diff --git a/docs/api/skills-placement.md b/docs/api/skills-placement.md
index ea0ce49..e8410af 100644
--- a/docs/api/skills-placement.md
+++ b/docs/api/skills-placement.md
@@ -16,7 +16,7 @@ interface ComposedSkills
 
 ### `composeSkillsForHarness`
 
-`function` — Compose {@link ComposedSkills} for `harness`: `mounted` delivery when the platform names a cwd skill dir for it, `inline` delivery (the automatic fallback that keeps every skill available on every ha…
+`function` — Compose {@link ComposedSkills} for `harness`.
 
 ```ts
 (input: ComposeSkillsForHarnessInput) => ComposedSkills
diff --git a/docs/codemap.json b/docs/codemap.json
index 02a5243..cc52cb8 100644
--- a/docs/codemap.json
+++ b/docs/codemap.json
@@ -15187,7 +15187,7 @@
           "name": "composeSkillsForHarness",
           "kind": "function",
           "signature": "(input: ComposeSkillsForHarnessInput) => ComposedSkills",
-          "doc": "Compose {@link ComposedSkills} for `harness`: `mounted` delivery when the platform names a cwd skill dir for it, `inline` delivery (the automatic fallback that keeps every skill available on every ha…"
+          "doc": "Compose {@link ComposedSkills} for `harness`."
         },
         {
           "name": "ComposeSkillsForHarnessInput",
diff --git a/docs/llms-full.txt b/docs/llms-full.txt
index bcd5404..b77b360 100644
--- a/docs/llms-full.txt
+++ b/docs/llms-full.txt
@@ -19778,7 +19778,7 @@ interface ComposedSkills
 
 ### `composeSkillsForHarness`
 
-`function` — Compose {@link ComposedSkills} for `harness`: `mounted` delivery when the platform names a cwd skill dir for it, `inline` delivery (the automatic fallback that keeps every skill available on every ha…
+`function` — Compose {@link ComposedSkills} for `harness`.
 
 ```ts
 (input: ComposeSkillsForHarnessInput) => ComposedSkills
diff --git a/src/skills-placement/index.test.ts b/src/skills-placement/index.test.ts
index b811792..4370f74 100644
--- a/src/skills-placement/index.test.ts
+++ b/src/skills-placement/index.test.ts
@@ -67,14 +67,63 @@ describe.skipIf(!available)('composeSkillsForHarness', () => {
     expect(result.promptSection).not.toContain('Strip the tells.')
   })
 
-  it('hermes -> inline: no refs, bodies rendered into the section', () => {
-    const result = mod!.composeSkillsForHarness({ skills, harness: 'hermes' })
+  it('hermes, default onNoSkillDir -> throws instead of silently inlining', () => {
+    expect(() => mod!.composeSkillsForHarness({ skills, harness: 'hermes' })).toThrow(
+      /no native skill-mount directory/,
+    )
+  })
+
+  it('amp, default onNoSkillDir -> throws instead of silently inlining', () => {
+    expect(() => mod!.composeSkillsForHarness({ skills, harness: 'amp' })).toThrow(
+      /no native skill-mount directory/,
+    )
+  })
+
+  it('amp, default onNoSkillDir -> throws AND the thrown message never contains a skill body', () => {
+    // Regression guard for the silent-inline bug: it must actually throw (not
+    // just fail to contain the body because it silently returned a prompt
+    // section as before), and the ERROR path itself must never leak skill
+    // bytes into anything a caller might log/display as if it were a normal
+    // composed prompt section.
+    expect(() => mod!.composeSkillsForHarness({ skills, harness: 'amp' })).toThrow()
+    let message = ''
+    try {
+      mod!.composeSkillsForHarness({ skills, harness: 'amp' })
+    } catch (err) {
+      message = (err as Error).message
+    }
+    expect(message).not.toBe('')
+    expect(message).not.toContain('Strip the tells.')
+  })
+
+  it('amp, default onNoSkillDir -> error names the harness, skill count, byte size, and mountable harnesses', () => {
+    expect(() => mod!.composeSkillsForHarness({ skills, harness: 'amp' })).toThrow(
+      /"amp".*1 skill\(s\).*\d+ bytes.*opencode/s,
+    )
+  })
+
+  it('amp, onNoSkillDir: "throw" (explicit) -> same as default', () => {
+    expect(() =>
+      mod!.composeSkillsForHarness({ skills, harness: 'amp', onNoSkillDir: 'throw' }),
+    ).toThrow(/no native skill-mount directory/)
+  })
+
+  it("amp, onNoSkillDir: 'inline' -> explicit opt-in still works: no refs, bodies rendered into the section", () => {
+    const result = mod!.composeSkillsForHarness({
+      skills,
+      harness: 'amp',
+      onNoSkillDir: 'inline',
+    })
     expect(result.refs).toEqual([])
     expect(result.promptSection).toContain('Strip the tells.')
   })
 
