diff --git a/packages/agents/content/skills/capture-event/SKILL.md b/packages/agents/content/skills/capture-event/SKILL.md
index ccccdd4b..90905522 100644
--- a/packages/agents/content/skills/capture-event/SKILL.md
+++ b/packages/agents/content/skills/capture-event/SKILL.md
@@ -6,7 +6,7 @@ user-invocable: true
 
 # Capture an event
 
-Append an event record to the shared knowledge substrate, or amend an existing one that has not yet been pushed. A bundled helper does the mechanical work — it resolves the event store by name, auto-fills the record's context (a ULID `id`, the capture timestamp, the session, the working directory, and a best-effort `repo`), validates the event record's required fields, and writes the record atomically. You supply the `summary` and the event body.
+Append an event record to the shared knowledge substrate, or amend an existing one that has not yet been pushed. A bundled helper does the mechanical work — it resolves the event store by name, auto-fills the record's context (a ULID `id`, the capture timestamp, the working directory, and a best-effort `session` and `repo`), validates the event record's required fields, and writes the record atomically. You supply the `summary` and the event body.
 
 This is a pure append. Unlike `kb-add`, it runs no survey, no `kb-retrieve` cross-referencing, and no dedup. The point is to capture the event cheaply and move on; recall and triage happen later via `kb-retrieve`.
 
@@ -38,7 +38,7 @@ A value-bearing flag accepts both `--summary text` and `--summary=text`; `--allo
 
 ### Auto-filled vs agent-supplied
 
-- **Auto-filled by the helper:** `recordType` (`event`), `id` (ULID), `captured-at`, `session` (`CLAUDE_CODE_SESSION_ID`), `cwd`, and `repo` (the `owner/name` git remote at `cwd`, best-effort — omitted silently when no remote resolves).
+- **Auto-filled by the helper:** `recordType` (`event`), `id` (ULID), `captured-at`, `cwd`, `session` (`CLAUDE_CODE_SESSION_ID`, best-effort — omitted silently on a harness that exposes no session id), and `repo` (the `owner/name` git remote at `cwd`, best-effort — omitted silently when no remote resolves).
 - **Template-injected:** `harness` — `codeassembly-agents` writes the agent platform (`claude` or `rovodev`) into the `--harness` flag when it installs this skill. Unlike `model`, which varies per session and is self-reported, the harness is fixed at install time; keep the injected `--harness` flag verbatim rather than filling in a value yourself.
 - **Agent-supplied:** `summary`, the optional `skill`/`model`/`tags`/`impact`, and the body.
 
diff --git a/packages/agents/src/capture-event/__tests__/cli.test.ts b/packages/agents/src/capture-event/__tests__/cli.test.ts
index f784cf2f..4a2a3086 100644
--- a/packages/agents/src/capture-event/__tests__/cli.test.ts
+++ b/packages/agents/src/capture-event/__tests__/cli.test.ts
@@ -213,6 +213,75 @@ describe(runCapture, () => {
     }
   });
 
+  it('captures an event with no session field when the harness exposes no session id', async () => {
+    const { home } = await makeStore('codeassembly');
+    const repo = await makeRepoWithRemote('git@github.com:williamthorsen/codeassembly.git');
+
+    const result = await runCapture({
+      argv: ['--store', '@default', '--summary', 'Noticed a thing'],
+      stdin: bodyStream('Body text.'),
+      cwd: repo,
+      env: {},
+      now: NOW,
+      home,
+    });
+
+    expect(result.ok).toBe(true);
+    if (result.ok) {
+      const written = await readFile(result.path, 'utf8');
+      expect(written).not.toMatch(/^session:/m);
+      expect(written).toContain('summary: Noticed a thing');
+    }
+  });
+
+  it('treats a blank session id as no session at all', async () => {
+    const { home } = await makeStore('codeassembly');
+    const repo = await makeRepoWithRemote('git@github.com:williamthorsen/codeassembly.git');
+
+    const result = await runCapture({
+      argv: ['--store', '@default', '--summary', 'Noticed a thing'],
+      stdin: bodyStream('Body text.'),
+      cwd: repo,
+      env: { CLAUDE_CODE_SESSION_ID: '' },
+      now: NOW,
+      home,
+    });
+
+    expect(result.ok).toBe(true);
+    if (result.ok) {
+      const written = await readFile(result.path, 'utf8');
+      expect(written).not.toMatch(/^session:/m);
+    }
+  });
+
+  it('amends an event stored with an empty session, dropping the empty field', async () => {
+    const { storePath, home } = await makeStore('codeassembly');
+    await mkdir(join(storePath, 'content', 'events'), { recursive: true });
+    await writeFile(
+      join(storePath, 'content', 'events', `${ID}.md`),
+      `---\nrecordType: event\nid: ${ID}\ncaptured-at: 2026-06-04T06:57:22Z\nsession: ''\ncwd: /tmp/work\nsummary: Original summary\nharness: rovodev\n---\n\nOriginal body.\n`,
+      'utf8',
+    );
+
+    const amended = await runCapture({
+      argv: ['--store', '@default', '--amend', ID, '--summary', 'Corrected summary'],
+      stdin: bodyStream('Corrected body.'),
+      cwd: '/tmp/different-cwd',
+      env: {},
+      now: NOW,
+      home,
+    });
+
+    expect(amended.ok).toBe(true);
+    if (amended.ok) {
+      const written = await readFile(amended.path, 'utf8');
+      expect(written).not.toMatch(/^session:/m);
+      expect(written).toContain('summary: Corrected summary');
+      expect(written).toContain('harness: rovodev');
+      expect(written).toContain('cwd: /tmp/work');
+    }
+  });
+
   it('writes the impact field when --impact is supplied', async () => {
     const { home } = await makeStore('codeassembly');
     const repo = await makeRepoWithRemote('git@github.com:williamthorsen/codeassembly.git');
diff --git a/packages/agents/src/capture-event/__tests__/prepare-event.test.ts b/packages/agents/src/capture-event/__tests__/prepare-event.test.ts
index 3ad5bc30..a2a1eaf8 100644
--- a/packages/agents/src/capture-event/__tests__/prepare-event.test.ts
+++ b/packages/agents/src/capture-event/__tests__/prepare-event.test.ts
@@ -102,6 +102,21 @@ describe(prepareEvent, () => {
     }
   });
 
