diff --git a/src/modules/aix/client/ContentReassembler.ts b/src/modules/aix/client/ContentReassembler.ts
index c7f895a29..2217280c2 100644
--- a/src/modules/aix/client/ContentReassembler.ts
+++ b/src/modules/aix/client/ContentReassembler.ts
@@ -238,11 +238,14 @@ export class ContentReassembler {
           case 'ii':
             await this.onAppendInlineImage(op);
             break;
+          case 'svs':
+            this.onSetVendorState(op);
+            break;
           case 'urlc':
             this.onAddUrlCitation(op);
             break;
           case 'vp':
-            this.onVoidPlaceholder(op);
+            this.onAppendVoidPlaceholder(op);
             break;
           default:
             // noinspection JSUnusedLocalSymbols
@@ -579,7 +582,7 @@ export class ContentReassembler {
     // This ensures we don't interrupt the text flow
   }
 
-  private onVoidPlaceholder(vp: Extract<AixWire_Particles.PartParticleOp, { p: 'vp' }>): void {
+  private onAppendVoidPlaceholder(vp: Extract<AixWire_Particles.PartParticleOp, { p: 'vp' }>): void {
     const { text, mot } = vp;
 
     // update the model op
@@ -603,6 +606,22 @@ export class ContentReassembler {
     // Placeholders don't affect text fragment indexing
   }
 
+  private onSetVendorState(vs: Extract<AixWire_Particles.PartParticleOp, { p: 'svs' }>): void {
+    // apply vendor state to the last created fragment
+    const lastFragment = this.accumulator.fragments[this.accumulator.fragments.length - 1];
+    if (!lastFragment) {
+      console.warn('[ContentReassembler] Vendor state particle without preceding content fragment');
+      return;
+    }
+
+    // attach vendor state
+    const { vendor, state } = vs;
+    lastFragment.vendorState = {
+      ...lastFragment.vendorState,
+      [vendor]: state,
+    }
+  }
+
   // Helper to remove placeholder when real content arrives
   private removePlaceholderIfAtIndex0(): void {
     if (this.accumulator.fragments.length > 0) {
diff --git a/src/modules/aix/server/api/aix.wiretypes.ts b/src/modules/aix/server/api/aix.wiretypes.ts
index 1e49595d2..bfaf29c20 100644
--- a/src/modules/aix/server/api/aix.wiretypes.ts
+++ b/src/modules/aix/server/api/aix.wiretypes.ts
@@ -213,6 +213,7 @@ export namespace AixWire_Parts {
 
   // Model Auxiliary Part (for thinking blocks)
 
+  // NOTE: not a _BasePart_schema for now, may become if we put the vndAnt attributes there
   export const ModelAuxPart_schema = z.object({
     pt: z.literal('ma'),
     aType: z.literal('reasoning'),
@@ -669,6 +670,7 @@ export namespace AixWire_Particles {
     | { p: 'ia', mimeType: string, a_b64: string, label?: string, generator?: string, durationMs?: number } // inline audio, complete
     | { p: 'ii', mimeType: string, i_b64: string, label?: string, generator?: string, prompt?: string } // inline image, complete
     | { p: 'urlc', title: string, url: string, num?: number, from?: number, to?: number, text?: string, pubTs?: number } // url citation - pubTs: publication timestamp
+    | { p: 'svs', vendor: string, state: Record<string, unknown> } // set vendor state - applies to the last emitted part (opaque protocol state)
     | { p: 'vp', text: string, mot: 'search-web' | 'gen-image' | 'code-exec' }; // void placeholder - temporary status text that gets wiped when real content arrives
 
 }
diff --git a/src/modules/aix/server/dispatch/chatGenerate/ChatGenerateTransmitter.ts b/src/modules/aix/server/dispatch/chatGenerate/ChatGenerateTransmitter.ts
index 1a02920ae..39c75464e 100644
--- a/src/modules/aix/server/dispatch/chatGenerate/ChatGenerateTransmitter.ts
+++ b/src/modules/aix/server/dispatch/chatGenerate/ChatGenerateTransmitter.ts
@@ -442,6 +442,19 @@ export class ChatGenerateTransmitter implements IParticleTransmitter {
     } satisfies Extract<AixWire_Particles.PartParticleOp, { p: 'vp' }>);
   }
 
+  /**
+   * Sends vendor-specific state modifier for the last emitted part.
+   * This attaches opaque protocol state (e.g., Gemini thoughtSignature) without polluting core part schemas.
+   */
+  sendSetVendorState(vendor: string, state: Record<string, unknown>) {
+    // queue vendor state particle immediately after the content part has been queued (and if text, it will be emitted sooner anyway)
+    this.transmissionQueue.push({
+      p: 'svs',
+      vendor,
+      state,
+    } satisfies Extract<AixWire_Particles.PartParticleOp, { p: 'svs' }>);
+  }
+
   /** Communicates the model name to the client */
   setModelName(modelName: string) {
     this.transmissionQueue.push({
diff --git a/src/modules/aix/server/dispatch/chatGenerate/parsers/IParticleTransmitter.ts b/src/modules/aix/server/dispatch/chatGenerate/parsers/IParticleTransmitter.ts
index fa463fdb7..41a1c4f0b 100644
--- a/src/modules/aix/server/dispatch/chatGenerate/parsers/IParticleTransmitter.ts
+++ b/src/modules/aix/server/dispatch/chatGenerate/parsers/IParticleTransmitter.ts
@@ -69,6 +69,12 @@ export interface IParticleTransmitter {
   /** Sends a void placeholder particle - temporary status that gets wiped when real content arrives */
   sendVoidPlaceholder(mot: 'search-web' | 'gen-image' | 'code-exec', text: string): void;
 
+  /**
+   * Sends vendor-specific state modifier for the last emitted part.
+   * Used to attach opaque protocol state (e.g., Gemini thoughtSignature) without polluting core part schemas.
+   */
+  sendSetVendorState(vendor: string, state: unknown): void;
+
   // Non-parts data //
 
   /** Communicates the model name to the client */