-  it('amp -> inline: no refs, bodies rendered into the section', () => {
-    const result = mod!.composeSkillsForHarness({ skills, harness: 'amp' })
+  it("hermes, onNoSkillDir: 'inline' -> explicit opt-in still works: no refs, bodies rendered into the section", () => {
+    const result = mod!.composeSkillsForHarness({
+      skills,
+      harness: 'hermes',
+      onNoSkillDir: 'inline',
+    })
     expect(result.refs).toEqual([])
     expect(result.promptSection).toContain('Strip the tells.')
   })
diff --git a/src/skills-placement/index.ts b/src/skills-placement/index.ts
index b203ac6..6b78aeb 100644
--- a/src/skills-placement/index.ts
+++ b/src/skills-placement/index.ts
@@ -18,17 +18,19 @@
  * works without it, falling back to `inline` delivery.
  */
 
-import type { Harness } from '../harness/index'
+import { KNOWN_HARNESSES, type Harness } from '../harness/index'
 import { skillDirForHarness, type HarnessId } from '@tangle-network/agent-profile-materialize'
-import { composeSkills, type ComposedSkills, type SkillEntry } from '../skills/index'
+import { composeSkills, renderInlineSkills, type ComposedSkills, type SkillEntry } from '../skills/index'
 
 /** agent-app `Harness` -> platform `HarnessId`, identity-mapped for exactly
  *  the harnesses the platform map covers. Harnesses absent here (`amp`,
  *  `factory-droids`, `forge`, `acp`, `cursor`, `cli-base`) resolve to `null` —
- *  callers fall back to `inline` delivery. `cursor`'s adapter supports
- *  `resources.skills` bespokely but isn't in the platform map yet; treating it
- *  as unbridged (inline fallback) is the safe posture until the map covers
- *  it, rather than guessing its cwd skill dir here. */
+ *  {@link composeSkillsForHarness} refuses by default rather than silently
+ *  delivering `inline`; a caller that genuinely needs `inline` on one of
+ *  these opts in explicitly (`onNoSkillDir: 'inline'`). `cursor`'s adapter
+ *  supports `resources.skills` bespokely but isn't in the platform map yet;
+ *  treating it as unbridged is the safe posture until the map covers it,
+ *  rather than guessing its cwd skill dir here. */
 const HARNESS_BRIDGE: Partial<Record<Harness, HarnessId>> = {
   opencode: 'opencode',
   'claude-code': 'claude-code',
@@ -72,20 +74,74 @@ export interface ComposeSkillsForHarnessInput {
   harness: Harness
   tier?: string
   heading?: string
+  /**
+   * What to do when `harness` has no native skill-mount directory (see
+   * {@link resolveSkillDir}). Default `'throw'`: refuse rather than silently
+   * inlining every skill BODY into the system prompt. Pass `'inline'` only as
+   * a deliberate, auditable opt-in — the caller has decided every skill must
+   * reach this harness even though it will be concatenated into the prompt.
+   */
+  onNoSkillDir?: 'throw' | 'inline'
+}
+
+/** `KNOWN_HARNESSES` filtered to those the platform materializer can mount
+ *  skill files onto natively (a non-null {@link resolveSkillDir}). Computed
+ *  fresh rather than cached — the platform map can grow without a release
+ *  here. Used only to build the actionable error in
+ *  {@link composeSkillsForHarness}; not exported because it duplicates
+ *  {@link unsupportedSkillHarnesses}'s complement and callers should reach
+ *  for that instead when they need the harness list at runtime. */
+function nativeSkillMountHarnesses(): Harness[] {
+  return KNOWN_HARNESSES.filter((harness) => resolveSkillDir(harness) !== null)
 }
 
