LLM error invalid_request_error: messages.N.content.M: thinking or
redacted_thinking blocks in the latest assistant message cannot be
modified. These blocks must remain as they were in the original response.

---

if (block.type === "thinking") {
    if (block.redacted) { /* ... */ }
    if (block.thinking.trim().length === 0) continue;
    if (!block.thinkingSignature || block.thinkingSignature.trim().length === 0) {
        blocks.push({
            type: "text",
            text: sanitizeTransportPayloadText(block.thinking)
        });
    } else {
        const thinking = sanitizeTransportPayloadText(block.thinking);  // ← BUG
        if (block.thinkingSignature === "reasoning_content") {
            // ... allowReasoningContentReplay path ...
        }
        blocks.push({
            type: "thinking",
            thinking,                              // ← mutated text
            signature: block.thinkingSignature     // ← original signature for unmutated text
        });
    }
}

---

function sanitizeTransportPayloadText(text) {
    return text.replace(/[\uD800-\uDBFF](?![\uDC00-\uDFFF])|(?<![\uD800-\uDBFF])[\uDC00-\uDFFF]/g, "");
}

---

if (block.type === "thinking") {
                    if (block.redacted) { /* ... */ }
                    if (block.thinking.trim().length === 0) continue;
                    if (!block.thinkingSignature || block.thinkingSignature.trim().length === 0) {
                        blocks.push({
                            type: "text",
                            text: sanitizeTransportPayloadText(block.thinking)
                        });
                    } else {
-                       const thinking = sanitizeTransportPayloadText(block.thinking);
+                       // Do NOT sanitize Anthropic-signed thinking blocks. Sanitization mutates
+                       // the text and invalidates the cryptographic signature attached by Anthropic.
+                       // Only sanitize the synthetic "reasoning_content" signature variant.
+                       const thinking = block.thinkingSignature === "reasoning_content"
+                           ? sanitizeTransportPayloadText(block.thinking)
+                           : block.thinking;
                        if (block.thinkingSignature === "reasoning_content") {
                            // ...
                        }
                        blocks.push({
                            type: "thinking",
                            thinking,
                            signature: block.thinkingSignature
                        });
                    }
                }

---

// tests/provider-stream-thinking-signature.test.ts
import { describe, it, expect } from "vitest";
import { transformTransportMessages } from "../src/agents/provider-stream";

describe("thinking block signature preservation", () => {
    it("does not mutate Anthropic-signed thinking content", () => {
        const input = {
            role: "assistant",
            content: [{
                type: "thinking",
                thinking: "Let me think about 🤔 this problem",  // emoji is a surrogate pair
                thinkingSignature: "ANTHROPIC_REAL_SIGNATURE_BASE64",
            }]
        };
        const output = transformTransportMessages([input], { provider: "anthropic" });
        expect(output[0].content[0].thinking).toBe("Let me think about 🤔 this problem");
    });

    it("does sanitize reasoning_content variant (non-Anthropic)", () => {
        const input = {
            role: "assistant",
            content: [{
                type: "thinking",
                thinking: "broken\uD800surrogate",  // lone surrogate
                thinkingSignature: "reasoning_content",
            }]
        };
        const output = transformTransportMessages([input], { provider: "xai" });
        expect(output[0].content[0].thinking).toBe("brokensurrogate");
    });
});