diff --git a/src/core/task/__tests__/appendEnvironmentDetails.spec.ts b/src/core/task/__tests__/appendEnvironmentDetails.spec.ts
new file mode 100644
index 00000000000..54ccb1ee1ed
--- /dev/null
+++ b/src/core/task/__tests__/appendEnvironmentDetails.spec.ts
@@ -0,0 +1,414 @@
+import { Anthropic } from "@anthropic-ai/sdk"
+import {
+	appendEnvironmentDetails,
+	removeEnvironmentDetailsBlocks,
+	stripAppendedEnvironmentDetails,
+	UserContentBlock,
+} from "../appendEnvironmentDetails"
+
+describe("appendEnvironmentDetails", () => {
+	const envDetails = "<environment_details>\n# Test\nSome details\n</environment_details>"
+
+	describe("empty content", () => {
+		it("should return a text block when content is empty", () => {
+			const result = appendEnvironmentDetails([], envDetails)
+
+			expect(result).toHaveLength(1)
+			expect(result[0].type).toBe("text")
+			expect((result[0] as Anthropic.Messages.TextBlockParam).text).toBe(envDetails)
+		})
+	})
+
+	describe("text block handling", () => {
+		it("should append to the last text block", () => {
+			const content: UserContentBlock[] = [{ type: "text", text: "User message" }]
+
+			const result = appendEnvironmentDetails(content, envDetails)
+
+			expect(result).toHaveLength(1)
+			expect(result[0].type).toBe("text")
+			expect((result[0] as Anthropic.Messages.TextBlockParam).text).toBe("User message\n\n" + envDetails)
+		})
+
+		it("should append to the last text block when multiple text blocks exist", () => {
+			const content: UserContentBlock[] = [
+				{ type: "text", text: "First message" },
+				{ type: "text", text: "Second message" },
+			]
+
+			const result = appendEnvironmentDetails(content, envDetails)
+
+			expect(result).toHaveLength(2)
+			expect((result[0] as Anthropic.Messages.TextBlockParam).text).toBe("First message")
+			expect((result[1] as Anthropic.Messages.TextBlockParam).text).toBe("Second message\n\n" + envDetails)
+		})
+
+		it("should not mutate the original content array", () => {
+			const content: UserContentBlock[] = [{ type: "text", text: "Original" }]
+
+			appendEnvironmentDetails(content, envDetails)
+
+			expect((content[0] as Anthropic.Messages.TextBlockParam).text).toBe("Original")
+		})
+	})
+
+	describe("tool_result block handling", () => {
+		it("should append to tool_result with string content", () => {
+			const content: UserContentBlock[] = [
+				{
+					type: "tool_result",
+					tool_use_id: "tool-123",
+					content: "Tool result text",
+				},
+			]
+
+			const result = appendEnvironmentDetails(content, envDetails)
+
+			expect(result).toHaveLength(1)
+			expect(result[0].type).toBe("tool_result")
+			const toolResult = result[0] as Anthropic.Messages.ToolResultBlockParam
+			expect(toolResult.content).toBe("Tool result text\n\n" + envDetails)
+			expect(toolResult.tool_use_id).toBe("tool-123")
+		})
+
+		it("should append to tool_result with undefined content", () => {
+			const content: UserContentBlock[] = [
+				{
+					type: "tool_result",
+					tool_use_id: "tool-123",
+				} as Anthropic.Messages.ToolResultBlockParam,
+			]
+
+			const result = appendEnvironmentDetails(content, envDetails)
+
+			expect(result).toHaveLength(1)
+			const toolResult = result[0] as Anthropic.Messages.ToolResultBlockParam
+			expect(toolResult.content).toBe(envDetails)
+		})
+
+		it("should append to tool_result with array content containing text", () => {
+			const content: UserContentBlock[] = [
+				{
+					type: "tool_result",
+					tool_use_id: "tool-123",
+					content: [{ type: "text", text: "Result line 1" }],
+				},
+			]
+
+			const result = appendEnvironmentDetails(content, envDetails)
+
+			expect(result).toHaveLength(1)
+			const toolResult = result[0] as Anthropic.Messages.ToolResultBlockParam
+			expect(Array.isArray(toolResult.content)).toBe(true)
+			const contentArray = toolResult.content as Anthropic.Messages.TextBlockParam[]
+			expect(contentArray).toHaveLength(1)
+			expect(contentArray[0].text).toBe("Result line 1\n\n" + envDetails)
+		})
+
+		it("should add text block to tool_result with array content containing only images", () => {
