diff --git a/src/api/providers/openrouter.ts b/src/api/providers/openrouter.ts
index 8f56cddc5c8..d435a05618e 100644
--- a/src/api/providers/openrouter.ts
+++ b/src/api/providers/openrouter.ts
@@ -220,7 +220,8 @@ export class OpenRouterHandler extends BaseProvider implements SingleCompletionH
 		// even if you don't request them. This is not the default for
 		// other providers (including Gemini), so we need to explicitly disable
 		// them unless the user has explicitly configured reasoning.
-		// Note: Gemini 3 models use reasoning_details format and should not be excluded.
+		// Note: Gemini 3 models use reasoning_details format with thought signatures,
+		// but we handle this via skip_thought_signature_validator injection below.
 		if (
 			(modelId === "google/gemini-2.5-pro-preview" || modelId === "google/gemini-2.5-pro") &&
 			typeof reasoning === "undefined"
@@ -250,8 +251,13 @@ export class OpenRouterHandler extends BaseProvider implements SingleCompletionH
 		const isNativeProtocol = toolProtocol === TOOL_PROTOCOL.NATIVE
 		const isGemini = modelId.startsWith("google/gemini")
 
-		// For Gemini with native protocol: inject fake reasoning.encrypted blocks for tool calls
-		// This is required when switching from other models to Gemini to satisfy API validation
+		// For Gemini with native protocol: inject fake reasoning.encrypted block for tool calls
+		// This is required when switching from other models to Gemini to satisfy API validation.
+		// Per OpenRouter documentation (conversation with Toven, Nov 2025):
+		// - Create ONE reasoning_details entry per assistant message with tool calls
+		// - Set `id` to the FIRST tool call's ID from the tool_calls array
+		// - Set `data` to "skip_thought_signature_validator" to bypass signature validation
+		// - Set `index` to 0
 		if (isNativeProtocol && isGemini) {
 			openAiMessages = openAiMessages.map((msg) => {
 				if (msg.role === "assistant") {
@@ -263,17 +269,19 @@ export class OpenRouterHandler extends BaseProvider implements SingleCompletionH
 						const hasEncrypted = existingDetails?.some((d) => d.type === "reasoning.encrypted") ?? false
 
 						if (!hasEncrypted) {
-							const fakeEncrypted = toolCalls.map((tc, idx) => ({
-								id: tc.id,
+							// Create ONE fake encrypted block with the FIRST tool call's ID
+							// This is the documented format from OpenRouter for skipping thought signature validation
+							const fakeEncrypted = {
 								type: "reasoning.encrypted",
 								data: "skip_thought_signature_validator",
+								id: toolCalls[0].id,
 								format: "google-gemini-v1",
-								index: (existingDetails?.length ?? 0) + idx,
-							}))
+								index: 0,
+							}
 
 							return {
 								...msg,
-								reasoning_details: [...(existingDetails ?? []), ...fakeEncrypted],
+								reasoning_details: [...(existingDetails ?? []), fakeEncrypted],
 							}
 						}
 					}