diff --git a/src/core/source-id.ts b/src/core/source-id.ts
new file mode 100644
index 000000000..d6da5e8d6
--- /dev/null
+++ b/src/core/source-id.ts
@@ -0,0 +1,54 @@
+/**
+ * src/core/source-id.ts — single canonical source_id validation.
+ *
+ * Dependency-free by design (no imports beyond TS stdlib). Imported by both
+ * engines, cycle, source-resolver, sources-ops, and any future site that
+ * needs to validate a source_id. Pre-v0.38 the regex was duplicated across
+ * three files (`utils.ts` had a permissive variant; `sources-ops.ts` and
+ * `source-resolver.ts` had the strict variant). Codex outside-voice flagged
+ * the drift; this module is the consolidation.
+ *
+ * **Canonical regex (strict):** `^[a-z0-9](?:[a-z0-9-]{0,30}[a-z0-9])?$`
+ *
+ * Rules enforced:
+ *   - 1-32 characters
+ *   - lowercase alphanumeric only (no underscores, no dots, no slashes)
+ *   - interior hyphens allowed
+ *   - first and last character must be alphanumeric (no edge hyphens)
+ *
+ * Single-character source IDs like `a` or `1` are valid. Underscored ids
+ * like `my_source` are rejected even though they passed the legacy
+ * permissive regex — `sources-ops` always rejected them at creation time,
+ * so no existing source IDs break.
+ *
+ * **Exports two validators:**
+ *   - `isValidSourceId(s)`: boolean — for tiers that silently fall back
+ *     to the next resolution step on invalid input (dotfile, brain_default).
+ *   - `assertValidSourceId(s)`: void, throws — for tiers that must reject
+ *     invalid input loudly (explicit `--source` flag, `GBRAIN_SOURCE` env,
+ *     `cycleLockIdFor` primitive defense-in-depth).
+ *
+ * Codex P1-F flagged the need for both shapes.
+ */
+
+export const SOURCE_ID_RE = /^[a-z0-9](?:[a-z0-9-]{0,30}[a-z0-9])?$/;
+
+/** Returns true if the string matches the canonical source_id regex. */
+export function isValidSourceId(s: unknown): s is string {
+  return typeof s === 'string' && SOURCE_ID_RE.test(s);
+}
+
+/**
+ * Throws if the input doesn't match the canonical source_id regex.
+ * Error message includes the offending value (JSON-stringified for
+ * non-string types so debugging weird input is fast).
+ */
+export function assertValidSourceId(s: unknown): asserts s is string {
+  if (!isValidSourceId(s)) {
+    throw new Error(
+      `Invalid source_id: ${JSON.stringify(s)}. ` +
+      `Must be 1-32 lowercase alnum chars with optional interior hyphens ` +
+      `(matches ${SOURCE_ID_RE}).`,
+    );
+  }
+}
diff --git a/test/source-id.test.ts b/test/source-id.test.ts
new file mode 100644
index 000000000..435b1a33d
--- /dev/null
+++ b/test/source-id.test.ts
@@ -0,0 +1,147 @@
+import { describe, test, expect } from 'bun:test';
+import {
+  SOURCE_ID_RE,
+  isValidSourceId,
+  assertValidSourceId,
+} from '../src/core/source-id.ts';
+
+describe('source-id canonical validator', () => {
+  describe('SOURCE_ID_RE', () => {
+    test('accepts single-character ids', () => {
+      expect(SOURCE_ID_RE.test('a')).toBe(true);
+      expect(SOURCE_ID_RE.test('1')).toBe(true);
+      expect(SOURCE_ID_RE.test('z')).toBe(true);
+      expect(SOURCE_ID_RE.test('0')).toBe(true);
+    });
+
+    test('accepts kebab-case ids with interior hyphens', () => {
+      expect(SOURCE_ID_RE.test('default')).toBe(true);
+      expect(SOURCE_ID_RE.test('portfolio')).toBe(true);
+      expect(SOURCE_ID_RE.test('my-source')).toBe(true);
+      expect(SOURCE_ID_RE.test('alpha-beta-gamma')).toBe(true);
+      expect(SOURCE_ID_RE.test('a-b')).toBe(true);
+    });
+
+    test('accepts max-length 32-char ids', () => {
+      const max = 'a' + 'b'.repeat(30) + 'c'; // 32 chars
+      expect(max.length).toBe(32);
+      expect(SOURCE_ID_RE.test(max)).toBe(true);
+    });
+
+    test('rejects 33+ char ids', () => {
+      const tooLong = 'a' + 'b'.repeat(31) + 'c'; // 33 chars
+      expect(SOURCE_ID_RE.test(tooLong)).toBe(false);
+    });
+
+    test('rejects underscores (P1-D blast radius case)', () => {
+      expect(SOURCE_ID_RE.test('snake_id')).toBe(false);
+      expect(SOURCE_ID_RE.test('my_source')).toBe(false);
+      expect(SOURCE_ID_RE.test('_leading')).toBe(false);