+			const content: UserContentBlock[] = [
+				{
+					type: "tool_result",
+					tool_use_id: "tool-123",
+					content: [
+						{
+							type: "image",
+							source: { type: "base64", media_type: "image/png", data: "abc123" },
+						},
+					],
+				},
+			]
+
+			const result = appendEnvironmentDetails(content, envDetails)
+
+			expect(result).toHaveLength(1)
+			const toolResult = result[0] as Anthropic.Messages.ToolResultBlockParam
+			expect(Array.isArray(toolResult.content)).toBe(true)
+			const contentArray = toolResult.content as Array<
+				Anthropic.Messages.TextBlockParam | Anthropic.Messages.ImageBlockParam
+			>
+			expect(contentArray).toHaveLength(2)
+			expect(contentArray[0].type).toBe("image")
+			expect(contentArray[1].type).toBe("text")
+			expect((contentArray[1] as Anthropic.Messages.TextBlockParam).text).toBe(envDetails)
+		})
+
+		it("should append to the last text block in tool_result array content", () => {
+			const content: UserContentBlock[] = [
+				{
+					type: "tool_result",
+					tool_use_id: "tool-123",
+					content: [
+						{ type: "text", text: "First text" },
+						{
+							type: "image",
+							source: { type: "base64", media_type: "image/png", data: "abc123" },
+						},
+						{ type: "text", text: "Last text" },
+					],
+				},
+			]
+
+			const result = appendEnvironmentDetails(content, envDetails)
+
+			const toolResult = result[0] as Anthropic.Messages.ToolResultBlockParam
+			const contentArray = toolResult.content as Array<
+				Anthropic.Messages.TextBlockParam | Anthropic.Messages.ImageBlockParam
+			>
+			expect(contentArray).toHaveLength(3)
+			expect((contentArray[0] as Anthropic.Messages.TextBlockParam).text).toBe("First text")
+			expect((contentArray[2] as Anthropic.Messages.TextBlockParam).text).toBe("Last text\n\n" + envDetails)
+		})
+
+		it("should preserve is_error flag on tool_result", () => {
+			const content: UserContentBlock[] = [
+				{
+					type: "tool_result",
+					tool_use_id: "tool-123",
+					content: "Error message",
+					is_error: true,
+				},
+			]
+
+			const result = appendEnvironmentDetails(content, envDetails)
+
+			const toolResult = result[0] as Anthropic.Messages.ToolResultBlockParam
+			expect(toolResult.is_error).toBe(true)
+			expect(toolResult.content).toBe("Error message\n\n" + envDetails)
+		})
+	})
+
+	describe("mixed content handling", () => {
+		it("should append to last text block when text comes after tool_result", () => {
+			const content: UserContentBlock[] = [
+				{
+					type: "tool_result",
+					tool_use_id: "tool-123",
+					content: "Tool result",
+				},
+				{ type: "text", text: "User comment" },
+			]
+
+			const result = appendEnvironmentDetails(content, envDetails)
+
+			expect(result).toHaveLength(2)
+			expect((result[0] as Anthropic.Messages.ToolResultBlockParam).content).toBe("Tool result")
+			expect((result[1] as Anthropic.Messages.TextBlockParam).text).toBe("User comment\n\n" + envDetails)
+		})
+
+		it("should append to last tool_result when no text block follows", () => {
+			const content: UserContentBlock[] = [
+				{ type: "text", text: "User message" },
+				{
+					type: "tool_result",
+					tool_use_id: "tool-123",
+					content: "Tool result",
+				},
+			]
+
+			const result = appendEnvironmentDetails(content, envDetails)
+
+			expect(result).toHaveLength(2)
+			expect((result[0] as Anthropic.Messages.TextBlockParam).text).toBe("User message")
+			expect((result[1] as Anthropic.Messages.ToolResultBlockParam).content).toBe("Tool result\n\n" + envDetails)
+		})
+
+		it("should add new text block when content only has images", () => {
+			const content: UserContentBlock[] = [
+				{
+					type: "image",
+					source: { type: "base64", media_type: "image/png", data: "abc123" },
+				},
+			]
+
+			const result = appendEnvironmentDetails(content, envDetails)
+
+			expect(result).toHaveLength(2)
+			expect(result[0].type).toBe("image")
+			expect(result[1].type).toBe("text")
+			expect((result[1] as Anthropic.Messages.TextBlockParam).text).toBe(envDetails)
