diff --git a/packages/core/src/enums.test.ts b/packages/core/src/enums.test.ts
new file mode 100644
index 0000000..f113a0d
--- /dev/null
+++ b/packages/core/src/enums.test.ts
@@ -0,0 +1,22 @@
+import { describe, expect, it } from "vitest";
+
+import {
+  ApprovalDecisionActionValues,
+  BackendValues,
+  CapabilityValues,
+  RiskLevelValues,
+} from "./enums.js";
+
+describe("core enums", () => {
+  it("keeps approval decisions separate from run pause controls", () => {
+    expect(ApprovalDecisionActionValues).toEqual(["approve", "reject", "request_changes", "abort"]);
+    expect(ApprovalDecisionActionValues).not.toContain("pause");
+  });
+
+  it("exports the locked backend, risk, and capability sets", () => {
+    expect(BackendValues).toEqual(["codex", "claude", "fake"]);
+    expect(RiskLevelValues).toEqual(["low", "medium", "high"]);
+    expect(CapabilityValues).toContain("test_first_development");
+    expect(CapabilityValues).toContain("backtest_run");
+  });
+});
diff --git a/packages/core/src/enums.ts b/packages/core/src/enums.ts
index fa98a33..56ef0d0 100644
--- a/packages/core/src/enums.ts
+++ b/packages/core/src/enums.ts
@@ -3,3 +3,88 @@ import { z } from "zod";
 export const BackendValues = ["codex", "claude", "fake"] as const;
 export const Backend = z.enum(BackendValues);
 export type Backend = z.infer<typeof Backend>;
+
+export const CapabilityValues = [
+  "spec_write",
+  "phase_planning",
+  "task_dag_planning",
+  "code_edit",
+  "test_first_development",
+  "code_review",
+  "evidence_check",
+  "command_execute",
+  "backtest_run",
+  "metric_extract",
+  "failure_mining",
+  "objective_eval",
+  "final_report_compose",
+] as const;
+export const Capability = z.enum(CapabilityValues);
+export type Capability = z.infer<typeof Capability>;
+
+export const RiskLevelValues = ["low", "medium", "high"] as const;
+export const RiskLevel = z.enum(RiskLevelValues);
+export type RiskLevel = z.infer<typeof RiskLevel>;
+
+export const ApprovalDecisionActionValues = [
+  "approve",
+  "reject",
+  "request_changes",
+  "abort",
+] as const;
+export const ApprovalDecisionAction = z.enum(ApprovalDecisionActionValues);
+export type ApprovalDecisionAction = z.infer<typeof ApprovalDecisionAction>;
+
+export const ApprovalStateValues = [
+  "pending",
+  "approved",
+  "rejected",
+  "changes_requested",
+  "aborted",
+  "paused",
+] as const;
+export const ApprovalState = z.enum(ApprovalStateValues);
+export type ApprovalState = z.infer<typeof ApprovalState>;
+
+export const RunStateValues = [
+  "created",
+  "bound",
+  "planning",
+  "awaiting_approval",
+  "executing",
+  "paused",
+  "completed",
+  "failed",
+  "aborted",
+] as const;
+export const RunState = z.enum(RunStateValues);
+export type RunState = z.infer<typeof RunState>;
+
+export const RunPhaseStateValues = [
+  "pending",
+  "running",
+  "awaiting_artifact",
+  "validating",
+  "awaiting_approval",
+  "completed",
+  "failed",
+  "skipped",
+] as const;
+export const RunPhaseState = z.enum(RunPhaseStateValues);
+export type RunPhaseState = z.infer<typeof RunPhaseState>;
+
+export const SessionStateValues = [
+  "CREATED",
+  "BOOTSTRAPPING",
+  "READY",
+  "BUSY",
+  "WAITING_FOR_APPROVAL",
+  "ARTIFACT_TIMEOUT",
+  "HUNG",
+  "CRASHED",
+  "RESUMING",
+  "REBOOTSTRAPPED",
+  "FAILED_NEEDS_HUMAN",