+  it('writes an event with session absent when the harness exposes no session id', () => {
+    const result = prepareEvent({
+      args: argsFor({}),
+      context: { cwd: '/tmp/work', repo: 'owner/name' },
+      id: ID,
+      capturedAt: CAPTURED_AT,
+      body: '',
+    });
+
+    expect(result.ok).toBe(true);
+    if (result.ok) {
+      expect(result.prepared.content).not.toMatch(/^session:/m);
+    }
+  });
+
   it('renders the agent-supplied skill, model, harness, and tags into the record', () => {
     const result = prepareEvent({
       args: argsFor({ skill: 'kb-retrieve', model: 'claude-opus-4-8', harness: 'claude', tags: ['recall', 'kb'] }),
diff --git a/packages/agents/src/capture-event/cli.ts b/packages/agents/src/capture-event/cli.ts
index bda2fef2..f084125b 100644
--- a/packages/agents/src/capture-event/cli.ts
+++ b/packages/agents/src/capture-event/cli.ts
@@ -71,8 +71,8 @@ if (isEntryPoint()) {
 /**
  * Runs the helper end to end: parses args, reads the event body from stdin, and resolves the target store (a concrete
  * `--store` by name, or the registry's `default_kb` via the `@default` sentinel; an omitted `--store` is refused). A
- * fresh capture fills in the auto-derived context (ULID `id`, `captured-at`, `session`, `cwd`, best-effort `repo`),
- * validates the event record's required spine, and writes `content/events/{id}.md` without
+ * fresh capture fills in the auto-derived context (ULID `id`, `captured-at`, `cwd`, and a best-effort `session` and
+ * `repo`), validates the event record's required spine, and writes `content/events/{id}.md` without
  * overwriting an existing id. With `--amend <id>`, it instead rewrites that existing event in place; see
  * {@link amendEvent}.
  *
@@ -143,9 +143,13 @@ export async function runCapture(input: {
     return amendEvent({ args, store, body });
   }
 
-  const session = input.env.CLAUDE_CODE_SESSION_ID ?? '';
+  const session = input.env.CLAUDE_CODE_SESSION_ID;
   const repo = await resolveRepo(input.cwd);
-  const context: CaptureContext = { session, cwd: input.cwd, ...(repo !== undefined && { repo }) };
+  const context: CaptureContext = {
+    cwd: input.cwd,
+    ...(session !== undefined && session.length > 0 && { session }),
+    ...(repo !== undefined && { repo }),
+  };
 