+		})
+
+		it("should handle multiple tool_results and append to the last one", () => {
+			const content: UserContentBlock[] = [
+				{
+					type: "tool_result",
+					tool_use_id: "tool-1",
+					content: "First result",
+				},
+				{
+					type: "tool_result",
+					tool_use_id: "tool-2",
+					content: "Second result",
+				},
+			]
+
+			const result = appendEnvironmentDetails(content, envDetails)
+
+			expect(result).toHaveLength(2)
+			expect((result[0] as Anthropic.Messages.ToolResultBlockParam).content).toBe("First result")
+			expect((result[1] as Anthropic.Messages.ToolResultBlockParam).content).toBe(
+				"Second result\n\n" + envDetails,
+			)
+		})
+	})
+})
+
+describe("removeEnvironmentDetailsBlocks", () => {
+	const envDetailsBlock: UserContentBlock = {
+		type: "text",
+		text: "<environment_details>\n# Test\nSome details\n</environment_details>",
+	}
+
+	it("should remove standalone environment_details text blocks", () => {
+		const content: UserContentBlock[] = [{ type: "text", text: "User message" }, envDetailsBlock]
+
+		const result = removeEnvironmentDetailsBlocks(content)
+
+		expect(result).toHaveLength(1)
+		expect((result[0] as Anthropic.Messages.TextBlockParam).text).toBe("User message")
+	})
+
+	it("should not remove text blocks that mention environment_details but aren't complete blocks", () => {
+		const content: UserContentBlock[] = [
+			{ type: "text", text: "Let me explain <environment_details> tags" },
+			{ type: "text", text: "The closing tag is </environment_details>" },
+		]
+
+		const result = removeEnvironmentDetailsBlocks(content)
+
+		expect(result).toHaveLength(2)
+	})
+
+	it("should preserve non-text blocks", () => {
+		const content: UserContentBlock[] = [
+			{
+				type: "tool_result",
+				tool_use_id: "tool-123",
+				content: "Result",
+			},
+			envDetailsBlock,
+			{
+				type: "image",
+				source: { type: "base64", media_type: "image/png", data: "abc123" },
+			},
+		]
+
+		const result = removeEnvironmentDetailsBlocks(content)
+
+		expect(result).toHaveLength(2)
+		expect(result[0].type).toBe("tool_result")
+		expect(result[1].type).toBe("image")
+	})
+
+	it("should handle empty content", () => {
+		const result = removeEnvironmentDetailsBlocks([])
+		expect(result).toHaveLength(0)
+	})
+
+	it("should handle whitespace around environment_details tags", () => {
+		const content: UserContentBlock[] = [
+			{
+				type: "text",
+				text: "  <environment_details>\n# Test\nSome details\n</environment_details>  ",
+			},
+		]
+
+		const result = removeEnvironmentDetailsBlocks(content)
+
+		expect(result).toHaveLength(0)
+	})
+})
+
+describe("stripAppendedEnvironmentDetails", () => {
+	const envDetails = "<environment_details>\n# Test\nSome details\n</environment_details>"
+
+	it("should strip environment details from the end of a text block", () => {
+		const content: UserContentBlock[] = [{ type: "text", text: "User message\n\n" + envDetails }]
+
+		const result = stripAppendedEnvironmentDetails(content)
+
+		expect(result).toHaveLength(1)
+		expect((result[0] as Anthropic.Messages.TextBlockParam).text).toBe("User message")
+	})
+
+	it("should strip environment details from tool_result string content", () => {
+		const content: UserContentBlock[] = [
+			{
+				type: "tool_result",
+				tool_use_id: "tool-123",
+				content: "Tool result\n\n" + envDetails,
+			},
+		]
+
+		const result = stripAppendedEnvironmentDetails(content)
+
+		expect(result).toHaveLength(1)
+		expect((result[0] as Anthropic.Messages.ToolResultBlockParam).content).toBe("Tool result")
+	})
+
+	it("should strip environment details from tool_result array content", () => {
+		const content: UserContentBlock[] = [
+			{
+				type: "tool_result",
+				tool_use_id: "tool-123",
+				content: [{ type: "text", text: "Result text\n\n" + envDetails }],
+			},
+		]
+
+		const result = stripAppendedEnvironmentDetails(content)
+
+		const toolResult = result[0] as Anthropic.Messages.ToolResultBlockParam
+		const contentArray = toolResult.content as Anthropic.Messages.TextBlockParam[]
