diff --git a/packages/rete/src/cycle.test.ts b/packages/rete/src/cycle.test.ts
new file mode 100644
index 0000000..ba8477f
--- /dev/null
+++ b/packages/rete/src/cycle.test.ts
@@ -0,0 +1,134 @@
+import { describe, it, expect } from "vitest";
+import { RecursionLimitExceededError } from "./cycle.js";
+import { Session } from "./session.js";
+
+describe("RecursionLimitExceededError", () => {
+  it("is an Error subclass", () => {
+    const err = new RecursionLimitExceededError(10, ["ruleA", "ruleB"]);
+    expect(err).toBeInstanceOf(Error);
+    expect(err.name).toBe("RecursionLimitExceededError");
+  });
+
+  it("stores depth and activationTrace", () => {
+    const err = new RecursionLimitExceededError(5, ["rule1", "rule2", "rule1"]);
+    expect(err.depth).toBe(5);
+    expect(err.activationTrace).toEqual(["rule1", "rule2", "rule1"]);
+  });
+
+  it("message includes rule name(s) from trace", () => {
+    const err = new RecursionLimitExceededError(10, ["cycleRule"]);
+    expect(err.message).toContain("cycleRule");
+  });
+
+  it("message includes the depth value", () => {
+    const err = new RecursionLimitExceededError(42, []);
+    expect(err.message).toContain("42");
+  });
+
+  it("handles empty activationTrace", () => {
+    const err = new RecursionLimitExceededError(7, []);
+    expect(err.activationTrace).toEqual([]);
+    expect(err.message).toContain("7");
+  });
+});
+
+describe("Session.fireRules() recursion limit", () => {
+  it("throws RecursionLimitExceededError when recursion depth exceeds limit", () => {
+    const session = new Session({ autoFire: false, recursionLimit: 3 });
+    expect(() => {
+      // eslint-disable-next-line @typescript-eslint/no-explicit-any
+      (session as any)._fireDepth = 4;
+      session.fireRules({ recursionLimit: 3 });
+    }).toThrow(RecursionLimitExceededError);
+  });
+
+  it("throws when depth equals limit (>=)", () => {
+    const session = new Session({ autoFire: false, recursionLimit: 3 });
+    expect(() => {
+      // eslint-disable-next-line @typescript-eslint/no-explicit-any
+      (session as any)._fireDepth = 3;
+      session.fireRules({ recursionLimit: 3 });
+    }).toThrow(RecursionLimitExceededError);
+  });
+
+  it("does not throw when depth is below limit", () => {
+    const session = new Session({ autoFire: false, recursionLimit: 64 });
+    expect(() => {
+      // eslint-disable-next-line @typescript-eslint/no-explicit-any
+      (session as any)._fireDepth = 0;
+      session.fireRules({ recursionLimit: 64 });
+    }).not.toThrow();
+  });
+
+  it("uses session default recursionLimit when opts omitted", () => {
+    const session = new Session({ autoFire: false, recursionLimit: 3 });
+    expect(() => {
+      // eslint-disable-next-line @typescript-eslint/no-explicit-any
+      (session as any)._fireDepth = 5;
+      session.fireRules();
+    }).toThrow(RecursionLimitExceededError);
+  });
+
+  it("resets recursion depth to 0 between independent fireRules() calls", () => {
+    const session = new Session({ autoFire: false, recursionLimit: 3 });
+    expect(() => session.fireRules()).not.toThrow();
+    expect(() => session.fireRules()).not.toThrow();
+    expect(() => session.fireRules()).not.toThrow();
+    // eslint-disable-next-line @typescript-eslint/no-explicit-any
+    expect((session as any)._fireDepth).toBe(0);
+  });
+
+  it("decrements depth even if fireRules body were to throw (finally)", () => {
+    const session = new Session({ autoFire: false, recursionLimit: 3 });
+    // Force recursion guard to throw; internal counter must not be left elevated
+    // eslint-disable-next-line @typescript-eslint/no-explicit-any
+    (session as any)._fireDepth = 5;
+    expect(() => session.fireRules({ recursionLimit: 3 })).toThrow(
+      RecursionLimitExceededError,
+    );
+    // After throw, depth should still be 5 (guard throws before increment)
+    // eslint-disable-next-line @typescript-eslint/no-explicit-any
+    expect((session as any)._fireDepth).toBe(5);
+  });
+
+  it("recursionLimit: 0 means no limit — does not throw", () => {
+    const session = new Session({ autoFire: false, recursionLimit: 0 });
+    expect(() => {
+      // eslint-disable-next-line @typescript-eslint/no-explicit-any
+      (session as any)._fireDepth = 1000;
+      session.fireRules({ recursionLimit: 0 });
+    }).not.toThrow();
+  });
+
+  it("recursionLimit: 0 at session level also means no limit", () => {
+    const session = new Session({ autoFire: false, recursionLimit: 0 });
+    expect(() => {
+      // eslint-disable-next-line @typescript-eslint/no-explicit-any
+      (session as any)._fireDepth = 1000;
+      session.fireRules();
+    }).not.toThrow();
+  });
+
+  it("thrown error carries the actual depth value", () => {
+    const session = new Session({ autoFire: false, recursionLimit: 3 });
+    try {
+      // eslint-disable-next-line @typescript-eslint/no-explicit-any
+      (session as any)._fireDepth = 7;
+      session.fireRules({ recursionLimit: 3 });
+      expect.fail("should have thrown");
+    } catch (err) {
+      expect(err).toBeInstanceOf(RecursionLimitExceededError);
+      expect((err as RecursionLimitExceededError).depth).toBe(7);
+    }
+  });
+
+  it("opts.recursionLimit overrides the session default", () => {
+    const session = new Session({ autoFire: false, recursionLimit: 64 });
+    // Session default is 64 — would not throw. But opts override to 2.
+    expect(() => {
+      // eslint-disable-next-line @typescript-eslint/no-explicit-any
+      (session as any)._fireDepth = 2;
+      session.fireRules({ recursionLimit: 2 });
+    }).toThrow(RecursionLimitExceededError);
+  });
+});
diff --git a/packages/rete/src/cycle.ts b/packages/rete/src/cycle.ts
new file mode 100644
index 0000000..b91264d
--- /dev/null
+++ b/packages/rete/src/cycle.ts
@@ -0,0 +1,19 @@
+/**
+ * RecursionLimitExceededError — thrown when fireRules() recursion depth exceeds the limit.
+ * Per SPEC.md §Cycle Detection.
+ */
+export class RecursionLimitExceededError extends Error {
+  constructor(
+    readonly depth: number,
+    readonly activationTrace: readonly string[],
+  ) {
+    const traceStr =
+      activationTrace.length > 0
+        ? ` Last activations: [${activationTrace.join(" → ")}]`
+        : "";
+    super(
+      `RecursionLimitExceededError: recursion depth ${depth} exceeded.${traceStr}`,
+    );
+    this.name = "RecursionLimitExceededError";
+  }
+}
diff --git a/packages/rete/src/index.ts b/packages/rete/src/index.ts
index 2c32497..d4b8718 100644
--- a/packages/rete/src/index.ts
+++ b/packages/rete/src/index.ts
@@ -25,11 +25,19 @@ export { v, defineRule } from "./builder.js";
 export type { HandlerFn, PredicateFn } from "./registry.js";
 export { HandlerRegistry, PredicateRegistry, UnknownHandlerError, UnknownPredicateError } from "./registry.js";
 
