diff --git a/docs/core-data-structures/core.md b/docs/core-data-structures/core.md
index 61f980ff73..5ec00acf45 100644
--- a/docs/core-data-structures/core.md
+++ b/docs/core-data-structures/core.md
@@ -24,6 +24,7 @@ Everything else is documented on a **sub-page**, not here. The rule that draws t
 | [compaction.md](compaction.md) | the compaction seam: the `compact/*` session events, `CompactionResult`, the `CompactService` interface |
 | [subagent.md](subagent.md) | the subagent seam: the named-provider registry, `SubagentStartRequest`/`Result`/`Run`, the start-time-vs-runtime capability split |
 | [web.md](web.md) | the web access seam: `WebSearchRequest`/`Result`, `WebFetchRequest`/`Result`, `WebFetchBody`, provider/capability status, `WebError` |
+| [spill.md](spill.md) | the spill storage seam: `SaveTextSpill`, `SpillOwner`/`SpillSource`, `SpillRef`, the branded `SpillPath` |
 
 > Type definitions on this page are pasted **verbatim** from source and drift-checked by `pnpm run verify-type-equiv` (see [development.md](../development.md#documenting-types-verbatim-ts-type-equiv)). Inline JSDoc is omitted for readability; follow the source link for the full contracts.
 
diff --git a/docs/core-data-structures/spill.md b/docs/core-data-structures/spill.md
new file mode 100644
index 0000000000..7586d70633
--- /dev/null
+++ b/docs/core-data-structures/spill.md
@@ -0,0 +1,55 @@
+# Spill Storage
+
+The spill storage seam — a [capability seam](../rfc/implemented/architecture/2026-07-08-tool-output-spill-files.md) that persists a tool's oversized text to a session-scoped path the model can later `read`, split across packages: interface ([dsh-spill](../../packages/spill/spill), `ctx.spillFiles`), implementation ([dsh-spill-local](../../packages/spill/spill-local), private session-scoped files on the host filesystem), and consumer ([dsh-spill-policy](../../packages/spill/spill-policy), the `tools/post-execute` policy). Spill is **one optional capability**, not part of the agent-loop spine — so its vocabulary lives here, not in [core.md](core.md). Preview mechanics stay in [dsh-retention](../../packages/util/retention); this seam only saves the final text the policy hands it.
+
+Source: [`packages/spill/spill/src/types.ts`](../../packages/spill/spill/src/types.ts)
+
+## The save request
+
+`saveText` is the whole seam: persist `content` verbatim, return a readable path plus the exact byte count. The request carries WHO the artifact belongs to (`owner`), WHERE it came from (`source`, descriptive provenance for the filename and future cleanup — not access control), and a `suggestedName` the backend sanitizes to one safe path segment before use (it is a hint, never a path).
+
+```ts type-equiv
+interface SaveTextSpill {
+  owner: SpillOwner
+  source: SpillSource
+  suggestedName: string
+  content: string
+}
+```
+
+```ts type-equiv
+interface SpillOwner {
+  sessionId: SessionId
+}
+```
+
+`SpillOwner` scopes storage to a `SessionId` — spill is inherently session-scoped (its directory layout and future cleanup unit are per session), so the seam imports `dsh-session`'s `SessionId` directly rather than minting a decoupled token like the bash executor's cross-session `OwnerToken` ([bash.md](bash.md)).
+
+```ts type-equiv
+interface SpillSource {
+  toolName: string
+  callId: CallId
+  label: string
+}
+```
+
+## The result
+
+```ts type-equiv
+interface SpillRef {
+  path: SpillPath
+  bytes: number
+}
+```
+
+`SpillPath` is a [branded](core.md#branded-ids) local filesystem path returned by the backend and intended for the model's `read` tool. The brand records that the path came from the spill seam (a runtime artifact, not a workspace file the model authored); it is still rendered to the model as an ordinary path string in v1. A future remote or virtual backend may replace it with a `spill://…` URI plus a read-only filesystem bridge, so consumers treat it as opaque.
+
+```ts type-equiv
+type SpillPath = Branded<'SpillPath'>
+```
+
+## The service
+
+`SpillFiles` (`ctx.spillFiles`, defined in [`packages/spill/spill/src/index.ts`](../../packages/spill/spill/src/index.ts)) is a one-method abstract service: `saveText(input) → Promise<SpillRef>`. It persists the FULL `content`, chooses a private (not world-readable) location and a collision-free name derived from — never equal to — `suggestedName`, and REJECTS on a real storage failure (permissions, ENOSPC, backend unavailable). The seam owns storage only: no retention policy, no tool-result replacement, no file inspection.
+
+The local backend ([dsh-spill-local](../../packages/spill/spill-local)) writes under `<root>/session-<hash>/<random>-<safeName>` — a configured or lazily-created private (0700) root, a `sha256(sessionId)` session subdir, and an exclusive owner-only (`open(path, 'wx', 0o600)`) write so a planted symlink cannot redirect it. The policy consumer ([dsh-spill-policy](../../packages/spill/spill-policy)) replaces an over-`maxInlineBytes` plain-text final result with a retention-library head/tail preview plus the spill path, best-effort: a save failure keeps the original inline result rather than turning a successful call into an `isError`.
diff --git a/packages/spill/spill-policy/README.md b/packages/spill/spill-policy/README.md
index 42c3334bf7..24e720c5c8 100644
--- a/packages/spill/spill-policy/README.md
+++ b/packages/spill/spill-policy/README.md
@@ -8,7 +8,7 @@ This plugin registers **no service** and owns no storage or preview mechanics: p
 