+      expect(SOURCE_ID_RE.test('trailing_')).toBe(false);
+    });
+
+    test('rejects edge hyphens (boundary-bad)', () => {
+      expect(SOURCE_ID_RE.test('-leading')).toBe(false);
+      expect(SOURCE_ID_RE.test('trailing-')).toBe(false);
+      expect(SOURCE_ID_RE.test('-')).toBe(false);
+      expect(SOURCE_ID_RE.test('--')).toBe(false);
+    });
+
+    test('rejects uppercase', () => {
+      expect(SOURCE_ID_RE.test('Default')).toBe(false);
+      expect(SOURCE_ID_RE.test('PORTFOLIO')).toBe(false);
+      expect(SOURCE_ID_RE.test('myID')).toBe(false);
+    });
+
+    test('rejects path-traversal shapes (P1-B security)', () => {
+      expect(SOURCE_ID_RE.test('../etc')).toBe(false);
+      expect(SOURCE_ID_RE.test('/abs')).toBe(false);
+      expect(SOURCE_ID_RE.test('a/b')).toBe(false);
+      expect(SOURCE_ID_RE.test('a.b')).toBe(false);
+    });
+
+    test('rejects whitespace', () => {
+      expect(SOURCE_ID_RE.test('A B')).toBe(false);
+      expect(SOURCE_ID_RE.test('a b')).toBe(false);
+      expect(SOURCE_ID_RE.test(' a')).toBe(false);
+      expect(SOURCE_ID_RE.test('a ')).toBe(false);
+      expect(SOURCE_ID_RE.test('\t')).toBe(false);
+      expect(SOURCE_ID_RE.test('\n')).toBe(false);
+    });
+
+    test('rejects empty string', () => {
+      expect(SOURCE_ID_RE.test('')).toBe(false);
+    });
+
+    test('rejects non-ASCII', () => {
+      expect(SOURCE_ID_RE.test('café')).toBe(false);
+      expect(SOURCE_ID_RE.test('日本')).toBe(false);
+      expect(SOURCE_ID_RE.test('𝕏')).toBe(false);
+    });
+  });
+
+  describe('isValidSourceId (boolean — for silent-fallback tiers per P1-F)', () => {
+    test('returns true for valid ids', () => {
+      expect(isValidSourceId('default')).toBe(true);
+      expect(isValidSourceId('portfolio')).toBe(true);
+      expect(isValidSourceId('a')).toBe(true);
+    });
+
+    test('returns false for invalid ids without throwing', () => {
+      expect(isValidSourceId('SnakeCase')).toBe(false);
+      expect(isValidSourceId('snake_case')).toBe(false);
+      expect(isValidSourceId('../etc')).toBe(false);
+      expect(isValidSourceId('')).toBe(false);
+    });
+
+    test('returns false for non-string inputs without throwing', () => {
+      expect(isValidSourceId(undefined)).toBe(false);
+      expect(isValidSourceId(null)).toBe(false);
+      expect(isValidSourceId(42)).toBe(false);
+      expect(isValidSourceId({})).toBe(false);
+      expect(isValidSourceId([])).toBe(false);
+    });
+
+    test('narrows type to string when true', () => {
+      const x: unknown = 'portfolio';
+      if (isValidSourceId(x)) {
+        // TS narrowing check — concat would fail if x weren't string
+        const _y: string = x + '-suffix';
+        expect(_y).toBe('portfolio-suffix');
+      } else {
+        throw new Error('narrowing failed');
+      }
+    });
+  });
+
+  describe('assertValidSourceId (throwing — for explicit/env tiers per P1-F)', () => {
+    test('returns void for valid ids', () => {
+      expect(() => assertValidSourceId('default')).not.toThrow();
+      expect(() => assertValidSourceId('portfolio')).not.toThrow();
+      expect(() => assertValidSourceId('a')).not.toThrow();
+    });
+
+    test('throws with offending value in message for invalid ids', () => {
+      expect(() => assertValidSourceId('snake_id')).toThrow(/snake_id/);
+      expect(() => assertValidSourceId('../etc')).toThrow(/\.\.\/etc/);
+      expect(() => assertValidSourceId('A B')).toThrow(/A B/);
+    });
+
+    test('throws on non-string inputs (JSON-stringified for debug clarity)', () => {
+      expect(() => assertValidSourceId(undefined)).toThrow(/undefined|Invalid source_id/);
+      expect(() => assertValidSourceId(null)).toThrow(/null/);
+      expect(() => assertValidSourceId(42)).toThrow(/42/);
+    });
+
+    test('error message includes regex for caller clarity', () => {
+      try {
+        assertValidSourceId('snake_id');
+        throw new Error('should have thrown');
+      } catch (e) {
+        const msg = (e as Error).message;
+        expect(msg).toMatch(/1-32 lowercase alnum/);
+        expect(msg).toMatch(/\^\[a-z0-9\]/);
+      }
+    });
+  });
+});