diff --git a/CHANGELOG.md b/CHANGELOG.md
index 8100eec1..ed8532ab 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -15,6 +15,8 @@ parallel copies under `docs/` or `scripts/notes/`. At cut time: rename
 
 ### Agent
 
+- **Thinking-only replay no longer collapses into an identical request.** Assistant turns with no text or tool_call (empty content, leftover thinking/citation) are replaced with a stable `[thinking-only turn omitted]` marker so the turn is kept, roles still alternate, and the next `buildRequest` body differs from the previous one.
+
 - **Compaction keeps scored work, not retry loops.** Errored tool results are no
   longer auto-pinned; identical errors collapse to one representative. Anchors
   are scored (writes, successful task completions, plan updates) and pair
diff --git a/src/provider/openai-compatible-adapter.ts b/src/provider/openai-compatible-adapter.ts
index ef007c0a..ed6bbf7a 100644
--- a/src/provider/openai-compatible-adapter.ts
+++ b/src/provider/openai-compatible-adapter.ts
@@ -28,23 +28,7 @@ export function createOpenAICompatibleAdapter(source: AdapterSource): ProviderAd
   };
 
   const buildRequest: ProviderAdapter["buildRequest"] = (messages, model, options) => {
-    // Strip assistant turns with no text or tool_call content (e.g. a turn that
-    // produced only thinking blocks). transform.ts should handle this but misses
-    // the case where filteredContent is non-empty; the API rejects such turns
-    // with HTTP 400. Fast-path: only allocate when a bad turn is actually found.
-    const needsSanitize = messages.some(
-      (msg) =>
-        msg.role === "assistant" &&
-        !msg.content.some((b) => b.type === "text" || b.type === "tool_call"),
-    );
-    const sanitized = needsSanitize
-      ? messages.filter(
-          (msg) =>
-            msg.role !== "assistant" ||
-            msg.content.some((b) => b.type === "text" || b.type === "tool_call"),
-        )
-      : messages;
-    const built = base.buildRequest(sanitized, model, options);
+    const built = base.buildRequest(messages, model, options);
     const providerOptions = options.providerOptions;
     const hasProviderOptions =
       providerOptions !== undefined && Object.keys(providerOptions).length > 0;
diff --git a/src/provider/replay-sanitizer.test.ts b/src/provider/replay-sanitizer.test.ts
index 2a8aea83..aa82a95c 100644
--- a/src/provider/replay-sanitizer.test.ts
+++ b/src/provider/replay-sanitizer.test.ts
@@ -1,7 +1,14 @@
 import { describe, expect, it } from "bun:test";
+import type { AdapterRegistry } from "@intx/inference";
 import { createBuiltinRegistry } from "@intx/inference/providers";
 import type { ConversationTurn, LastCycleSource } from "@intx/types/runtime";
-import { sanitizeReplayTurns, withReplaySanitizer } from "./replay-sanitizer.js";
+import { createGrokResponsesAdapter } from "./grok-responses-adapter.js";
+import { createOpenAICompatibleAdapter } from "./openai-compatible-adapter.js";
+import {
+  sanitizeReplayTurns,
+  THINKING_ONLY_OMITTED,
+  withReplaySanitizer,
+} from "./replay-sanitizer.js";
 
 const GROK_SIGNATURE = "grok-opaque-signature-blob";
 
@@ -33,6 +40,59 @@ function resolveSanitized(source: LastCycleSource) {
   return withReplaySanitizer(createBuiltinRegistry()).resolve(source);
 }
 