 | Key | Default | Meaning |
 |---|---|---|
-| `maxInlineBytes` | *(omitted)* | Model-facing context cap for a plain-text result, in UTF-8 bytes. **Omitted disables the policy entirely** (the plugin registers nothing). When set, a larger result is spilled and replaced with a preview derived from the same budget (head/tail split). |
+| `maxInlineBytes` | *(omitted)* | Model-facing context cap for a plain-text result, in UTF-8 bytes (a non-negative integer; validated at load). **Omitted disables the policy entirely** (the plugin registers nothing). When set, a larger result is spilled and replaced with a preview derived from the same budget (head/tail split). |
 
 ## Behavior
 
diff --git a/packages/spill/spill-policy/src/index.ts b/packages/spill/spill-policy/src/index.ts
index f4672a4751..57c651d70c 100644
--- a/packages/spill/spill-policy/src/index.ts
+++ b/packages/spill/spill-policy/src/index.ts
@@ -101,6 +101,12 @@ export function apply(ctx: Context, config: Config): void {
   const maxInlineBytes = config.maxInlineBytes
   // Omitted ⇒ no automatic spill policy: register nothing at all.
   if (maxInlineBytes === undefined) return
+  // Validate at LOAD, not per call: a negative/fractional cap would reach
+  // TextRetainer's assertBudget and throw, turning every oversized-result call
+  // into an isError. A bad config must fail the deployment, not the tool.
+  if (!Number.isInteger(maxInlineBytes) || maxInlineBytes < 0) {
+    throw new Error(`spill-policy: maxInlineBytes must be a non-negative integer (got ${maxInlineBytes})`)
+  }
 
   ctx.on('tools/post-execute', async (exec, result, next): Promise<PostToolDecision> => {
     // Delegate first so a downstream listener (e.g. a hook) settles the result;
diff --git a/packages/spill/spill-policy/tests/spill-policy.spec.ts b/packages/spill/spill-policy/tests/spill-policy.spec.ts
index 1a2592cb53..61da0abe76 100644
--- a/packages/spill/spill-policy/tests/spill-policy.spec.ts
+++ b/packages/spill/spill-policy/tests/spill-policy.spec.ts
@@ -82,6 +82,16 @@ describe('disabled mode', () => {
   })
 })
 
+describe('config validation', () => {
+  it('rejects a negative maxInlineBytes at load', async () => {
+    await expect(setup({ maxInlineBytes: -1 })).rejects.toThrow(/non-negative integer/)
+  })
+
+  it('rejects a fractional maxInlineBytes at load', async () => {
+    await expect(setup({ maxInlineBytes: 1.5 })).rejects.toThrow(/non-negative integer/)
+  })
+})
+
 describe('oversized plain-text replacement', () => {
   it('spills the full text and replaces the result with a preview + path', async () => {
     const { ctx, spill } = await setup({ maxInlineBytes: 20 })
diff --git a/scripts/type-equiv.manifest.json b/scripts/type-equiv.manifest.json
index b1c3163782..8dcf5c4715 100644
--- a/scripts/type-equiv.manifest.json
+++ b/scripts/type-equiv.manifest.json
@@ -77,6 +77,12 @@
     { "doc": "docs/core-data-structures/web.md", "symbol": "WebFetchRequest", "source": "packages/web/web/src/types.ts" },
     { "doc": "docs/core-data-structures/web.md", "symbol": "WebFetchResult", "source": "packages/web/web/src/types.ts" },
     { "doc": "docs/core-data-structures/web.md", "symbol": "WebFetchBody", "source": "packages/web/web/src/types.ts" },
-    { "doc": "docs/core-data-structures/web.md", "symbol": "WebProviderStatus", "source": "packages/web/web/src/types.ts" }
+    { "doc": "docs/core-data-structures/web.md", "symbol": "WebProviderStatus", "source": "packages/web/web/src/types.ts" },
+
+    { "doc": "docs/core-data-structures/spill.md", "symbol": "SaveTextSpill", "source": "packages/spill/spill/src/types.ts" },
+    { "doc": "docs/core-data-structures/spill.md", "symbol": "SpillOwner", "source": "packages/spill/spill/src/types.ts" },
+    { "doc": "docs/core-data-structures/spill.md", "symbol": "SpillSource", "source": "packages/spill/spill/src/types.ts" },
+    { "doc": "docs/core-data-structures/spill.md", "symbol": "SpillRef", "source": "packages/spill/spill/src/types.ts" },
+    { "doc": "docs/core-data-structures/spill.md", "symbol": "SpillPath", "source": "packages/spill/spill/src/types.ts" }
   ]
 }