+] as const;
+export const SessionState = z.enum(SessionStateValues);
+export type SessionState = z.infer<typeof SessionState>;
diff --git a/packages/core/src/errors.test.ts b/packages/core/src/errors.test.ts
new file mode 100644
index 0000000..96fd639
--- /dev/null
+++ b/packages/core/src/errors.test.ts
@@ -0,0 +1,17 @@
+import { describe, expect, it } from "vitest";
+
+import { DevflowError } from "./errors.js";
+
+describe("DevflowError", () => {
+  it("carries stable classification metadata", () => {
+    const error = new DevflowError("blocked", {
+      class: "human_required",
+      code: "destructive_command_blocked",
+      recoveryHint: "Ask for approval before running rm -rf",
+    });
+
+    expect(error.class).toBe("human_required");
+    expect(error.code).toBe("destructive_command_blocked");
+    expect(error.recoveryHint).toContain("approval");
+  });
+});
diff --git a/packages/core/src/errors.ts b/packages/core/src/errors.ts
new file mode 100644
index 0000000..8632e7e
--- /dev/null
+++ b/packages/core/src/errors.ts
@@ -0,0 +1,30 @@
+export type ErrorClass = "recoverable" | "human_required" | "fatal";
+
+export interface DevflowErrorOptions {
+  class: ErrorClass;
+  code: string;
+  runId?: string;
+  phaseId?: string;
+  recoveryHint?: string;
+  cause?: unknown;
+}
+
+export class DevflowError extends Error {
+  readonly class: ErrorClass;
+  readonly code: string;
+  readonly runId: string | undefined;
+  readonly phaseId: string | undefined;
+  readonly recoveryHint: string | undefined;
+  override readonly cause: unknown;
+
+  constructor(message: string, options: DevflowErrorOptions) {
+    super(message, { cause: options.cause });
+    this.name = "DevflowError";
+    this.class = options.class;
+    this.code = options.code;
+    this.runId = options.runId;
+    this.phaseId = options.phaseId;
+    this.recoveryHint = options.recoveryHint;
+    this.cause = options.cause;
+  }
+}
diff --git a/packages/core/src/hash.test.ts b/packages/core/src/hash.test.ts
new file mode 100644
index 0000000..f8c5e0d
--- /dev/null
+++ b/packages/core/src/hash.test.ts
@@ -0,0 +1,26 @@
+import { describe, expect, it } from "vitest";
+
+import { canonicalize, hash } from "./hash.js";
+
+describe("content hashing", () => {
+  it("canonicalizes object keys lexicographically while preserving array order", () => {
+    expect(canonicalize({ z: 1, a: [{ b: true, a: null }] })).toBe(
+      '{"a":[{"a":null,"b":true}],"z":1}',
+    );
+  });
+
+  it("hashes equivalent object key orders to the same sha256 hex", () => {
+    const left = hash({ z: 1, a: 2 });
+    const right = hash({ a: 2, z: 1 });
+
+    expect(left).toBe(right);
+    expect(left).toMatch(/^[a-f0-9]{64}$/);
+  });
+
+  it("rejects values that are not JSON-safe", () => {
+    expect(() => canonicalize({ date: new Date("2026-05-09T00:00:00Z") })).toThrow(
+      /non-plain object/,
+    );
+    expect(() => canonicalize({ missing: undefined })).toThrow(/undefined/);
+  });
+});
diff --git a/packages/core/src/hash.ts b/packages/core/src/hash.ts
new file mode 100644
index 0000000..ded3a1b
--- /dev/null
+++ b/packages/core/src/hash.ts
@@ -0,0 +1,74 @@
+import { createHash } from "node:crypto";
+
+type JsonValue = null | boolean | number | string | JsonValue[] | { [key: string]: JsonValue };
+
+export function canonicalize(value: unknown): string {