+export type { FilterSpec } from "./condition.js";
+export { FilterNode } from "./condition.js";
+
 export type { SessionOptions } from "./session.js";
 export { Session } from "./session.js";
 
+export { RecursionLimitExceededError } from "./cycle.js";
+
 export type { SerializedRule } from "./serialize.js";
 export { serialize, deserialize, RULE_SCHEMA_V1 } from "./serialize.js";
 
 export type { Match } from "./query.js";
 export { ProductionNode, query, queryAll, NoMatchError } from "./query.js";
+
+export type { DerivedFact, DerivedHandler } from "./derived.js";
+export { DerivedFactProduction } from "./derived.js";
diff --git a/packages/rete/src/session.ts b/packages/rete/src/session.ts
index df90283..9d06455 100644
--- a/packages/rete/src/session.ts
+++ b/packages/rete/src/session.ts
@@ -7,6 +7,7 @@ import { WorkingMemory, type AttrKey, type FactValue } from "./wm.js";
 import { AlphaNetwork } from "./alpha.js";
 import type { EntityId } from "./schema.js";
 import type { RuleDefinition } from "./builder.js";
+import { RecursionLimitExceededError } from "./cycle.js";
 
 export interface SessionOptions {
   /** If true, fireRules() is called automatically after each insert/retract. Default: true. */
@@ -22,6 +23,13 @@ export class Session {
   readonly #opts: Required<SessionOptions>;
   #idCounter = 0;
 
+  /**
+   * Internal recursion depth counter for fireRules().
+   * Underscore prefix signals package-internal use; not exported from index.
+   * Per SPEC.md §Cycle Detection.
+   */
+  _fireDepth = 0;
+
   constructor(opts: SessionOptions = {}) {
     this.#opts = {
       autoFire: opts.autoFire ?? true,
@@ -83,11 +91,22 @@ export class Session {
    * Fire all pending rule activations in deterministic order.
    * Returns the count of rule firings.
    * Per SPEC.md §Conflict Resolution: salience desc → specificity desc → insertion order asc.
+   * Per SPEC.md §Cycle Detection: throws RecursionLimitExceededError when depth exceeds limit.
+   * A limit of 0 disables the check.
    */
-  fireRules(_opts?: { recursionLimit?: number }): number {
-    // Stub: no production nodes wired yet (Phase 1.7+)
-    // Returns 0 until beta network + production nodes are integrated.
-    return 0;
+  fireRules(opts?: { recursionLimit?: number }): number {
+    const limit = opts?.recursionLimit ?? this.#opts.recursionLimit;
+    if (limit > 0 && this._fireDepth >= limit) {
+      throw new RecursionLimitExceededError(this._fireDepth, []);
+    }
+    this._fireDepth++;
+    try {
+      // Stub: no production nodes wired yet (Phase 1.7+)
+      // Returns 0 until beta network + production nodes are integrated.
+      return 0;
+    } finally {
+      this._fireDepth--;
+    }
   }
 
   /** Expose AlphaNetwork for wiring by beta/join nodes (package-internal). */