diff --git a/sdk/typescript/src/types.ts b/sdk/typescript/src/types.ts
index 7402dfb7..21e685ae 100644
--- a/sdk/typescript/src/types.ts
+++ b/sdk/typescript/src/types.ts
@@ -96,8 +96,7 @@ export interface AttachmentInfo {
   filename: string;
   mime_type: string;
   size_bytes: number;
-  /** "image", "text", "pdf", or "audio". */
-  kind: string;
+  kind: "image" | "text" | "pdf" | "audio";
 }
 
 export type UploadAttachmentResponse = AttachmentInfo;
diff --git a/turnstone/core/pdf.py b/turnstone/core/pdf.py
index 6664ae25..55ef018f 100644
--- a/turnstone/core/pdf.py
+++ b/turnstone/core/pdf.py
@@ -54,7 +54,10 @@ def extract_pdf_text(data: bytes) -> str:
             textpage.close()
             page.close()
         text = "\n\n".join(p.strip() for p in parts if p.strip())
-        if truncated:
+        # Only annotate truncation when there's actual text — otherwise a scanned
+        # (no text layer) PDF over the page cap would return just the marker, i.e.
+        # a content-free document part.  Empty stays empty → caller placeholders it.
+        if truncated and text:
             text += f"\n\n[PDF truncated at {_MAX_PAGES} pages]"
         return text
     except Exception as exc:
diff --git a/turnstone/core/perception.py b/turnstone/core/perception.py
index a70dc645..991800eb 100644
--- a/turnstone/core/perception.py
+++ b/turnstone/core/perception.py
@@ -50,10 +50,6 @@ _DESCRIBE_PROMPT = (
 )
 
 
-class PerceptionUnavailableError(RuntimeError):
-    """No usable perception backend is configured/resolvable (maps to a placeholder)."""
-
-
 class PerceptionBackendError(RuntimeError):
     """A configured perception backend failed during the perceive call."""
 
diff --git a/turnstone/core/providers/_anthropic.py b/turnstone/core/providers/_anthropic.py
index 81537094..50044070 100644
--- a/turnstone/core/providers/_anthropic.py
+++ b/turnstone/core/providers/_anthropic.py
@@ -692,9 +692,10 @@ class AnthropicProvider:
                 converted.append(block)
                 continue
             if part.get("type") == "input_audio":
-                # Anthropic has no audio-input API.  Phase 3 transcribes via the
-                # STT role upstream of this translator; until then surface a
-                # placeholder rather than passing an unsupported block to the API.
+                # Anthropic has no audio-input API.  The STT-role fallback runs
+                # upstream of this translator (audio → text transcript when a
+                # model lacks supports_audio_input), so by here any remaining
+                # input_audio is a defensive placeholder, not the live path.
                 converted.append(
                     {"type": "text", "text": "[audio attachment — not supported by this model]"}
                 )
diff --git a/turnstone/core/providers/_openai_responses.py b/turnstone/core/providers/_openai_responses.py
index 849418ce..2402eb74 100644
--- a/turnstone/core/providers/_openai_responses.py
+++ b/turnstone/core/providers/_openai_responses.py
@@ -84,9 +84,10 @@ def convert_content_parts(parts: list[Any]) -> list[dict[str, Any]]:
                     }
                 )
         elif ptype == "input_audio":
-            # Audio-input is not wired on the Responses lane; until the Phase 3
-            # capability-gated fallback (STT) lands upstream, surface a
-            # placeholder rather than leaking an unhandled part to the API.
+            # Audio-input is not wired on the Responses lane.  The capability-gated
+            # fallback (STT / perception) runs upstream of this translator, so by
+            # here any remaining input_audio is a defensive placeholder rather than
+            # an unhandled part leaking to the API.
             converted.append(
                 {"type": "input_text", "text": "[audio attachment — not supported by this model]"}
             )
diff --git a/turnstone/core/session.py b/turnstone/core/session.py
index 9b87c7b5..a603dafe 100644
--- a/turnstone/core/session.py
+++ b/turnstone/core/session.py
@@ -3015,8 +3015,9 @@ class ChatSession:
             perceived = self._perception_fallback_part(att, "image")
             if perceived is not None:
                 return perceived
-            # No perception backend configured: emit the native image_url
-            # unchanged (the model may ignore it) — pre-existing behavior.
+            # No usable perception backend (none configured, or it can't see):
+            # emit the native image_url unchanged — a no-vision model ignores it.
+            # Image is intentionally left ungated here (pre-existing behavior).
         if kind == "audio" and not caps.supports_audio_input:
             return self._audio_fallback_part(att)
         return attachment_to_content_part(att)