+  return renderCanonical(assertJsonValue(value));
+}
+
+export function hash(value: unknown): string {
+  return createHash("sha256").update(canonicalize(value)).digest("hex");
+}
+
+function renderCanonical(value: JsonValue): string {
+  if (value === null || typeof value === "boolean" || typeof value === "string") {
+    return JSON.stringify(value);
+  }
+
+  if (typeof value === "number") {
+    return JSON.stringify(value);
+  }
+
+  if (Array.isArray(value)) {
+    return `[${value.map((item) => renderCanonical(item)).join(",")}]`;
+  }
+
+  return `{${Object.keys(value)
+    .sort()
+    .map((key) => `${JSON.stringify(key)}:${renderCanonical(value[key] as JsonValue)}`)
+    .join(",")}}`;
+}
+
+function assertJsonValue(value: unknown): JsonValue {
+  if (
+    value === null ||
+    typeof value === "boolean" ||
+    typeof value === "string" ||
+    Array.isArray(value)
+  ) {
+    if (Array.isArray(value)) {
+      return value.map((item) => assertJsonValue(item));
+    }
+
+    return value;
+  }
+
+  if (typeof value === "number") {
+    if (!Number.isFinite(value)) {
+      throw new TypeError("Cannot canonicalize non-finite numbers");
+    }
+
+    return value;
+  }
+
+  if (typeof value === "object") {
+    const prototype = Object.getPrototypeOf(value);
+    if (prototype !== Object.prototype && prototype !== null) {
+      throw new TypeError("Cannot canonicalize non-plain object");
+    }
+
+    const objectValue: Record<string, JsonValue> = {};
+
+    for (const [key, childValue] of Object.entries(value as Record<string, unknown>)) {
+      if (childValue === undefined) {
+        throw new TypeError(`Cannot canonicalize undefined at key ${key}`);
+      }
+
+      objectValue[key] = assertJsonValue(childValue);
+    }
+
+    return objectValue;
+  }
+
+  throw new TypeError(`Cannot canonicalize ${typeof value}`);
+}
diff --git a/packages/core/src/index.ts b/packages/core/src/index.ts
index 896ec92..46a3aa5 100644
--- a/packages/core/src/index.ts
+++ b/packages/core/src/index.ts
@@ -1,2 +1,6 @@
 export * from "./config.js";
 export * from "./enums.js";
+export * from "./errors.js";
+export * from "./hash.js";
+export * from "./prompt-envelope.js";
+export * from "./run-event.js";
diff --git a/packages/core/src/prompt-envelope.test.ts b/packages/core/src/prompt-envelope.test.ts
new file mode 100644
index 0000000..54a592b
--- /dev/null
+++ b/packages/core/src/prompt-envelope.test.ts
@@ -0,0 +1,25 @@
+import { describe, expect, it } from "vitest";
+
+import { PromptEnvelope, renderPromptEnvelope } from "./prompt-envelope.js";
+
+describe("prompt envelope", () => {
+  it("validates and renders the locked wire markers", () => {
+    const envelope = PromptEnvelope.parse({
+      uuid: "00000000-0000-4000-8000-000000000000",
+      runId: "11111111-1111-4111-8111-111111111111",
+      roleId: "planner",
+      phaseKey: "plan",
+      attempt: 0,
+      expectedArtifact: "/tmp/devflow/spec.json",
+      expectedSchema: "dev/spec@1",
+      dedupKey: "a".repeat(64),
+      instructions: "Write the spec.",
+    });
+
+    expect(renderPromptEnvelope(envelope)).toContain(
+      "DEVFLOW_PROMPT_BEGIN 00000000-0000-4000-8000-000000000000",
+    );
+    expect(renderPromptEnvelope(envelope)).toContain("Phase: plan");
