diff --git a/packages/agent-core/src/agent/turn/kosong-llm.ts b/packages/agent-core/src/agent/turn/kosong-llm.ts
index 893ea13fd9..86dc14f1cf 100644
--- a/packages/agent-core/src/agent/turn/kosong-llm.ts
+++ b/packages/agent-core/src/agent/turn/kosong-llm.ts
@@ -301,7 +301,19 @@ export function downgradeUnsupportedMedia(
   messages: readonly Message[],
   capability: ModelCapability | undefined,
 ): Message[] {
-  if (capability === undefined || isUnknownCapability(capability)) return [...messages];
+  // Leave history alone when the host omitted a matrix (undefined) or only
+  // knows the model is uncatalogued (UNKNOWN_CAPABILITY). UNKNOWN means "we
+  // do not know", not "text-only" — a multimodal custom model can still use
+  // that marker (SingleModelProvider default, KimiForCodingProvider).
+  //
+  // Explicit matrices still strip: ProviderManager merges declared
+  // capabilities with the catalog into a plain ModelCapability object, so a
+  // text-only custom model configured as capabilities = ["thinking",
+  // "tool_use"] gets image_in: false without the UNKNOWN marker and is
+  // protected against image_url 400s that brick later turns (#2669).
+  if (capability === undefined || isUnknownCapability(capability)) {
+    return [...messages];
+  }
   const dropImage = !capability.image_in;
   const dropVideo = !capability.video_in;
   const dropAudio = !capability.audio_in;
diff --git a/packages/agent-core/test/agent/kosong-llm.test.ts b/packages/agent-core/test/agent/kosong-llm.test.ts
index eee3388092..cf0dd8a77f 100644
--- a/packages/agent-core/test/agent/kosong-llm.test.ts
+++ b/packages/agent-core/test/agent/kosong-llm.test.ts
@@ -440,12 +440,40 @@ describe('downgradeUnsupportedMedia', () => {
     expect(out[0]?.content).toEqual([imagePart, videoPart]);
   });
 
-  it('does not downgrade for UNKNOWN_CAPABILITY or an undefined capability', () => {
+  it('does not downgrade when capability is undefined (host omitted matrix)', () => {
     const input = [mediaMessage([videoPart])];
-    expect(downgradeUnsupportedMedia(input, UNKNOWN_CAPABILITY)[0]?.content).toEqual([videoPart]);
     expect(downgradeUnsupportedMedia(input, undefined)[0]?.content).toEqual([videoPart]);
   });
 
+  it('preserves media for UNKNOWN_CAPABILITY (uncatalogued ≠ text-only)', () => {
+    // UNKNOWN is the SingleModelProvider / unresolved default: the model may
+    // still be multimodal. Explicit text-only matrices are covered below.
+    const input = [mediaMessage([imagePart, videoPart, audioPart])];
+    expect(downgradeUnsupportedMedia(input, UNKNOWN_CAPABILITY)[0]?.content).toEqual([
+      imagePart,
+      videoPart,
+      audioPart,
+    ]);
+  });
+
+  it('strips images when an explicit matrix has no image_in (#2669)', () => {
+    // ProviderManager resolves declared capabilities into a plain object
+    // (not the UNKNOWN marker), e.g. capabilities = ["thinking", "tool_use"].
+    const capability: ModelCapability = {
+      image_in: false,
+      video_in: false,
+      audio_in: false,
+      thinking: true,
+      tool_use: true,
+      max_context_tokens: 128_000,
+    };
+    const input = [mediaMessage([{ type: 'text', text: 'see this' }, imagePart])];
+    expect(downgradeUnsupportedMedia(input, capability)[0]?.content).toEqual([
+      { type: 'text', text: 'see this' },
+      { type: 'text', text: '[image omitted: current model has no image input]' },
+    ]);
+  });
+
   it('returns a new array and never mutates the caller input', () => {
     const capability = makeCapability(1000); // all media dropped
     const message = mediaMessage([videoPart]);