+		expect(contentArray[0].text).toBe("Result text")
+	})
+
+	it("should also remove standalone environment_details blocks", () => {
+		const content: UserContentBlock[] = [
+			{ type: "text", text: "User message" },
+			{ type: "text", text: envDetails },
+		]
+
+		const result = stripAppendedEnvironmentDetails(content)
+
+		expect(result).toHaveLength(1)
+		expect((result[0] as Anthropic.Messages.TextBlockParam).text).toBe("User message")
+	})
+
+	it("should handle content without environment details", () => {
+		const content: UserContentBlock[] = [
+			{ type: "text", text: "User message" },
+			{
+				type: "tool_result",
+				tool_use_id: "tool-123",
+				content: "Tool result",
+			},
+		]
+
+		const result = stripAppendedEnvironmentDetails(content)
+
+		expect(result).toEqual(content)
+	})
+
+	it("should handle empty content", () => {
+		const result = stripAppendedEnvironmentDetails([])
+		expect(result).toHaveLength(0)
+	})
+
+	it("should preserve is_error flag when stripping from tool_result", () => {
+		const content: UserContentBlock[] = [
+			{
+				type: "tool_result",
+				tool_use_id: "tool-123",
+				content: "Error\n\n" + envDetails,
+				is_error: true,
+			},
+		]
+
+		const result = stripAppendedEnvironmentDetails(content)
+
+		const toolResult = result[0] as Anthropic.Messages.ToolResultBlockParam
+		expect(toolResult.is_error).toBe(true)
+		expect(toolResult.content).toBe("Error")
+	})
+})
diff --git a/src/core/task/appendEnvironmentDetails.ts b/src/core/task/appendEnvironmentDetails.ts
new file mode 100644
index 00000000000..09495d89e7f
--- /dev/null
+++ b/src/core/task/appendEnvironmentDetails.ts
@@ -0,0 +1,238 @@
+import { Anthropic } from "@anthropic-ai/sdk"
+
+/**
+ * Type for content block params that can appear in messages.
+ * Using the broader ContentBlockParam type to handle all possible block types.
+ */
+export type UserContentBlock = Anthropic.Messages.ContentBlockParam
+
+/**
+ * Appends environment details to the last text block or tool_result block in user content.
+ * This avoids creating a standalone trailing text block, which can break interleaved-thinking
+ * models like DeepSeek reasoner that expect specific message shapes.
+ *
+ * Priority:
+ * 1. If the last block is a text block, append to it
+ * 2. If the last block is a tool_result, append to its content
+ * 3. If no suitable block found, add as a new text block (fallback)
+ *
+ * @param content - Array of content blocks from a user message
+ * @param environmentDetails - The environment details string to append
+ * @returns New array with environment details appended to the appropriate block
+ */
+export function appendEnvironmentDetails(content: UserContentBlock[], environmentDetails: string): UserContentBlock[] {
+	if (content.length === 0) {
+		// No existing content, just return the environment details as a text block
+		return [{ type: "text" as const, text: environmentDetails }]
+	}
+
+	// Create a shallow copy so we don't mutate the original array
+	const result = [...content]
+
+	// Find the last suitable block (text or tool_result)
+	let lastSuitableIndex = -1
+	for (let i = result.length - 1; i >= 0; i--) {
+		const block = result[i]
+		if (block.type === "text" || block.type === "tool_result") {
+			lastSuitableIndex = i
+			break
+		}
+	}
+
+	if (lastSuitableIndex === -1) {
+		// No text or tool_result block found (content only has images?), add new text block
+		result.push({ type: "text" as const, text: environmentDetails })
+		return result
+	}
+
+	const lastBlock = result[lastSuitableIndex]
+
+	if (lastBlock.type === "text") {
+		// Append to existing text block
+		result[lastSuitableIndex] = {
+			type: "text" as const,
+			text: lastBlock.text + "\n\n" + environmentDetails,
+		}
+	} else if (lastBlock.type === "tool_result") {
+		// Append to tool_result content
+		result[lastSuitableIndex] = appendToToolResult(lastBlock, environmentDetails)
+	}
+
+	return result
+}
+
+/**
+ * Appends text to a tool_result block's content.
+ * Tool result content can be a string or an array of content blocks.