+    expect(renderPromptEnvelope(envelope)).toContain("DEVFLOW_PROMPT_END");
+  });
+});
diff --git a/packages/core/src/prompt-envelope.ts b/packages/core/src/prompt-envelope.ts
new file mode 100644
index 0000000..4ccf304
--- /dev/null
+++ b/packages/core/src/prompt-envelope.ts
@@ -0,0 +1,31 @@
+import { z } from "zod";
+
+export const PromptEnvelope = z.object({
+  uuid: z.string().uuid(),
+  runId: z.string().uuid(),
+  roleId: z.string().min(1),
+  phaseKey: z.string().min(1),
+  attempt: z.number().int().nonnegative(),
+  expectedArtifact: z.string().min(1),
+  expectedSchema: z.string().min(1),
+  dedupKey: z.string().regex(/^[a-f0-9]{64}$/),
+  instructions: z.string(),
+});
+
+export type PromptEnvelope = z.infer<typeof PromptEnvelope>;
+
+export function renderPromptEnvelope(envelope: PromptEnvelope): string {
+  return [
+    `DEVFLOW_PROMPT_BEGIN ${envelope.uuid}`,
+    `Run: ${envelope.runId}`,
+    `Role: ${envelope.roleId}`,
+    `Phase: ${envelope.phaseKey}`,
+    `Attempt: ${envelope.attempt}`,
+    `Expected artifact: ${envelope.expectedArtifact}`,
+    `Expected schema: ${envelope.expectedSchema}`,
+    `Dedup-Key: ${envelope.dedupKey}`,
+    "Instructions:",
+    envelope.instructions,
+    `DEVFLOW_PROMPT_END ${envelope.uuid}`,
+  ].join("\n");
+}
diff --git a/packages/core/src/run-event.test.ts b/packages/core/src/run-event.test.ts
new file mode 100644
index 0000000..623dead
--- /dev/null
+++ b/packages/core/src/run-event.test.ts
@@ -0,0 +1,9 @@
+import { describe, expect, it } from "vitest";
+
+import { RunEventPayloadSchemas, RunEventTypeValues } from "./run-event.js";
+
+describe("run events", () => {
+  it("keeps a payload schema for every closed run event type", () => {
+    expect(Object.keys(RunEventPayloadSchemas).sort()).toEqual([...RunEventTypeValues].sort());
+  });
+});
diff --git a/packages/core/src/run-event.ts b/packages/core/src/run-event.ts
new file mode 100644
index 0000000..4a79a8c
--- /dev/null
+++ b/packages/core/src/run-event.ts
@@ -0,0 +1,54 @@
+import { z } from "zod";
+
+export const RunEventTypeValues = [
+  "run.created",
+  "run.started",
+  "run.paused",
+  "run.resumed",
+  "run.completed",
+  "run.failed",
+  "run.aborted",
+  "phase.started",
+  "phase.completed",
+  "phase.failed",
+  "phase.skipped",
+  "prompt.sent",
+  "prompt.repaired",
+  "artifact.expected",
+  "artifact.validated",
+  "artifact.invalid",
+  "artifact.timeout",
+  "approval.requested",
+  "approval.resolved",
+  "session.created",
+  "session.ready",
+  "session.busy",
+  "session.idle",
+  "session.crashed",
+  "session.recovered",
+  "session.failed",
+  "command.started",
+  "command.completed",
+  "command.failed",
+  "review.batch_recorded",
+  "finding.verifier_resolved",
+  "backtest.iteration_started",
+  "backtest.iteration_completed",
+  "backtest.objective_evaluated",
+] as const;
+
+export const RunEventType = z.enum(RunEventTypeValues);
+export type RunEventType = z.infer<typeof RunEventType>;
+
+const payloadSchema = z.record(z.unknown());
+
+export const RunEventPayloadSchemas = Object.freeze(
+  Object.fromEntries(RunEventTypeValues.map((type) => [type, payloadSchema])),
+) as Readonly<Record<RunEventType, typeof payloadSchema>>;
+
+export const RunEvent = z.object({
+  type: RunEventType,
+  payload: payloadSchema,
+});
+
+export type RunEvent = z.infer<typeof RunEvent>;