fix(provider): substitute space for empty text between signed reasoning blocks

Anthropic adaptive thinking (Opus 4.6+) emits empty text parts between
thinking blocks. Dropping them invalidates thinking block signatures
('thinking blocks cannot be modified'), but sending them as '' fails
validation ('text content blocks must be non-empty'). Substitute a
single space so the part survives all filters and Anthropic accepts it.

Gated on reasoning having an Anthropic signature (metadata.anthropic.
signature != null) so other providers' reasoning — which don't
position-validate — continue to have empty text filtered normally.

Closes #16748

Author: edevil

packages/opencode/src/session/message-v2.ts

@@ -825,13 +825,21 @@ export const toModelMessagesEffect = Effect.fnUntraced(function* (
         role: "assistant",
         parts: [],
       }
+      // Substitute a space for empty text between signed Anthropic reasoning
+      // blocks to keep thinking block positions (and signatures) valid without
+      // triggering Anthropic's empty-content rejection or the AI SDK's filter.
+      const hasSignedReasoning = msg.parts.some(
+        (p) => p.type === "reasoning" && (p.metadata as any)?.anthropic?.signature != null,
+      )
       for (const part of msg.parts) {
-        if (part.type === "text")
+        if (part.type === "text") {
+          const text = part.text === "" && hasSignedReasoning ? " " : part.text
           assistantMessage.parts.push({
             type: "text",
-            text: part.text,
+            text,
             ...(differentModel ? {} : { providerMetadata: part.metadata }),
           })
+        }
         if (part.type === "step-start")
           assistantMessage.parts.push({
             type: "step-start",

packages/opencode/test/session/message-v2.test.ts

@@ -947,6 +947,82 @@ describe("session.message-v2.toModelMessage", () => {
       },
     ])
   })
+
+  test("substitutes space for empty text between signed reasoning blocks", async () => {
+    // Reproduces the bug pattern: [reasoning(sig), text(""), reasoning(sig), text(full)]
+    const assistantID = "m-assistant"
+    const input: MessageV2.WithParts[] = [
+      {
+        info: assistantInfo(assistantID, "m-parent"),
+        parts: [
+          { ...basePart(assistantID, "p1"), type: "step-start" },
+          {
+            ...basePart(assistantID, "p2"),
+            type: "reasoning",
+            text: "thinking-one",
+            metadata: { anthropic: { signature: "sig1" } },
+          },
+          { ...basePart(assistantID, "p3"), type: "text", text: "" },
+          { ...basePart(assistantID, "p4"), type: "step-start" },
+          {
+            ...basePart(assistantID, "p5"),
+            type: "reasoning",
+            text: "thinking-two",
+            metadata: { anthropic: { signature: "sig2" } },
+          },
+          { ...basePart(assistantID, "p6"), type: "text", text: "the answer" },
+        ] as MessageV2.Part[],
+      },
+    ]
+
+    const result = await MessageV2.toModelMessages(input, model)
+
+    // step-start splits into two assistant messages; SDK's groupIntoBlocks merges them later
+    expect(result).toHaveLength(2)
+    expect((result[0].content as any[]).find((p) => p.type === "text").text).toBe(" ")
+    expect((result[1].content as any[]).find((p) => p.type === "text").text).toBe("the answer")
+  })
+
+  test("leaves empty text alone when reasoning has no Anthropic signature", async () => {
+    // Non-Anthropic providers' reasoning doesn't position-validate, so empty text
+    // should be filtered normally rather than substituted.
+    const assistantID = "m-assistant-unsigned"
+    const input: MessageV2.WithParts[] = [
+      {
+        info: assistantInfo(assistantID, "m-parent"),
+        parts: [
+          { ...basePart(assistantID, "p1"), type: "reasoning", text: "thinking" },
+          { ...basePart(assistantID, "p2"), type: "text", text: "" },
+          { ...basePart(assistantID, "p3"), type: "text", text: "answer" },
+        ] as MessageV2.Part[],
+      },
+    ]
+
+    const result = await MessageV2.toModelMessages(input, model)
+
+    expect(result).toHaveLength(1)
+    const texts = (result[0].content as any[]).filter((p) => p.type === "text")
+    expect(texts.map((t) => t.text)).toStrictEqual(["", "answer"])
+  })
+
+  test("leaves empty text alone in assistant messages without reasoning", async () => {
+    const assistantID = "m-assistant-no-reasoning"
+    const input: MessageV2.WithParts[] = [
+      {
+        info: assistantInfo(assistantID, "m-parent"),
+        parts: [
+          { ...basePart(assistantID, "p1"), type: "text", text: "" },
+          { ...basePart(assistantID, "p2"), type: "text", text: "hello" },
+        ] as MessageV2.Part[],
+      },
+    ]
+
+    const result = await MessageV2.toModelMessages(input, model)
+
+    expect(result).toHaveLength(1)
+    const texts = (result[0].content as any[]).filter((p) => p.type === "text")
+    expect(texts.map((t) => t.text)).toStrictEqual(["", "hello"])
+  })
 })
 
 describe("session.message-v2.fromError", () => {