+}
+
+/** Record a failure on the span, if the tracer supports it. */
+export function recordError(span: Span, error: unknown): void {
+  try {
+    span.recordException?.(error);
+    span.setStatus?.({
+      code: STATUS_ERROR,
+      message: error instanceof Error ? error.message : String(error),
+    });
+  } catch {
+    // As above.
+  }
+}
+
+/** End a span, swallowing anything the tracer throws. */
+export function endSpan(span: Span): void {
+  try {
+    span.end();
+  } catch {
+    // As above.
+  }
+}
diff --git a/packages/webdecoy/src/types.ts b/packages/webdecoy/src/types.ts
index 2f74703..0ce8b0e 100644
--- a/packages/webdecoy/src/types.ts
+++ b/packages/webdecoy/src/types.ts
@@ -8,6 +8,7 @@ import type { AgentVerifierOptions } from './agent/types';
 import type { Characteristic } from './characteristics';
 import type { DecisionCacheOptions } from './decision-cache';
 import type { Logger } from './logger';
+import type { Tracer } from './tracing';
 
 export type { ProtectResult, Conclusion, RuleState, RuleOutcome } from './decision';
 
@@ -109,6 +110,21 @@ export interface WebDecoyConfig {
    * silently drops every structured field.
    */
   logger?: Logger;
+
+  /**
+   * An OpenTelemetry tracer, if you run one.
+   *
+   * Injected rather than imported so this package stays dependency-free and
+   * edge-safe. The interface is a structural subset of OpenTelemetry's, so
+   * `trace.getTracer('webdecoy')` works directly:
+   *
+   * ```ts
+   * new WebDecoy({ tracer: trace.getTracer('webdecoy') });
+   * ```
+   *
+   * Omit it and there are no spans, no dependency and no behaviour change.
+   */
+  tracer?: Tracer;
 }
 
 /**