   const prep = prepareEvent({
     args,
diff --git a/packages/agents/src/capture-event/prepare-event.ts b/packages/agents/src/capture-event/prepare-event.ts
index 5f78312d..8c3b489c 100644
--- a/packages/agents/src/capture-event/prepare-event.ts
+++ b/packages/agents/src/capture-event/prepare-event.ts
@@ -31,10 +31,11 @@ export type PrepareOutcome = PrepareSuccess | PrepareFailure;
 /**
  * Composes a `KbEvent` from agent-supplied args and auto-filled context, renders it through the record module's
  * `renderEvent`, and validates the serialized note as an `event` record by re-parsing through `parseEvent`. The record
- * carries the stored `recordType: event` discriminant and the typed event spine (`id`, `captured-at`, `session`, `cwd`,
- * `summary`, plus any supplied `tags`/`impact`); `repo`/`skill`/`model`/`harness` have no typed field and ride in
- * `extra`, which `renderEvent` emits after the spine. No `updated`/`last-verified` field is written: an event carries a
- * single canonical state, editable in place via `capture-event --amend` until it is pushed and immutable after.
+ * carries the stored `recordType: event` discriminant and the typed event spine (`id`, `captured-at`, `cwd`, `summary`,
+ * plus `session` when the harness exposes one and any supplied `tags`/`impact`); `repo`/`skill`/`model`/`harness` have
+ * no typed field and ride in `extra`, which `renderEvent` emits after the spine. No `updated`/`last-verified` field is
+ * written: an event carries a single canonical state, editable in place via `capture-event --amend` until it is pushed
+ * and immutable after.
  *
  * Rendering the composed record through the same `renderEvent`/`renderNote` path the amend path uses keeps a fresh
  * capture and its later amendments identical in field order. Validation round-trips the serialized note through
@@ -68,7 +69,7 @@ export function prepareEvent(input: {
     recordType: 'event',
     id,
     capturedAt,
-    session: context.session,
+    ...(context.session !== undefined && { session: context.session }),
     cwd: context.cwd,
     summary: args.summary,
     tags: args.tags,
diff --git a/packages/agents/src/capture-event/types.ts b/packages/agents/src/capture-event/types.ts
index 5b196c5a..160eaeba 100644
--- a/packages/agents/src/capture-event/types.ts
+++ b/packages/agents/src/capture-event/types.ts
@@ -34,8 +34,8 @@ export interface ParsedArgs {
 
 /** The auto-filled context an event carries beyond the agent-supplied fields. */
 export interface CaptureContext {
-  /** Session identifier read from `CLAUDE_CODE_SESSION_ID`. */
-  session: string;
+  /** Session identifier read from `CLAUDE_CODE_SESSION_ID`; omitted when the harness exposes none. */
+  session?: string;
   /** Absolute working directory the capture ran from. */
   cwd: string;
   /** `owner/name` git remote at `cwd`, best-effort; omitted when unresolvable. */
diff --git a/packages/kb/src/records/__tests__/event.test.ts b/packages/kb/src/records/__tests__/event.test.ts
index 74faf648..387ed1da 100644
--- a/packages/kb/src/records/__tests__/event.test.ts
+++ b/packages/kb/src/records/__tests__/event.test.ts
@@ -1,6 +1,6 @@
 import { describe, expect, it } from 'vitest';
 
-import { EVENT_IMPACT_LEVELS, isEventImpact, parseEvent, renderEvent } from '../event.ts';
+import { EVENT_IMPACT_LEVELS, isEventImpact, type KbEvent, parseEvent, renderEvent } from '../event.ts';
 
 const validFields = {
   recordType: 'event',
@@ -33,6 +33,29 @@ describe(parseEvent, () => {
     expect(result.ok).toBe(false);
   });
 
+  it('parses an event captured with no session', () => {
+    const { session: _session, ...withoutSession } = validFields;
+    const result = parseEvent(withoutSession, '');
+    expect(result.ok).toBe(true);
+    if (!result.ok) return;
+    expect(result.record.session).toBeUndefined();
+  });
+
+  it('reads a stored empty session as absent rather than as an empty string', () => {
+    const result = parseEvent({ ...validFields, session: '' }, '');
+    expect(result.ok).toBe(true);
+    if (!result.ok) return;
+    expect(result.record.session).toBeUndefined();
+    expect(result.record.extra).toEqual({});
+  });
+
+  it('reports a non-string session', () => {
+    const result = parseEvent({ ...validFields, session: 42 }, '');
+    expect(result.ok).toBe(false);
+    if (result.ok) return;
+    expect(result.errors.join(' ')).toContain('session');
+  });
+
   it('reads tags and addressed-by as typed fields, not extra', () => {
     const result = parseEvent({ ...validFields, tags: ['fix'], 'addressed-by': ['#849'] }, '');
     expect(result.ok).toBe(true);
@@ -110,6 +133,38 @@ describe(renderEvent, () => {
     expect(parseEvent(fields, body)).toEqual(parsed);
   });
 