 /**
- * Compose {@link ComposedSkills} for `harness`: `mounted` delivery when the
- * platform names a cwd skill dir for it, `inline` delivery (the automatic
- * fallback that keeps every skill available on every harness) otherwise. The
- * one function a product calls instead of hand-checking `resolveSkillDir`
+ * Compose {@link ComposedSkills} for `harness`.
+ *
+ * `mounted` delivery is the standard path: whenever the platform names a cwd
+ * skill dir for `harness`, skills ride the typed `resources.skills` channel
+ * and the platform materializer writes each one to that directory as a real
+ * file — the agent reads it on demand with its own file tools.
+ *
+ * `inline` delivery — every skill BODY rendered straight into the system
+ * prompt — is a NARROW fallback for harnesses with no native skill dir, never
+ * a co-equal mode. An `AgentProfile` is a RECIPE that materializes into a
+ * sandbox, not a prompt template: skill bodies belong in a file mount, and
+ * `prompt.systemPrompt` should carry only what the agent must obey without a
+ * tool call (identity, tool-call contract, safety rules, output format) plus
+ * a short index of what's mounted. Concatenating skill bodies into the prompt
+ * degrades the model toward empty answers once it grows large enough — one
+ * product reproduced a 151,882-byte prompt exactly this way, from an
+ * unnoticed inline fallback.
+ *
+ * Because of that, `onNoSkillDir` defaults to `'throw'`: a harness with no
+ * skill dir refuses instead of silently inlining. The thrown error names the
+ * harness, the skill count, the byte size that would have been inlined, and
+ * the harnesses that DO support native mounting. Pass `onNoSkillDir:
+ * 'inline'` only as a deliberate, auditable opt-in when every skill genuinely
+ * must reach this harness regardless of prompt cost.
+ *
+ * The one function a product calls instead of hand-checking `resolveSkillDir`
  * and branching between {@link composeSkills}'s two modes itself.
  */
 export function composeSkillsForHarness(input: ComposeSkillsForHarnessInput): ComposedSkills {
-  const { skills, harness, tier, heading } = input
+  const { skills, harness, tier, heading, onNoSkillDir = 'throw' } = input
   const skillDir = resolveSkillDir(harness)
   if (skillDir) return composeSkills({ skills, mode: 'mounted', skillDir, tier, heading })
-  return composeSkills({ skills, mode: 'inline', tier, heading })
+  if (onNoSkillDir === 'inline') return composeSkills({ skills, mode: 'inline', tier, heading })
+
+  const relevant = tier ? skills.filter((s) => s.tier === tier) : skills
+  const promptSection = renderInlineSkills({ skills, tier, heading })
+  const bytes = new TextEncoder().encode(promptSection).byteLength
+  const mountable = nativeSkillMountHarnesses()
+  throw new Error(
+    `composeSkillsForHarness: "${harness}" has no native skill-mount directory. ` +
+      `Inlining ${relevant.length} skill(s) (${bytes} bytes) into its system prompt would silently ` +
+      `convert mounted skills into prompt text — a 151,882-byte prompt shipped this way once, and ` +
+      `oversized prompts degrade toward empty answers. Mount the skills instead: pick a harness with ` +
+      `native skill support (${mountable.join(', ')}), or pass onNoSkillDir: 'inline' to opt in ` +
+      `explicitly and accept the inlined bytes.`,
+  )
 }
 
 export type { ComposedSkills, SkillEntry } from '../skills/index'
diff --git a/src/skills/index.ts b/src/skills/index.ts
index dcda5cd..ef98943 100644
--- a/src/skills/index.ts
+++ b/src/skills/index.ts
@@ -637,7 +637,8 @@ export interface ComposeSkillsInput {
  * directory); a null/absent one throws rather than silently falling back, so
  * the caller resolves the fallback deliberately (see
  * `@tangle-network/agent-app/skills-placement`'s `composeSkillsForHarness`,
- * which does exactly that).
+ * whose own default is to throw rather than choose `inline` for you — inline
+ * is a narrow, explicitly opted-into fallback, not a co-equal mode).
  */
 export function composeSkills(input: ComposeSkillsInput): ComposedSkills {
   const { skills, mode, tier, heading } = input