+function corbitsRegistry(): AdapterRegistry {
+  const builtin = createBuiltinRegistry();
+  return {
+    has: (provider) =>
+      provider === "openai-compatible" || provider === "grok-responses" || builtin.has(provider),
+    resolve(source, quirks) {
+      if (source.provider === "openai-compatible") {
+        return createOpenAICompatibleAdapter(source);
+      }
+      if (source.provider === "grok-responses") {
+        return createGrokResponsesAdapter(source);
+      }
+      return builtin.resolve(source, quirks);
+    },
+  };
+}
+
+function thinkingOnlyHistory(): ConversationTurn[] {
+  return [
+    {
+      role: "user",
+      content: [{ type: "text", text: "hello" }],
+      timestamp: 1,
+    },
+    {
+      role: "assistant",
+      model: "grok-4",
+      content: [{ type: "thinking", thinking: "pondering" }],
+      timestamp: 2,
+    },
+    {
+      role: "user",
+      content: [{ type: "text", text: "continue" }],
+      timestamp: 3,
+    },
+  ];
+}
+
+const USER_ONLY: ConversationTurn[] = [
+  {
+    role: "user",
+    content: [{ type: "text", text: "hello" }],
+    timestamp: 1,
+  },
+];
+
+const THINKING_ONLY_TAIL: ConversationTurn = {
+  role: "assistant",
+  model: "grok-4",
+  content: [{ type: "thinking", thinking: "pondering" }],
+  timestamp: 2,
+};
+
 describe("sanitizeReplayTurns", () => {
   it("strips foreign thinking blocks and signatures", () => {
     const turns = sanitizeReplayTurns(grokThinkingHistory(), "claude-opus-4");
@@ -101,6 +161,68 @@ describe("sanitizeReplayTurns", () => {
     expect(results).toHaveLength(1);
     expect(results[0]).toMatchObject({ callId: "call_1", isError: true });
   });
+
+  it("replaces a thinking-only assistant between users with a marker and keeps roles", () => {
+    const turns = sanitizeReplayTurns(thinkingOnlyHistory(), "claude-opus-4");
+    expect(turns.map((t) => t.role)).toEqual(["user", "assistant", "user"]);
+    expect(turns[1]?.content).toEqual([{ type: "text", text: THINKING_ONLY_OMITTED }]);
+  });
+
+  it("replaces empty and leftover-only assistant turns with the same marker", () => {
+    const empty = sanitizeReplayTurns(
+      [
+        {
+          role: "assistant",
+          model: "grok-4",
+          content: [],
+          timestamp: 1,
+        },
+      ],
+      "claude-opus-4",
+    );
+    expect(empty[0]?.content).toEqual([{ type: "text", text: THINKING_ONLY_OMITTED }]);
+
+    const leftovers = sanitizeReplayTurns(
+      [
+        {
+          role: "assistant",
+          model: "claude-opus-4",
+          content: [
+            { type: "thinking", thinking: "pondering" },
+            { type: "redacted_thinking", data: "opaque" },
+            { type: "citation", citedText: "quote", source: {} },
+          ],
+          timestamp: 1,
+        },
+      ],
+      "gemini-2.5-pro",
+    );
+    expect(leftovers[0]?.content).toEqual([{ type: "text", text: THINKING_ONLY_OMITTED }]);
+    expect(JSON.stringify(leftovers)).not.toContain("pondering");
+    expect(JSON.stringify(leftovers)).not.toContain("opaque");
+  });
+
+  it("leaves assistant turns with text and/or tool_call unchanged", () => {
+    const withText = sanitizeReplayTurns(grokThinkingHistory(), "grok-4");
+    expect(withText[1]?.content).toEqual([
+      { type: "thinking", thinking: "pondering", signature: GROK_SIGNATURE },
+      { type: "text", text: "answer", signature: GROK_SIGNATURE },
+    ]);
+
+    const withTool = [
+      {
+        role: "assistant" as const,
+        model: "grok-4",
+        content: [
+          { type: "thinking" as const, thinking: "need a tool" },
+          { type: "tool_call" as const, id: "call_1", name: "ls", arguments: {} },
+        ],
+        timestamp: 1,
+      },
+    ];
+    const kept = sanitizeReplayTurns(withTool, "grok-4");
+    expect(kept[0]?.content).toEqual(withTool[0]?.content);
+  });
 });
 