+  it('omits session when the event carries none', () => {
+    const { session: _session, ...withoutSession } = validFields;
+    const parsed = parseEvent(withoutSession, '');
+    expect(parsed.ok).toBe(true);
+    if (!parsed.ok) return;
+    const keys = Object.keys(renderEvent(parsed.record).fields);
+    expect(keys).toEqual(['recordType', 'id', 'captured-at', 'cwd', 'summary']);
+  });
+
+  it('drops the empty session of a record stored before session became optional', () => {
+    const parsed = parseEvent({ ...validFields, session: '' }, '');
+    expect(parsed.ok).toBe(true);
+    if (!parsed.ok) return;
+    expect(Object.keys(renderEvent(parsed.record).fields)).not.toContain('session');
+  });
+
+  it('omits an empty session on a record composed directly rather than parsed', () => {
+    const record: KbEvent = {
+      recordType: 'event',
+      id: '01HZCEVENTAAAAAAAAAAAAAAAA',
+      capturedAt: '2026-06-18T09:41:02Z',
+      session: '',
+      cwd: '/tmp/work',
+      summary: 'Noticed a phantomwidget glitch',
+      tags: [],
+      addressedBy: [],
+      extra: {},
+      body: '',
+    };
+    expect(Object.keys(renderEvent(record).fields)).not.toContain('session');
+  });
+
   it('omits tags and addressed-by when empty', () => {
     const parsed = parseEvent(validFields, '');
     expect(parsed.ok).toBe(true);
diff --git a/packages/kb/src/records/event.ts b/packages/kb/src/records/event.ts
index 0b79f0b4..707d2b25 100644
--- a/packages/kb/src/records/event.ts
+++ b/packages/kb/src/records/event.ts
@@ -24,7 +24,8 @@ export interface KbEvent {
   recordType: 'event';
   id: string;
   capturedAt: string;
-  session: string;
+  /** Harness-dependent provenance: absent when the harness that captured the event exposes no session id. */
+  session?: string;
   cwd: string;
   summary: string;
   tags: string[];
@@ -58,7 +59,7 @@ export function parseEvent(fields: Record<string, unknown>, body: string): Parse
   }
 
   const id = requireString(fields.id, 'id', errors);
-  const session = requireString(fields.session, 'session', errors);
+  const session = readOptionalNonEmptyString(fields.session, 'session', errors);
   const cwd = requireString(fields.cwd, 'cwd', errors);
   const summary = requireString(fields.summary, 'summary', errors);
 
@@ -80,7 +81,6 @@ export function parseEvent(fields: Record<string, unknown>, body: string): Parse
     errors.length > 0 ||
     id === undefined ||
     capturedAt === undefined ||
-    session === undefined ||
     cwd === undefined ||
     summary === undefined ||
     tags === undefined ||
@@ -102,7 +102,7 @@ export function parseEvent(fields: Record<string, unknown>, body: string): Parse
       recordType: 'event',
       id,
       capturedAt,
-      session,
+      ...(session !== undefined && { session }),
       cwd,
       summary,
       tags,
@@ -114,13 +114,17 @@ export function parseEvent(fields: Record<string, unknown>, body: string): Parse
   };
 }
 
-/** Projects an event back to a frontmatter field map (declared fields first, then preserved `extra`) plus its body. */
+/**
+ * Projects an event back to a frontmatter field map (declared fields first, then preserved `extra`) plus its body. An
+ * empty `session` is omitted like an absent one, mirroring {@link parseEvent}: the two spellings of "no session" have a
+ * single representation on both edges of the module, so no record can reacquire the empty field on a write.
+ */
 export function renderEvent(record: KbEvent): { fields: Record<string, unknown>; body: string } {
   const fields: Record<string, unknown> = {
     recordType: record.recordType,
     id: record.id,
     'captured-at': record.capturedAt,
-    session: record.session,
+    ...(record.session !== undefined && record.session.length > 0 && { session: record.session }),
     cwd: record.cwd,
     summary: record.summary,
   };
@@ -167,6 +171,22 @@ function readListField(value: unknown, field: string, errors: string[]): string[
   return list;
 }
 
+/**
+ * Reads an optional string field for which the empty string carries no more meaning than absence: both yield
+ * `undefined`, so the record holds one representation of "not supplied". A present non-string value records an error and
+ * yields `undefined`.
+ */
+function readOptionalNonEmptyString(value: unknown, field: string, errors: string[]): string | undefined {
+  if (value === undefined || value === null || value === '') {
+    return undefined;
+  }
+  if (typeof value === 'string') {
+    return value;
+  }
+  errors.push(`${field}: expected a string`);
+  return undefined;
+}
+
 /** Reads a required string field, pushing an error when it is absent or empty; returns the value or `undefined`. */
 function requireString(value: unknown, field: string, errors: string[]): string | undefined {
   if (typeof value === 'string' && value.length > 0) {