+ */
+function appendToToolResult(
+	toolResult: Anthropic.Messages.ToolResultBlockParam,
+	textToAppend: string,
+): Anthropic.Messages.ToolResultBlockParam {
+	const { content, ...rest } = toolResult
+
+	if (content === undefined || content === null) {
+		// No existing content, just set the text
+		return {
+			...rest,
+			content: textToAppend,
+		}
+	}
+
+	if (typeof content === "string") {
+		// String content, just concatenate
+		return {
+			...rest,
+			content: content + "\n\n" + textToAppend,
+		}
+	}
+
+	if (Array.isArray(content)) {
+		// Array content - find the last text block and append, or add new text block
+		const contentCopy = [...content]
+		let lastTextIndex = -1
+
+		for (let i = contentCopy.length - 1; i >= 0; i--) {
+			if (contentCopy[i].type === "text") {
+				lastTextIndex = i
+				break
+			}
+		}
+
+		if (lastTextIndex >= 0) {
+			// Append to last text block in array
+			const lastTextBlock = contentCopy[lastTextIndex] as Anthropic.Messages.TextBlockParam
+			contentCopy[lastTextIndex] = {
+				type: "text" as const,
+				text: lastTextBlock.text + "\n\n" + textToAppend,
+			}
+		} else {
+			// No text block in array, add new one
+			contentCopy.push({ type: "text" as const, text: textToAppend })
+		}
+
+		return {
+			...rest,
+			content: contentCopy,
+		}
+	}
+
+	// Unknown content type, return with text appended as new content
+	return {
+		...rest,
+		content: textToAppend,
+	}
+}
+
+/**
+ * Removes any existing environment_details blocks from the content array.
+ * A block is considered an environment_details block if it's a text block
+ * that starts with <environment_details> and ends with </environment_details>.
+ *
+ * @param content - Array of content blocks to filter
+ * @returns New array with environment_details blocks removed
+ */
+export function removeEnvironmentDetailsBlocks(content: UserContentBlock[]): UserContentBlock[] {
+	return content.filter((block) => {
+		if (block.type === "text" && typeof block.text === "string") {
+			const trimmed = block.text.trim()
+			const isEnvironmentDetailsBlock =
+				trimmed.startsWith("<environment_details>") && trimmed.endsWith("</environment_details>")
+			return !isEnvironmentDetailsBlock
+		}
+		return true
+	})
+}
+
+/**
+ * Strips environment details from the last text block or tool_result in the content.
+ * This handles the case where environment details were appended to an existing block
+ * rather than added as a standalone block.
+ *
+ * @param content - Array of content blocks
+ * @returns New array with environment details stripped from the last suitable block
+ */
+export function stripAppendedEnvironmentDetails(content: UserContentBlock[]): UserContentBlock[] {
+	if (content.length === 0) {
+		return content
+	}
+
+	// First, remove any standalone environment_details blocks
+	let result = removeEnvironmentDetailsBlocks(content)
+
+	if (result.length === 0) {
+		return result
+	}
+
+	// Then, strip appended environment details from the last block
+	const lastIndex = result.length - 1
+	const lastBlock = result[lastIndex]
+
+	if (lastBlock.type === "text") {
+		const strippedText = stripEnvDetailsFromText(lastBlock.text)
+		if (strippedText !== lastBlock.text) {
+			result = [...result]
+			result[lastIndex] = { type: "text" as const, text: strippedText }
+		}
+	} else if (lastBlock.type === "tool_result") {
+		const strippedToolResult = stripEnvDetailsFromToolResult(lastBlock)
+		if (strippedToolResult !== lastBlock) {
+			result = [...result]
+			result[lastIndex] = strippedToolResult
+		}
+	}
+
+	return result
+}
+
+/**
+ * Strips environment details from the end of a text string.
+ */
+function stripEnvDetailsFromText(text: string): string {
+	// Match environment details at the end of the string, with optional preceding newlines
+	const envDetailsPattern = /\n*<environment_details>[\s\S]*<\/environment_details>\s*$/
+	return text.replace(envDetailsPattern, "")
+}
+
+/**
+ * Strips environment details from a tool_result block's content.
+ */
+function stripEnvDetailsFromToolResult(
+	toolResult: Anthropic.Messages.ToolResultBlockParam,
+): Anthropic.Messages.ToolResultBlockParam {
+	const { content, ...rest } = toolResult
+
+	if (content === undefined || content === null) {
+		return toolResult
+	}
+
+	if (typeof content === "string") {
+		const strippedContent = stripEnvDetailsFromText(content)
+		if (strippedContent === content) {
+			return toolResult
+		}
+		return { ...rest, content: strippedContent }
+	}
+
+	if (Array.isArray(content)) {
+		let changed = false
+		const newContent = content.map((block) => {
+			if (block.type === "text") {
+				const strippedText = stripEnvDetailsFromText(block.text)
+				if (strippedText !== block.text) {
+					changed = true
+					return { type: "text" as const, text: strippedText }
+				}
+			}
+			return block
+		})
+
+		if (!changed) {
+			return toolResult
+		}
+
+		return { ...rest, content: newContent }
+	}
+
+	return toolResult
+}