 describe("withReplaySanitizer", () => {
@@ -186,4 +308,53 @@ describe("withReplaySanitizer", () => {
     expect(request.body).toContain("tool_result");
     expect(request.body).toContain("call_1");
   });
+
+  it("changes buildRequest bodies after a thinking-only turn for builtin and Corbits adapters", () => {
+    const sanitized = withReplaySanitizer(corbitsRegistry());
+    const cases: LastCycleSource[] = [
+      { sourceId: "s1", provider: "anthropic", model: "claude-opus-4" },
+      { sourceId: "s1", provider: "google-genai", model: "gemini-2.5-pro" },
+      { sourceId: "s1", provider: "openai", model: "gpt-5" },
+      { sourceId: "s1", provider: "openai-compatible", model: "kimi-k2" },
+      { sourceId: "s1", provider: "grok-responses", model: "grok-4.5" },
+    ];
+    for (const source of cases) {
+      const adapter = sanitized.resolve(source);
+      const without = adapter.buildRequest(USER_ONLY, source.model, {});
+      const withThinking = adapter.buildRequest(
+        [...USER_ONLY, THINKING_ONLY_TAIL],
+        source.model,
+        {},
+      );
+      expect(withThinking.body).not.toEqual(without.body);
+      expect(withThinking.body).toContain(THINKING_ONLY_OMITTED);
+    }
+  });
+
+  it("builds requests for thinking-only and leftover-only assistant turns", () => {
+    const adapter = resolveSanitized({
+      sourceId: "s1",
+      provider: "anthropic",
+      model: "claude-opus-4",
+    });
+    const leftoverHistory: ConversationTurn[] = [
+      {
+        role: "user",
+        content: [{ type: "text", text: "hello" }],
+        timestamp: 1,
+      },
+      {
+        role: "assistant",
+        model: "grok-4",
+        content: [
+          { type: "thinking", thinking: "pondering" },
+          { type: "redacted_thinking", data: "opaque" },
+          { type: "citation", citedText: "quote", source: {} },
+        ],
+        timestamp: 2,
+      },
+    ];
+    expect(() => adapter.buildRequest(thinkingOnlyHistory(), "claude-opus-4", {})).not.toThrow();
+    expect(() => adapter.buildRequest(leftoverHistory, "claude-opus-4", {})).not.toThrow();
+  });
 });
diff --git a/src/provider/replay-sanitizer.ts b/src/provider/replay-sanitizer.ts
index a4d3639b..9da27f01 100644
--- a/src/provider/replay-sanitizer.ts
+++ b/src/provider/replay-sanitizer.ts
@@ -11,6 +11,8 @@ import type { ContentBlock, ConversationTurn } from "@intx/types/runtime";
 // audio, video, code execution) that make adapter builders throw, and opaque
 // provider signatures that a foreign provider rejects when echoed back.
 
+export const THINKING_ONLY_OMITTED = "[thinking-only turn omitted]";
+
 // Output-only shapes a foreign provider cannot round-trip; adapter builders
 // throw on them, so they are dropped from foreign-model turns before build.
 const FOREIGN_UNMAPPABLE_TYPES = new Set<ContentBlock["type"]>([
@@ -43,6 +45,21 @@ function stripForeignBlocks(turn: ConversationTurn): ConversationTurn {
   return { ...turn, content };
 }
 
+function hasTextOrToolCall(content: ContentBlock[]): boolean {
+  return content.some((block) => block.type === "text" || block.type === "tool_call");
+}
+
+// transformMessages drops an assistant turn only when stripping thinking
+// leaves empty content. Same-model thinking-only and leftover-only turns
+// survive with no text/tool_call; adapters then 400 or used to drop them,
+// producing an identical next request and a thinking-only loop. Replace
+// the unusable turn with a stable text marker so the turn stays, roles
+// alternate, and the wire body changes.
+function replaceUnusableAssistantTurn(turn: ConversationTurn): ConversationTurn {
+  if (turn.role !== "assistant" || hasTextOrToolCall(turn.content)) return turn;
+  return { ...turn, content: [{ type: "text", text: THINKING_ONLY_OMITTED }] };
+}
+
 /**
  * Repair persisted turns for replay against `targetModel`. Assistant turns
  * produced by a different model lose blocks the target provider cannot
@@ -52,10 +69,11 @@ export function sanitizeReplayTurns(
   turns: ConversationTurn[],
   targetModel: string,
 ): ConversationTurn[] {
-  const repaired = turns.map((turn) =>
+  const stripped = turns.map((turn) =>
     turn.role === "assistant" && turn.model !== targetModel ? stripForeignBlocks(turn) : turn,
   );
-  return transformMessages(repaired, { targetModel });
+  const marked = stripped.map(replaceUnusableAssistantTurn);
+  return transformMessages(marked, { targetModel });
 }
 
 /**